Optically and non-optically excited thermography for composites: A review

Infrared Physics & Technology 75 (2016) 26–50

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Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared

Review

Optically and non-optically excited thermography for composites: A review Ruizhen Yang a,⇑,1, Yunze He b,⇑ a b

Department of Civil and Architectural Engineering, Changsha University, Changsha 410003, PR China College of Mechatronics Engineering and Automation, National University of Defense Technology, Changsha 410073, PR China

h i g h l i g h t s
A fully and comprehensive review of thermography for composites was conducted.
Optic, laser, eddy current, microwave, and ultrasound thermography were reviewed.
Some case studies for scanning thermography were reviewed and analyzed.
Strengths and limitations of thermography were concluded through comparison study.
Some research trends for thermography were predicted.

a r t i c l e

i n f o

Article history: Received 28 August 2015 Available online 23 January 2016 Keywords: Frequency modulated thermography Laser thermography Microwave thermography Eddy current thermography Ultrasound thermography Thermal wave radar

a b s t r a c t Composites, such as glass fiber reinforced polymer (GFRP) and carbon fiber reinforced polymer (CFRP), and adhesive bonding are being increasingly used in fields of aerospace, renewable energy, civil and architecture, and other industries. Flaws and damages are inevitable during either fabrication or lifetime of composites structures or components. Thus, nondestructive testing (NDT) are extremely required to prevent failures and to increase reliability of composite structures or components in both manufacture and in-service inspection. Infrared thermography techniques including pulsed thermography, pulsed phase thermography, and lock-in thermography have shown the great potential and advantages. Besides conventional optical thermography, other sources such as laser, eddy current, microwave, and ultrasound excited thermography are drawing increasingly attentions for composites. In this work, a fully, in-depth and comprehensive review of thermography NDT techniques for composites inspection was conducted based on an orderly and concise literature survey and detailed analysis. Firstly, basic concepts for thermography NDT were defined and introduced, such as volume heating thermography. Next, the developments of conventional optic, laser, eddy current, microwave, and ultrasound thermography for composite inspection were reviewed. Then, some case studies for scanning thermography were also reviewed. After that, the strengths and limitations of thermography techniques were concluded through comparison studies. At last, some research trends were predicted. This work containing critical overview, detailed comparison and extensive list of references will disseminates knowledge between users, manufacturers, designers and researchers involved in composite structures or components inspection by means of thermography NDT techniques. Ó 2016 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Basic concepts of thermography NDT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Passive thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Classification of active thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. Tel.: +86 13467698133. 1

E-mail addresses: [email protected] (R. Yang), [email protected] (Y. He). Tel.: +86 731 84261208.

http://dx.doi.org/10.1016/j.infrared.2015.12.026 1350-4495/Ó 2016 Elsevier B.V. All rights reserved.

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R. Yang, Y. He / Infrared Physics & Technology 75 (2016) 26–50

3.

4.

5.

6.

7.

2.2.1. Classification by heating function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Classification by excitation sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. Classification by heating style . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4. Classification by relative position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5. Classification by relative motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Development of thermography NDT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Optical thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Pulsed thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Lock-in thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3. Pulsed phase thermography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.4. Frequency/phase modulated thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Laser thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Laser pulsed thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Laser pulsed phase thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3. Laser lock-in thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.4. Laser frequency modulated thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Eddy current thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. Basic knowledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2. Eddy current pulsed thermography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3. Eddy current lock-in thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.4. Eddy current pulsed phase thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.5. Eddy current thermography involving cross correlation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Microwave thermography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1. Basic knowledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2. Microwave pulsed thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.3. Microwave time-resolved infrared radiometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.4. Microwave lock-in thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Vibrothermography and ultrasound thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1. Basic knowledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.2. Ultrasound lock-in thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.3. Ultrasound burst phase thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.4. Ultrasound thermography involving frequency modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scanning thermography NDT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Line scanning thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Scanning eddy current thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Comparison between thermography NDTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1. Different heating functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2. Different excitation sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Comparison with other NDTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. New physics and multiple physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Simulation and modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Signal processing algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4. Standard specimen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5. Integrated inspection system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6. Standards for composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conflict of interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A. Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Present day, more and more composites such as glass fiber reinforced polymer (GFRP) and carbon fiber reinforced polymer (CFRP) are being used in the aerospace [1], renewable energy [2], civil and architecture [3,4], and other industries, due to their excellent advantages such as low cost, light weight, high strength/weight and high stiffness/weight ratios compared with traditional metals. And, longer, greater and more complex composite components are being fabricated, such as wind turbine blade, which increases manufacturing and maintenance difficulty. For most polymer matrix composites (PMC), only adhesive bonding and mechanical fastening can be utilized [5]. Adhesive bonding is preferable to mechanical fastening because of the continuous connection, whereas in drilling holes for bolts or rivets, fibers or other reinforcements

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are cut, and large stress concentrations occur at each discrete fastener hole. Inevitably, flaws such as delamination, disbond, random porosity and undesirable material may occur during composites’ manufacture process. What’ more, composite components are facing harsh and complex service environment and could be damaged in service. Theses damages, while often difficult or even impossible to look from the surface, severely degrades the loadbearing capacity of the structures. Thus, it is extremely necessary to avoid composite components failure in both manufactory and in service. Aside from developing more advanced composites to improve the availability, another effective way would be to apply reliable and cost-effective nondestructive testing (NDT) [6] and structural health monitoring (SHM). Nowadays, a plenty of NDT techniques are investigated for composite inspection, such as ultrasonic testing, acoustic emission,

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R. Yang, Y. He / Infrared Physics & Technology 75 (2016) 26–50

Fig. 1. Excitation functions of (a) PT and SPT, (b) ST, (c) LT, and (d) FMT. Table 1 Major thermography NDT techniques. Thermal sources Optical excitation

Conventional optical excitation, flash, lamp Laser

Electromagnetic excitation

Induced eddy current

Mechanical wave (ultrasound or sonic wave)

Stress/strain

Term of technology

Heating principle

Heating style

Conventional (optical) thermography

Optical-thermal effect

SHT

Laser thermography

Optical-thermal effect

SHT

Joule heating/eddy current loss

Electrical current

Eddy current thermography, Induction thermography Conduction thermography

Magnetic field Microwave

Magnetic induction thermography Microwave thermography

Hysteresis loss Dielectric loss

SHT, VHT, AHT SHT, VHT, AHT VHT VHT, AHT

Vibration, sonic wave

Vibrothermography, Sonic-IR

AHT

Ultrasound

Ultrasound thermography

Friction, thermoelastic effect and hysteresis effect Friction, thermoelastic effect and hysteresis effect

Stress

Thermoelastic stress analysis

Thermoelastic effect

VHT

Joule heating

AHT

Fig. 2. Heating styles (a) SHT, (b) VHT, and (c) AHT.

fiber Bragg grating (FBG), electrical resistivity measurement, eddy current, microwave, T-ray, X-ray, speckle shearing interferometry, shearography, and infrared (IR) thermography. Among them, IR thermography has shown the great potential and advantages, which has greater inspection speed, higher resolution and sensitivity, and detectability of inner defect due to heat conduction. Unlike ultrasonic testing, it doesn’t require the couplant and is total non-contact. Besides conventional thermography using optical excitation, other thermal sources including eddy current, laser, microwave, and ultrasound, excited thermography NDTs have been developed. They are built on the different physics so that having different advantages and disadvantages, which may increase the difficulty in investigation and utilization. By now, there are some review works [7–19], but, they are focused on conventional optical thermography NDT and barely mention the applications in specific fields such as renewable energy, civil and architecture. Thus, a deep review work including the principle, developments, strengths and limitations, and research trends for thermography NDT with different excitation sources is required. In this work, a fully, in-depth and comprehensive review of optical and non-

optical thermography techniques for composite inspection was reported based on an orderly and concise literature survey. The rest of the paper is organized as follows. Firstly, the basic concepts of thermography NDT were introduced in Section 2. Next, the developments of thermography NDT including optical thermography, laser thermography, microwave thermography, eddy current thermography, and ultrasound thermography for composite and structures inspection were reviewed in Section 3. Then, some scanning thermography case studies were reviewed in Section 4. After that, the comparisons between thermography NDT techniques and with other NDT techniques were concluded in Section 5, which was followed by predicting some trends in Section 6. Finally, conclusions were outlined in Section 7. 2. Basic concepts of thermography NDT 2.1. Passive thermography IR thermography can be divided into two approaches: passive thermography and active thermography. Passive thermography

R. Yang, Y. He / Infrared Physics & Technology 75 (2016) 26–50

measures thermal variations of a material using an infrared vision device without external thermal sources. In passive thermography, the features of interest are naturally at a higher or lower temperature than the background. With passive thermography, it is possible to monitor health condition of composite structures, such as monitoring rotor blades from the ground level without stopping their operation. Periodic loads occurring in the process of blade rotation cause its heating and defective areas can be characterized by specific dynamics of heat flows [20]. What’s more, the damage and thermo mechanical behavior of the composite in mechanical tests can be achieved using passive thermography [21]. 2.2. Classification of active thermography 2.2.1. Classification by heating function Contrary to passive thermography, active thermography requires an external heat sources to stimulate the materials under tests. The original heat sources can be a heat gun, hot water jets, hot air jets, or a hot water bag. However, optical thermal sources are normally used, such as high power cinematographic lamps, quartz line IR lamps, high power photographic flash, and laser beam. The common types of active thermography are pulsed thermography (PT), stepped thermography (ST) [22], lock-in thermography (LT) or called modulated thermography (MT) [23], pulsed phase thermography (PPT) [11,24], and frequency modulated thermography (FMT) [25,26]. As shown the solid line in Fig. 1(a), PT warms or cools the material with a short duration energy pulse and a measurement of the temporal evolution of the surface temperature is performed with an IR camera. Sometimes, PT is also called as burst thermography, flash thermography, and square pulse thermography (SPT) that uses a square pulse as excitation, shown in dotted line in Fig. 1(a). As shown in Fig. 1(b), with ST, a long pulse is used to step heat the sample and the temperature’s rising process is observed. As shown in Fig. 1(c), LT uses periodic thermal excitation in order to derive information on reflected thermal wave phase and magnitude even at considerably low peak powers. PPT used the pulse as excitation and phase analysis in frequency domain, which is a link between PT and LT [24]. Similarly, the phase analysis of SPT and BT are called as square pulse phase thermography (SPPT) and burst phase thermography (BPT). As shown in Fig. 1(d), FMT uses frequency modulated excitation in order to derive phase information on many thermal waves at one inspection. 2.2.2. Classification by excitation sources Thermal source used in active thermography testing is various, such as optical excitation, electromagnetic excitation, acoustic excitation, and stress/strain excitation, as shown in Table 1. According to excitation sources, thermography can be classified as: (1) Optical thermography using optical excitation such as flash and lamp, which is named laser thermography if using laser beam as thermal source; (2) Eddy current thermography (ECT), which uses induced eddy current as thermal sources to heat conductive materials; (3) Conduction thermography which uses electrical current as sources; (4) Magnetic induction thermography [27], which uses magnetic field as heat source for ferromagnetic materials; (5) Microwave thermography (MWT) [28], which uses microwave as heat source for dielectric materials; (6) Vibrothermography using mechanical variation as excitation, which is named as ultrasound thermography (UT) if using ultrasound as excitation; and (7) Thermoelastic stress analysis, which use strain or stress as heat sources. Every thermography mentioned above can be applied as PT, ST, LT, PPT, and FMT. Among them, eddy current thermography, microwave thermography, and ultrasound thermography are being increasingly investigated in composite inspection, which will be reviewed in next section.

