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Non-destructive testing of CFRP by laser excited thermography
Accepted Manuscript Non-destructive testing of CFRP by laser excited thermography Waldemar Swiderski PII: DOI: Reference:
S0263-8223(18)32788-0 https://doi.org/10.1016/j.compstruct.2018.11.013 COST 10378
To appear in:
Composite Structures
Received Date: Revised Date: Accepted Date:
2 August 2018 1 November 2018 5 November 2018
Please cite this article as: Swiderski, W., Non-destructive testing of CFRP by laser excited thermography, Composite Structures (2018), doi: https://doi.org/10.1016/j.compstruct.2018.11.013
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.
Non-destructive testing of CFRP by laser excited thermography Waldemar Swiderski Military Institute of Armament Technology, 05-220 Zielonka, Wyszynskiego 7 Str., Poland, [email protected] Abstract The possibilities for detecting defects in multilayer carbon composites occurring as a combination of layers of carbon fiber fabric with thin layers of epoxy resin (CFRP) are presented in this paper. The most common defect in this type of structure is delamination. For non-destructive testing of CFRP samples with deliberately introduced defects, infrared thermography with laser thermal stimulation was used and both computer numerical simulations (using the ThermoCalc program) and experimental tests were carried out. In the experimental testing, a semiconductor laser with a wavelength of 808 nm and a maximum power of 32 W was used for thermal stimulation. Changes of temperature field on the surface of the test sample were recorded with a thermal camera, FLIR 7600 SC. Both pulsed and lock-in thermography methods were used, and this paper presents selected results, together with their analysis. The results presented in this paper show that it is possible to use a laser as a heating source in thermographic non-destructive testing of CFRP composite. Key words: non-destructive testing, IR thermography, composite 1. Introduction The interest in composites results from their excellent mechanical and strength parameters at low specific weight. With the simultaneous occurrence of these qualities, we can deal in principle only with composites, hence, the rapid growth in recent years in their use in structures for which these characteristics are of paramount importance. These are mainly aerospace structures, automobiles, and sports equipment (boats, skis, tennis rackets, bicycles), in which composites are required for thin-walled structures. After developing the technology for producing carbon fibers (low-modulus and later high-modulus) in the middle of the last century, they have become the most commonly used structural composites. Methods for active infrared thermography require thermal excitation of the tested object. For this purpose, various types of heat sources are used, the most popular of which is the use of optical heat sources; various types of heating lamps and lasers. Defects which occur in the internal structure of the object are detected based on changes in the surface temperature field of the object. On the basis of images (thermograms) recorded by the thermal camera, both the dimensions of the defect and the depth of its position below the surface of the object can be determined. The use of laser beams to heat the object enable direct heating of a selected, relatively small surface. The heat generated by the laser heats the test surface with an asymmetrical distribution beam. In the structure of material in which
there are no defects, the heat flow from the heated surface should also be symmetrical. Disturbances in this flow symmetry testify to inhomogeneities (defects) in the internal structure of the tested material. Heating by means of a laser is most often used in thermography to detect microcracks in metals [1-4]. So far, mainly gas lasers [8] and solid bodies [9] have been used as the heating source in active thermography. In his work [9], Broberg states that the advantage of laser heating of the material to be tested by spot method in thermographic studies is the ease of control in terms of the size of the heated area and the length of heating time. As a disadvantage, however, he states that with high-power gas lasers (6 kJ) there are large dimensions and high costs. Broberg also suggests that it is easier to direct the laser by using optical fibers to transmit the laser beam. Keo et al. [8], compares heating with a long laser pulse with optical lock-in thermography (heating with halogen lamps) as the main advantage of laser heating and considers unidirectional beam characteristics, which allows the sample to be heated in a specific position and area. The advantage of laser heating is that it causes a spherical flow of heat in the material to be tested, enabling the detection of material voids in any orientation, as indicated by Schlichting et. al. [2] and Roemer et al. [4]. In a paper by Pech-May et al. [10], the advantage of using laser heating is stated as the speed of measurement. The potential of laser lock-in thermography in the localization of cracks in specific regions of metal samples difficult to achieve by means of conventional non-destructive testing methods (eddy currents, ultrasounds) is indicated by Fedala et al. in their paper [11]. The use of laser line scanning