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Active thermographic inspection of carbon fiber reinforced plastic laminates using laser scanning heating
Accepted Manuscript Active thermographic inspection of carbon fiber reinforced plastic laminates using laser scanning heating Masashi Ishikawa, Masaki Ando, Masashi Koyama, Hideo Nishino PII: DOI: Reference:
S0263-8223(18)32576-5 https://doi.org/10.1016/j.compstruct.2018.10.113 COST 10365
To appear in:
Composite Structures
Received Date: Revised Date: Accepted Date:
21 July 2018 18 October 2018 31 October 2018
Please cite this article as: Ishikawa, M., Ando, M., Koyama, M., Nishino, H., Active thermographic inspection of carbon fiber reinforced plastic laminates using laser scanning heating, Composite Structures (2018), doi: https:// doi.org/10.1016/j.compstruct.2018.10.113
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Active thermographic inspection of carbon fiber reinforced plastic laminates using laser scanning heating
Masashi Ishikawa 1*, Masaki Ando 1, Masashi Koyama 2, Hideo Nishino 1
1
Graduate School of Technology, Industrial and Social Sciences, Tokushima University, 2-1 Minamijosannjima-cho, Tokushima 770-8506, Japan
2
School of Science and Engineering, Meisei University 2-1-1 Hodokubo, Hino, Tokyo, 191-8506, Japan
*
Corresponding author TEL: +81-88-656-7358, E-mail: [email protected]
Abstract In this study, inspection capability of active thermographic nondestructive inspection using the laser scanning method was examined. Carbon fiber reinforced plastic (CFRP) specimens were heated using a CO2 laser scanning device and their inspection capability was investigated. The experimental results indicate that the temperature and phase behavior after laser scanning heating were similar to those after conventional instantaneous flash heating. We also investigated the effect of the relationship between the scanning direction and fiber direction of the tested CFRPs on defect detection. The numerical and experimental results showed that the observed temperature contrast appearing on the defective surface varied depending on the scanning direction, and the temperature contrast increased when the scanning direction was the same as the fiber direction of the laminates. In addition, it was found that defects in a CFRP specimen located 10 m from the heat source could be detected using the laser scanning heating. These results suggest that using the laser scanning heating method is effective to inspect large objects located at far distances.
Keywords Non-destructive testing; Infrared thermography; CFRP laminate; CO2 laser
1 Introduction Various non-destructive inspection (NDI) techniques have been used for inspection of carbon fiber reinforced plastics (CFRPs), such as ultrasonic testing, radiography method, acoustic emission testing, and eddy current testing [1-4]. In the NDI techniques, the active thermography method is one of the promising inspection methods because it is a non-contact testing method and can inspect large areas in a single test. In active thermographic inspection, the surface of the test object is heated by heat sources and the temperature on the surface is observed by infrared cameras. When there is a defect in the test object (such as delaminations in CFRPs), a temperature contrast appears on the surface because of the change in heat flow from the surface. In conventional active thermographic methods, optical lamps such as halogen lamps or xenon flash lamps are frequently used as heat sources [5-7]. However, when using optical lamp heating, the heat sources must be located near the tested objects in order to input enough heat energy to the test objects. This implies that it is difficult to inspect objects located far from the observers. Although one method to overcome the limitation is to equip light-focusing reflectors around the lamp [8, 9], well-designed high-accuracy reflectors are required for heating distant objects. Based on the above-mentioned backgrounds, the aim of this study is to achieve inspection of CFRPs located at far distances, and we suggest using the laser scanning (LS) heating method in order to achieve this. Active thermographic inspection using laser spot or laser scanning heating has been reported in some papers [10-15]; most of the reports focus on detecting surface cracks by observing in-plane heat diffusion from a heat spot. In this study, we focused on the inspection of internal defects in CFRP laminates using the laser remote heating method. Because the diffusion attenuation by distance of the laser beam is much smaller than that of conventional optical heaters, effective remote heating can be achieved. In the present paper, we first examine the inspection capability of the thermographic method using LS through experiments (section 2). The inspection capability is compared with that of the conventional optical flash lamp heating method. The
comparison was applied not only to the conventional thermal data but also to the phase data obtained by applying Fourier transformation to the thermal data. When an optical flash lamp is used, the inspection using the phase data is known as pulse phase thermography (PPT) [16-18]; using the phase data is effective in enhancing the inspection capability [19-21]. In section 3, the influence of the anisotropy of the CFRP laminates on the inspection results obtained by the LS method is investigated. Finally, inspection for a CFRP specimen located 10 m from the laser source is performed and the defect detectability of the suggested method is examined in section 4.
2 Comparison between the laser scanning and flash heating method The inspection capability of active thermographic inspection using the LS method was compared to inspections using the conventional flash heating method.
2.1 Experiments Figure 1 shows a CFRP specimen used in the experiments. The specimen was a laminate unidirectionally reinforced with polyacrylonitrile-based carbon fiber (T800, Toray Industries, Inc.). The thermal properties of the specimen are summarized in Table 1. The specimen has 5- and 10mm-diameter flat-bottomed holes as artificial defects, and the defect depth from the heated surface, d, is 0.2–1.0 mm. In the experiments for the LS method, the specimen was heated using a CO2 laser scanning device (Multiscan VS, Rofin-Baasel, Inc.). The laser beam excited by the device was focused 147mm away from the laser head, and the beam diameter expanded beyond the focus point. The heat input Q in LS can be obtained approximately by the following equation: Q =
σ 2π ρ v
(1)
where σ is the laser diameter (determined based on the standard deviation of the laser intensity distribution), v is the scanning speed, and ρ is the energy density of the laser beam. In the experiments, the CFRP specimen was located at 580 mm from the laser head, and the resulting σ = 18 mm. v, and ρ were adjusted in order to match the heat input of LS with that of the flash heating experiments. They were finally adjusted to 150 mm/s, and 228 kW/m2, respectively. The specimen was heated from the surface without holes and the temperature distribution on the same surface during and after scanning heating was monitored using an infrared camera (A315, FLIR Systems, Inc.) with a sampling frequency of 15 Hz. The obtained thermal data were processed using a commercial thermal data processing software (IR Phase, KJTD Co., Ltd. [22]). In contrast, two xenon flash lamps (1000 J/F each) were used for the flash heating experiments. The surface of the specimen was uniformly heated instantaneously, and the temperature distribution after heating was observed by the same infrared camera.
2.2 Experimental results Experimentally observed temperature images obtained after LS and flash heating are compared in Fig. 2. In both the images, defects are detected as local hot spots, and all the defects are identified in both the images. The temperature contrasts ΔT appearing on the defect with each d is shown in Fig. 3 as a function of time after heating. By comparing the ΔT values, temperature behavior after LS heating is similar to that of the conventional flash heating. This is because the heating duration by LS at a certain area is as short as that by flash heating (estimating from the values of σ and v, a laser beam passes across a point in approximately 0.12 s). These results suggest that the inspection capability of LS is similar to that of flash heating when the heat input is equal.