29

2.2.3. Classification by heating style Normally, these optical thermal sources lead to the surface heating of the material under tests (MUT), as shown in Fig. 2(a). And, the defect depth quantification methods under reflection mode are built on the heat conduction from surface to inside and its reverse reflection [29–33]. Thus, they can be named as surface heating thermography (SHT). Other SHT examples include eddy current thermography and conduction thermography for ferromagnetic metal material, where the skin depth is very small due to the great permeability. Thus, it can be classified into SHT family. On the contrary, when induction current is used as excitation, CFRP with small conductivity are volumetric heated. And, when microwave is used as excitation, GFRP are volumetric heated. As shown in Fig. 2(b), these methods are named as volume heating thermography (VHT) [34]. Defects which do not generate heat or generate more heat will lead to the heat abnormal. And, the related defect depth quantification methods are built on the heat abnormal conduction from inside to surface. The most advantage of VHT is that the thermal waves only have to travel half the distance (from the defect to the surface) than with optical methods in reflection mode (from the surface to the defect and back to the surface). With VHT, the characterization methods under transmission and reflection modes are similar due to volume heating [35]. In some cases, just inside defects or interesting objects are heated but the host material is not. We call these methods as abnormal heating thermography (AHT) or selectively heating thermography, as shown in Fig. 2 (c). For example, microwave thermography is a kind of IHT for water detection in concrete structures. And, in vibrothermography, heat is generated mainly from the friction of crack tip. Thermography methods with different thermal sources can be classified as SHT or VHT, as shown in Table 1. 2.2.4. Classification by relative position According to relative position of excitation and camera, there are two configurations: (1) reflection mode, where excitation and camera are located on the same side, and (2) transmission mode, where excitation and camera are located on the opposite side. Usually, there is no direct access to both sides of the components. Hence reflection mode configuration is more practical in NDT field. And, for surface heated optical thermography, defect depth can be quantified under reflection mode while not under transmission mode. However, transmission mode generally yields more accurate results than reflection mode for some components (like thin plate) [36]. Another advantage of transmission mode is that the excitation does not block the camera view to sample surface. 2.2.5. Classification by relative motion According to relative motion of excitation, camera and MUT, thermography can be classified into static and scanning configuration. The major limitation of static configuration is that the inspection area is limited by heating area and camera view. Scanning configuration is a valuable solution for this problem. Scanning configuration means at least one member out of excitation, IR camera and MUT moves in the inspection. Scanning configuration can be applied: (1) Point source moves in camera view. Today, with the development of IR camera, its view is large enough for some samples. So, just heating source need scanning sometimes. A classic example is flying spot laser thermography, which uses a laser beam to local heat the sample surface, and uses a fixed IR camera to capture the temperature field of scanning area [37–39]. In [40], a flying spot laser lock-in thermography (LLT) technique was proposed for detection of surface-breaking fatigue cracks on uncoated steel structures

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analysis (ICA), and differential absolute contrast (DAC) [60]. In [15], basics of pulsed thermography including theoretical solutions, data processing algorithms and practical implementation were summarized and applications in the aerospace industry were discussed briefly. Here, some case studies in recently will be introduced. In [57], a novel onesided reference-free technique based on the conversion of the decaying front-surface temperature signals into an artificial function having clear maxima/minima was proposed to determine the material diffusivity. One problem in PT is that non-uniform backgrounds commonly observed in thermal images often reduce the detection power of PT. In [61], mathematical morphology (MM) was proposed for the analysis of geometrical structures. By subtracting the backgrounds from the original data, improved NDT was achieved. Experiment results show the effectiveness of the proposed method, where the defects in the CFRP specimen are more clearly identified. (2) Wind turbine blade. In [62], a GFRP with embedded defects (i.e. foreign matter and air inclusions) in different depth was tested by PT. In [63], a GFRP blade with surface defects, such as air bubbles and edge bonding were easily and clearly detected by PT. The analyses of the original, 1st and 2nd derivatives were used in quantitatively calculation. The results indicate that it is better to use the 1st derivative than the original to analyze the deep defects. In [64], the glue faults between supporting spars and GFRP shells with different thickness were inspected by using PT in laboratory. The test results indicated that PT has potential for the detection of glue faults in at least about 15 mm thickness GFRP shell. In [65], the glue defects in GFRP blades was also detected. The results indicate that the shape and size of deficiency glue defects in the specimens show good agreement with the real situation. In [66], a PT production-ready system was designed for inspection of wind turbine blades. The system can be used as an end line manufacturing quality control. Some damages in manufacturing process, such as structure inhomogeneity, improper resin impregnation, have been detected. And the internal structure defects that have not been prepared during manufacturing process, but have occurred during exploitation of the blade, can also be detected. (3) Fiber reinforced polymer (FRP) reinforced structures. Thermography was used to inspect subsurface defects such as disbonds and delaminations in FRP bridge decks [67]. Airfilled and water-filled disbonds between wearing surface and the underlying FRP bridge deck could be readily detected [68]. Thermography has also been used to detect debonding in FRP strengthened concrete [69–71]. Besides debonding, defects such as air voids, moisture zones and other types of anomalies are also detected with thermography.

with low surface emissivity. Scanning laser spot thermography is mainly used for crack detection in metal and semiconductor based on the lateral heat conduction [37,40–42]. (2) Line source move in camera view. A more effective heating than spot source is line source, which heats the sample in linear scanning style in the camera view [43,44]. In [43], a linearly-focused continuous wave laser beam was scanned toward a direction perpendicular to the beam covering whole surface of a specimen and the thermal image was observed by a fixed thermo-tracer. In [45], a new line laser lock-in thermography (LLT) technique for instantaneous inspection of surface cracks in semiconductor chips. In these set-ups, the mechanical scanning devices were needed. In [44], a whole-electronic line-focus light-scanning apparatus was proposed, designed and fabricated through a multiple column light-emitting diode (LED) array coupled with a focusing lens. These set-ups are still limited by camera view for large components or on-line inspection. (3) Both excitation and camera move along material [46,47]. These configurations are still limited for large-scale components. In order solve this problem, both excitation and camera can be relative moved to sample [46,47]. Line scanning thermography (LST), where IR detector moves in tandem with the heat source along the sample is a classic example [46,48–51]. In addition, other kinds of thermography like eddy current thermography can be configured as scanning configuration [52–54]. 3. Development of thermography NDT 3.1. Optical thermography 3.1.1. Pulsed thermography In PT, a thermal pulse is applied to the material to be inspected. Following application of this thermal pulse, a measurement of the temporal evolution of the specimen surface temperature is performed with an IR camera allowing subsurface defects to be revealed. The temperature of the material changes rapidly after the initial thermal perturbation because the thermal front propagates by diffusion under the surface. The presence of a defect reduces the diffusion rate so that when observing the surface temperature, defects appear as areas of different temperatures with respect to surrounding sound areas once the thermal front has reached them. Consequently, deeper defects will be observed later and with a reduced contrast. In fact, the observation time t is a function (in a first approximation) of the square of the depth z and the loss of contrast c is proportional to the inverse of the cube of the depth z [23].

t

z2

a

and c 

1 z3

ð1Þ

where a is thermal diffusivity. These two relations show two limitations of PT: observable defects will generally be shallow and the contrasts will be weak. An empirical rule says that the radius of the smallest detectable defect should be at least one to two times larger than its depth under the surface. In addition, the surface temperature gradients are not only caused by hidden defects but also affected by local variations of emissivity on the material surface as well as due to non-uniform heating. (1) Aerospace structures. PT has been commonly used for assessing composites in aerospace industries, there are a lot of works demonstrating this topic [30,31,55–58]. Some classical advances include thermal signal reconstruction (TSR) [59], dynamic thermal tomography (DTT) [32], principal components analysis (PCA), independent components

3.1.2. Lock-in thermography Lock-in thermography is based on the application of a periodic thermal energy input to the surface of an object. When the resulting heat wave encounters a defect, some of it is reflected, causing a phase shift with respect to the input heat wave. The depth range for defect detection depends on the thermal diffusion length, which can be presented as

lt

sffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffi 2k a ¼ ¼ xqc pf

ð2Þ

where k is thermal conductivity, q is density, C is heat capacity, a is thermal diffusivity, and f is frequency of thermal wave. The

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advantage of LT is that the phase is less sensitive to local variations of illumination and/or of surface emissivity. However, because of its mono-frequency excitation, the depth resolution of a test is fixed by fixed thermal diffusion length. To detect defects located at various depths in test sample, repetition of the test at various frequencies becomes a time consuming process. (1) Aerospace structures. A lot of studies using LT for aerospace structures can be found [72–74]. In [18], the use of PT and LT were discussed. Many applications were described which involve several different materials (metals, plastics, plaster, composites, hybrid composites and sandwiches) and different types of bonds (coatings, sandwiches and joints). In [75], some specimens made of CFRP were tested by LT. These specimens were fabricated with the infusion technique. The thin KaptonÒ foils of different shape (circular or elliptical) and dimensions were embedded, during fabrication, in each sample at different depths. In some cases, two or more concentric disks, of different diameters, were overlapped. Specimens were all non-destructively inspected with optical lock-in thermography. All the inserts were detected and well outlined also in presence of thin overlapped foils which, of course, made more difficult their discrimination. A quantitative data analysis, involving measurement of both size and depth of the detected slag inclusions, was also performed. (2) Wind turbine blade. In [76–79, LT was proposed to detect skin–skin delaminations and skin–core delaminations presenting in a 9 m CX-100 wind turbine blade. In sample design, 16 defects (skin–skin delaminations and skin–core delaminations) ranging in size between 12.7 and 50.8 mm were considered. The depths of the defects were in the range of 0.7–1.3 mm from the blade surface. Since ‘blind frequencies’ affect defect detection, lock-in tests were performed at multiple (five) different frequencies at each defect location, to ensure that any defect could be detected by at least one of the lock-in frequencies. A set of image processing algorithms including binarization and morphological operations and multivariate outlier analysis were used in conjunction with the classical lock-in thermography technique to counter the ‘blind frequency’ effects and to improve the defect contrast. The results demonstrate that the approach of sweeping multiple lock-in frequencies and combining the phase image results provides the best overall defect detection performance compared to any single lock-in frequency. In addition, the detection of defects using lock-in thermography is a slow process. A bulk of the time is spent in the data acquisition process. This could be a concern for deeper defects since very low lock-in frequencies are needed to achieve the required thermal diffusion length. Hence, there is a trade-off between depth of defects targeted and inspection time. (3) FRP reinforced structures. In [80], FRP strengthened concrete structures were tested by LT, which provides a clear, albeit somewhat qualitative, method for determining the depth at which a defect exists beneath the surface of the FRP composite. The primary advantage of this method is that large areas can be inspected during a single observation period with relatively inexpensive heat sources. One downside to this method is that different FRP systems will require different heating periods in order to determine defect depth. If information about the specific FRP system is unknown, some degree of trial and error will also be necessary to determine the appropriate heating periods.

