Characterizing damage in CFRP structures using flash thermography in reflection and transmission configurations

Composites: Part B 57 (2014) 35–46

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Characterizing damage in CFRP structures using flash thermography in reflection and transmission configurations Christiane Maierhofer a,⇑, Philipp Myrach a, Mercedes Reischel a, Henrik Steinfurth a, Mathias Röllig a, Matthias Kunert b a b

BAM Federal Institute for Materials Research and Testing, Division 8.4, Unter den Eichen 87, D-12205 Berlin, Germany DGZfP German Society for Non-Destructive Testing e. V., Schillerplatz 3, D-19322 Wittenberge, Germany

a r t i c l e

i n f o

Article history: Received 18 April 2013 Received in revised form 8 August 2013 Accepted 16 September 2013 Available online 26 September 2013 Keywords: D. Non-destructive testing D. Thermal analysis B. Thermal properties B. Delamination Flash thermography

a b s t r a c t Carbon fiber reinforced polymer (CFRP) specimens with artificial delaminations and with impact damage have been characterized using active thermography with flash excitation. Systematic investigations have been performed in four different experimental configurations of flash lamps and infrared (IR) camera in transmission as well as in reflection alignment. It is shown here that the diffusivities determined for the sound and for the damaged areas give a good measure for damage characterization. Although reflection measurements also give information about defect depth, reflection measurements from only one side are not sufficient for assessing the whole cross section of the specimens. Thus, depending on sample thickness the lateral size of damage could only be determined from reflections measurements from both sides or from transmission measurements. In this paper, measurement accuracy and limits of flash thermography for the investigation of CFRP specimens are presented in detail together with quantitative data concerning the defects. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The area of application of carbon fiber reinforced polymers (CFRP) has strongly increased during the last years with the main aims of reducing weight of vehicles in transport industry (e.g. aerospace, automotive), for energy consumption and of exploiting the large strength- and stiffness-to-weight ratios for larger constructions (e.g. wind turbines and even bridges) [1–4]. Due to the inhomogeneous structure, a good bonding between fiber and matrix is required and delamination between the layers is the most critical damage that might lead to failure of the whole structure [5]. Delamination occurs due to different influences which can be attributed to the fiber/matrix materials, their structure and alignment, manufacturing processes and environmental conditions and loading in service. In several cases including low velocity impacts, delamination and further damage is not directly visible at the surface. Thus, non-destructive testing (NDT) methods are required for periodical and incident correlated testing of CFRP structures. NDT methods which currently have been further developed and which are applied for testing these structures are X-ray, ultrasonics, eddy-current, electrical conductivity, shearography and active thermography [6]. X-ray computed tomography is very well suited to resolve internal structures like single carbon fibers and ⇑ Corresponding author. Tel.: +49 30 8104 1441; fax: +49 30 8104 1847. E-mail address: [email protected] (C. Maierhofer). 1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.09.036

bundles, resin, macro- and micropores and cracks [7]. Although time consuming, the method is appropriate as a reference for further NDT techniques. As the whole sample is positioned into the measurement system, restrictions in geometry have to be considered. With ultrasonics, the resolution is limited by scattering effects due to the inhomogeneous structure. Large success in locating delaminations has been obtained using ultrasonic phased array techniques [8] and ultrasonic techniques with air coupling [9,10]. Both methods can be easily automated. Disadvantages are the required surface contact by water coupling for single transducers and arrays, and a set-up in transmission configuration for air coupling. Although low, the electric conductivity of the carbon fibers allows the investigation of fiber orientation by eddy current techniques [11]. A high resolution of the outer layers enabling imaging of single fiber bundles can be achieved by applying high frequency techniques [12]. Delaminations close to the surface can be detected, too. The conductivity of the fibers is also exploited by electrical resistivity measurements, where the longitudinal resistance is measured during fatigue experiments by a four wire technique [13]. Here, a direct access to the samples is required and reference data are needed. Shearography is based on the analysis of the difference of two speckle pattern at different deformations of the object and has already been proven for detecting delamination and further inhomogeneities in aircraft industry [14,15]. A comparison of lock-in thermography and lock-in speckle interferometry shows that although the latter one is able to detect