2.3 Comparison in phase data
In the conventional PPT method, Fourier transformation is applied to the temperature–time data obtained by the flash heating thermography, and the phase data for each frequency can be calculated from the following equation [16, 17]: I(f) φ(f) = tan - 1 R(f)
(2)
where f is the frequency, and R(f) and I (f) are the values of the real and imaginary parts of the complex number obtained by the Fourier transformation, respectively. By using the calculated φ(f) at each pixel of the images, two-dimensional phase images at each frequency can be constructed, and abnormalities can be identified by detecting the phase contrast between the defective area and sound area in the phase images. Similar to the PPT processing, the temperature–time data obtained from the experiments using LS were also transformed into the phase data, and the phase images obtained by LS were compared with that obtained by the flash heating experiments. Note that, in the PPT processing, Fourier transform is applied to the thermal data in the cooling stage after instantaneous flash heating. Thus, in order to transform in a similar manner to the PPT, Fourier transformation for LS was applied to the temperature data immediately after irradiating with the laser beam (i.e., the start time of the Fourier transform was varied depending on the positions within the specimen because of the time delay of scanning). By using the transformed phase–frequency data, phase contrasts appearing on the surfaces of the defects were obtained. The phase images and phase contrasts Δφ appearing on the defects as a function of frequency obtained using the LS and flash heating methods are presented in Figs. 4 and 5, respectively. In the phase obtained by experiment using flash heating (i.e., conventional PPT), Δφ exhibits a local minimum peak whose frequency decreases with increasing defect depth (see Fig. 5(b)). Meanwhile, as seen in Fig. 4(b), the Δφ obtained from LS also exhibits similar behaviors as Δφ in Fig. 5(b). The relationships between the frequency when the Δφ reaches its peak value and d obtained from both heating methods are compared in Fig. 6. These results show that the phase data obtained by both the
heating methods are consistent, and suggest that using the phase data obtained by the LS method is also effective for inspection similar to the PPT method.
3 Influence of the anisotropy of CFRP on laser scanning method The thermal anisotropy of CFRP laminates could affect the results of the LS inspections. In order to investigate the influence of anisotropy, we performed numerical and experimental studies, particularly focusing on the relationship between the direction of laser scanning and fiber orientation in the CFRP.
3.1 Numerical studies 3.1.1 Numerical model Figure 7 shows a schematic of the numerical model. Two types of models were used in the calculations. One assumes a unidirectionally reinforced laminate (UD) and the other assumes a cross-ply laminate (CP). The UD model assumes a CFRP reinforced with carbon fibers oriented only in the 0° direction (see Fig. 7); thus, the thermal conductivity in the 0° direction is higher than that in the 90° direction. The thermal properties used in the calculations are identical to those presented in Table 1. In the CP model, unidirectionally reinforced 0.2-mm-thick layers are stacked with varying direction of kL (see Table 1) in 0° and 90° alternately (thus, the number of laminated layers is 25 because the thickness of the model is 5 mm). Both models have a 10-mm diameter flatbottomed hole as an artificial defect, and the defect depth d from the heated surface varies by 0.2– 1.0 mm. In the calculations, scanning heating was applied to the surface of the models. The calculations were performed under the conditions of the scanning directions of 0° and 90°, and the temperature contrasts ΔT between the defective and non-defective area obtained from both
conditions were compared. In addition, the heat flux in the in-plane direction at the defect edge was calculated. 3.1.2 Numerical results Figure 8 shows the calculated ΔT as a function of time and the in-plane heat flux in the specimen surface obtained from the UD model when d = 0.4 mm. In Fig. 8(a), ΔT is higher when the scanning direction is 0° (i.e., the scanning direction is parallel to the fiber direction). In contrast, the in-plane heat flux (see Fig. 8(b)) is larger when the scanning direction is 90° (i.e., the scanning direction is vertical to the fiber direction). This implies that the heat deposited on the defect dissipates more easily when the scanning direction is vertical to the fiber direction. This should be the reason for the difference in ΔT caused by different scanning directions. These results suggest that scanning along the fiber direction is more effective for defect detection. Table 2 presents the maximum values of ΔT for d = 0.2–0.8 mm when the scanning directions are 0° (ΔT0) and 90° (ΔT90). In the same table, the calculated values of ΔT90/ΔT0 with d are also presented. It is found that the ΔT90/ΔT0 decreases with d, indicating that the influence of the scanning direction with respect to the fiber direction on defect detection is enhanced with increasing defect depth. Figure 9 shows ΔT in the in-plane heat fluxes calculated in the CP model when d = 0.2 and 0.4 mm, and ΔT0 and ΔT90 for each d are presented in Table 3. It is found from Fig. 9 and Table 3 that, although ΔT0 is larger than ΔT90 when d = 0.2 and 0.6 mm, similar to the results from the UD model, there is little difference when d = 0.4 and 0.8 mm. Figure 10 shows the in-plane heat fluxes calculated in the CP model. In Figs. 9(b) and 9(d), the heat flux on the specimen surface and defect surface (i.e., in-plane heat flux appearing immediately above the defect boundary) is compared. When d = 0.2 mm (Fig. 9(b)), the heat flux in the direction of 90° is higher both at the specimen surface and defect surface. In contrast, when d = 0.4 mm (Fig. 9(d)), the heat flux in the direction of 0° is higher at the defect surface. This should be because the thickness of the single layers in the CP model is 0.2 mm, and the fiber direction of the layer above the 0.4-mm-deep defect is rotated 90° to
the fiber direction in the specimen surface. Because of this effect, the influence of the thermal anisotropy could be reduced, decreasing the difference between ΔT0 and ΔT90. The same effect should occur in the result for d = 0.8 mm. Therefore, these results suggest that the inspection capability is almost independent of the scan direction when detecting defects located beneath several layers in the cross-ply laminates. Note that even when inspecting the cross-plied laminates, it should be effective to scan along the fiber direction of the uppermost layer when detecting shallow defects or defects in the CFRPs reinforced with carbon fibers with higher thermal conductivities.
3.2 Experimental verification The numerical results were verified by experiments for a UD and CP CFRP laminate. The UD laminate was the same specimen used in section 2 (see Fig. 1). The configuration of the CP specimen was also identical to that shown in Fig. 1, although approximately 0.2-mm-thick, unidirectionally reinforced thin layers were stacked alternately while changing the fiber direction. In the experiments, the surface of the specimens was heated by the same laser scanning device used in section 2. Scanning heating was performed in directions both parallel and orthogonal to the carbon fiber direction of the outermost layer. The LS conditions (laser diameter, scan speed, and energy density) were also identical to those used in section 2. Figure 10 shows the experimentally obtained temperature contrast ΔT observed on the defects in the specimens with depths of 0.2 and 0.4 mm. As seen in the results for the UD specimen (Fig. 10(a) and 10(b)), ΔT is larger when the scan direction is parallel to the fiber direction, regardless of the defect depth. In contrast, in the results for the CP specimen (Fig. 10(c) and 10(d)), although ΔT obtained when scanning parallel to the fiber direction of the outermost layer is larger than that obtained when scanning orthogonal to the fiber when d = 0.2 mm, those values are almost the same when d = 0.4 mm. These tendencies agree with the numerical results.