3.1.3. Pulsed phase thermography Pulsed phase thermography (PPT), which simultaneously combines advantages of PT and LT was originally developed in 1996 by Maldague and Marinetti [24]. PPT is phase analysis in frequency domain of PT built on thermal wave propagation. It is well-known that any waveform, periodic or not, can be approximated by the sum of purely harmonic waves oscillating at different frequencies. The frequency content of an ideal temporal pulse of null duration has a frequency spectrum with uniform energy distribution between all frequencies from 0 to 1. Although the thermal pulse in CFRP is different from that of an ideal temporal pulse, it can be recognized as a sum of thermal waves. Due to surface heating style, thermal waves propagate to inside. Each of them has a different frequency x, thermal diffusion length lt, and speed v. According to thermal wave theory, the thermal diffusion length lt and propagation speed v can be expressed by (3)

lt

sffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffi 2k 2a ¼ ¼ x xqC

and

v¼

pffiffiffiffiffiffiffiffiffiffi 2xa

ð3Þ

where k is thermal conductivity, q is density, C is heat capacity, a is thermal diffusivity. The above two equations indicate that higher frequency thermal waves propagate nearer but faster, while lower frequency thermal wave propagates farther but more slowly. If the thermal diffusion length lt is greater than d, the variation of thermal wave at this frequency can show defect. PPT phase analysis is based on the discrete Fourier transform (DFT). Differential process is usually used in the thermography fields. The differential phase spectra are obtained by subtracting the defect-free phase spectrum from defect phase spectra. Then, several features are extracted from phase spectra or differential phase spectra. One is blind frequency, which is defined as the frequency at which the defect becomes visible [11]. After differential process, blind frequency is the frequency at which the differential spectrum arrives at zero. Another two features are extracted from the differential phase spectra. One is min phase, which is the minimum value of spectra; the other is frequency to min phase, which means the frequency when spectrum is the smallest. Combines characteristics and benefits of PT and LT, PPT is a common NDT technique in aerospace industry for inspection of composite structures [7,81]. In [7], the applications of PPT for aeronautical sandwich structures was reviewed. Recently, PPT has been investigated for other industrial fields. (1) Wind turbine blade. In [82], PPT was used for the evaluation of GFRP wind turbine blades. In the phasegrams, the plastic, composite, and foam material can be identified through different phase values. In the trials, it was also made clear that pulsed phase thermography could be limited in some cases of defect detection (i.e. impact damage detection, near surface defects, relatively large defects-greater in diameter than depth). (2) Nuclear reactor. In [83], the effectiveness of PPT as a means of inspection for the bond between the beryllium (Be) tiles and the copper alloy (CuCrZr) heat sink of the ITER NHF FW components was evaluated. The rapid and non-contact nature of PPT gives the potential for in-service inspections as well as a quality measure for as-manufactured components. The technique has been appraised via experimental trials using ITER first wall mockups with pre-existing disbonds and finite element simulations. The results suggest that PPT is an effective tool to identify defective tiles prior to detailed characterization of suspected defects via ultrasonics.

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Fig. 3. (a) Correlation image obtained at 8.5 s. (b) Phase image obtained at 0.014 Hz [87]. Reprinted/reproduced with permission from AIP Publishing LLC [87].

(3) FRP reinforced concrete. In [80], FRP strengthened concrete structures were tested by PPT, which provides reliable defect detection. The general trend between defect depth and blind frequency was observed, but the longer heating period of 30 s (as opposed to flash heating) made precise identification of the blind frequency difficult. 3.1.4. Frequency/phase modulated thermography PT, LT and PPT have their advantages and disadvantages. PT uses high power short-pulse sources to ensure detection of high signal-to-noise ratio, which may lead to damage to samples. And, the surface temperature gradients are not only caused by hidden defects but also affected by local variations of emissivity on the material surface as well as due to non-uniform heating. With LT, the phase angle has the advantage of being less sensitive to local variations of illumination and/or of surface emissivity. However, because of its mono-frequency excitation, the depth resolution of a test is fixed by ‘thermal wavelength’. To detect defects located at various depths in the test sample, repetition of the test at various frequencies becomes a time consuming process [25]. In addition, there is a compromise one needs to make between maximum detection depth and depth resolution. While low frequency thermal waves can ‘see’ deep into the sample, they lack the desired depth resolution. High frequency thermal waves, on the other hand, display exactly the opposite behavior due to the reduced thermal diffusion length and thermal wavelength [84]. With PPT, the phase images show all the merits of the phase images obtained with LT. However, the energy amplitude damps with the increase of frequency. To overcome some of the traditional limitations of PT, LT and PPT, such as inspection depth, depth resolution, broadband frequency, sensitivity, and elimination of emissivity and nonuniform heating, frequency modulation (FM) was introduced in thermography. FM was proposed in photo-acoustic and photothermal field in 90s [85]. After 2000, linear frequency modulated (LFM) thermography methods were proposed by various researchers and they used different signal processing methods [25,26]. In addition, binary phase coded (BPC) modulation was proposed to achieve more energy localization by means of pulse compression techniques through reducing the width of the CC peak while increasing its height [86]. In [87], a novel 7 bit Barker coded binary phase excitation was proposed. In [88], non-stationary nonlinear

(Quadratic) frequency modulation was proposed in thermal wave imaging, which can provide more energy deposition, better depth resolution, improved dynamic range and side lobe reduction. The signal processing methods are various. In [25], the amplitude and phase at various frequency components in captured sequence were extracted using the Fourier transform on each pixel of the thermogram sequence. In [89], the pulse compression in radio detection and ranging was proposed to process the reference signal with the received signal to get the time delay through correlation algorithm. In [26], the well-known cross correlation matched filter technique in frequency domain was used to process the thermogram sequence. And, thermal wave radar was proposed. Cross correlation peak delay times can built a relationship with the thickness or the defect depth. Cross correlation peak delay times decreases as the thickness increases. In [84], an emissivity-normalized, higher-dynamic-range contrast parameter known as cross correlation phase (time domain phase) was proposed based on cross correlation matched filter and Hilbert transform. In [90], PT, LT, and FMT were compared for the analysis of a given CFRP containing four flat bottom holes of dimensions. It is observed that FMT produces relatively more signal noise ratio (SNR) for larger diameter defects than PT and LT. In [87], a CFRP specimen containing flat bottom holes located at different depths was tested by thermography using a Barker coded excitation. Capability of the proposed excitation scheme was highlighted with correlation based post processing approach and compared with the existing phase based analysis by taking the signal to noise ratio into consideration. Correlation image extracted at a delayed instant of 8.5 s, is as shown in Fig. 3(a), all the defects at different depths and sizes have been detected with a variable contrast (SNR) according to their depth and size has been observed. Defects of larger diameter are providing more contrast than that of the smaller ones even at the same depth (example defect ‘a’ with respect to ‘e’), which illustrates the effect of defect size on correlation processing. The best phase image obtained at a frequency of 0.014 Hz, shown in Fig. 3(b), has been detecting all the defects but the smaller and deeper defect ‘h’ is exhibiting low phase contrast with respect to background. Defects’ SNR illustrates that, the constructed correlated image at a chosen time instant exhibits higher SNR due to the adopted correlated approach which reduces random noise floor than the phase image.

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Fig. 4. LPT thermograms at 41 s. (a) CFRP without defect; (b) CFRP with defect; (c) Thermal contrast between the specimens; (d) Thermal contrast compared to initial instant [93]. Reprinted/reproduced with permission from [93].

Fig. 5. Phase images after FFT (a) at 0.00586 Hz, (b) at 0.5 Hz and (c) at 0.03516 Hz [94]. Reprinted/reproduced with permission from Elsevier Ltd [94].

In [91], a CFRP sample with flat bottom holes was inspected using non-stationary digitized frequency modulated thermal wave imaging technique. Furthermore, depth scanning performance using frequency domain-based phase approach has been compared with time domain phase approach. Results clearly show that time domain phase has enhanced depth scanning with improved resolution and sensitivity for defect detection than frequency domain phase. In [92], a combined theoretical and experimental approach was reported using thermal wave radar imaging (TWRI) for CFRP with subsurface defects inspection. Experimental results indicate that TWRI is available for detecting the subsurface defect. For the

shallow defect (defect depth 6 1 mm), the delay time image of CC exhibits high contrast, and the phase image of FMT has high SNR at the right frequency component. For the deep defect (defect depth 2.0 mm), the phase images have both high contrast and large SNR value. The development and case studies of optical thermography have been reviewed. It is concluded that: (1) optical thermography can be classified as pulsed thermography (including square pulse thermography), step heating thermography, lock-in thermography, pulsed phase thermography, and frequency modulated thermography; (2) optical thermography has a lot of advantages, like being non-contact, fast, full-field, and high efficient. Especially, FMT has

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most advantages including: improved inspection depth and depth resolution; higher sensitivity (SNR) and better detectability (contrast); quantification of depth or thickness; and elimination of emissivity and non-uniform heating; (3) optical thermography has been proved as an effective NDT method for composites in aerospace and is drawing more and more attentions in field of wind energy and civil engineering. 3.2. Laser thermography Laser thermography is an optical thermography [38,39], which use laser beam as thermal sources. It can be also applied as laser pulsed thermography (LPT), laser lock-in thermography (LLT), laser pulsed phase thermography (LPPT), and laser frequency modulated thermography (LFMT). 3.2.1. Laser pulsed thermography In [93], a laser pulse thermography using a CO2 laser with the wavelength of 10.6 lm was applied to defect detection in CFRP. A deviation device was used to change the direction of the laser beam. A concrete plate, with size of 40  40  4.5 cm3, was reinforced by the CFRP with size of 22  40 cm2. The CFRP layer consists of the Sikadur adhesive and the bidirectional carbon fiber. The defect is the bonding failure made by the absence of adhesive on 10  10 cm2 in the middle of the specimen. The sample was tested by laser pulse thermography system. The thermograms obtained at the 41 s are shown in Fig. 4. The thermogram of the sample without defect in Fig. 4(a) shows the circular form and the non-uniformity of the laser beam on the surface of specimen. The thermogram of the sample with defect in Fig. 4(b) shows the hotter area and the non-uniformity of the laser beam. Thermal contrast between the specimens in Fig. 4(c) and the thermal contrast thermogram compared to initial instant in Fig. 4(d) shows clearly the defect in the middle of the specimen, which is hotter than other healthy area. 3.2.2. Laser pulsed phase thermography In [94], the LPT system was enlarged as a laser pulse phase thermography system by analyzing with the fast Fourier Transform (FFT) to get the phase images in frequency domain. Some phase images at different frequency are shown in Fig. 5. These phase images show that at the frequency of 0.00586 Hz, the defected area does not appear clearly because the excitation penetrates too deeply into the sample. At the frequency of 0.5 Hz, which is a high value, the excitation has too small penetration depth that does not allow us to see the defected area in the phase image. Otherwise, we see clearly the defected area on the phase image at the frequency of 0.03516 Hz. It is also observed that the inhomogeneous heating by the laser beam does not have the effect on the phase images. 3.2.3. Laser lock-in thermography In [95], laser lock-in thermography was applied to determine the thermal contact conductance of a W-layer (140 mm) on a CFC-substrate. The experimental setup with the pyrometer system and the pulsed repetition rate Nd:YAG laser was developed. The phase shift measurements have demonstrated the possibility to determine qualitatively the layer/substrate thermal contact in different zones of the sample. In [96], a series of CFRP specimens with artificial flat-bottom holes (FBHs) were tested using a laser lock-in thermography. Phase decision threshold (PDT) values criterion between defective regions and healthy ones were implanted to determine the defect. The probability of detection (POD) curves of continuous phase response data and the hit/miss data processing were used for estimation of the detection capability and reliability of LLT phase