thermography, when compared to the use of laser spot thermography, significantly reduces the time of inspection of the tested object. In previous work [13] it was shown that the scanning time in thermographic studies using a laser line is over an order of magnitude smaller than with the spot technique, while still producing images of irregularities of similar quality. For optical heating of the tested samples in non-destructive thermographic tests, which also include laser heating, there are two main research methods: pulse thermography and lock-in thermography. Pulse thermography is one of the most popular methods of active thermography in the study of composite materials. In this method, a single or series of pulses generated by lamps, lasers etc. are used to heat the material being tested, which last from a few milliseconds (materials with high thermal conductivity - e.g. metals) to a few seconds (materials with low thermal conductivity - e.g. artificial materials). Pulse thermography can be realized by reflection and transmission techniques. During testing, in both the heating and cooling phases, temperature field changes on the surface of the test object are recorded by a thermal camera [14]. Zones of higher or lower temperatures at the surface will indicate areas where there may be material defects of the test object. To detect a defect, the defect temperature signal must have a signal-to-noise ratio (SNR) greater than 1. Physical principles, theory, and data processing in pulsed thermography have all been described in detail by Vavilova et al. [14]. Lock-in thermography (modulatory, synchronous) is a method during which the surface of the test sample is heated with a heat stream. Based on the recorded temperature field over time, the surface amplitude and phase distributions can be determined on the sample surface. The heating of the sample surface changes harmonically. In cases when the thermal properties of the defect and the material without a defect are different, at the given frequency of the excitation signal, the system response is registered based on which its amplitude and phase can be
determined. Defects can be visualized on the basis of differences between wave phases for the surface of the defective sample and no defect [8, 12, 14, 15]. 2. Simulation of defect detection in CFRP In non-destructive testing (NDT), using the thermographic technique to perform a diagnostic task consisting of estimating the size and depth of a defect in the tested object, it is necessary to use an appropriate mathematical model describing the relationship between time-space temperature distribution and the features of the tested object. Numerical modelling of the heat transfer phenomenon is a complicated problem, therefore, in most cases modelling of heat exchange in thermal testing of composite materials using infrared thermography methods is limited to issues related to heat conduction. For simulation of defect detection in CFRP with laser heating, the Thermo-Calc6LTM program was used to simulate conditions of free (natural) convection, during which the temperature between the body and its surroundings is equalized [5]. ThermoCalc-6LTM program solves heat conduction equations using the numerical method of finite elements. The basis for the development of the ThermoCalc-6LTM program were simulations of non-destructive testing processes for which signals correspond to transition states of surface temperature over a subsurface defect. The ThermoCalc-6LTM uses a unique algorithm that allows modelling very thin defects in thick materials without losing computational accuracy. The program assumes that both the sample under test and subsurface defects have the shape of a parallelepiped. Heating or cooling with an external "heat" pulse is made on the side of the front surface of the sample. It is assumed that the heat flux on this side is homogeneous or distribution of its density in cross-sections x and y describes the Gaussian function (Fig.1). In the second case, the point of maximum flux density can be located anywhere in the heated surface. In addition to stimulated heating or cooling, front and rear surfaces are subject to cooling in accordance with Newton's law (a process involving the exchange of heat by convection and radiation). For this purpose, appropriate heat transfer coefficients are introduced. The thermal parameters of the sample and any defects can be determined independently in all three planes of space, thanks to which these elements can be characterized by full anisotropy. The model assumes that lateral surfaces of the sample are adiabatically insulated. The temperature continuity is maintained between boundaries of the sample layers and between defects and their surroundings. The program includes capacitive defects. This means that, unlike many other models used in nondestructive testing, calculations take into account both diffusivity and thermal conductivity of defects. As a result, a more accurate description of thermal phenomena related to the defect and its surroundings is possible. The sample model is shown in Figure 1, and Table 1 presents thermophysical parameters of the sample material and defects used for numerical calculations. The thermal properties of defects (Teflon, steel) and CFRP are taken from the literature (Table 1) [6, 7].