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inhomogeneities, quantification is difficult due to the gradients in the data [16]. In comparison to these techniques active thermography has the following advantages: First, the measurements can be performed without any direct contact and at a suitable distance to the sample. Second, larger areas (up to 1 m2) can be tested fast and without scanning. And third, as will be shown below, a number of quantitative information can be obtained. For active thermography, there are several different excitation sources and techniques available, e.g. flash lamps for pulsed heating, halogen lamps for step and periodic heating, infrared (IR) radiators, fan heaters, etc. Thermal excitation can also be achieved indirectly using ultrasonics or inductive heating. Already in 1998 and 2000, round robin tests have been realized at CFRP test specimens with impact damage for comparing different active thermography techniques like optical excitation with flash lamps, static and modulated halogen lamps and periodic ultrasonic excitation [17]. Here, the detection of the impact damage from the side of the impact was only possible using ultrasonic excitation. As up to now flash excitation is the fastest excitation technique, further research was focused optimizing this technique for investigating CFRP structures. Systematic investigations have been performed concerning the appropriate selection of measurement parameters [18–20]. In order to improve the signal to noise ratio of defect indications, signal processing techniques like TSR (thermal signal reconstruction) [21] and PPT (pulse phase thermography) [22] have been developed (among others). In 2007 an ASTM standard was published appointing the procedures for thermal testing of composite panels and repair patches used in aerospace applications [23]. Flash thermography in transmission and in reflection configuration is well suited for quantification of diffusivity and thus porosity in CFRP materials [24]. First systematic investigations concerning the quantification of impact damage in CFRP with optical step impulse and ultrasonic excited thermography have shown that the signature of impact depth can be compared with that of flat bottom holes [25]. In this paper, for the first time a systematic comparison is given on flash thermography using reflection and transmission configuration for the characterization of inhomogeneities in different CFRP structures. The main aim of this study was to demonstrate the limits as well as the advantages of both configurations for a detailed damage assessment. Penetration depth, depth resolution and the determination of lateral defect size have been assessed using different CFRP structures with artificial delaminations and impact damage. Although the results of reflection measurements give information about the position, lateral size and depth (coverage with sound material) of damage, transmission measurements can be used for a more detailed quantification of the amount of damage along the whole cross section of the sample.

2. Principle and theory of active thermography Flash thermography measurements are generally performed by heating the sample with a short flash from a Xenon flash lamp with a duration of 1 ms to several tens of ms and by recording the surface temperature as a function of time. During the flash, a thin layer of the specimen is heated up. Afterwards, the heat diffuses into the material. If there are any inhomogeneities inside the material, e.g. delamination or inclusions, the heat diffusion is altered depending on the thermal properties of the sound material as well as on the inhomogeneity. In case of delamination or voids, heat diffusion is reduced and the temperature becomes higher in front of the defect and lower behind it in comparison to the surrounding material. Flash thermography can be well described by 1D analyt-

ical models [26] assuming that a Dirac pulse with an energy q0 is heating up the surface. Here, the temperature T as a function of time t at the surface (x = 0, reflection) as well as at the backside (x = L, transmission) of a plate having a thickness L, a diffusivity a, a density q and a specific heat capacity c is given by

"  2 2 # 1 X q0 n p Tð0; tÞ ¼ 1þ2 exp  2 at qcL L n¼1

ð1Þ

and

TðL; tÞ ¼

"  2 2 # 1 X q0 n p 1 þ 2 ð1Þn exp  2 at qcL L n¼1

ð2Þ

If the material parameters are known, these equations are well suited for estimating maximum surface temperatures and temporal temperature changes and thus for selecting optimum measurement parameters like temperature range, temperature resolution and frame rate of the IR camera as well as the required measurement duration. The other way around, if experimental data have been already recorded, a fit of the experimental temperature evolution using Eq. (1) or (2) enables estimation of the absorbed energy and of material parameters and their changes due to defects. Although the thermal contrast of the defect also depends on the difference of the effusivities of defect and sound material, on the thermal contact resistance of the defect and on its lateral size, in case of larger delamination or voids, the temperature above these can be calculated from Eq. (1) replacing L by the defect depth d. It has to be considered that lateral heat flow is neglected in these 1D analytical calculations. Lateral heat flow is induced due to inhomogeneous heating, inhomogeneous absorption of heat due to inhomogeneous reflectivity at the surface and lateral temperature gradients above defects. In the latter case, temperature increase above defects like delamination or voids is reduced by the heat flow around the defect. Therefore, the temperature above a defect and its time of maximum contrast also depends on its aspect ratio (ratio of lateral size of defect versus coverage) and on lateral diffusivity of the surrounding material. This behavior can be described analytical by enhancing the 1D model [27]. In the description above, it is assumed that the defect is characterized by an air gap representing a high absolute value for the reflection coefficient R for the heat impulse. Thus, the heat diffusion through the defect can be neglected. In case of real delamination, it is possible that along the defect, some thermal contact still remains enabling a reduced diffusion through the defect. This can be described by a thermal resistance and the reflection coefficient at the interface becomes complex leading to a phase shift between reflected and transmitted wave [28]. This complex reflection coefficient depends on the thermal resistance and on the frequency of the reflected thermal wave in the way that the absolute value of R decreases if any parameter is decreasing. As dispersion of thermal waves is very high, in reflection configuration interfaces with weak thermal resistance can only be detected if they are located close to the surface. As already mentioned above, flash thermography can be performed in reflection as well as in transmission configuration. For the first configuration, flash lamps and IR camera are on the same side of the specimen and for the latter, on opposite sides. Measurements in transmission configuration are only possible if the specimen is accessible from both sides. Up to now, there are only a few comparisons between both configurations. Only in transmission configuration, flash thermography has been proven to quantify the quality of spot welds reliably [29]. For the investigation of porosity content in CFRP, the results of transmission measurements show better correspondence with analytical data than those