image for NDT of CFRP composites, and the effects of PDT value and modulation frequency on POD curves were compared. It was found that the r90 and r90/95 are increased with increasing the PDT value, and there exists an optimal modulated frequency to minimize both the r90 and r90/95 for a given PDT value. In [97], the inverse heat transfer approach using laser lock-in thermography was developed to estimate the size and depth of the subsurface defects in CFRP materials, using the phase obtained by finite element method and from experimental results. The hybrid method that integrates simulation annealing algorithm (SA) into Nelder–Mead simplex search method (NM) allows searching the optimal solutions of the objective function, and the experimental results show that the size and depth of subsurface defects are effectively obtained through inverse solving the constructed objective function by the hybrid method. The estimated maximum errors for the size and depth of subsurface defects are less than 5% and 4% for the defect diameter of 10 mm and the defect depth ranging from 0.62 mm to 2.0 mm for the given CFRP composite by the hybrid method, respectively. 3.2.4. Laser frequency modulated thermography In [26], thermal wave radar (TWR) for depth profile and depth selective method based on a laser linear frequency modulated excitation and matched filter signal processing method was introduced. The method was based on radar principles and exhibits enhanced dynamic imaging range up to 150% over conventional frequency-scanned thermal wave imaging. In [84], cross correlation phase in TWR was proposed with advantages of emissivitynormalized and higher-dynamic-range contrast. In order to experimentally investigate the capabilities of the TWR imaging method, three samples (a black plastic step wedge placed inside a scattering medium, an thick Fisher Scientific borosilicate microscope cover slips covered with commercial green and black paints, and an extracted human tooth that was locally demineralized using an acidic gel) were tested. 3.3. Eddy current thermography 3.3.1. Basic knowledge ECT, also named as induction thermography, is a combination of eddy current and thermography, which is based on electromagnetic induction and Joule effect heating. ECT has a lot of advantages, such as non-contact, fast, full-field and high resolution [98,99]. ECT is a kind of volume heating thermography for CFRP due to small conductivity and great skin depth [34,35]. In 1990s, the thermographic NDT based on the transient temperature distribution under the Joule effect heating by an electric current or induction current was proposed [100,101]. In 1999, a new thermographic NDT technique based on the singular method combined with the lock-in thermography technique was proposed [102]. After 2000, more and more ECT methods were proposed by researchers, such as thermal-inductive [103], electromagneticthermal [104], tone burst eddy current thermography (TBET) [105], eddy current pulsed thermography (ECPT) [106,107], eddy current step thermography (ECST) [108] and eddy current lock-in thermography (ECLT) [109]. ECT is built on induction heating, heat conduction and IR imaging. Firstly, the electric current passing through the coil will induce eddy currents and generate resistive heat in materials. These eddy currents are governed by penetration depth (or called skin depth based on skin effect) [110–113], which can be calculated by (4)

1 d ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffi plrf

ð4Þ

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Fig. 6. (a) Schematic diagram of ECPT; (b) Excitation signal; (c) Temperature response for one pixel.

where f is frequency of excitation signal, r is electrical conductivity (S/m) and l is magnetic permeability (H/m) for materials. It is concluded that skin depths vary with the electromagnetic materials. For CFRP (r  1000 S/m and no magnetic), the skin depth is significantly great (about 50 mm under 100 kHz excitation). Thus, the heating style for CFRP is volumetric heating [35]. At the same time of induction heating, the IR camera captures all pixels of IR radiation as an image with a sampling frequency. And then the image sequence is transformed to a PC for analysis. Defects in the sample heat up more than homogeneous material due to locally enhanced eddy current density or interupt the heat conduction. Then, surface temperature will be different. According to heating function and signal processing, ECT can be applied in terms of eddy current pulsed thermography (ECPT) [98,114], eddy current step thermography (ECST), eddy current lock-in thermography (ECLT) [115], and eddy current pulsed phase thermography (ECPPT) [116]. Among them, ECPT, ECLT, and ECPPT have been investigated for composite inspection. 3.3.2. Eddy current pulsed thermography ECPT’s principle is shown in previous works [98,117–119]. Fig. 6 (a) shows the principle schematic of ECPT for inspecting CFRP blade. Excitation signal generated by the excitation module is a small period of high frequency alternating current, as shown in Fig. 6(b). Then, the current passing through the coil will induce eddy currents and generate resistive heat pulse in materials. One typical temperature response for a pixel is shown in Fig. 6(c). It can be divided into two phases: heating phase and cooling phase. Due to volume heating, the temperature increases linearly in heating phase and then it decreases slowly in cooling phase. At the beginning, ECPT was used for crack and corrosion detection in metallic materials [103–105,114]. Logarithmic analysis of ECPT was proposed to quantify the depth of subsurface defects in aluminum and steel [120]. The variation of surface emissivity can be reduced by a two heat balance states-based method [121]. Recently, ECPT has been investigated for CFRP inspection [6,122]. In modeling of ECT on CFRPs, a methodology based on shell elements was presented to model the electromagnetic-thermal behavior of multi-layered conductive composite materials [123]. A model taking into account the influence of different fiber orientations on the electromagnetic parameters was presented [124]. Then, a multiscale approach was used to calculate the

electromagnetic and thermal field distribution. The relevance of this technique was then discussed for different positions of flaws and the optimal frequency was estimated [125]. In experimental investigation, ECPT was first investigated for surface-breaking artificial crack evaluation in CFRP in 2012 [126]. An ECPT inspection system for CFRP materials was studied and optimized. Using the system, the directional electrical conductivity of the CFRP material was observed through the surface heating pattern. Then, the normalized temperature rise and decay were investigated through the inspection of notches with varied depths and widths. The position invariance of the coil with respect to the notch along the fiber direction was also studied in the experiments. The work shows that ECPT can be used for defect detection and characterization through analysis of the surface heating pattern and the transient temperature change. In [106], the transmission mode, where the inductor and IR camera are on opposite sides of component was investigated for defect characterization through the analytical analysis and experimental studies. The studies have shown that the detection mechanisms for impact and delamination in CFRP are totally different. Carbon structure can be observed on the early stage of heating phase and impact leading to decreasing conductivity can be also detected in heating phase. However, delamination can be characterized using late stage of cooling phase. Combing the detection mechanisms, principal components analysis and independent components analysis, image reconstruction method was used to improve the sensitivity. In [127], eddy current pulsed thermography was investigated for CFRP testing and impact evaluation. Laminates impacted with different energies from 4 to 12 J were characterized. The qualitative and quantitative conclusions for impact behavior understanding were outlined, which was helpful to develop the reliable instruments for quality control and in-service inspection of CFRP blade. The main contribution of this work includes: (1) impact shows the different hot spot shapes at the thermograms. The impact behavior for real damages are drawn. The hot area by impacts with 10 J and 12 J is like circle shape; the hot area by impact with 6 J and 8 J is concentrated; both 2 and 4 J impacts cannot be detected; (2) two detection modes are compared. Reflection mode is more suitable for in-situ inspection, because there is no direct access to both sides for many practical components. However, the transmission mode is more suitable for manufacturing

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Fig. 7. (a) Thermal images at 20, 200, and 500 ms; (b) Phase images formed by phase at 7.7 Hz, min phase and frequency to min phase [35]. Reprinted/reproduced with permission from AIP Publishing LLC [35].

and testing, because the coil doesn’t affect the camera view to object under this mode. In [128], the idea of divergence has been applied to NDT in CFRP composites using ECPT in order to describe heat flow around impact damaged areas. Extraction of the divergence value was implemented at the selected time points. The relationships between the divergence and heat transfer process, as well as divergence and impact energy, have been analyzed. Impacts above positive and below negative threshold values of divergence have been linked to the resulting material state i.e. compressive and tensile stress caused by impact damages has been discussed. The results show that both the divergence and area of a defect increases with an increase of impact energy. In [106,129], principal components analysis (PCA) and independent components analysis (ICA) were proposed to process transient temperature responses in order to reconstruct thermogram and improve the sensitivity of delamination and impact in CFRP. In [6], optical flow was applied for capturing thermal flow patterns. In addition to the PCA methods for specifically and manually selecting the pixel-level transient thermal behavior from a highlighted area, optical flow for heat flow pattern evaluation over a relatively larger range was proposed. The results have shown that the flow area increases as the impact energy increases, which can be applied to quantitative characterization of impact damage. 3.3.3. Eddy current lock-in thermography ECLT uses amplitude modulated inductive heating [109,115, 130,131]. The electric current is amplitude modulated at the lockin frequency of typically around 0.01–1 Hz. The modulation of eddy current results in periodical heat generation in material. This heat at a instant frequency propagates into the sample as a thermal wave, whose thermal diffusion length is governed by Eq. (2). Thus, the depth range for defect detection not only depends on the eddy current penetration depth but also on the thermal diffusion length. After temperature is recorded by IR camera, Fourier analysis performed at this lockin frequency on the temperature image sequence so that providing amplitude and phase images with a significantly better signal to noise ratio than temperature images. Phase images have further advantages like the suppression of temperature gradients caused by the inductive heating. In [130], carbon fiber reinforced ceramic, a modern material particularly used in high tech applications like heat shields of reentry vehicles or high end brake disks, with delaminations was tested. The phase image shows a black stripe on front of the sample which indicates the edge of the delamination. The air in the delamination acts like an additional boundary layer which obstructs the heat transport. A CFRP sample with impact damages was also

tested. In temperature amplitude image, the heavily inhomogeneous temperature distribution is due to the laterally inhomogeneous induction heating. The actually interesting structures the impact damages are not visible. However, the impact damages are visible in the phase image as three horizontal stripes with high contrast. Additionally the phase image shows the structure of the laminate (45/45). In [115], Glare, a laminate consisting of GFRP and aluminum plates, with deaminations simulated by an additional circular PTFE foil embedded between two adjacent laminate layers, was tested by ECLT under the transmission mode. The damage in the ECLT phase image appears obviously. As the delamination is an obstacle to the induced eddy currents they are condensed at the edge of the delamination thereby causing the bright ring around a black circle which indicates the real delamination. 3.3.4. Eddy current pulsed phase thermography ECPT can be achieved as eddy current pulsed phase thermography (ECPPT) through frequency domain phase analysis. The experimental results for subsurface defect evaluation in steel have illustrated that the non-uniform heating effect by the shape of induction coil can be eliminated [132,133] and the variation of surface emissivity can be also reduced [116]. The principle of ECPPT is built on thermal wave propagation. The above subsection illustrated that there is a heat pulse generated in CFRP blade. The resistive heat will diffuse as the time delay till the heat balances in blade. The thermal pulse in CFRP can be recognized as a sum of thermal waves. Due to volumetric heating style, thermal waves are generated inside and propagate to surrounding area. Each of them has a different frequency x, thermal diffusion length lt, and speed v. According to thermal wave theory, higher frequency thermal wave propagate nearer but faster, while lower frequency thermal wave propagate farther but more slowly. If there is a defect at depth d, the parameters of thermal waves will be changed. However, not all variations of thermal waves can propagate to surface. If the thermal diffusion length lt is greater than d, the variation of thermal wave can reach the surface of CFRP. That is to say, the defect with greater depth will cause variation on the low frequency components observed from surface. ECPPT data analysis is based on the Fourier transform [24]. After getting phase spectra for all pixels, phases at some frequency are extracted to obtain phase image (phasegram). And, several features can be extracted to form phase images. Min phase is minimum value of phase spectrum; frequency to min phase means frequency when spectrum is the smallest [132,133]. In 2012, a CFRP plate with several impact damages was inspected using induction burst phase thermography. Fourier