4 Inspection from 10-m distance Although the inspection capability of the LS method is similar to that of the flash heating method, as shown in section 2, the LS method is expected to be effective for inspecting objects located a far distance from the observers because the diffusion attenuation of the laser beam is much smaller than that of the conventional optical heaters. In order to examine the remote inspection capability of the laser heating method, inspection of the CFRP specimen located 10 m from the laser source was performed.
4.1 Experimental setup Figure 11 shows the experimental setup and a CFRP specimen used in the experiment. The specimen (Fig. 11(b)) is a 980 × 980× 6 mm cross-ply laminate (reinforced with T700 carbon fiber), and has 20 × 20, 50 × 50, and 100 × 100 mm square, flat-bottomed holes as artificial defects. The defect depth from the heated surface d is 0.5–2.0 mm for 20 × 20 mm defect, and 0.5–3.0 mm for 50 × 50 and 100 × 100 mm defects. The specimen was placed 10 m from the laser scanning device, the same used in previous sections (see Fig. 11(a)), and was heated from the surface without holes. The scanning direction was parallel to the fiber direction of the uppermost layer. The laser diameter σ, scanning speed v, and laser density ρ on the specimen surface were approximately 380 mm, 570 mm/s, and 714 W/m2, respectively. The temperature distribution on the heated surface during and after the scanning heating was monitored using an infrared camera with a telephoto lens with a sampling frequency of 7.5 Hz.
4.2 Experimental results 4.2.1 Using temperature data
Figure 12 presents the temperature image observed after laser heating. In Fig. 12, defects with depths up to 1.0 mm are identified for the 50 × 50 mm defect, and those with depths up to 2.0–2.5 mm are identified for the 100 × 100 mm defect. In the temperature images, the 50 × 50 mm defect could not been detected. Figure 13 shows the temperature contrasts ΔT as a function of time (after heating) obtained on the 100 × 100 mm defect with depths of 0.5, 1.0, and 2.0 mm (ΔT was calculated as an average value of 10 data points on each depth defect). The maximum ΔT for each depth (determined as the peak value of the fifth-order approximation curve presented in Fig. 13 as a dashed line) is approximately 0.81, 0.40, and 0.15 °C; these values are higher than the temperature resolution of the infrared camera (0.07 K). These results suggest that the laser heating method enables remote thermographic inspection for CFRP structures.
4.2.2 Using phase data Figure 14 presents a phase image transformed from the temperature data in Fig. 12. In the phase image, all the defects are identified for the 50 × 50 and 100 × 100 mm defects. Moreover, the 20 × 20 mm defects, which could not be detected in the temperature images, are also detected in Fig. 14. As presented in section 2, the phase behavior observed after laser scanning heating is similar to that observed in the flash heating method. Further, in the conventional PPT method with flash heating, using phase images is effective in improving defect detectability. Thus, these results indicate that using phase images is also effective for inspecting distant objects using laser scanning heating. Note that, in the present setup, the detection of defects smaller than 20 mm will be difficult because of the coarse spatial resolution of the obtained images (the pixel size at 10 m distance is approximately 5 mm). Using a higher-resolution infrared camera or a higher-magnification telephoto lens should enable us to detect defects as small as practical defects in the CFRP structures.
5 Conclusions The active thermographic inspection of CFRP laminates using the laser scanning heating method was examined. The main findings obtained from the experimental and numerical studies are as follows: 1) The temperature and phase behavior obtained from the laser scanning heating was similar to that obtained from conventional flash lamp heating.
2) The temperature contrast caused by defects is higher when the scanning direction is the same as the fiber direction of the laminates. Note that the influence of the scanning direction decreases when inspecting cross-ply laminates. 3) The conjunction of the laser scanning method with phase transformation is effective for detecting defects in a CFRP laminate located at 10 m from the laser source. Although the distance between the laser source and the specimen in the present study was 10 m, the laser heating method could heat and inspect objects located at further distances by adjusting the focal length of the laser beam. In addition, the scanning method should be a convenient way to inspect large objects because it can easily expand the heating area by expanding the scanning area. These features should contribute toward improvement of inspection efficiency, and therefore, the suggested method could be an effective way to inspect large CFRP structures. Note that, although inspection of plate specimens were discussed in this paper, further investigation of the inspection capability of practical structures, such as structures with curved surface, is also necessary; it will be the future work of this study.
Acknowledgement This work was supported by JSPS KAKENHI Grant Number 26282099.
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Figure captions
Fig. 1 (a) CFRP specimen with artificial defects (5- and 10-mm-diameter flat-bottomed holes) and (b) schematic of the specimen (the numerals in the figure denote the defect depth from the heated surface).
Fig. 2 Temperature images observed 1 s after heating obtained by (a) LS and (b) flash heating (area enclosed by dashed lines shows the specimen, and the numerals in the figure denote defect depth).
Fig. 3 Temperature contrasts ΔT appearing on the defect with each d obtained by (a) LS and (b) flash heating.
Fig. 4 (a) Phase image obtained by LS at a frequency of 0.044 Hz and (b) phase contrasts Δφ as a function of frequency observed on the defect with d.
Fig. 5 (a) Phase image obtained by flash heating at a frequency of 0.044 Hz and (b) phase contrasts Δφ as a function of frequency observed on the defect with d.
Fig. 6 Comparison of relationships between defect depth and frequency at the local minimum peak of the phase contrast obtained by laser scanning and flash heating.
Fig. 7 Schematic of numerical model.
Fig. 8 Numerically calculated (a) temperature contrast ΔT and (b) in-plane heat flux appearing on the specimen surface obtained from the UD model with a 0.4-mm depth defect.
Fig. 9 Temperature contrasts ΔT and in-plane heat flux appearing on the specimen and defect surface calculated from the CP model: (a) ΔT and (b) heat flux for 0.2-mm depth defect, and (c) ΔT and (d) heat flux for 0.4-mm depth defect.
Fig. 10 Experimentally obtained temperature contrasts, (a) UD specimen, d = 0.2 mm; (b) UD specimen, d = 0.4 mm; (c) CP specimen, d = 0.2 mm; and (d) CP specimen, d = 0.4 mm.
Fig. 11 (a) Setup of the experiment for the CFRP specimen located at 10 m distance, (b) CFRP specimen (the numerals denote the defect depth from the heated surface in millimeters).
Fig. 12 Experimentally obtained temperature image after laser scanning heating for CFRP specimen located at 10 m distance.
Fig. 13 Temperature contrasts ΔT as a function of time (after heating) obtained on the 100 × 100 mm defect with depths of 0.5, 1.0, and 2.0 mm.
Fig. 14 Phase image obtained by applying Fourier transform to temperature data in Fig. 12.
Table captions
Table 1 Thermal properties of CFRP specimen.
Table 2 Maximum values of the temperature contrasts calculated from UD model.
Table 3 Maximum values of the temperature contrasts calculated from CP model.
Table 2 Maximum values of the temperature contrasts calculated from UD model.