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of the reflection measurements [30]. For detecting and evaluating wall thinning with inductive heating, it was found that transmission mode is more suitable than reflection mode as some defects with small aspect ratio could only be detected in transmission mode [31]. Apart from the accessibility, both configurations have its advantages and disadvantages. In reflection configuration:  defects close to the surface can be detected with high sensitivity and high lateral resolution,  only the upper surface of defects can be examined and no information is gained on the material underneath,  the thermal contrast of the defect depends on the difference of the effusivities of defect and sound material, on the contact resistance between both and on the aspect ratio of the defect,  the penetration depth is less than in transmission configuration as the heat has to travel twice the distance,  depth resolution can be obtained by applying TSR [19] or PPT [20], In transmission configuration:  an integral information of the sample along the whole cross section is gained,  all information about depth resolution is nearly lost,  thicker samples as in reflection configuration can be investigated,  defects with smaller aspect ratio and with larger absolute coverage can be detected, in comparison to reflection configuration,  fitting with the analytical solution in Eq. (2) is easier than for reflection measurements, as no initial surface temperature has to be estimated (which might be difficult due to the superpositioning of the emitted radiation with reflected light and heat from the lamp) and temperature changes occur slower.

3. Experimentals For all CFRP samples described below, reflection and transmission measurements have been performed from both sides, thus at least four data sets have been recorded for each specimen. Details are described in the following.

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3.1. Test specimens 3.1.1. Steps with artificial delaminations Two stepbar specimens have been made of aviation approved epoxy resin impregnated carbon filamentary materials (prepregs) with a layer thickness of 0.13 mm. The specimens have been assembled according to DIN EN 2374 [32] using prepreg technology and autoclave method with a pressure of 7 bar at 180 °C for about 20 min. The thickness of the steps (St) was 5.4, 4.9, 4.5, 4.0, and 3.6 mm for specimen no. S1 and 3.3, 2.7, 2.3, 1.9, and 1.4 mm for specimen no. S2. S1 has been built by 48 prepreg layers with different lateral sizes and quasi isotropic orientation [+45/ 45/0/90°]12 and for S2, 28 layers have been applied by 7 times replicating the quasi isotropic arrangement. In both stepbars, double-laid sheets of Polytetrafluoroethylene (PTFE) with a thickness of approx. 0.1 mm each and with sizes D between 20  20 and 2  2 mm2 have been inserted on a pile of 8 layers which corresponds to a thickness of about 1 mm. Thus the coverage of these artificial delaminations measured from the flat side of the bars was always about 1 mm. From the stepped sides, the coverage of these defects varied between 0.4 and 4.4 mm. Sketches of both specimens are shown in Fig. 1.

3.1.2. Plates with impacts For the test specimens with low velocity impact damage, plates with sizes of 100  150  3 mm3 have been constructed according to the description of test specimen in DIN 65561 [33] containing 24 layers of unidirectional non-woven fabric (unidirectional prepreg) ordered symmetrically in direction of 45°, +45°, 90° and 0° to the mid layer. Low velocity impacts have been introduced by a drop weight with two different impactors with radii of 8 and 10 mm. These have been mounted onto two different slides which resulted in weights from 2.26 to 5.8 kg. Velocities of 1.7–3.4 m/s have been reached. In total, impact energies between 4 and 25 J have been realized. Here, the investigations comprises three samples damaged with impact energies of 4.4 J (no. 24L01), 14.4 J (no. 24L02) and 24.5 J (no. 24L03). At the front side, the impact was only hardly visible while at the backside of 24L02 and 24L03, a spread off of fiber bundles was observed with an increased damaged area with increasing impact energy, as visualized in the photos in Fig. 2.

Fig. 1. Sketches of stepped specimens with double layer PTFE inserts of different sizes. (a) Specimen S1 with step sizes from 3.6 to 5.4 mm and coverages from 2.6 to 4.4 mm. (b) Specimen S2 with step sizes from 1.4 to 3.3 mm and coverages from 0.4 to 2.3 mm. All values are belonging to mm.