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Fig. 8. (a) Front side of 12 J impacted areas; (b) Raw thermograms at 1 s; (c) CC phasegrams at 0.6 s.

transformation of the cooling sequence suppresses the temperature gradients. The signal-to-noise ratio was improved as compared to the image of highest contrast in temperature response, the impacts stand out clearly [134]. In 2014, CFRPs was inspected using induction heating thermography and phase images was obtained by applying Fourier transform to the temperature data in order to achieve higher defect detectability for delamination [135]. In [35], ECPPT based on volumetric heating was proposed for CFRP inspection and the inner delamination with 100 mm2 in 3.5 mm thickness can be evaluated effectively using phasegram. Fig. 7(a) shows thermal images at 20, 200, and 500 ms. The temperature is normalized to [0,1]. The coil is along the y axis, so there is no non-uniform heating effect in direction of y axis. However, the temperature gradient caused by non-uniform heating effect is dominated in x axis direction in all three images, which affects delamination identification. In image at 20 ms, fiber woven structure can be observed through hot and dark patterns. In detail, carbon fiber is highlighted, because the eddy currents directly heat the carbon fiber. In image at 200 ms, carbon fiber structures blur but still are visible. In image at 500 ms, a low temperature area appears. The data in cool phase was processed by DFT. Fig. 7(b) shows the phase images at 7.7 Hz and the phase images formed by min phase and frequency to min phase after de-noising the phase spectra. It is noticeable that not only non-uniform heating effect but also carbon fiber structures can be eliminated from phase images and phase profiles, and then the delamination detectability can be improved than ECPT results. In [34], eddy current volume heating thermography (ECVHT) and phase analysis based on ECPPT was proposed for delamination inspection in CFRP blade, which has been verified through experimental studies under both transmission and reflection modes. After discrete Fourier transform (DFT) of temperature responses, the phasegram and phase spectra can be used to image and characterize interface delamination in CFRP due to elimination of non-uniform heating effect and carbon fiber structures. With the whole temperature response processed by DFT, carbon fiber structures and delamination can be differentiated due to periodic oscillation of phase spectra. With temperature response in cooling phase processed by DFT, some characteristic features can be extracted to construct the new phase images according to the shape of phase spectra. 3.3.5. Eddy current thermography involving cross correlation Cross correlation matched filtering (CCMF) was originated from radar sciences in the early 1940s to detect deterministic signals within highly noised channels and to augment range resolution. This methodology localizes the energy of the received signal under a single peak located at a delay time and has been used in FMT [84,92,136,137]. Recently, pulsed inductive thermal wave radar (PI-TWR) was proposed by introducing the CCMF in eddy current pulsed ther-

mography. The results illustrate a significant improvement in the dynamic range, depth resolution, emissivity variation reduction and detectability of subsurface defects and inside delamination for the nondestructive testing applications [138]. CFRP samples having 12 layers of 5HS carbon fiber woven with balanced woven fabric [139] were tested. Fig. 8(a) shows the front side of 12 J impacted laminate using optical microscope. Clearly, some protruding structures are around the concavity on the front side. Fig. 8(b) shows the raw thermograms using ECPT for front side of 12 J impact areas at 1 s. The impact damage is invisible due to the non-uniform heating effect especially in y direction and lateral blurring effect as the heat diffuses laterally. Fig. 8(c) shows the CC phase images at 0.6 s for front side of 12 J impact areas. Comparing the results in Fig. 8, it is noticeable that not only non-uniform heating effect and lateral blurring effect, but also carbon fiber structures can be eliminated from CCMF images. Then, the damage shape can be identified more easily. Surface broken damage in shape of ring and invisible inner damage caused by impact can be detected on the front side through different colors. And, an incidental damage area can be found in the left lower corner, as marked in Fig. 8(c), which is not found in Fig. 8(b). The development and case studies of eddy current thermography have been reviewed. It is concluded that: (1) eddy current thermography can be accomplished through eddy current pulsed thermography, eddy current step heating thermography, eddy current lock-in thermography, and eddy current pulsed phase thermography. Predictably, eddy current frequency modulated thermography will also be realized soon; (2) eddy current thermography combines advantages of eddy current testing and thermography, like being non-contact, fast, full-field, and high efficiency; (3) eddy current thermography has shown the potential as an effective NDT method for conductive composites, especially for CFRP; (4) although barely seen in literature review, radio frequency eddy current thermography should be an effective NDT method for CFRP. 3.4. Microwave thermography Microwave heating is a new heating technique due to dielectric loss, which is a process within a family of electromagnetic heat techniques, such as induction, radio frequency, direct resistance or IR heating, all of which utilize specific parts of the electromagnetic spectrum. Major advantages of using microwaves for industrial processing are rapid heat transfer, relatively uniform heating pattern, volumetric and selective heating, compactness of equipment, speed of switching on and off, and pollution-free environment as there are no products of combustion. Microwave leakage can certainly be kept well below government recommended levels. In industry, microwave heating is performed at a frequency either close to 900 MHz or to 2450 MHz, which are chosen by international agreement with the principal aim of minimizing interference

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Fig. 9. (a) Specimen without defect, (b) specimen with defect, (c) subtraction between the sample with defect and the sound sample, and (d) the subtraction of thermogram in Fig. 8(c) and its initial thermogram [93]. Reprinted/reproduced with permission from [93].

3.4.1. Basic knowledge The physics behind microwave thermography is dielectric loss. Due to the dielectric loss of medium, the dissipated power per unit volume P in dielectric media is given by the following equation:

heating function, microwave thermography can be achieved through microwave pulse thermography (MWPT), microwave time-resolved thermography (MWTRT), and microwave lock-in thermography (MWLT). With microwave pulse thermography, one small period of microwave is used to heat the MUT and the temperature rise in heating phase and temperature decrease in cool phase are observed. For microwave time-resolved thermography, a long pulse is used to step heat the MUT and the temperature rise in heating phase is observed. In microwave lock-in thermography, a periodic amplitude modulated microwave is used to heat MUT and the periodic temperature change is observed.

P ¼ 2pf e0 e00 E2

3.4.2. Microwave pulsed thermography

with communication services [140]. Microwaves have already been used associated with thermography in different applications. Some scientists used the microwave thermography (MWT) to detect mine and buried objects [141,142]. Recently, microwave thermography has also been investigated in NDT for composites inspection [93,143–146].

ð5Þ

where f and E is frequency and electric field of microwave, e0 is the permittivity of air, and e00 is the loss factor, which clearly quantifies the power dissipation. The temperature change per unit at heating time t by power dissipation of continuous microwave is given as follow:

TðtÞ ¼

Pt xe0 e00 E2 ¼ t qCp qCp

ð6Þ

where q and Cp is the density and heat capacity of material. Clearly, the temperature rise is approximately linear with time if the properties of material and parameters of microwave are constant. The general heat conduction within the dielectric material is a timedependent heat diffusion equation. In 1990s, Leveque and Ambrosio made a preliminary research on microwave thermography [147,148]. Sakagami et al. with Osaka University proposed microwave thermography for detection of cracks in concrete structures. When microwave is applied to the concrete structure with wet cracks, water in the cracks can be selectively heated by microwave. Thus cracks can be detected from the thermal images [149]. By now, various microwave thermography techniques have been investigated. According to the microwave

(1) Aircraft components. In 2002, scientists in Military Institute of Armament Technology and Air Force Institute of Technology in Poland investigated microwave pulsed thermography. They used microwave antennas which transmit a frequency of 2 GHz at a beam width of 30° to illuminate the surface of a honeycomb sample with power density of 30 mW/cm2. Both camera and microwave source (antenna) were located on the same side of the sample (reflection mode). One antenna was positioned at 1 m from the sample, and the camera was positioned at 0.7 m. The sample was irradiated by microwaves for 5 s, and then the sequence of images was registered for 20 s at a rate of 1 Hz. The results shown the places with water are well visible [150]. (2) Wind turbine blade. In [143], two kinds of microwave pulsed thermography systems have been developed. Flann 18 094SF40 waveguide adaptor and open-ended waveguide were used to heat a GFRP wind turbine blade with 4 holes at the root. Rohde & Schwarz SMF 100A signal generator was used and linked to the waveguide adaptor with the maximum output power of 1 W at 18 GHz excitation frequency. The distance between waveguide and sample is 5 mm, and the

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Fig. 10. (a) Microwave thermography setup, (b) sample photo, (c) the result before heating, (d) the result after 5 s heating and (e) results under the 45 cm standoff measurement [145]. Reprinted/reproduced with permission from IEEE [145].

illumination direction is roughly 45° with the normal direction of the sample. A thermal camera Flir SC7500 was placed to capture thermal images and videos of surface temperature distribution. Due to the cable loss, only 115.2 mW power reaches the port of waveguide adaptor during the experiment, when the output power from signal generator was 1 W. It can be noticed that temperature rise rate around the hole (heating area) is larger than surrounding area. In high power microwave pulsed thermography setup, a 2.45 GHz magnetron with maximum power 1 kW was used for microwave excitation. WR 430 waveguide was implemented for microwave transmission and linked with openended aperture. The output power was controlled by Dipolar Magdrive 1000. The standoff of waveguide-sample distance was set as 22.7 mm for impedance matching in order to maximize the transmitted power from waveguide to sample and to minimize the reflected power from sample back to waveguide. Their experiment results show the temperature discontinuities appear at the defect region, mainly the edges [143]. (3) Civil Engineering. In [93], University Institute of Technology of Bethune in French has developed microwave pulsed thermography. In their tests, microwaves were generated by a commercial magnetron at the frequency of 2.45 GHz. A pyramidal horn antenna with an opening of 40° served to guide the microwave beam onto the tested samples. An IR camera sensitive to medium waves in a range of 3–5 lm, with a detector of 320  256 matrix detector in InSb was placed at 1.5 m from the sample in 55° direction so as to detect the whole area heated by the microwave beam. The antenna was placed in the 45° direction. Thermograms were recorded at 1 Hz by a computer, synchronized to the IR camera, using the ALTAIR program. The CFRP specimen was heated with a power of 360 W for 150 s. The tests on both samples (with defect, and without defect) had the same

procedure. Fig. 9(a) shows the thermogram of a specimen without defect at the instant 100 s. Fig. 9(b) shows the thermogram of a specimen with defect. Fig. 9(c) is the subtraction between the thermogram of the sample with defect and the thermogram of the sound sample. Thermogram in Fig. 9(d) is the subtraction of thermogram in Fig. 9(c) and its initial thermogram. Clearly, defect can be found in the middle of the sample. The absence of adhesive made it hotter than other area without defect [93]. In [145], Missouri University of Science & Technology investigated to inspect CFRP rehabilitated cement structures in 2014. Fig. 10(a) shows the setup that is capable of transmitting 50 W and operates at 2.4 GHz. High power microwave energy is generated using an HP8690B Sweep Oscillator and an Ophir RF 5303084 power amplifier. A horn antenna was used to illuminate the sample with this microwave energy. A DRS Tamarisk 320 thermal camera with a sensitivity of 50 mK was used to measure the thermal profile on the surface of the sample. As shown in Fig. 10 (b), the sample was a mortar sample with dimensions of 20  20  4 cm3 covered with a 13  13 cm2 CFRP (attached using adhesive). A delamination was created (through a lack of adhesive) with dimensions of 2  2 cm2, located offset from the center of the sample [145]. Measurements were made at standoff distances (distance between the horn aperture and SUT) of 6 cm with a corresponding 5 s heating time, and 45 cm with a corresponding 15 s heating time. Thermal images were obtained before and after heating. Fig. 10(c) shows the result before heating. Fig. 10(d) shows the result after 5 s heating. As can be seen, the presence of the delamination is clear. In addition, edge effects are also evident for the 45 cm standoff measurement. 3.4.3. Microwave time-resolved infrared radiometry Scientists in The John Hopkins University proposed microwave time-resolved infrared radiometry. Hewlett–Packard 6890B