Figure 1. The sample model
Table 1. Thermophysical parameters of the sample material and defects Material
Specific heat
Density
[J·kg-1·K-1]
[kg·m-3]
Thermal conductivity [W·m-1·K-1]
CFRP
406
1500
0 .55()* 2.33 ()*
Teflon
1050
2210
0.23
Steel
470
7850
50
*() - perpendicular to the fibers () - parallel to the fibers
The results of simulation of temperature change in the time over defect D3 (steel) are presented in the graph of Fig.2. Fig. 3 presents temperature changes on the surface of the sample at the point of heating by the laser. As you can see, the maximum temperature rise is approximately 90°C. This does not cause destructive changes on the surface of the sample. During simulation, the axis of the laser beam was located 1 cm from the edge of the defect, and diameter of the beam divergence on the surface of the sample was 5 mm. The results obtained for other analysed defects are presented in Table 2.
Output time step = 0.01 s
Figure 3. The results of simulation of temperature change in the time over defect D3 (steel)
Output time step = 0.01 s
Figure 4. Temperature changes on the surface of the sample at the point of heating by the laser.
Table 2. The results obtained for all defects Material Number of defect ΔT [ºC]
Teflon
Steel
D1
D2
D3
D1
D2
D3
0.34
0.09
0.68
0.59
0.17
1.02
3. Experimental testing The experiments at the Military Institute of Armament Technology (MIAT) were made using a FLIR SC 7600 IR imager (image format 640×512) in a sequence of 300 thermograms. Thermal stimulation was performed with a semiconductor laser with a wavelength of 808 nm and a beam diameter of 4.5 mm. Maximum output power was 120 W. The heating signal was generated for 0.1 sec while the registration time was 1 second; Figure 4 presents the set-up used for the thermographic tests containing a laser thermal stimulation. Tests were carried out on a sample (Fig.5) composite plate made of three carbon fibres (150x350 mm, having a thickness of 1 mm) connected to a layer of epoxy resin, having a thickness of approximately 0.1 mm. Six defects were placed between the plates: three steel sheets (two squares with lengths of 20 and 10 mm and a circle with a diameter of 20 mm) and three Teflon sheets of the same dimensions.
Figure.4. Set-up
Figure 5. Test sample Selected results from experimental tests are shown in Figures 6 and 7. They present detected defect (steel) located at a depth of 2.1 mm below the surface of the sample.
Figure 6. Detected defect D2 (steel) located at a depth of 2.1 mm below the surface of the sample – thermogram (spot laser pulsed thermography).
Figure 7. Detected defect D2 (steel) located at a depth of 2.1 mm below the surface of the sample – phase image ( laser lock-in thermography). Figures 8 and 9 present detected defect (Teflon) located at a depth of 2.1 mm below the surface of the sample.
Figure 8. Detected defect D2 (Teflon) located at a depth of 1 mm below the surface of the sample – thermogram (spot laser pulsed thermography).
Figure 9. Detected defect D2 (Teflon) located at a depth of 1 mm below the surface of the sample – phase image ( laser lock-in thermography).
4. Discussions and Summary In a computer simulation, the weave of fibers in all layers of the fabric is of a plain type, whereas, the weave varies in the sample to be tested, which is twill type. This difference should not have a major impact on discrepancies resulting from the obtained experimental results and computer simulation. Of course, this assumption could be wrong. Therefore, it will be checked in planned future work. IRNDT software from Automation Technology was used to analyze the recorded sequences of thermograms. This software, in addition to standard algorithms for the analysis of thermograms, also allows the use of a lock-in function