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Fig. 2. Plates with sizes of 100  150  3 mm3 and impacts with different energies from 4.4 to 24.5 J. The photos show the backside of the impact where for 14.4 and 24.5 J, some fiber delaminations are recognizable. (a) Specimen 24L01 with 4.4 J. (b) Specimen 24L02 with 14.4 J. (c) Specimen 24L03 with 24.5 J.

Fig. 3. Three sections of a side rudder with stringers, shown from the stringer side. Impacts have been introduced from this side at the marked positions. (a) SR07 with main sizes of 410  380 mm2. (b) SR08 with main sizes of 450  375 mm2. (c) SR10 with main sizes of 365  315 mm2.

3.1.3. Aircraft rudder with impacts Sections of the side rudder of an airplane have been cut off and low velocity impacts have been introduced afterwards with an energy of about 4 J. Three of these specimens (SR07, SR08 and SR10) shown in Fig. 3 have been examined. The impacts have been introduced from the stringer side, as it was easier to position the samples with the stringers upwards. At the stringer side (front side of the impact), a very weak indent is visible. At the front side (back side of the impact), a larger buckling with some small cracks has appeared, especially for SR10. It has to be considered that at the front side, a thin but dense copper mesh for lightning protection was included.

3.2. Experimental set-up The experimental investigations concerning reflection as well as transmission measurements have been carried out using four synchronized flash lamps with a 2.6 ms long flash and a nominal energy of 6 kJ each and an IR camera with a cooled InSb focal plane array. The duration of the flash at 6 kJ was measured with a Si photodiode by analyzing the recorded signal width at half maximum. For reducing the disturbing IR radiation of the hot lamps after flash, Polymethylmethacrylate (PMMA) sheets have been positioned in front of each lamp. The detector of the IR camera consists of 640  512 pixels and works in a wavelength range between 1.6 and 5 lm. The maximum full frame rate of 93 Hz in the integrate then read modus has been used, thus all pixels in one thermogram have been recorded at once as a snapshot. The IR camera was operated in a temperature calibrated mode ranged from 10 to 50 °C, which corresponds to an integration time of 0.63 ms. The field of view of the 29 mm lens is 18.8  15.1°.

3.2.1. Reflection configuration The four flash lamps have been positioned on the same side of the specimen as the IR camera. The distance of the lamps to the specimens was varied between 0.25 and 0.8 m and the lamps have been adjusted to illuminate the specimen homogeneously and avoiding direct reflections into the camera, see Fig. 4a). Thermal sequences containing 1050 images have been recorded including a few images before the flash. 3.2.2. Transmission configuration The four flash lamps have been positioned on the opposite side of the IR camera, see Fig. 4b). For avoiding direct flashes into the IR camera, the smaller samples have been surrounded by a thick paperboard (the sample contour was cut from the paperboard). Due to small slits, it was still possible to detect the flash, which was required for the zero point on time scale. Thermal sequences with 1050 up to 1200 images have been recorded. The four different configurations for reflection and transmission and its indications are shown in Fig. 4c). 4. Results and discussion 4.1. Stepped test specimen with artificial delaminations 4.1.1. Transmission configuration Thermograms as well as phase images of data recorded in the two transmission configurations TF and TB (IR camera at the stepped/front side and at the plain/back side) of both specimens (nos. S1 and S2) are shown in Figs. 5 and 6, respectively. For each step, the thermogram with the optimum contrast has been selected. Thus, the thermograms represent compound images opti-

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Fig. 4. Experimental set-up in reflection (a) and transmission (b) configuration. In (c), the different set-ups are shown schematically (only for two flash lamps each).

Fig. 5. Compound thermograms (a and b) (time after flash: St1:0.3 s, St2: 0.62 s, St3: 0.9 s, St4: 1.16 s and St5: 1.7 s) and phase images (c and d) (frequency: St1: 0.73 Hz, St2: 0.54 Hz, St3: 0.36 Hz, St4: 0.27 Hz, St5: 0.27 Hz) recorded in two different transmission configurations at specimen no. S2 with steps from 1.4 to 3.3 mm. (a and c) Configuration TF. (b and d) Configuration TB.

mized to each step. The phase images have been calculated by applying a Fast Fourier Transformation (FFT) to each pixel to the whole cooling down time recorded after the flash. Also the phase images have been optimized for each step. For the transmission configuration TB at S2 (Fig. 5b), slightly more defects can be detected as in Fig. 5a). In the phase images, this is shown even clearer: with the IR camera at the plain side of the specimen (Fig. 5d), nearly all defects can be detected inside all steps. For the other configuration TF (Fig. 5c), the resolution is much less and for a coverage of 1.3 mm (corresponding to St3) and larger, only the larger defects are recognizable.