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Oscillator (5–10 GHz) was used to produce microwaves at a frequency of 9 GHz. This signal was amplified by a Hughes 1277 X-band traveling wave tube amplifier and fed into a single flare horn antenna through rectangular waveguide. The horn antenna has a beam width of about 50° and was placed about 15 cm from the sample. The input power at the horn was 2.3 W and created a 20 mW/cm2 power density for the typical experiment. Both the angle of incidence and the polarization of the microwave field relative to the sample are controlled. The specimen surface temperature was monitored by an IR camera, a 128  128 InSb focal plane array operating in the 3–5 lm band. Time-dependent measurements were made by recording a series of frames before, during and after the application of the microwave step heating pulse. Two carbon fibers in fiberglass–epoxy at two different depths of 0.25 mm and 0.75 mm were tested. The time dependence of the temperature at the center of the fibers has an obvious difference [151]. 3.4.4. Microwave lock-in thermography Scientists in Politecnico di Bari in have developed a microwave lock-in thermography. A power relay controlled by a function generator commands the switch off/on of the oven. The excitation frequency was set as 0.1 Hz. The temperature evolution over time presents a strong drift due to the low velocity of heat diffusion and to low thermal conductivity of composite material [146]. Thermographic data were processed with a fast Fourier Transform based algorithm, which acquires frame by frame the thermal signal on the surface of the specimen and processes the thermal signal over time by means a Fourier transform in order to obtain the amplitude and the phase signal. These obtained images present some problem on the edges of the specimen. In fact, the interaction among electromagnetic field and specimen geometry provides a high temperature near the edge of the specimen. However, the impact damaged zone is clearly evident [146]. The principles of microwave thermography have been introduced. After this, the development and case studies of microwave thermography have been reviewed. It is concluded that: (1) Microwave thermography can be accomplished through microwave pulsed thermography, microwave time-resolved thermography, and microwave lock-in thermography. Predictably, microwave pulsed phase thermography and microwave frequency modulated thermography will also be realized soon; (2) Microwave thermography combines advantages of microwave and thermography, like being non-contact, fast, full-field, high efficiency, and uniform heating or selective heating; (3) Microwave thermography has shown the potential to be an effective NDT method for composites, especially for water detection in structures [28]. 3.5. Vibrothermography and ultrasound thermography 3.5.1. Basic knowledge In contrast to classical thermography, in which the energy is delivered to the sample surface by optical means, in vibrothermography arrangements the sample is mechanically excited by elastic wave from mechanical vibration, such as sonic or ultrasonic oscillations. Especially, if ultrasound is used as the excitation, vibrothermography is named as ultrasound thermography. In general, the propagation of the damped acoustic waves along the material converts mechanical energy into thermal energy, but in the vicinity of the defects the energy dissipation is bigger due to friction between the faces of the defect and/or stress concentration at the surrounding area. This mechanical excitation acts as a selective inner heat source, located just at the defect, which diffuses inside the material and can be detected as a temperature variation on its surface by means of an IR camera [152–154]. Attenuation of elastic waves in a solid due to internal mechanical losses was reported for the first time in the late 70s by

Henneke and his colleagues both for continuous and pulsed ultrasound injection, and then vibrothermography was introduced as a new type of excitation in the field of thermographic NDT [155– 157]. In 90s, the lock-in principle of modulating the excitation and analyzing the response with respect to the modulation frequency was applied to vibrothermography. And, lock-in vibrothermography was proposed [152]. In 21 century, ultrasound burst phase thermography (UBPT) [158,159] and ultrasound frequency modulated thermography (UFMT) [160] were proposed. The known ultrasound thermography methods are ultrasound lock-in thermography (ULT), ultrasound burst phase thermography (UBPT), and ultrasound frequency modulated thermography (UFMT). 3.5.2. Ultrasound lock-in thermography The lock-in principle of modulating the excitation and analyzing the response with respect to the modulation frequency can be applied to ultrasound thermography to achieve considerable improvement [161]. The basic principle of ULT is shown in [162]. The ultrasound amplitude modulated at a lock-in frequency is derived to sample through transducer. Modulation of the elastic wave results in periodical heat generation so that the defect is turned into a local transmitter of thermal wave, which propagates to the surface where it is detected after the sequence of images from IR camera has been processed. The signal processing method is fast Fourier transformation (FFT) performed at the modulation frequency. Both amplitude and phase can be obtained. Phasegram contains the propagation time of the thermal wave from the defect to the surface, so it indicates the depth where the defect is located [162]. In [163], ULT was performed on various kinds of typical defects in aerospace structures. An example is the cover of an access hole for aircraft maintenance. Due to an impact, there was damage at the outer edge next to the screws. The phase image of optical lock-in thermography (OLT) is dominated by the holes for the screws while the ULT image shows mostly the defect area where boundaries are rubbing against each other. Another example is impact detection. OLT image displays mainly the fiber directions ±45° with no indication of the impact, while ULT image shows only the impact. With ULT, the obtained phase angle images can reveal areas of hidden corrosion, cracks in rows of rivets, disbonds, impacts, and delaminations in aerospace structures. 3.5.3. Ultrasound burst phase thermography Ultrasound burst phase thermography (UBPT) was derived from the UBT and ULT. UBPT uses only short ultrasound bursts as excitation like UBT and the heating up followed by a cooling down period is recorded by IR camera. While, UBPT derive phase angle images like ULT using FFT [158]. Therefore, UBPT combines the advantages of both ULT and UBT. Due to the phase information, the signal to noise ratio of UBPT images (and hence defect detectability) is significantly better than temperature image taken from the recorded sequence. The local spectral components of that signal provide information about defects in a way similar to the lock-in technique but with an improved robustness against coupling problems and with a reduced measuring duration. As the characteristic defect signal is contained in a limited spectral range while the noise typically is distributed over the whole spectrum, one can reduce noise as well. UBPT allows for faster measurements than ULT while the advantages of phase images are the same: depth resolved recognition of defects, suppression of inhomogeneous emissivity and of temperature gradients [158]. In [158], UBPT was applied for inspection of adhesive joints. In modern automobiles, more and more crash optimized bonds are applied. These safety relevant adhesive bonded joints over 100 m per automobile have to be inspected during production process. Defects including entrapped air, poor adhesion, kissing bonds,

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Fig. 11. (a) Phase of OLT at 0.01 Hz, (b) phase of ULT at 0.05 Hz, (c) phase of UFMLT at 0.05 Hz [166]. Reprinted/reproduced with permission from [166].

and non-cured or missing adhesive can be effectively detected by UBPT. In [162], depth resolved defect selective imaging was achieved by UBPT. The highest frequency has the smallest depth range. 3.5.4. Ultrasound thermography involving frequency modulation There are two kinds of frequency modulation in ultrasound thermography. One is mono-frequency ultrasonic wave modulated by frequency modulated lock-in signal named as ultrasound frequency modulation thermography (UFMT) [164], which is derived from optical frequency modulated thermography (OFMT); the other is frequency modulated ultrasound as excitation named as frequency modulated ultrasound thermography (FMUT) [160,165], which can be applied as (1) burst phase thermography with pulse modulation and phase analysis, and (2) lock-in thermography with periodical modulation and phase analysis. They are respectively named as frequency modulated ultrasound burst phase thermography (FMUBPT) and frequency modulated ultrasound lock-in thermography (FMULT). UFMT has the similar advantage like OFMT, while FMUT is used to avoid the standing wave pattern. By applying a mono-frequent excitation to a sample it is not unlikely that this frequency matches to a resonance of the vibrating system. The result is probably a standing wave pattern. Due to hysteretic losses in the elongation maximum, these standing elastic waves can appear as temperature patterns causing misinterpretations: In the worst case the defect could be hidden in a node (‘blind spot’) while the standing wave maximum might appear as a defect. In order to avoid this problem, ultrasound frequency modulation thermography (UFMT) was proposed whose excitation is a frequency modulation ultrasound. In these cases the standing wave pattern is superimposed by a field of propagating waves giving sensitivity also where only nodes existed before [160,165]. In [160,166], a CFRP plate with nine areas of heat damage at the rear side of the sample was tested by OLT, ULT and UFMT. Due to the limited depth range of optically lock-in thermography, all damage remains undetected shown in Fig. 11(a). Using the ULT with its enlarged depth range, five of the nine damaged areas could be found, shown in Fig. 11(b). However, the monofrequent excitation (20 kHz) generates a temperature pattern caused by standing elastic waves. In particular, the smaller defects remain still undetected. A significant enhancement was achieved by wobbling the excitation frequency from 15 to 25 kHz with a modulation frequency of 20 Hz. The pattern was reduced substantially and eight of the thermal damage areas are visible, shown in Fig. 11(c). The development and case studies of vibrothermography and ultrasound thermography have been reviewed. It is concluded that: (1) ultrasound thermography can be accomplished through ultrasound pulsed thermography, and ultrasound lock-in thermography,

ultrasound burst phase thermography, and ultrasound frequency modulated thermography; (2) ultrasound thermography combines advantages of ultrasound and thermography, like full-field, fast, high resolution, selective heating. However, it is contact due to couplant; (3) ultrasound thermography has shown the positional to be an effective NDT method for composites. 4. Scanning thermography NDT 4.1. Line scanning thermography Line scanning thermography (LST) is a dynamic thermography technique patented by NASA, which is successfully applied to the inspection of metallic surfaces and composites. In LST, the heat source is moved across the sample’s surface at a constant speed and an IR imager is used to record the temperature changes on the surface during the heating protocol. The IR detector moves in tandem with the heat source and the imager’s field of view is set to contain a region above and below the location where heat is deposited. Temperature recorded from the unheated region provides information related to the baseline or initial temperature distribution of the sample. The temperature recorded in the heated region is used to analyze the cooling behavior after heat deposition detecting the presence of defects or material property changes [48,49]. In [50,51], the feasibility of using LST to inspect the bond quality in sections of wind turbine blades, and samples with similar geometry, material composition and layer structure used in the fabrication of wind turbine blades was explored. Three types of samples: a flat GFRP laminate with fabricated defects, sandwich panels with fabricated defects, and a section of a blade were tested. The results have shown that LST technique has provided a quick and efficient methodology to scan large composite wind turbine blades. 4.2. Scanning eddy current thermography Eddy current thermography can be configured as scanning configuration, where the induction coil moves over the sample at optimal speeds and the temperature on the sample is captured using an IR camera in the form of video images. From the recorded video images, the pixel columns are extracted to create a new image sequence. After adjusting the images according to the shift between two consecutive recorded images, the sequence represents the temporal change of the temperature after a short heating pulse. And then, the algorithms such as FFT can be applied to process the new image sequences. In [52,54], a scanning eddy current thermography was developed. The linear induction coil and the IR camera were fixed and the inspected sample was moved along them with a linear actuator. A conversion algorithm was developed by extracting always

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Fig. 12. (a) Sketch of the two adhesively bonded plates; (b) Thermal image and (c) phase image [54]. Reprinted/reproduced with permission from Taylor & Francis [54].