because the sequence of thermograms recorded during the tests is synchronized with the source of thermal excitation in the case of these tests with a semiconductor laser. Thanks to this, you can have data in the course of one measurement enabling the analysis of both the pulsed thermography as well as lock-in thermography. In the presented thermograms obtained by pulsed method and phase images obtained by the lock-in method (Fig. 5-8), it is clearly visible that with deeply located defects under the surface of the sample (in this case 2.1 mm), a better quality display and increased possibility of detecting a defect are obtained in the lock-in method. This method can be particularly useful when the temperature signal from the defect is close to the level of interference. In the pulsed method, it is sometimes difficult to select (from a long sequence of thermograms - often from several hundred or more) the best imaging that will allow detection of defects using special algorithms for thermogram processing. It seems that in the applied test method using the CFRP sample for heating, it is possible to detect subsurface defects located at a depth of approx. 2.1 mm. Increasing the laser beam density could increase the probability of detecting even deeper defects, but there is an increased risk that permanent damage to the sample could occur. The results presented in this paper show that it is possible to use a laser as a heating source in thermographic non-destructive testing of CFRP composite. When testing real samples with internal structural damage, it is recommended to use a laser beam to scan the surface of the sample because even relatively shallow defects located under the surface of the sample require that the axis of the laser beam is close to the defect. In addition, they can then cause distortion of the symmetry of temperature field distribution resulting from heating by laser beam so that they can be detected. Further work will be directed towards the use of the laser beam scanning area of the tested object. References 1. Y.-K. An, J. M. Kim, H. Sohn, Laser lock-in thermography for detection of surface-breaking fatigue cracks on uncoated steel structures, NDT&E International, 65, 2014, pp. 54 - 63 2. J. Schlichting, Ch. Maierhofer, M. Kreutzbruck, Crack sizing by laser excited thermography, NDT&E International, 45, 2012, pp. 133–140 3. N. Puthiyaveettil, S. Krishna, R. Kidangan, S. Unnikrishnakurup, C. V. Krishnamurthy, M. Zeigler, P. Myrach and K. Balasubramaniam, In-line laser thermography for crack detection at elevated temperature: A Numerical modeling study, Proc. of QIRT, 2016 4. J. Roemer, T. Uhl, Ł. Pieczonka, Laser spot thermography for crack detection in aluminum structures, 7th International Symposium on NDT in Aerospace – We.5.A.5 5. ThermoCalc-6LTM, Operation Manual, Innovation Ltd., Tomsk, 2014. 6. V. Vavilov, Infra-red non-destructive testing of bondem structures: aspects of theory and practice. Br J NDT (Jul) 1980, pp. 175-183
7. X. P. V. Maldague, Theory and practice of infrared technology for nondestructive testing. John Wiley&Sons, Inc. New York, 2001 8. S. A. Keo, F. Brachelet, F. Breaban, D. Defer, Defect detection in CFRP by infrared thermography with CO2 Laser excitation compared to conventional lock-in infrared thermography, Composites: Part B 69 (2015) 1–5 9. Patrik Broberg ,Surface crack detection in welds using thermography, NDT&E International 57 (2013) 69–73 10. N. W. Pech-May, A. Oleaga, A. Mendioroz, A. Salazar, Fast Characterization of theWidth of Vertical Cracks Using Pulsed Laser Spot Infrared Thermography, J Nondestruct Eval (2016) 35:22 11. Y. Fedala · M. Streza · F. Sepulveda · J.-P. Roger, G. Tessier · C. Boué, Infrared Lock-in Thermography Crack Localization on Metallic Surfaces for Industrial Diagnosis, Journal of Nondestructive Evaluation · December 2013 12. F. Ciampa , P. Mahmoodi, F. Pinto and M. Meo, Recent Advances in Active Infrared Thermography for Non-Destructive Testing of Aerospace Components, Sensors 2018, 18, 609 13. Vladimir P.Vavilov, DouglasD.Burleigh , Review of pulsed thermal NDT: Physical principles, theory and data processing, NDT&E International 73 (2015) 28–52 14. W. Swiderski W., „Lock-in Thermography to rapid evaluation of destruction area in composite materials used in military applications” SPIE vol. 5132, pp. 506-517, 2003
S0263-8223(18)32788-0 https://doi.org/10.1016/j.compstruct.2018.11.013 COST 10378
To appear in:
Composite Structures
Received Date: Revised Date: Accepted Date:
2 August 2018 1 November 2018 5 November 2018
Please cite this article as: Swiderski, W., Non-destructive testing of CFRP by laser excited thermography, Composite Structures (2018), doi: https://doi.org/10.1016/j.compstruct.2018.11.013
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.