Fig. 6. Compound thermograms (a and b) (time after flash: St1: 3.0 s, St2: 3.42 s, St3: 3.8 s, St4: 4.12 s and St5: 5.19 s) and phase images (c and d) (frequency: St1: 0.27 Hz, St2: 0.27 Hz, St3: 0.18 Hz, St4: 0.18 Hz, St5: 0.18 Hz) recorded in two different transmission configurations at specimen no. S1 with steps from 3.6 to 5.4 mm. (a and c) Configuration TF. (b and d) Configuration TB.

For S1, the results are similar as for S2: Mainly all defects can be detected in the phase images only in the transmission configuration TB (Fig. 6d). In the other configuration TF, already for the step with the smallest thickness of 3.6 mm (St1), the smaller defects are not visible any more (Fig. 6c). Thus in summary, with the configuration TB artificial delaminations having sizes down to 3  3 mm2 can be detected in 5.4 mm thick CFRP specimens. For demonstrating the influence of the artificial delamination on the thermal diffusivity and thus on temporal behavior, in

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Fig. 7. Temperature as a function of time for the transmission configuration TB of specimen S2 (a) and S1 (b). The temperatures above undisturbed areas of each step St1 and St2 are compared to the temperatures above defects of step St1.

Fig. 7 the temperature is shown as a function of time for configuration TB of specimens S1 and S2. For S1 in Fig. 7b), the temperature curves for undisturbed areas above step St1 (thickness l = 3.6 mm) and step St2 (l = 4.0 mm) clearly show how the increased thickness results in an increase of the travel time of the heating impulse. It is also evident that the travel time increases with increasing defect size from D4 (8  8 mm2) to D1 (20  20 mm2) in step St1. This is due to the lateral heat flow as described in chapter 2. But this effect is smaller in comparison to the influence of the sample thickness (see St2). The same is valid for sample S2. By fitting the temperature as a function of time for the undisturbed areas using Eq. (2) and using the known step thicknesses, mean diffusivities of about (7.0 ± 0.1)  107 m/s2 for S1 and of about (8.2 ± 0.1)  107 m/s2 for specimen S2 have been calculated in depth direction. 4.1.2. Reflection configuration By recording thermal sequences in reflection configuration at the plain side of both specimens (configuration RB), all defects at a depth of 1 mm can be detected clearly. By data recording from the stepped side (configuration RF), only for the thinner specimen S2 delaminations up to a coverage of 1.7 mm (St4, l = 2.7 mm) could be detected. This is shown in the thermogram and the phase image in Fig. 8. In the phase image, the contrast of the defects is slightly enhanced, thus more defects can be identified in steps St3 and St4. The detectability of the defects is limited to the 2  2 mm2 defect at coverage of 0.4 mm to the 8  8 mm2 defect at coverage of 1.7 mm as demonstrated by the circles in Fig. 8b). Thus, the aspect ratio of the defects at the detection limit is between 2 and 3. Fig. 9 shows the double logarithmic presentation of the normalized temperatures (normalized to the temperatures at 10 ms after the flash) above undisturbed areas and above defect D1 of all steps of specimen S2. It can be clearly seen that for each curve with

Fig. 8. Compound thermogram (a) (time after flash: St1: 0.19 s, St2: 0.68 s, St3: 1.54 s, St4: 1.54 s, St5: 1.54 s) and phase image (b) (frequency: St1: 1.36 Hz, St2: 0.45 Hz, St3: 0.27 Hz, St4: 0.27 Hz and St5: 0.18 Hz) of data recorded in reflection configuration RF for specimen S2. The circles are marking the limits of detection for each step.

Fig. 9. Double logarithmic presentation of the normalized temperature as a function of time above undisturbed areas and above defect D1 of all steps of specimen S2, configuration RF.