Table 2 Comparison between PT, LT, PPT, and FMT. Techniques

Strength

Limitation

Pulsed thermography Lock-in thermography Pulsed phase thermography

Full-field, high resolution, high sensitivity, quantification, fast, easy deployment

Small depth, long time for thick material, emissivity and non-uniform heating dependence, high power Compromise between depth and depth resolution, time consuming, Post signal processing

Frequency modulated thermography

Full-field, high resolution, higher sensitivity, quantification, low power, emissivity independence, elimination of non-uniform heating Full-field, high resolution, high sensitivity, quantification, fast, emissivity independence, elimination of non-uniform heating, greater depth and resolution, better detectability Full-field, high resolution, high sensitivity, quantification, fast, emissivity independence, elimination of non-uniform heating, greater depth, better detectability

one column of pixels parallel to the coil and then an image in a specified distance from the heating was created. Using all that pixel columns and creating a separate image from each of them, a new IR sequence was created. The generated image sequences were evaluated with the technique of PPT. The system was used to detect air gaps in adhesive bonded plates. Two steel plates with a thickness of 0.75 mm have been partially bonded, but the glue has been only applied in the shape of four circles with different diameters, see Fig. 12(a). The measurement has been carried out in transmission mode. The surface of the sample has not been blackened. Therefore, in the thermal image in Fig. 12(b) a small label sticking to the surface can be well recognized, as it has a different emissivity value as the metal surface. In contrast, in the phase image in Fig. 12(c) generated with FFT, the emissivity differences become negligible and the bounded areas can be very well recognized. In 1990s, inductive line heating has been used to detect delaminations in CFRP [167]. In [53], a real component made of CFRP was investigated. The results show that major challenge faced while scanning real component is the behavior of certain structural features such as holes and corners. These features are behaving like big defects and produce high thermal contrast which misleads us from the real defects. An image processing algorithm should be developed to minimize the effects of structural features and to enhance the results.

System complex, post signal processing

5. Comparison and discussion 5.1. Comparison between thermography NDTs 5.1.1. Different heating functions According to literature review [90,168–171] and authors’ experience, the comparisons between PT, LT, PPT, and FMT are listed in Table 2. Because of using IR camera, they are all full-field, high resolution, high sensitivity and can achieve quantification of thickness and depth. (1) PT is fast and easy to deploy but need long time for thick material. With PT, surface temperature gradients are not only caused by hidden defects but also affected by local variations on surface emissivity and non-uniform heating. At last, PT may cause the damage on the material due to the high energy. (2) The energy required to perform LT is generally less than PT. Due to lock-in technique, LT has higher sensitivity than PT. The phase angle has the advantage of being less sensitive to local variations of illumination and/or of surface emissivity. However, there is a compromise between depth and depth resolution. To detect defects located at various depths, repetition of the test at various frequencies becomes a time consuming process.

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Table 3 Summary and comparison for thermography using different thermal sources with major NDT methods for composites inspection. NDT techniques

Strength

Limitation

Ultrasound-echo/ phased array/linear array Guide wave Acoustic emission Shearography

Great depth, high resolution, many deployment options

Sound attenuation, coupling for contact testing, non-sensitive to surface defects

Large areas In-service, passive, large areas, Non-contact, full-field, fast, high sensitivity

Sound attenuation, coupling for contact testing Noise, bad quantitation, non-sensitive to static defects Sensitive to part movement, small thickness/stiffness, require unique test set-ups, expensive, hard to quantitatively analyze Conductive material, scanner required, sensitive to lift-off, low resolution Scanner required, near-field, lift-off influence Surface heating, small depth, non-sensitive to crack

Eddy current Microwave Optical thermography Laser thermography Eddy current thermography Microwave thermography Ultrasound thermography X-ray/Gamma-ray

Non-contact, low-cost, no surface treatment Non-contact, high resolution, suitable to dielectric material Non-contact, full-field, high resolution, high sensitivity, quantification, fast, low-cost Non-contact, full-field, high resolution, high sensitivity, quantification, fast, far distance Non-contact, full-field, high resolution, high sensitivity, quantification, fast, inner heating Non-contact, full-field, high resolution, high sensitivity, quantification, fast, uniform heating, selective heating Full-field, high resolution, high sensitivity, quantification, fast, selective heating High resolution, non-contact

(3) PPT combines the advantages of PT and LT. Like LT, PPT is independent on emissivity and non-uniform heating. And, PPT is faster than LT because of applying short pulse as excitation and containing wide frequency spectra. However, the energy decreases with the increase of frequency. PPT has greater depth with better resolution, and better detectability than PT, but needs post processing algorithm. (4) FMT has all the advantages mentioned above, like fast, emissivity independence, elimination of non-uniform heating, greater depth and resolution, better detectability (SNR). However, its system is the most complex. 5.1.2. Different excitation sources Some comparison works were conducted by other researchers [150]. In [172], the capabilities of different approaches such as optical pulsed and lock-in thermography, vibrothermography and LED illumination have been compared in order to detect artificial Teflon inserts between CFRP plies using Tanimoto criterion. In [168], a comparative study involving pulsed thermography, lockin thermography and vibrothermography was offered through experimental results for two typical aerospace parts: honeycomb structures and Glare. In [173], optical, ultrasonic, and inductive excited lock-in thermography were compared in aerospace applications. Summary and comparison for major thermography methods with different excitation including optical thermography, laser thermography, eddy current thermography, microwave thermography, and ultrasound thermography are provided in Table 3. They all are full-field, high resolution, high sensitivity, fast, and can achieve quantification of thickness and depth. They have some distinguishes: (1) Optical thermography and laser thermography are SHT for composite, while, eddy current thermography and microwave thermography are VHT for composites. Ultrasound thermography is abnormal heating thermography. VHT and AHT have an advantage that thermal waves only have to travel half the distance (from the defect to the surface) than SHT in reflection mode (from the surface to the defect and back to the surface); (2) Ultrasound thermography is sensitive to inner friction and therefore defect-selective. However, it is contact and need holding the specimen, while optical thermography, laser thermography, eddy current thermography, and microwave

Small heating area, scanner required, more suitable for surface crack Conductive or semi conductive material, non-uniform heating effect, excitation system complex, near-field heating, small heating area Heating system complex, electromagnetic radiation Contact, need holding specimen, lacking of quantitative studies X-ray radiation hazards, operation complex, scanner required

(3)

(4)

(5)

(6)

thermography are non-contact. And, UT is still lacking of quantitative studies, and very often optimal inspection parameters must be found experimentally. Laser thermography can be applied from long distance. However, due to small heating area, laser thermography usually needs scanning mechanism for spot scan or line scan. It is sensitive to surface crack due to lateral heat conduction. Optical thermography, laser thermography, microwave thermography, and ultrasound thermography are suitable for almost all composites, while eddy current thermography works for conductive composites, like CFRP. Microwave thermography is the most immature in theory, system development, and application comparing with other thermography NDTs. Microwave thermography is very useful for water-filled defect detection. Optical thermography is normally used to detect lateral defects such as delamination and disbond. Laser thermography is especially sensitive to surface crack. Ultrasound thermography is sensitive to all types of flaws and damages. Eddy current thermography is sensitive to both crack and lateral defects, and it can be used to observe the carbon fiber pattern. Just like eddy current thermography, microwave thermography is sensitive to both crack and lateral defects. And it is especially sensitive to inclusion such as water and conductive objects.

5.2. Comparison with other NDTs Thermography and other NDT techniques were compared in composite inspection [174–176], including wind turbine blade [177], low energy impact detection in CFRP laminates [127,178], and 3D carbon fiber polymer matrix composites. Two case studies will be introduced. (1) Wind turbine blade [179]. In [177], the detection capabilities and performance of ultrasound, shearography, thermography and X-ray CT techniques for the inspection of wind turbine blades with delamination defects have been analyzed. Two specimens of E-glass/polyester and E-glass/epoxy with several Teflon inserts have been tested. Finally, the efficacy of each NDT technique has been analyzed and a comparison among their capabilities for such application has been

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carried out. Thermography has shown to be a potential technique for wind turbine blade inspection. The inspection time is relatively low (150 s) and it has been proven to be highly sensible to delaminations. Furthermore, it is contactless, the implementation is easy and low-cost and the required user formation is low. However its limitations for highthickness measurements should be taken into account. The performance of X-ray CT is largely dependent on the thickness of delamination and on the part size. Thus, dedicated X-ray CT systems for big parts measurements must be analyzed in order to adequately evaluate its performance in wind turbine blade inspection. Ultrasonic inspection is able to detect delamination damage and provide in-depth information, even in thick parts. But complex signal processing, longer acquisition times and the need of contact reduce drastically its potential for big parts inspection such as wind turbine blades. Although being the fastest technique, Shearography has shown limited sensitivity to delamination defects in thick parts and furthermore, the required optical equipment is relatively complex and expensive. (2) Low energy impact in CFRP laminates. Low energy impacts in CFRP laminates from 4 J to 12 J were evaluated by pulsed eddy current [178] and eddy current pulsed thermography [127]. Eddy current pulsed thermography shows the advantages including high-speed. (3) 3D carbon fiber polymer matrix composite. 3D Carbon fiber polymer matrix composites are increasingly used for aircraft construction due to their exceptional stiffness and strengthto-mass ratios. However, defects are common in the 3D combining areas and are challenging to inspect. In [180], different NDT techniques including ultrasound C-scan, pulsed thermography, vibrothermography, laser spot thermography and laser line thermography were used to detect microporosities in 3D carbon fiber composite. Microscopic inspection allows the sample’s interior to be examined in a clear manner, but this inspection technique is time consuming, and it is thus impossible to conduct a large-scale inspection. Micro-laser spot thermography has the same problem as microscopic inspection. Ultrasound testing does not allow micro defects to be detected because there is noise and coupling. Classical pulsed thermography is also not suitable for micro-defects inspection because of noise, coupling and resolution. Finally the authors defined a micro-laser line thermography and it is a promising technique for micro defects detection. The micro-laser line thermography results clearly indicate a few micro porosities on the surface and also near the surface. However the inspected depth was limited because the energy diffusion of the laser line was changing. According to the literature review and our experiences, summary and comparison for major NDT methods are provided in Table 3.