Defect depth ΔT0 ΔT90
0.2 mm 24.6 24.1
0.4 mm 10.2 9.91
0.6 mm 5.93 5.68
0.8 mm 5.71 4.76
ΔT90/ΔT0
0.98
0.97
0.96
0.83
Table 1 Thermal properties of CFRP specimen.
Density ρ [kg/m3] Specific heat capacity c [J/(kg·K)] Along fiber direction kL Thermal conductivity k [W/(m·K)] Perpendicular to fiber direction kT
1536 865 4.2 0.56
Table 3 Maximum values of the temperature contrasts calculated from CP model.
Defect depth ΔT0
0.2 mm 24.7
0.4 mm 10.2
0.6 mm 5.75
0.8 mm 3.59
ΔT90
24.0
10.2
5.62
3.58
ΔT90/ΔT0
0.97
1.00
0.98
0.99
(a)
(b) 0.2 mm
0.8 mm
0.4 mm
1.0 mm
0.8 mm 0.6 mm 0.4 mm
0.6 mm
1.0 mm
φ10 mm
Fig. 1
0.2 mm
φ5 mm
(a) CFRP specimen with artificial defects (5- and 10-mm-diameter flat-bottomed
holes) and (b) schematic of the specimen (the numerals in the figure denote the defect depth from the heated surface).
(a)
(b) 0.8 mm 0.2 mm
0.2 mm
0.8 mm
0.2 mm
1.0 mm
0.4 mm
1.0 mm
0.6 mm
0.8 mm 0.4 mm 0.6 mm
1.0 mm 0.4 mm 0.6 mm
0.4 mm 1.0 mm 0.6 mm
0.8 mm
0.2 mm
Fig. 2
Temperature images observed 1 s after heating obtained by (a) LS and (b) flash
heating (area enclosed by dashed lines shows the specimen, and the numerals in the figure denote defect depth).
(a)
(b) Temperature contrast T (ºC)
Temperature contrast T (ºC)
10.0 Defect depth d
8.0
0.2 mm 0.4 mm 0.6 mm 0.8 mm 1.0 mm
6.0 4.0 2.0
10.0 Defect depth d
8.0
0.2 mm 0.4 mm 0.6 mm 0.8 mm 1.0 mm
6.0 4.0 2.0 0.0
0.0 0
2
4
6
Time t (s)
8
10
0
2
4
6
8
Time t (s)
Fig. 3 Temperature contrasts ΔT appearing on the defect with each d obtained by (a) LS and (b) flash heating.
10
(b)
15 10 5 0 -5 -10 -15 -20 -25
Phase contrasts (deg)
[deg]
(a)
10 0
-10 -20 Defect depth d
-30 -40
0.2 mm 0.4 mm 0.6 mm 0.8 mm 1.0 mm
-50 0.01
0.1
1
Frequency f (Hz)
Fig. 4
(a) Phase image obtained by LS at a frequency of 0.044 Hz and (b) phase
contrasts Δφ as a function of frequency observed on the defect with d.
(b)
[deg] -25 -30 -35 -40 -45 -50 -55
20
Phase contrasts (deg)
(a)
10 0 -10 -20 -30 -40
Defect depth d 0.2 mm 0.4 mm 0.6 mm 0.8 mm 1.0 mm
-50 0.01
0.1
1
Frequency f (Hz)
Fig. 5
(a) Phase image obtained by flash heating at a frequency of 0.044 Hz and (b)
phase contrasts Δφ as a function of frequency observed on the defect with d.
Frequency f (Hz)
0.6 0.5
Laser scanning heating Flash heating
0.4 0.3 0.2 0.1 0.0 0.0
0.2
0.4
0.6
0.8
1.0
Defect depth d (mm)
Fig. 6
Comparison of relationships between defect depth and frequency at the local
minimum peak of the phase contrast obtained by laser scanning and flash heating.
Defect (φ10 mm flat-bottomed hole)
Fig. 7
Schematic of numerical model.
(b) 10
Scanning direction 0º (Parallel) 90º (Orthogonal)
10
Scanning direction Heat flux (kW/m²)
Temperature contrast T (ºC)
(a) 12
8 6
4 2
0
0
2
4
6
Time t (s)
8
10
8
0º (Parallel) 90º (Orthogonal)
6 4 2 0
0
2
4
6
8
Time t (s)
Fig. 8 Numerically calculated (a) temperature contrast ΔT and (b) in-plane heat flux appearing on the specimen surface obtained from the UD model with a 0.4-mm depth defect.
10
Temperature contrast T (ºC)
(a)
(b)
25
Scanning direction
20
0º 90º
15
10
5
0 0
2
4
6
8
10
Time t (s)
Temperature contrast T (ºC)
(c)
(d)
12
Scanning direction 0º 90º
10 8 6
4 2
0
0
2
4
6
8
10
Time t (s)
Fig. 9 Temperature contrasts ΔT and in-plane heat flux appearing on the specimen and defect surface calculated from the CP model: (a) ΔT and (b) heat flux for 0.2-mm depth defect, and (c) ΔT and (d) heat flux for 0.4-mm depth defect.
(b)
6.0
Temperature contrast T (ºC)
Temperature contrast T (ºC)
(a)
Scanning direction 4.0
Parallel Orthogonal
2.0
0.0
0
2
4
6
8
10
3.0 2.5
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2.0 1.5 1.0 0.5 0.0
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Time t (s)
Temperature contrast T (ºC)
Temperature contrast T (ºC)
Scanning direction 4.0
Parallel Orthogonal
2.0
2
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6
Time t (s)
Fig. 10
8
8
10
3.0 2.5
Scanning direction Parallel Orthogonal
2.0 1.5 1.0 0.5 0.0
0
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4
6
8
Time t (s)
Experimentally obtained temperature contrasts, (a) UD specimen, d = 0.2 mm;
(b) UD specimen, d = 0.4 mm; (c) CP specimen, d = 0.2 mm; and (d) CP specimen, d = 0.4 mm.
10
(d)
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(c)
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(a) CFRP specimen
(b) Laser head
2.0
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1.0
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IR camera
(with telephoto lens)
3.0
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100×100 mm
50×50 mm 20×20 mm
Fig. 11
(a) Setup of the experiment for the CFRP specimen located at 10 m distance,
(b) CFRP specimen (the numerals denote the defect depth from the heated surface in millimeters).
Fig. 12
0.5 mm
0.5 mm
2.0 mm
1.0 mm
1.0 mm
2.5 mm
1.5 mm
3.0 mm
Experimentally obtained temperature image after laser scanning heating for
CFRP specimen located at 10 m distance.
Fig. 13
Temperature contrasts ΔT as a function of time (after heating) obtained on the
100 × 100 mm defect with depths of 0.5, 1.0, and 2.0 mm.
[deg]
20×20 mm defect
0.5 mm 2.0 mm
0.5 mm
2.0 mm
66.0 0 60
0.5 mm
40 1.0 mm
1.0 mm
2.5 mm
1.0 mm
2.5 mm
20 0
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3.0 mm
- 20 - 40 - 59.92
Fig. 14 Phase image obtained by applying Fourier transform to temperature data in Fig. 12.