increasing step thickness, the kink to a horizontal line occurs later. By fitting these data using Eq. (1), a very good agreement between experimental and theoretical data was obtained using the given samples thicknesses and diffusivities determined from the transmission measurements (see Table 1). The influence of defect D1 is only marginal: a slight increase in temperature above the defects in comparison to the undisturbed steps is observed at early times (before the kink). After longer cooling down times, the temperature above the defects is slightly lower than above the undisturbed area of the steps. In total, the defects can be regarded as a small increase in mean diffusivity of the cross section. 4.2. Samples with impact damage 4.2.1. Transmission configuration In Fig. 10, thermograms and phase images of the three samples with impact damage recorded in the two transmission configurations are shown. The thermograms have been captured 2.2 s after the flash, where the maximum contrast of the defect appeared. A reference thermogram recorded before the flash was subtracted, therefore here only the temperature increase is shown. The phase images are always the first image of the FFT of the whole data set corresponding to the entire measurement period of 11.3 s. From these images, the following conclusions can be drawn:  In the thermograms, the damaged areas appear cooler as the sound material and show the typical butterfly characteristic [34].  With increasing impact energy, the size of the main damaged area increases as well, from 290 mm2 (24L01) to 630 mm2 (24L02) and 1370 mm2 (24L03). These areas have been measured manually in the thermograms of Fig. 10 of configuration

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C. Maierhofer et al. / Composites: Part B 57 (2014) 35–46 Table 1 Thicknesses, evaluated diffusivities of sound area and impact area (measured in depth direction) and sizes of impact areas of the investigated CFRP specimens. Specimen S1 S2 24L01 24L02 24L03 SR07 SR08a SR10b a

Thickness in mm 3.6–5.4 1.4–3.3 2.97 3.05 2.96 3.3 5.9 3.3

Diffusivity of sound area in m/s2 7

(7.0 ± 0.1)  10 (8.2 ± 0.1)  107 (5.3 ± 0.1)  107 (5.3 ± 0.1)  107 (5.9 ± 0.1)  107 (5.1 ± 0.1)  107 (6.4 ± 0.1)  107 (4.7 ± 0.1)  107

Diffusivity of impact area in m/s2

Size of impact areaa in mm2

– – (3.4 ± 0.3)  107 (3.0 ± 0.3)  107 (1.4 ± 0.2)  107 (4.5 ± 0.2)  107 (5.4 ± 0.3)  107 (3.4 ± 0.2)  107

– – 290 ± 20 630 ± 50 1370 ± 100 80 ± 15 80 ± 15 80 ± 15

As determined from transmission measurements.

Fig. 10. Thermograms (time after flash: 2.2 s) and phase images (frequency: 0.18 Hz) of both transmission configurations of the samples 24L01 to 24L03 with impact damage from 4.4 to 24.4 J.









TF for all temperatures equal or below the lower value of the temperature span around the defect. The temperature span is indicated below the thermograms. The whole damaged area can be detected in both transmission configurations. In configuration TB, the delaminated part is shown with higher spatial resolution. For the samples 24L02 and 24L03, in addition to the main damaged area at the center of the defect, larger areas with less damage can be seen, which cover nearly 30% of each sample. In all images, inhomogeneities due the pores and voids can be detected, which are originated from the manufacturing process of the samples. In comparison to the thermograms, the phase images show no additional information (in contradiction to Figs. 5 and 6).

Fig. 11a shows the temperatures as a function of time recorded in configuration TF above the sound area of sample 24L01 (as a reference) and above the middle of the impact areas of all three sam-

ples. The mean values of an area of 10  10 pixels have been analyzed each. It can be clearly seen that with increasing impact energy, the temperature increase at the damaged areas is slowing down. These curves can be fitted well using Eq. (2) by keeping the sample thickness L fixed and by increasing the diffusivity. The resulting diffusivities are summarized in Table 1. For the sound material, diffusivities of 5.2  107 to 5.9  107 m/s2 are determined. At the damaged areas, these values decrease remarkably with increased impact energy to (3.4 ± 0.3)  107 m/s2 for an impact energy of 4.4 J and to (1.4 ± 0.3)  107 m/s2 for an impact energy of 24.4 J. Therefore, from these data the amount of impact damage can not only be described by its lateral size as in Fig. 10, but also by its decreased diffusivity, which might be drawn back to an increased number of cracks and delaminations in depth. 4.2.2. Reflection configuration The results of both reflection configurations are displayed in Fig. 12. The thermograms have been recorded 0.7 and 1.3 s after

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Fig. 11. (a) Temperature as a function of time above sound and above impact areas for all samples 24L01 to 24L03 measured in transmission configuration TF. (b) Temperature as a function of time measured in reflection configuration RF. Here, the difference curves between impact area and sound area are shown, also in comparison to the difference curve of defect D1 of step St1 of specimen S2 (as an example of a delamination with coverage of 0.4 mm).

the flash, which was considerably later as the maximum contrast for getting more information from deeper structures. For the phase images, in most cases the 2nd harmonic from the FFT of the whole sequence has been selected. This phase image includes similar information about the impact as the 1st, but more information about the pores. The observations in these images can be summarized as follows:  In the thermograms, the damaged areas appear warmer than the sound area. This contrast increases with impact energy. This increase is also shown in Fig. 11b, where the temperature differ-

ence between damaged and sound areas is depicted as a function of time for all samples. Here again, the data from the reference area and from the middle of the impact area have been averaged along 10  10 pixels.  The increased size of the damaged areas can only be recognized by the reflection measurements from the back side.  As already seen in the images recorded in transmission configuration, in addition to the main damaged area at the center of the defect, larger areas with less damage are indicated. But here, these are only visible in the phase images from the sequences recorded in RB configuration.