Non-destructive testing of CFRP by laser excited thermography Waldemar Swiderski Military Institute of Armament Technology, 05-220 Zielonka, Wyszynskiego 7 Str., Poland, [email protected] Abstract The possibilities for detecting defects in multilayer carbon composites occurring as a combination of layers of carbon fiber fabric with thin layers of epoxy resin (CFRP) are presented in this paper. The most common defect in this type of structure is delamination. For non-destructive testing of CFRP samples with deliberately introduced defects, infrared thermography with laser thermal stimulation was used and both computer numerical simulations (using the ThermoCalc program) and experimental tests were carried out. In the experimental testing, a semiconductor laser with a wavelength of 808 nm and a maximum power of 32 W was used for thermal stimulation. Changes of temperature field on the surface of the test sample were recorded with a thermal camera, FLIR 7600 SC. Both pulsed and lock-in thermography methods were used, and this paper presents selected results, together with their analysis. The results presented in this paper show that it is possible to use a laser as a heating source in thermographic non-destructive testing of CFRP composite. Key words: non-destructive testing, IR thermography, composite 1. Introduction The interest in composites results from their excellent mechanical and strength parameters at low specific weight. With the simultaneous occurrence of these qualities, we can deal in principle only with composites, hence, the rapid growth in recent years in their use in structures for which these characteristics are of paramount importance. These are mainly aerospace structures, automobiles, and sports equipment (boats, skis, tennis rackets, bicycles), in which composites are required for thin-walled structures. After developing the technology for producing carbon fibers (low-modulus and later high-modulus) in the middle of the last century, they have become the most commonly used structural composites. Methods for active infrared thermography require thermal excitation of the tested object. For this purpose, various types of heat sources are used, the most popular of which is the use of optical heat sources; various types of heating lamps and lasers. Defects which occur in the internal structure of the object are detected based on changes in the surface temperature field of the object. On the basis of images (thermograms) recorded by the thermal camera, both the dimensions of the defect and the depth of its position below the surface of the object can be determined. The use of laser beams to heat the object enable direct heating of a selected, relatively small surface. The heat generated by the laser heats the test surface with an asymmetrical distribution beam. In the structure of material in which
there are no defects, the heat flow from the heated surface should also be symmetrical. Disturbances in this flow symmetry testify to inhomogeneities (defects) in the internal structure of the tested material. Heating by means of a laser is most often used in thermography to detect microcracks in metals [1-4]. So far, mainly gas lasers [8] and solid bodies [9] have been used as the heating source in active thermography. In his work [9], Broberg states that the advantage of laser heating of the material to be tested by spot method in thermographic studies is the ease of control in terms of the size of the heated area and the length of heating time. As a disadvantage, however, he states that with high-power gas lasers (6 kJ) there are large dimensions and high costs. Broberg also suggests that it is easier to direct the laser by using optical fibers to transmit the laser beam. Keo et al. [8], compares heating with a long laser pulse with optical lock-in thermography (heating with halogen lamps) as the main advantage of laser heating and considers unidirectional beam characteristics, which allows the sample to be heated in a specific position and area. The advantage of laser heating is that it causes a spherical flow of heat in the material to be tested, enabling the detection of material voids in any orientation, as indicated by Schlichting et. al. [2] and Roemer et al. [4]. In a paper by Pech-May et al. [10], the advantage of using laser heating is stated as the speed of measurement. The potential of laser lock-in thermography in the localization of cracks in specific regions of metal samples difficult to achieve by means of conventional non-destructive testing methods (eddy currents, ultrasounds) is indicated by Fedala et al. in their paper [11]. The use of laser line scanning thermography, when compared to the use of laser spot thermography, significantly reduces the time of inspection of the tested object. In previous work [13] it was shown that the scanning time in thermographic studies using a laser line is over an order of magnitude smaller than with the spot technique, while still producing images of irregularities of similar quality. For optical heating of the tested samples in non-destructive thermographic tests, which also include laser heating, there are two main research methods: pulse thermography and lock-in thermography. Pulse thermography is one of the most popular methods of active thermography in the study of composite materials. In this method, a single or series of pulses generated by lamps, lasers etc. are used to heat the material being tested, which last from a few milliseconds (materials with high thermal conductivity - e.g. metals) to a few seconds (materials with low thermal conductivity - e.g. artificial materials). Pulse thermography can be realized by reflection and transmission techniques. During testing, in both the heating and cooling phases, temperature field changes on the surface of the test object are recorded by a thermal camera [14]. Zones of higher or lower temperatures at the surface will indicate areas where there may be material defects of the test object. To detect a defect, the defect temperature signal must have a signal-to-noise ratio (SNR) greater than 1. Physical principles, theory, and data processing in pulsed thermography have all been described in detail by Vavilova et al. [14]. Lock-in thermography (modulatory, synchronous) is a method during which the surface of the test sample is heated with a heat stream. Based on the recorded temperature field over time, the surface amplitude and phase distributions can be determined on the sample surface. The heating of the sample surface changes harmonically. In cases when the thermal properties of the defect and the material without a defect are different, at the given frequency of the excitation signal, the system response is registered based on which its amplitude and phase can be