S0263-8223(18)32576-5 https://doi.org/10.1016/j.compstruct.2018.10.113 COST 10365
To appear in:
Composite Structures
Received Date: Revised Date: Accepted Date:
21 July 2018 18 October 2018 31 October 2018
Please cite this article as: Ishikawa, M., Ando, M., Koyama, M., Nishino, H., Active thermographic inspection of carbon fiber reinforced plastic laminates using laser scanning heating, Composite Structures (2018), doi: https:// doi.org/10.1016/j.compstruct.2018.10.113
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Active thermographic inspection of carbon fiber reinforced plastic laminates using laser scanning heating
Masashi Ishikawa 1*, Masaki Ando 1, Masashi Koyama 2, Hideo Nishino 1
1
Graduate School of Technology, Industrial and Social Sciences, Tokushima University, 2-1 Minamijosannjima-cho, Tokushima 770-8506, Japan
2
School of Science and Engineering, Meisei University 2-1-1 Hodokubo, Hino, Tokyo, 191-8506, Japan
*
Corresponding author TEL: +81-88-656-7358, E-mail: [email protected]
Abstract In this study, inspection capability of active thermographic nondestructive inspection using the laser scanning method was examined. Carbon fiber reinforced plastic (CFRP) specimens were heated using a CO2 laser scanning device and their inspection capability was investigated. The experimental results indicate that the temperature and phase behavior after laser scanning heating were similar to those after conventional instantaneous flash heating. We also investigated the effect of the relationship between the scanning direction and fiber direction of the tested CFRPs on defect detection. The numerical and experimental results showed that the observed temperature contrast appearing on the defective surface varied depending on the scanning direction, and the temperature contrast increased when the scanning direction was the same as the fiber direction of the laminates. In addition, it was found that defects in a CFRP specimen located 10 m from the heat source could be detected using the laser scanning heating. These results suggest that using the laser scanning heating method is effective to inspect large objects located at far distances.
Keywords Non-destructive testing; Infrared thermography; CFRP laminate; CO2 laser
1 Introduction Various non-destructive inspection (NDI) techniques have been used for inspection of carbon fiber reinforced plastics (CFRPs), such as ultrasonic testing, radiography method, acoustic emission testing, and eddy current testing [1-4]. In the NDI techniques, the active thermography method is one of the promising inspection methods because it is a non-contact testing method and can inspect large areas in a single test. In active thermographic inspection, the surface of the test object is heated by heat sources and the temperature on the surface is observed by infrared cameras. When there is a defect in the test object (such as delaminations in CFRPs), a temperature contrast appears on the surface because of the change in heat flow from the surface. In conventional active thermographic methods, optical lamps such as halogen lamps or xenon flash lamps are frequently used as heat sources [5-7]. However, when using optical lamp heating, the heat sources must be located near the tested objects in order to input enough heat energy to the test objects. This implies that it is difficult to inspect objects located far from the observers. Although one method to overcome the limitation is to equip light-focusing reflectors around the lamp [8, 9], well-designed high-accuracy reflectors are required for heating distant objects. Based on the above-mentioned backgrounds, the aim of this study is to achieve inspection of CFRPs located at far distances, and we suggest using the laser scanning (LS) heating method in order to achieve this. Active thermographic inspection using laser spot or laser scanning heating has been reported in some papers [10-15]; most of the reports focus on detecting surface cracks by observing in-plane heat diffusion from a heat spot. In this study, we focused on the inspection of internal defects in CFRP laminates using the laser remote heating method. Because the diffusion attenuation by distance of the laser beam is much smaller than that of conventional optical heaters, effective remote heating can be achieved. In the present paper, we first examine the inspection capability of the thermographic method using LS through experiments (section 2). The inspection capability is compared with that of the conventional optical flash lamp heating method. The
comparison was applied not only to the conventional thermal data but also to the phase data obtained by applying Fourier transformation to the thermal data. When an optical flash lamp is used, the inspection using the phase data is known as pulse phase thermography (PPT) [16-18]; using the phase data is effective in enhancing the inspection capability [19-21]. In section 3, the influence of the anisotropy of the CFRP laminates on the inspection results obtained by the LS method is investigated. Finally, inspection for a CFRP specimen located 10 m from the laser source is performed and the defect detectability of the suggested method is examined in section 4.
2 Comparison between the laser scanning and flash heating method The inspection capability of active thermographic inspection using the LS method was compared to inspections using the conventional flash heating method.
2.1 Experiments Figure 1 shows a CFRP specimen used in the experiments. The specimen was a laminate unidirectionally reinforced with polyacrylonitrile-based carbon fiber (T800, Toray Industries, Inc.). The thermal properties of the specimen are summarized in Table 1. The specimen has 5- and 10mm-diameter flat-bottomed holes as artificial defects, and the defect depth from the heated surface, d, is 0.2–1.0 mm. In the experiments for the LS method, the specimen was heated using a CO2 laser scanning device (Multiscan VS, Rofin-Baasel, Inc.). The laser beam excited by the device was focused 147mm away from the laser head, and the beam diameter expanded beyond the focus point. The heat input Q in LS can be obtained approximately by the following equation: Q =
σ 2π ρ v
(1)
where σ is the laser diameter (determined based on the standard deviation of the laser intensity distribution), v is the scanning speed, and ρ is the energy density of the laser beam. In the experiments, the CFRP specimen was located at 580 mm from the laser head, and the resulting σ = 18 mm. v, and ρ were adjusted in order to match the heat input of LS with that of the flash heating experiments. They were finally adjusted to 150 mm/s, and 228 kW/m2, respectively. The specimen was heated from the surface without holes and the temperature distribution on the same surface during and after scanning heating was monitored using an infrared camera (A315, FLIR Systems, Inc.) with a sampling frequency of 15 Hz. The obtained thermal data were processed using a commercial thermal data processing software (IR Phase, KJTD Co., Ltd. [22]). In contrast, two xenon flash lamps (1000 J/F each) were used for the flash heating experiments. The surface of the specimen was uniformly heated instantaneously, and the temperature distribution after heating was observed by the same infrared camera.
2.2 Experimental results Experimentally observed temperature images obtained after LS and flash heating are compared in Fig. 2. In both the images, defects are detected as local hot spots, and all the defects are identified in both the images. The temperature contrasts ΔT appearing on the defect with each d is shown in Fig. 3 as a function of time after heating. By comparing the ΔT values, temperature behavior after LS heating is similar to that of the conventional flash heating. This is because the heating duration by LS at a certain area is as short as that by flash heating (estimating from the values of σ and v, a laser beam passes across a point in approximately 0.12 s). These results suggest that the inspection capability of LS is similar to that of flash heating when the heat input is equal.