Fig. 12. Thermograms (time after flash: 24L01: 0.7 s, 24L02: 1.3 s and 24L03: 1.3 s, similar for both configurations) and phase images (frequency: 24L01: 0.18 Hz (for both configurations); 24L02: 0.36 Hz for RF, 0.18 Hz for RB; 24L03: 0.36 Hz for RF, 0.18 Hz for RB) of both reflection configurations of the samples 24L01 to 24L03 with impact damage from 4.4 to 24.4 J.

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Fig. 13. Thermograms of sample 24L02 for a direct comparison of results obtained in both transmissions and both reflection configurations (time after flash: 2.2 s (TF, TB); 1.3 s (RF, RB).

Fig. 14. Thermograms of transmission configuration TB at the side rudder elements SR07 (time after flash: 1.83 s), SR08 (time after flash: 6.45 s) and SR10 (time after flash: 2.15 s). (a–c) Overviews, the impacts investigated below are marked with a circle. (d–f) Detailed thermograms of the impacts with a size of 5 cm  4 cm.

In Fig. 11b, additionally to the impact specimens, the difference curve of the large defect D1 of specimen S2 at step St1 (delamination below a coverage of 0.4 mm) is included in the diagram for comparison. It becomes evident that the maximum contrast of all defects appear early and approximately around the same time, thus the defects are very close to the surface. The contrast of the impact damages is higher than for the delamination and increases considerably with impact energy. For the delamination, the contrast decreases fast with time. For the impact damages, this decrease is much slower, which is a hint for several successive delaminations due to the impact. 4.2.3. Comparison of transmission and reflection configuration For a comparison of results from transmission and reflection configurations, in Fig. 13 thermograms of sample 24L02 are collected. The lateral size of the total damage can be determined either from the thermograms of both reflection configurations (RF and RB) or from the thermograms of one transmission configuration (TF or TB), as the results of both transmission configurations are more or less similar. From the thermograms in reflection configuration, also information about the depth of damage might be gained. But it is not possible to gather the total damage from a reflection measurement from the front surface of the impact alone.

4.3. Side rudder with impact damage 4.3.1. Transmission configuration In Fig. 14, the thermograms of transmission configuration TB of the impacts of the side rudder elements SR07, SR08 and SR10 are displayed. As the thermograms of the other configuration and all phase images show no additional information, these are omitted here. The position of the investigated impact of each element was marked by a circle inside the thermograms of Fig. 14a–c, showing an overview of each element. In these overviews, the impacts are shown as dark (cold) spots. Additionally, the stringer structure as well as further changes in the thickness of the structures can be detected. In the sections of the impacts in Fig. 14d–f, all thermograms show circular shaped defects having similar sizes of approx. 80 mm2, thus the damages are much smaller than for the samples 24L01 to 24L03. Only in the thermogram of the impact of SR10, a small crack can be seen. Here again the diffusivities at the sound as well as at the impact areas have been calculated from curve fitting of the temperature versus time relationships. These values are depicted in Table 1. The diffusivities of the impacts are only 12–30% less than those of the sound areas, thus the damage is not as pronounced as in the case of specimens L2401 to L2403.

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Fig. 15. Thermograms of reflection measurements at the side rudder elements SR07 (time after flash: 0.27 s), SR08 (time after flash: 0.27 s) and SR10 (time after flash: 0.75 s for (c and f), 0.21 s for (i)). (a–c) Overviews, configuration RB. The impacts shown below are marked with a circle. (d–f) Sections in configuration RB. (g–i) Sections in configuration RF. Section size: 5 cm  4 cm.