determined. Defects can be visualized on the basis of differences between wave phases for the surface of the defective sample and no defect [8, 12, 14, 15]. 2. Simulation of defect detection in CFRP In non-destructive testing (NDT), using the thermographic technique to perform a diagnostic task consisting of estimating the size and depth of a defect in the tested object, it is necessary to use an appropriate mathematical model describing the relationship between time-space temperature distribution and the features of the tested object. Numerical modelling of the heat transfer phenomenon is a complicated problem, therefore, in most cases modelling of heat exchange in thermal testing of composite materials using infrared thermography methods is limited to issues related to heat conduction. For simulation of defect detection in CFRP with laser heating, the Thermo-Calc6LTM program was used to simulate conditions of free (natural) convection, during which the temperature between the body and its surroundings is equalized [5]. ThermoCalc-6LTM program solves heat conduction equations using the numerical method of finite elements. The basis for the development of the ThermoCalc-6LTM program were simulations of non-destructive testing processes for which signals correspond to transition states of surface temperature over a subsurface defect. The ThermoCalc-6LTM uses a unique algorithm that allows modelling very thin defects in thick materials without losing computational accuracy. The program assumes that both the sample under test and subsurface defects have the shape of a parallelepiped. Heating or cooling with an external "heat" pulse is made on the side of the front surface of the sample. It is assumed that the heat flux on this side is homogeneous or distribution of its density in cross-sections x and y describes the Gaussian function (Fig.1). In the second case, the point of maximum flux density can be located anywhere in the heated surface. In addition to stimulated heating or cooling, front and rear surfaces are subject to cooling in accordance with Newton's law (a process involving the exchange of heat by convection and radiation). For this purpose, appropriate heat transfer coefficients are introduced. The thermal parameters of the sample and any defects can be determined independently in all three planes of space, thanks to which these elements can be characterized by full anisotropy. The model assumes that lateral surfaces of the sample are adiabatically insulated. The temperature continuity is maintained between boundaries of the sample layers and between defects and their surroundings. The program includes capacitive defects. This means that, unlike many other models used in nondestructive testing, calculations take into account both diffusivity and thermal conductivity of defects. As a result, a more accurate description of thermal phenomena related to the defect and its surroundings is possible. The sample model is shown in Figure 1, and Table 1 presents thermophysical parameters of the sample material and defects used for numerical calculations. The thermal properties of defects (Teflon, steel) and CFRP are taken from the literature (Table 1) [6, 7].
Figure 1. The sample model
Table 1. Thermophysical parameters of the sample material and defects Material
Specific heat
Density
[J·kg-1·K-1]
[kg·m-3]
Thermal conductivity [W·m-1·K-1]
CFRP
406
1500
0 .55()* 2.33 ()*
Teflon
1050
2210
0.23
Steel
470
7850
50
*() - perpendicular to the fibers () - parallel to the fibers
The results of simulation of temperature change in the time over defect D3 (steel) are presented in the graph of Fig.2. Fig. 3 presents temperature changes on the surface of the sample at the point of heating by the laser. As you can see, the maximum temperature rise is approximately 90°C. This does not cause destructive changes on the surface of the sample. During simulation, the axis of the laser beam was located 1 cm from the edge of the defect, and diameter of the beam divergence on the surface of the sample was 5 mm. The results obtained for other analysed defects are presented in Table 2.
Output time step = 0.01 s
Figure 3. The results of simulation of temperature change in the time over defect D3 (steel)
Output time step = 0.01 s
Figure 4. Temperature changes on the surface of the sample at the point of heating by the laser.
Table 2. The results obtained for all defects Material Number of defect ΔT [ºC]
Teflon
Steel
D1
D2
D3
D1
D2
D3
0.34
0.09
0.68
0.59
0.17
1.02
3. Experimental testing The experiments at the Military Institute of Armament Technology (MIAT) were made using a FLIR SC 7600 IR imager (image format 640×512) in a sequence of 300 thermograms. Thermal stimulation was performed with a semiconductor laser with a wavelength of 808 nm and a beam diameter of 4.5 mm. Maximum output power was 120 W. The heating signal was generated for 0.1 sec while the registration time was 1 second; Figure 4 presents the set-up used for the thermographic tests containing a laser thermal stimulation. Tests were carried out on a sample (Fig.5) composite plate made of three carbon fibres (150x350 mm, having a thickness of 1 mm) connected to a layer of epoxy resin, having a thickness of approximately 0.1 mm. Six defects were placed between the plates: three steel sheets (two squares with lengths of 20 and 10 mm and a circle with a diameter of 20 mm) and three Teflon sheets of the same dimensions.