6. Trends 6.1. New physics and multiple physics Physics is foundation of NDT techniques and systems. If the materials under test are different, the physics and NDT will also be different. Conventional NDT techniques are mainly developed for metal and its alloy. However, composites’ parameters are totally different from metals. For example, they are anisotropic in parameters due to fiber reinforcement. And, they are multilayered. What’s more, a composite structure includes several mate-

rials, such as sandwich structures. Therefore, the physics and NDT for composites are different from that for metal. In recent years, some new physics and multiple physics based NDT were proposed for composites. One example is eddy current thermography based on volume heating. With eddy current thermography, the induced eddy currents have a skin depth, which varies with different materials. For ferromagnetic materials with large electrical conductivity and permeability, skin depth is very small on the order of lm. Given 100 kHz excitation frequency, the skin depth for cast iron is about 45 lm. Thus, the heating style for ferromagnetic materials is considered as surface heating. Under this condition, heat conducts from surface to inside. If there is a defect that block the heat conduction, heat will reflect from the defect to surface [181]. For CFRPs (r = 1000 S/m and no magnetic), the skin depth is significantly large (50 mm when excitation frequency is 100 kHz). Thus, the heating style for CFRPs is volumetric heating. If there is a defect, thermal abnormity caused by defect will directly transfer from defect to outside. This is the physics behind eddy current thermography based on volume heating [34,35,106]. In addition, sound propagation, electromagnetic field/wave, and other multi-physical field such as thermo-sonics field, thermoelastic effect, in CFRP/GFRP are also different from that in metal. Thus, more attention should be paid to new physics and related NDT for composites.

6.2. Simulation and modeling In order to understand the physics (such as heat generation and heat transfer) behind NDT techniques in CFRP and GFRP, simulation and modeling such as finite element method (FEM), finite difference method (FDM) were used. In addition, simulation and modeling are useful to optimize the sensor design and technique parameters. What’s more, accurate models will significantly reduce experimental costs and aid in the understanding of the influence of parameters such as flaw location, flaw size, flaw orientation, specimen size, and specimen shape on the NDT. In [123], a methodology based on shell elements was presented to model the electromagnetic-thermal behavior of multi-layered conductive CFRP. The standard 3D finite-element formulation was accomplished by a multilevel shell element representation. This leads to a reduced number of unknowns which improve the system convergence and consequently the computational time. The model developed was validated with classical 3-D finiteelement method and experimental results. The results show that the approach is suitable and relatively easy to implement. In [124], an electromagnetic and thermal model of CFRP composite material was introduced. This model takes into account the influence of different fiber orientations on the electromagnetic parameters. These parameters are defined in microscopic scale and introduced in finite-element-method model in macroscopic scale. In [182], the use of the finite element analysis (FEA) method to model vibrothermography systems was investigated. Physical samples of glass epoxy composites that contained large delaminations were tested using vibrothermography to create an empirical data set that was used to verify an FEA model of a similar system. The resulting surface temperatures in the FEA model were compared to those observed in the physical test. Both the strengths and shortcomings of using FEA to model these systems were discussed and the proposals for how to improve the model accuracy were provided. The use of FEA to create thermal models of vibrothermography systems was described in order to generate a detection model that could indicate the excitation time required to detect a flaw according to its size, depth, and heat generation rate. The model also accounts for measurement noise and camera

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R. Yang, Y. He / Infrared Physics & Technology 75 (2016) 26–50 Table 4 Standards in relation to thermography NDT for composite inspection. Document #

Title

Developer

ISO 10878:2013 ISO/AWI 10880 ISO/AWI 10881 ISO/CD 18251-1 ASTM E2533-09 ASTM E2582-07(2014) TAPPI TIP 0402-28 MIL-HDBK-731 NOT 1 BS ISO 10878:2013

Non-destructive testing – infrared thermography – vocabulary Non-destructive testing – infrared thermographic testing – general principles Non-destructive testing – infrared thermography – guidelines for examination of electrical installations Non-destructive testing – infrared thermography – system and equipment – Part 1: description of characteristics Standard guide for nondestructive testing of polymer matrix composites used in aerospace applications Standard practice for infrared flash thermography of composite panels and repair patches used in aerospace applications Best practice for inspecting used fiber-reinforced plastics (FRP) equipment Nondestructive testing methods of composite materials – thermography Non-destructive testing – infrared thermography – vocabulary (British Standard)

ISO ISO ISO ISO ASTM ASTM TAPPI US DoD BSI

distance, and was found to be accurate with short detection times and less reliable for those with longer times. 6.3. Signal processing algorithms Signal processing (SP) is a crucial means to extract useful information from raw data captured from sensors. More and more signal processing algorithms including thermographic signal reconstruction (TSR) [59], principal components analysis (PCA), independent components analysis (ICA) [98,106,129,183], wavelet transform [184], tucker decomposition [185], Support vector machine [186], and pattern recognition [187,188] are being and will be used in thermography for composites. Take cross correlation matched filtering (CCMF) for example. CCMF is a classical algorithm in radar field and has been introduced in thermography field. With the help of CCMF, a lot of new methods and applications were developed. Mandelis et al. proposed thermal wave radar (TWR) combining linear frequency modulated continuous wave excitation and CCMF signal processing based on Hilbert transform to detect human dental demineralization lesions and osteoporotic bone loss [84], which suggested a significant improvement in depth-resolution dynamic range of subsurface defect. In 2014, Liu used thermal wave radar imaging based on CCFM for CFRP inspection [92]. Also in 2014, Mulaveesala highlighted the Hilbert transform-based time domain phase analysis scheme introduced for testing and evaluation of sub-surface defects in a mild steel sample [136]. Mandelis et al. proposed truncated-correlation photothermal coherence tomography (TC-PCT) based on Hilbert transform and CCMF, which enabled three-dimensional visualization of subsurface features [137]. Recently, pulsed inductive thermal wave radar (PI-TWR) was proposed by introducing the CCMF in eddy current pulsed thermography. The results illustrate a significant improvement in the dynamic range, depth resolution, emissivity variation reduction and detectability of subsurface defects and inside delamination for the NDT applications [138]. It can be concluded that non-uniform heating effect and lateral blurring effect can be all suppressed through CC phasegrams, and then, damage shape identification can be improved. 6.4. Standard specimen With the goal to explore new NDT technologies and innovations which could potentially improve the efficiency and effectiveness of wind energy, the unique WTB specimens with variety of flaws are increasingly acquired. Take wind turbine blade as example. In [189], a specific wind turbine blade was built using the CX-100 design developed by TPI Composites, Inc. and Sandia National Laboratory (SNL). The 9-m blade was constructed with several embedded defects that

represent the most common manufacturing defects typically found, such as out-of-plane waviness, composite delamination, and adhesive disbond. The defects were embedded during the manufacturing process by using similar methods developed by both TPI and SNL for simulating actual defect characteristics. Though the blade is small in comparison to the average utility sized blade of around 40 m, the blade features similar materials and manufacturing methods, allowing for several NDT techniques to be studied on a representative platform. In [190], the test specimens used to evaluate the maturity and viability of a wide range of NDT methods contained an array of different, representative flaw types and wind turbine blade construction types including: snow flaking, voids, inter-ply delaminations, resin-starved regions, spar and shear web disbonds, ply waviness, adhesive voids, fiber fracture, erosion, impact, lightning strike, and fluid ingress. 6.5. Integrated inspection system Thermography NDTs are based on multi-physics effect such as optical-thermal effect, eddy current heating, microwave heating, and thermoelastic effect. By now, electromagnetic induction, microwave, and ultrasound have only been used to create thermal waves in the specimen in order to carry out the inspection with an IR camera. However, original physical fields have been rarely used. For example, with eddy current thermography, the large amount of eddy current induced in the material can also be used to perform eddy current inspection. This gives us an opportunity for development of integrated inspection system. In [191], a new hybrid system that can perform eddy current thermography and eddy current inspection simultaneously was developed. In [192], a system combining laser spot imaging thermography and ultrasonic measurements was developed for crack detection. 6.6. Standards for composites At present, there are published standards on the thermography NDT. The subcommittee TC 135/SC 8 titled ‘Infrared thermography for non-destructive testing’ of International Organization for Standardization (IOS) has developed ISO 10878:2013. And, there are three standards about thermography under development. ASME’s standard E2533-09 covers the established NDT methods including thermography as applied to polymeric matrix composites. ASME’s E2582-07(2014) describes a procedure for detecting subsurface flaws in composite panels and repair patches using flash thermography. The current state of standards in relation to composite inspection is summarized in Table 4. British Standard Institute (BSI) has developed the related standards according to IOS. However, these standards for NDT of composites are not sufficient. Thus, there is a need to develop the standards for NDT of composites components, particularly for in-service NDT.

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7. Conclusions Composites, such as GFRP and CFRP, are being increasingly used in the aerospace, renewable energy, civil and architecture, and other industries. Flaws and damages such as impact, delamination, and disbond are inevitable during either fabrication or lifetime of a composite structure and/or component. Thus, NDT are required to prevent failures and to increase reliability of composites structures and/or components in both manufacturing and in service. IR thermography NDTs have shown the great potential and advantages, which has greater inspection speed, higher resolution and sensitivity, detectability of inner defect due to heat conduction and requiring no couplant. Besides conventional optically excited thermography, laser thermography, eddy current thermography, microwave thermography, and ultrasound thermography are drawing increasingly attentions. They are based on the totally different mechanisms. In this work, a fully, in-depth and comprehensive review of these thermography techniques for composites inspection was reported based on an orderly and concise literature survey. Firstly, basic concepts for thermography NDT were defined and introduced in case of heating function, excitation source, relative position, heating style, and relative motion. Specially, volume heating thermography was proposed. Next, the developments of thermography NDTs in the order of optical thermography, laser thermography, eddy current thermography, microwave thermography, and ultrasound thermography for composite inspection were reviewed. Then, some case studies for scanning thermography such as line scanning thermography, and scanning eddy current thermography were reviewed. After that, the strengths and limitations of thermography NDT techniques were concluded through comparison studies between different heating functions and excitation sources, and with other NDT techniques. At last, some research trends in thermography NDT were predicted, such as new physics and NDT such as Material enabled thermography [193], simulation and modeling, specimen with variety of defects, signal processing algorithms, inspection systems, and standards for composites inspection. This work containing a critical overview, a detailed comparison, and an extensive list of references will disseminates knowledge between users, manufacturers, designers and researchers involved in composites science and engineering. Conflict of interest There is no conflict of interest. Acknowledgements The work was supported by National Natural Science Foundation of China (Grant Nos. 51408071 and 61501483). The authors are also grateful to China Scholarship Council for sponsoring Dr. Yunze He to Newcastle University, UK and Dr. Ruizhen Yang to University of British Columbia, Canada for joint study. Appendix A. Abbreviations

AHT CFRP BPT ECLT ECPPT ECPT ECST

abnormal heating thermography carbon fiber reinforced polymer burst phase thermography eddy current lock-in thermography eddy current pulsed phase thermography eddy current pulsed thermography eddy current step thermography

ECT ECVHT FMT FMUBPT FMULT FMUT FRP GFRP LFMT LLT LPPT LPT LST LT MUT MWLT MWPT MWT MWTRT MT NDT OLT POD PPT PT SHT SPT SPPT ST TWR UBPT UFMT ULT UT VHT

eddy current thermography eddy current volume heating thermography frequency modulated thermography frequency modulated ultrasound burst phase thermography frequency modulated ultrasound lock-in thermography frequency modulated ultrasound thermography fiber reinforced polymer glass fiber reinforced polymer laser frequency modulated thermography laser lock-in thermography laser pulsed phase thermography laser pulse thermography line scanning thermography lock-in thermography material under test microwave lock-in thermography microwave pulse thermography microwave thermography microwave time-resolved thermography modulated thermography nondestructive testing optical lock-in thermography probability of detection pulsed phase thermography pulsed thermography surface heating thermography square pulse thermography square pulse phase thermography stepped thermography thermal wave radar ultrasound burst phase thermography ultrasound frequency modulated thermography ultrasound lock-in thermography ultrasound thermography volume heating thermography

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