4.3.2. Reflection configuration Overview thermograms and thermograms of the single impacts recorded in both reflection configurations are shown in Fig. 15. Here again, in configuration RB the damage appeared larger as in TF. The crack at SR10 can be seen more clearly as compared to the transmission image. For SR07 and SR08, the impacts at the front side appear colder than the surrounding area. This might be explained by the compression of the material in this area, which enhances the diffusivity at the surface very locally. 5. Conclusions and summary The results recorded in transmission and reflection configuration at the stepbar specimens with the artificial delaminations as well as at the specimens with impact damage are leading to the following more general conclusions:  In reflection configuration, different sample thicknesses can be distinguished up to a thickness of 3.3 mm with an accuracy of approx. 0.1–0.2 mm. But artificial delaminations can only be detected to a maximum coverage of 1.7 mm. The aspect ratio of the defects at the detection limit is between 2 and 3. Also in case of impact damage, for samples with 3 mm thickness and even thicker, the lateral damage size cannot be assessed in reflection mode, if IR camera and flash lamps are positioned at the front side of the impact. This is only possible by performing reflection measurements from both sides.  In transmission configuration, most artificial delaminations and the lateral sizes of the impact damages can be detected well in both configurations up to a material thickness of 5.9 mm. The

smaller the coverage of the defect is at that side, where the IR camera is positioned, the better is the lateral resolution of this defect. A 3  3 mm2 large defect can already be detected inside a 5.4 mm thick plate, if the coverage is equal or less than 1 mm.  The diffusivities of the sound material as well as of the damaged material in depth direction can be determined well by fitting the temperature data recorded in transmission configuration using Eq. (2). For the damaged areas, these values are more related to an effective diffusivity, which is reduced in comparison to the sound material. This effective diffusivity is composed of reduced heat diffusion through the delaminated layers due to an enhanced thermal contact resistance and heat diffusion around the defects. The diffusivities of the sound materials of the different specimens differ from 8.2  107 m/s2 for S2 (stepbar specimen) to 4.7  107 m/s2 for SR10 (side rudder), although similar basic raw materials and production technologies have been used.  The impact damages can be characterized well by two parameters: first its lateral size and second by its effective diffusivity. Both values can be determined from transmission measurements and are summarized in Table 1. With increasing impact energy, the lateral size of the damage is increasing as well, while the diffusivity is decreasing. This decrease in diffusivity can be well explained with an increased number of successive delaminations, but also with the increased lateral size of these delaminations, which impedes the heat flow in depth direction as described above.  The investigations of the elements of the side rudder show that in both configurations, the different structures (stringers and different material thicknesses) can be visualized well, while

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the impact damages again can only be investigated along the whole cross section by transmission measurements. There was no noticeable influence of the thin copper mesh on the experimental data. In summary, seven different CFRP specimens have been investigated with flash thermography in reflection as well as in transmission configuration. All specimens have been constructed according to aerospace requirements and material thicknesses from 1.4 to 5.9 mm have been realized. For each specimen, a minimum of four data sets in different configurations has been recorded. The evaluation of reflection and transmission measurements clearly shows that although reflection measurements enable a high depth resolution for the determination of defect depth, only delaminations close to the surface (maximum coverage < 1.7 mm) could be detected reliably. The analysis of phase images enhances this penetration depth slightly. Thus, the lateral size of defects along the whole cross section of the samples could only be determined by two reflection measurements from both sides or by transmission measurements. For the latter ones, the two configurations lead to approximately the same results. Also for the transmission measurements, the detectability was slightly enhanced by analyzing the phase images. The elements cut off from the side rudder demonstrate the high sensitivity of flash thermography to thickness variations and that a thin copper mesh used for lightning protection does not influence data remarkably. In several cases due to the accessibility of only one side of the structure, only reflection measurements are possible. Therefore, an optimization of excitation techniques is required. As here already four flash lamps with an energy of 6 kJ each have been used, an increase of the energy might be appropriate, but has its limits. It might be possible to optimize the excitation by using several flashes in fixed time intervals. But with a double pulse technique, up to now only an increase of the signal to noise ratio of a few percentages has been reached [35]. A comparison of flash thermography and lock-in thermography using matched energies for testing CFRP structures has shown that lock-in thermography does not provide larger penetration depths than flash thermography [36]. But here, only matched energies have been compared. Therefore, for the future a systematic comparison of the performance of flash and lock-in thermography for characterizing CFRP structures is planned. Alternatively, an internal heat system might be considered which could be used for transmission measurements, where the IR camera is positioned outside the structure. But this has to be considered before construction and assembling of the CFRP structure.

Acknowledgements Part of this work was performed within the research project Development of standards for active thermography with flash excitation, which was funded by the research program Innovation with Norms and Standards by the Federal Ministry of Economics and Technology in cooperation with DIN e. V. The CFRP test specimens have been constructed by J. Häberle and J. Schulz from ZFL Zentrum für Faserverbunde und Leichtbau Haldensleben. P. Wossidlo from BAM 9.1 supported us with the equipment for generating the low-velocity impact damages. The side rudder elements have been provided by German Society for Non-Destructive Testing e. V. Sincere thanks are given to our colleagues Mathias Ziegler and Rainer Krankenhagen for constructive proof reading.

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