Figure.4. Set-up
Figure 5. Test sample Selected results from experimental tests are shown in Figures 6 and 7. They present detected defect (steel) located at a depth of 2.1 mm below the surface of the sample.
Figure 6. Detected defect D2 (steel) located at a depth of 2.1 mm below the surface of the sample – thermogram (spot laser pulsed thermography).
Figure 7. Detected defect D2 (steel) located at a depth of 2.1 mm below the surface of the sample – phase image ( laser lock-in thermography). Figures 8 and 9 present detected defect (Teflon) located at a depth of 2.1 mm below the surface of the sample.
Figure 8. Detected defect D2 (Teflon) located at a depth of 1 mm below the surface of the sample – thermogram (spot laser pulsed thermography).
Figure 9. Detected defect D2 (Teflon) located at a depth of 1 mm below the surface of the sample – phase image ( laser lock-in thermography).
4. Discussions and Summary In a computer simulation, the weave of fibers in all layers of the fabric is of a plain type, whereas, the weave varies in the sample to be tested, which is twill type. This difference should not have a major impact on discrepancies resulting from the obtained experimental results and computer simulation. Of course, this assumption could be wrong. Therefore, it will be checked in planned future work. IRNDT software from Automation Technology was used to analyze the recorded sequences of thermograms. This software, in addition to standard algorithms for the analysis of thermograms, also allows the use of a lock-in function
because the sequence of thermograms recorded during the tests is synchronized with the source of thermal excitation in the case of these tests with a semiconductor laser. Thanks to this, you can have data in the course of one measurement enabling the analysis of both the pulsed thermography as well as lock-in thermography. In the presented thermograms obtained by pulsed method and phase images obtained by the lock-in method (Fig. 5-8), it is clearly visible that with deeply located defects under the surface of the sample (in this case 2.1 mm), a better quality display and increased possibility of detecting a defect are obtained in the lock-in method. This method can be particularly useful when the temperature signal from the defect is close to the level of interference. In the pulsed method, it is sometimes difficult to select (from a long sequence of thermograms - often from several hundred or more) the best imaging that will allow detection of defects using special algorithms for thermogram processing. It seems that in the applied test method using the CFRP sample for heating, it is possible to detect subsurface defects located at a depth of approx. 2.1 mm. Increasing the laser beam density could increase the probability of detecting even deeper defects, but there is an increased risk that permanent damage to the sample could occur. The results presented in this paper show that it is possible to use a laser as a heating source in thermographic non-destructive testing of CFRP composite. When testing real samples with internal structural damage, it is recommended to use a laser beam to scan the surface of the sample because even relatively shallow defects located under the surface of the sample require that the axis of the laser beam is close to the defect. In addition, they can then cause distortion of the symmetry of temperature field distribution resulting from heating by laser beam so that they can be detected. Further work will be directed towards the use of the laser beam scanning area of the tested object. References 1. Y.-K. An, J. M. Kim, H. Sohn, Laser lock-in thermography for detection of surface-breaking fatigue cracks on uncoated steel structures, NDT&E International, 65, 2014, pp. 54 - 63 2. J. Schlichting, Ch. Maierhofer, M. Kreutzbruck, Crack sizing by laser excited thermography, NDT&E International, 45, 2012, pp. 133–140 3. N. Puthiyaveettil, S. Krishna, R. Kidangan, S. Unnikrishnakurup, C. V. Krishnamurthy, M. Zeigler, P. Myrach and K. Balasubramaniam, In-line laser thermography for crack detection at elevated temperature: A Numerical modeling study, Proc. of QIRT, 2016 4. J. Roemer, T. Uhl, Ł. Pieczonka, Laser spot thermography for crack detection in aluminum structures, 7th International Symposium on NDT in Aerospace – We.5.A.5 5. ThermoCalc-6LTM, Operation Manual, Innovation Ltd., Tomsk, 2014. 6. V. Vavilov, Infra-red non-destructive testing of bondem structures: aspects of theory and practice. Br J NDT (Jul) 1980, pp. 175-183
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