2.3 Comparison in phase data
In the conventional PPT method, Fourier transformation is applied to the temperature–time data obtained by the flash heating thermography, and the phase data for each frequency can be calculated from the following equation [16, 17]: I(f) φ(f) = tan - 1 R(f)
(2)
where f is the frequency, and R(f) and I (f) are the values of the real and imaginary parts of the complex number obtained by the Fourier transformation, respectively. By using the calculated φ(f) at each pixel of the images, two-dimensional phase images at each frequency can be constructed, and abnormalities can be identified by detecting the phase contrast between the defective area and sound area in the phase images. Similar to the PPT processing, the temperature–time data obtained from the experiments using LS were also transformed into the phase data, and the phase images obtained by LS were compared with that obtained by the flash heating experiments. Note that, in the PPT processing, Fourier transform is applied to the thermal data in the cooling stage after instantaneous flash heating. Thus, in order to transform in a similar manner to the PPT, Fourier transformation for LS was applied to the temperature data immediately after irradiating with the laser beam (i.e., the start time of the Fourier transform was varied depending on the positions within the specimen because of the time delay of scanning). By using the transformed phase–frequency data, phase contrasts appearing on the surfaces of the defects were obtained. The phase images and phase contrasts Δφ appearing on the defects as a function of frequency obtained using the LS and flash heating methods are presented in Figs. 4 and 5, respectively. In the phase obtained by experiment using flash heating (i.e., conventional PPT), Δφ exhibits a local minimum peak whose frequency decreases with increasing defect depth (see Fig. 5(b)). Meanwhile, as seen in Fig. 4(b), the Δφ obtained from LS also exhibits similar behaviors as Δφ in Fig. 5(b). The relationships between the frequency when the Δφ reaches its peak value and d obtained from both heating methods are compared in Fig. 6. These results show that the phase data obtained by both the
heating methods are consistent, and suggest that using the phase data obtained by the LS method is also effective for inspection similar to the PPT method.
3 Influence of the anisotropy of CFRP on laser scanning method The thermal anisotropy of CFRP laminates could affect the results of the LS inspections. In order to investigate the influence of anisotropy, we performed numerical and experimental studies, particularly focusing on the relationship between the direction of laser scanning and fiber orientation in the CFRP.
3.1 Numerical studies 3.1.1 Numerical model Figure 7 shows a schematic of the numerical model. Two types of models were used in the calculations. One assumes a unidirectionally reinforced laminate (UD) and the other assumes a cross-ply laminate (CP). The UD model assumes a CFRP reinforced with carbon fibers oriented only in the 0° direction (see Fig. 7); thus, the thermal conductivity in the 0° direction is higher than that in the 90° direction. The thermal properties used in the calculations are identical to those presented in Table 1. In the CP model, unidirectionally reinforced 0.2-mm-thick layers are stacked with varying direction of kL (see Table 1) in 0° and 90° alternately (thus, the number of laminated layers is 25 because the thickness of the model is 5 mm). Both models have a 10-mm diameter flatbottomed hole as an artificial defect, and the defect depth d from the heated surface varies by 0.2– 1.0 mm. In the calculations, scanning heating was applied to the surface of the models. The calculations were performed under the conditions of the scanning directions of 0° and 90°, and the temperature contrasts ΔT between the defective and non-defective area obtained from both
conditions were compared. In addition, the heat flux in the in-plane direction at the defect edge was calculated. 3.1.2 Numerical results Figure 8 shows the calculated ΔT as a function of time and the in-plane heat flux in the specimen surface obtained from the UD model when d = 0.4 mm. In Fig. 8(a), ΔT is higher when the scanning direction is 0° (i.e., the scanning direction is parallel to the fiber direction). In contrast, the in-plane heat flux (see Fig. 8(b)) is larger when the scanning direction is 90° (i.e., the scanning direction is vertical to the fiber direction). This implies that the heat deposited on the defect dissipates more easily when the scanning direction is vertical to the fiber direction. This should be the reason for the difference in ΔT caused by different scanning directions. These results suggest that scanning along the fiber direction is more effective for defect detection. Table 2 presents the maximum values of ΔT for d = 0.2–0.8 mm when the scanning directions are 0° (ΔT0) and 90° (ΔT90). In the same table, the calculated values of ΔT90/ΔT0 with d are also presented. It is found that the ΔT90/ΔT0 decreases with d, indicating that the influence of the scanning direction with respect to the fiber direction on defect detection is enhanced with increasing defect depth. Figure 9 shows ΔT in the in-plane heat fluxes calculated in the CP model when d = 0.2 and 0.4 mm, and ΔT0 and ΔT90 for each d are presented in Table 3. It is found from Fig. 9 and Table 3 that, although ΔT0 is larger than ΔT90 when d = 0.2 and 0.6 mm, similar to the results from the UD model, there is little difference when d = 0.4 and 0.8 mm. Figure 10 shows the in-plane heat fluxes calculated in the CP model. In Figs. 9(b) and 9(d), the heat flux on the specimen surface and defect surface (i.e., in-plane heat flux appearing immediately above the defect boundary) is compared. When d = 0.2 mm (Fig. 9(b)), the heat flux in the direction of 90° is higher both at the specimen surface and defect surface. In contrast, when d = 0.4 mm (Fig. 9(d)), the heat flux in the direction of 0° is higher at the defect surface. This should be because the thickness of the single layers in the CP model is 0.2 mm, and the fiber direction of the layer above the 0.4-mm-deep defect is rotated 90° to
the fiber direction in the specimen surface. Because of this effect, the influence of the thermal anisotropy could be reduced, decreasing the difference between ΔT0 and ΔT90. The same effect should occur in the result for d = 0.8 mm. Therefore, these results suggest that the inspection capability is almost independent of the scan direction when detecting defects located beneath several layers in the cross-ply laminates. Note that even when inspecting the cross-plied laminates, it should be effective to scan along the fiber direction of the uppermost layer when detecting shallow defects or defects in the CFRPs reinforced with carbon fibers with higher thermal conductivities.
3.2 Experimental verification The numerical results were verified by experiments for a UD and CP CFRP laminate. The UD laminate was the same specimen used in section 2 (see Fig. 1). The configuration of the CP specimen was also identical to that shown in Fig. 1, although approximately 0.2-mm-thick, unidirectionally reinforced thin layers were stacked alternately while changing the fiber direction. In the experiments, the surface of the specimens was heated by the same laser scanning device used in section 2. Scanning heating was performed in directions both parallel and orthogonal to the carbon fiber direction of the outermost layer. The LS conditions (laser diameter, scan speed, and energy density) were also identical to those used in section 2. Figure 10 shows the experimentally obtained temperature contrast ΔT observed on the defects in the specimens with depths of 0.2 and 0.4 mm. As seen in the results for the UD specimen (Fig. 10(a) and 10(b)), ΔT is larger when the scan direction is parallel to the fiber direction, regardless of the defect depth. In contrast, in the results for the CP specimen (Fig. 10(c) and 10(d)), although ΔT obtained when scanning parallel to the fiber direction of the outermost layer is larger than that obtained when scanning orthogonal to the fiber when d = 0.2 mm, those values are almost the same when d = 0.4 mm. These tendencies agree with the numerical results.
4 Inspection from 10-m distance Although the inspection capability of the LS method is similar to that of the flash heating method, as shown in section 2, the LS method is expected to be effective for inspecting objects located a far distance from the observers because the diffusion attenuation of the laser beam is much smaller than that of the conventional optical heaters. In order to examine the remote inspection capability of the laser heating method, inspection of the CFRP specimen located 10 m from the laser source was performed.
4.1 Experimental setup Figure 11 shows the experimental setup and a CFRP specimen used in the experiment. The specimen (Fig. 11(b)) is a 980 × 980× 6 mm cross-ply laminate (reinforced with T700 carbon fiber), and has 20 × 20, 50 × 50, and 100 × 100 mm square, flat-bottomed holes as artificial defects. The defect depth from the heated surface d is 0.5–2.0 mm for 20 × 20 mm defect, and 0.5–3.0 mm for 50 × 50 and 100 × 100 mm defects. The specimen was placed 10 m from the laser scanning device, the same used in previous sections (see Fig. 11(a)), and was heated from the surface without holes. The scanning direction was parallel to the fiber direction of the uppermost layer. The laser diameter σ, scanning speed v, and laser density ρ on the specimen surface were approximately 380 mm, 570 mm/s, and 714 W/m2, respectively. The temperature distribution on the heated surface during and after the scanning heating was monitored using an infrared camera with a telephoto lens with a sampling frequency of 7.5 Hz.
4.2 Experimental results 4.2.1 Using temperature data
Figure 12 presents the temperature image observed after laser heating. In Fig. 12, defects with depths up to 1.0 mm are identified for the 50 × 50 mm defect, and those with depths up to 2.0–2.5 mm are identified for the 100 × 100 mm defect. In the temperature images, the 50 × 50 mm defect could not been detected. Figure 13 shows the temperature contrasts ΔT as a function of time (after heating) obtained on the 100 × 100 mm defect with depths of 0.5, 1.0, and 2.0 mm (ΔT was calculated as an average value of 10 data points on each depth defect). The maximum ΔT for each depth (determined as the peak value of the fifth-order approximation curve presented in Fig. 13 as a dashed line) is approximately 0.81, 0.40, and 0.15 °C; these values are higher than the temperature resolution of the infrared camera (0.07 K). These results suggest that the laser heating method enables remote thermographic inspection for CFRP structures.
4.2.2 Using phase data Figure 14 presents a phase image transformed from the temperature data in Fig. 12. In the phase image, all the defects are identified for the 50 × 50 and 100 × 100 mm defects. Moreover, the 20 × 20 mm defects, which could not be detected in the temperature images, are also detected in Fig. 14. As presented in section 2, the phase behavior observed after laser scanning heating is similar to that observed in the flash heating method. Further, in the conventional PPT method with flash heating, using phase images is effective in improving defect detectability. Thus, these results indicate that using phase images is also effective for inspecting distant objects using laser scanning heating. Note that, in the present setup, the detection of defects smaller than 20 mm will be difficult because of the coarse spatial resolution of the obtained images (the pixel size at 10 m distance is approximately 5 mm). Using a higher-resolution infrared camera or a higher-magnification telephoto lens should enable us to detect defects as small as practical defects in the CFRP structures.
5 Conclusions The active thermographic inspection of CFRP laminates using the laser scanning heating method was examined. The main findings obtained from the experimental and numerical studies are as follows: 1) The temperature and phase behavior obtained from the laser scanning heating was similar to that obtained from conventional flash lamp heating.
2) The temperature contrast caused by defects is higher when the scanning direction is the same as the fiber direction of the laminates. Note that the influence of the scanning direction decreases when inspecting cross-ply laminates. 3) The conjunction of the laser scanning method with phase transformation is effective for detecting defects in a CFRP laminate located at 10 m from the laser source. Although the distance between the laser source and the specimen in the present study was 10 m, the laser heating method could heat and inspect objects located at further distances by adjusting the focal length of the laser beam. In addition, the scanning method should be a convenient way to inspect large objects because it can easily expand the heating area by expanding the scanning area. These features should contribute toward improvement of inspection efficiency, and therefore, the suggested method could be an effective way to inspect large CFRP structures. Note that, although inspection of plate specimens were discussed in this paper, further investigation of the inspection capability of practical structures, such as structures with curved surface, is also necessary; it will be the future work of this study.
Acknowledgement This work was supported by JSPS KAKENHI Grant Number 26282099.
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Figure captions
Fig. 1 (a) CFRP specimen with artificial defects (5- and 10-mm-diameter flat-bottomed holes) and (b) schematic of the specimen (the numerals in the figure denote the defect depth from the heated surface).
Fig. 2 Temperature images observed 1 s after heating obtained by (a) LS and (b) flash heating (area enclosed by dashed lines shows the specimen, and the numerals in the figure denote defect depth).
Fig. 3 Temperature contrasts ΔT appearing on the defect with each d obtained by (a) LS and (b) flash heating.
Fig. 4 (a) Phase image obtained by LS at a frequency of 0.044 Hz and (b) phase contrasts Δφ as a function of frequency observed on the defect with d.
Fig. 5 (a) Phase image obtained by flash heating at a frequency of 0.044 Hz and (b) phase contrasts Δφ as a function of frequency observed on the defect with d.
Fig. 6 Comparison of relationships between defect depth and frequency at the local minimum peak of the phase contrast obtained by laser scanning and flash heating.
Fig. 7 Schematic of numerical model.
Fig. 8 Numerically calculated (a) temperature contrast ΔT and (b) in-plane heat flux appearing on the specimen surface obtained from the UD model with a 0.4-mm depth defect.
Fig. 9 Temperature contrasts ΔT and in-plane heat flux appearing on the specimen and defect surface calculated from the CP model: (a) ΔT and (b) heat flux for 0.2-mm depth defect, and (c) ΔT and (d) heat flux for 0.4-mm depth defect.
Fig. 10 Experimentally obtained temperature contrasts, (a) UD specimen, d = 0.2 mm; (b) UD specimen, d = 0.4 mm; (c) CP specimen, d = 0.2 mm; and (d) CP specimen, d = 0.4 mm.
Fig. 11 (a) Setup of the experiment for the CFRP specimen located at 10 m distance, (b) CFRP specimen (the numerals denote the defect depth from the heated surface in millimeters).
Fig. 12 Experimentally obtained temperature image after laser scanning heating for CFRP specimen located at 10 m distance.
Fig. 13 Temperature contrasts ΔT as a function of time (after heating) obtained on the 100 × 100 mm defect with depths of 0.5, 1.0, and 2.0 mm.
Fig. 14 Phase image obtained by applying Fourier transform to temperature data in Fig. 12.
Table captions
Table 1 Thermal properties of CFRP specimen.
Table 2 Maximum values of the temperature contrasts calculated from UD model.
Table 3 Maximum values of the temperature contrasts calculated from CP model.
Table 2 Maximum values of the temperature contrasts calculated from UD model.
Defect depth ΔT0 ΔT90
0.2 mm 24.6 24.1
0.4 mm 10.2 9.91
0.6 mm 5.93 5.68
0.8 mm 5.71 4.76
ΔT90/ΔT0
0.98
0.97
0.96
0.83
Table 1 Thermal properties of CFRP specimen.
Density ρ [kg/m3] Specific heat capacity c [J/(kg·K)] Along fiber direction kL Thermal conductivity k [W/(m·K)] Perpendicular to fiber direction kT
1536 865 4.2 0.56
Table 3 Maximum values of the temperature contrasts calculated from CP model.
Defect depth ΔT0
0.2 mm 24.7
0.4 mm 10.2
0.6 mm 5.75
0.8 mm 3.59
ΔT90
24.0
10.2
5.62
3.58
ΔT90/ΔT0
0.97
1.00
0.98
0.99
(a)
(b) 0.2 mm
0.8 mm
0.4 mm
1.0 mm
0.8 mm 0.6 mm 0.4 mm
0.6 mm
1.0 mm
φ10 mm
Fig. 1
0.2 mm
φ5 mm
(a) CFRP specimen with artificial defects (5- and 10-mm-diameter flat-bottomed
holes) and (b) schematic of the specimen (the numerals in the figure denote the defect depth from the heated surface).
(a)
(b) 0.8 mm 0.2 mm
0.2 mm
0.8 mm
0.2 mm
1.0 mm
0.4 mm
1.0 mm
0.6 mm
0.8 mm 0.4 mm 0.6 mm
1.0 mm 0.4 mm 0.6 mm
0.4 mm 1.0 mm 0.6 mm
0.8 mm
0.2 mm
Fig. 2
Temperature images observed 1 s after heating obtained by (a) LS and (b) flash
heating (area enclosed by dashed lines shows the specimen, and the numerals in the figure denote defect depth).
(a)
(b) Temperature contrast T (ºC)
Temperature contrast T (ºC)
10.0 Defect depth d
8.0
0.2 mm 0.4 mm 0.6 mm 0.8 mm 1.0 mm
6.0 4.0 2.0
10.0 Defect depth d
8.0
0.2 mm 0.4 mm 0.6 mm 0.8 mm 1.0 mm
6.0 4.0 2.0 0.0
0.0 0
2
4
6
Time t (s)
8
10
0
2
4
6
8
Time t (s)
Fig. 3 Temperature contrasts ΔT appearing on the defect with each d obtained by (a) LS and (b) flash heating.
10
(b)
15 10 5 0 -5 -10 -15 -20 -25
Phase contrasts (deg)
[deg]
(a)
10 0
-10 -20 Defect depth d
-30 -40
0.2 mm 0.4 mm 0.6 mm 0.8 mm 1.0 mm
-50 0.01
0.1
1
Frequency f (Hz)
Fig. 4
(a) Phase image obtained by LS at a frequency of 0.044 Hz and (b) phase
contrasts Δφ as a function of frequency observed on the defect with d.
(b)
[deg] -25 -30 -35 -40 -45 -50 -55
20
Phase contrasts (deg)
(a)
10 0 -10 -20 -30 -40
Defect depth d 0.2 mm 0.4 mm 0.6 mm 0.8 mm 1.0 mm
-50 0.01
0.1
1
Frequency f (Hz)
Fig. 5
(a) Phase image obtained by flash heating at a frequency of 0.044 Hz and (b)
phase contrasts Δφ as a function of frequency observed on the defect with d.
Frequency f (Hz)
0.6 0.5
Laser scanning heating Flash heating
0.4 0.3 0.2 0.1 0.0 0.0
0.2
0.4
0.6
0.8
1.0
Defect depth d (mm)
Fig. 6
Comparison of relationships between defect depth and frequency at the local
minimum peak of the phase contrast obtained by laser scanning and flash heating.
Defect (φ10 mm flat-bottomed hole)
Fig. 7
Schematic of numerical model.
(b) 10
Scanning direction 0º (Parallel) 90º (Orthogonal)
10
Scanning direction Heat flux (kW/m²)
Temperature contrast T (ºC)
(a) 12
8 6
4 2
0
0
2
4
6
Time t (s)
8
10
8
0º (Parallel) 90º (Orthogonal)
6 4 2 0
0
2
4
6
8
Time t (s)
Fig. 8 Numerically calculated (a) temperature contrast ΔT and (b) in-plane heat flux appearing on the specimen surface obtained from the UD model with a 0.4-mm depth defect.
10
Temperature contrast T (ºC)
(a)
(b)
25
Scanning direction
20
0º 90º
15
10
5
0 0
2
4
6
8
10
Time t (s)
Temperature contrast T (ºC)
(c)
(d)
12
Scanning direction 0º 90º
10 8 6
4 2
0
0
2
4
6
8
10
Time t (s)
Fig. 9 Temperature contrasts ΔT and in-plane heat flux appearing on the specimen and defect surface calculated from the CP model: (a) ΔT and (b) heat flux for 0.2-mm depth defect, and (c) ΔT and (d) heat flux for 0.4-mm depth defect.
(b)
6.0
Temperature contrast T (ºC)
Temperature contrast T (ºC)
(a)
Scanning direction 4.0
Parallel Orthogonal
2.0
0.0
0
2
4
6
8
10
3.0 2.5
Scanning direction Parallel Orthogonal
2.0 1.5 1.0 0.5 0.0
0
2
Time t (s)
Temperature contrast T (ºC)
Temperature contrast T (ºC)
Scanning direction 4.0
Parallel Orthogonal
2.0
2
4
6
Time t (s)
Fig. 10
8
8
10
3.0 2.5
Scanning direction Parallel Orthogonal
2.0 1.5 1.0 0.5 0.0
0
2
4
6
8
Time t (s)
Experimentally obtained temperature contrasts, (a) UD specimen, d = 0.2 mm;
(b) UD specimen, d = 0.4 mm; (c) CP specimen, d = 0.2 mm; and (d) CP specimen, d = 0.4 mm.
10
(d)
6.0
0
6
Time t (s)
(c)
0.0
4
10
(a) CFRP specimen
(b) Laser head
2.0
0.5
0.5 2.0
0.5 1.0
2.5
Laser source
1.0
2.5
1.0 1.5
IR camera
(with telephoto lens)
3.0
1.5
3.0
1.5
2.0
100×100 mm
50×50 mm 20×20 mm
Fig. 11
(a) Setup of the experiment for the CFRP specimen located at 10 m distance,
(b) CFRP specimen (the numerals denote the defect depth from the heated surface in millimeters).
Fig. 12
0.5 mm
0.5 mm
2.0 mm
1.0 mm
1.0 mm
2.5 mm
1.5 mm
3.0 mm
Experimentally obtained temperature image after laser scanning heating for
CFRP specimen located at 10 m distance.
Fig. 13
Temperature contrasts ΔT as a function of time (after heating) obtained on the
100 × 100 mm defect with depths of 0.5, 1.0, and 2.0 mm.
[deg]
20×20 mm defect
0.5 mm 2.0 mm
0.5 mm
2.0 mm
66.0 0 60
0.5 mm
40 1.0 mm
1.0 mm
2.5 mm
1.0 mm
2.5 mm
20 0
1.5 mm
1.5 mm 3.0 mm
1.5 mm
3.0 mm
- 20 - 40 - 59.92
Fig. 14 Phase image obtained by applying Fourier transform to temperature data in Fig. 12.