Inspection of PVC, Stainless Steel and Mild Steel Specimens with Long Pulse Thermography

Transcription

1 Inspection of PVC, Stainless Steel and Mild Steel Specimens with Long Pulse Thermography Bandile Jama 1, Graham.J. Wilson 2 Jasson Gryzagoridis 3 1 MEng candidate, Cape Peninsula University of Technology, Department of Mechanical Engineering, and Employee at Eskom, Cape Town, Jamab@eskom.co.za, 2 Eskom, Cape Town, WilsonG@eskom.co.za 3 Cape Peninsula University of Technology, Department of Mechanical Engineering GryzagoridisJ@cput.ac.za Abstract This part of the research evaluates the use and effectiveness of Long Pulsed Thermography as a non-contacting, non-destructive technique that has been reported to provide a fast and efficient inspection for subsurface defects within a wide range of applications. Experiments with PVC, Stainless Steel and Mild Steel specimens with flat bottom holes resembling subsurface defects (delamination) were subjected to long pulse heating excitation. The time duration to obtain some positive results was 1s when the indication of the presence of certain subsurface defects on the material was clear, but for some defects, there was no indication of their presence. Keywords: Long Pulsed Thermography, Non-contacting, Non-distractive, Subsurface defects, 1

2 1. Introduction Long Pulsed Thermography is an active thermography non-contacting non-destructive testing (NDT) technique used to detect subsurface defects in monolithic materials and delamination in composites [1, 2 and 3]. It is one of the most popular thermal stimulation methods due to its quickness of the inspection. According to Steven [4], Long Pulse Thermography contests successfully with the conventional inspection technologies due to its sensitivity and speed. This study evaluated the effectiveness of Long Pulse Thermography by performing experiments on PVC, Stainless Steel and Mild Steel specimens with flat bottom holes (FBH) as subsurface defects resembling (according to industry s standards) delamination. In principle, a specified quantity of energy in the form of heat is applied to an object for a short time interval while using an infrared camera to monitor the temperature decay of the surface as the heat travels through the material [5, 6 and 7]. Figure 1, is a typical setup of Long Pulse Thermography. The heat source for Long Pulse Thermography can be halogen lights, heating gun, hair dryer and laser beams. It is a fast inspection and is easy to operate. The drawback of the technique is that; non-uniform application of heat and variation of surface emissivity might affect the results [7]. Specimen with subsurface defects Heating source Infrared Camera Figure 1. Long Pulse Thermography set up According to Graegorz [8], hot spots in areas above the Flat Bottom Holes (FBH) (subsurface defect) are observed as an indication of heat trapped between the surface and subsurface defect (see Figure 2). The defect beneath the surface restricts the homogeneous heat propagation through the component which causes a difference in the cooling rate of the surface above the discontinuity and forms a hot spot [8]. IRT takes advantage of the energy emitted from the surface and convert it to thermal contrast images. Normally, the contrast is approximately in the shape of the subsurface defect [8]. 2

3 Heat trapped Defect/Discontinuity Heat around the defect Figure 2. Trapped heat eventually flows around the defect however the hot spot on the surface has been created already 2. Theory of Long Pulse Thermography According to Sheregii [9], the heat propagation through the thickness of the specimen after excitation can be explained by the one-dimension solution of the Fourier's law of heat conduction, hence the time depended surface temperature response to heat pulsation is given by equation 1 [7, 9 and 10]. T t T 0 e Q 1 t 2 (1) where Q ( J ) is energy absorbed by the surface of the test pieces, T 0 ( 0 C ) is the initial temperature, t ( s ) rate of heat propagation, e ( W. s 1 2. m. 2 1 K ) is the material s effusivity which is defined by [5] as an ability of the material to exchange heat with the surrounding see equation 2. 1 k 2 e (2) c p where, k ( Wm 1 1 K ) is the thermal conductivity of the material to be tested, kg. m 3 is 1 1 the density of the material and c p ( Jkg K ) is a specific heat of the material Sherengii [9] mentioned that any change in temperature of the material is related to thermal diffusivity (α), which is the thermal property of the material that measures the material's ability to conduct heat in relation to its capacity to store it. Wrobel at el [11] defines it as an indicator that can be used to check how quickly a material will change temperature in response to the application of heat. Thermal diffusion is a thermo-physical property that is directly proportional to thermal conductivity and inversely proportional to material density and specific heat (see equation 3). k (3) c p 3

4 Constrast According to Almond et al [10], equation 1 could be modified to equation 4 specifically to simulate thermal response (contrast) on materials with circular subsurface defects, T C t 2Q 0 c pkt n1 2 2 D nd 2 t 4t e 1 e (4) where Q 0 is the initial heat energy applied on the surface of the specimen, d ( mm ) is the depth of the defect, D ( mm) is the diameter of the defect, t is the response time in seconds. Using equation 4, figure 3 illustrates the numerical response of PVC, Stainless Steel and Mild Steel specimens with 12mm diameter FBH, subjected to estimated heat 2 energy of J. m for a period of 1s. 1,6E+12 1,4E+12 1,2E+12 1E+12 8E+11 6E+11 4E+11 2E ,5 1 1,5 2 2,5 3 3,5 4 TIme Figure 3. Thermal contrast vs time The results indicate that PVC has an outstanding response compared to the other materials, stainless steel being the second and mild steel having the least. Figure 3 reveal an approximation of the early contrast time at each point in the graph, peak contrast and the time contrast peak is also observed. Almond et al, [10] stated that the ability of the above expression (equation 4) to provide estimated peak contrast and peak time is highly valuable because peak contrast and peak time validate the ability of the technique to detect subsurface defects (provided the defect s and its depth from the surface are known!). Thus the methodology of this study was to set the camera to video mode to capture the early peak contrast of each material. 4

5 3. Experimental Methodology A heating gun was used as the source to thermally excite the test specimens. The gun can produce a maximum power of 2000W and discharges a stream of hot air with a temperature range between 60 0 C C. Three specimens manufactured from PVC, Stainless Steel, and Mild Steel material respectively of identical geometry (see Figure 5 and 6) were fabricated with FBH, of various sizes and depths, at the far side of each specimen. The near surface of each specimen was coated with matt black paint to ensure uniform emissivity. A thermal camera (in video mode) was used to monitor the surface temperature of each specimen while cooling after having been excited by the hot air from the heating gun. Heating gun Thermal Camera 0.5m 0.3m Specimen Wooden table Figure 4. Schematic of the experimental set up of Long Pulse Thermography Figure 5. Geometry of the test specimen (units in mm) 5

6 Figure 6. Rear view of the specimen with machined defects 3.1. Experimental Procedure of Long Pulse Thermography Figure 7 illustrates the experimental set up of Long Pulse Thermography. The specimen was secured vertically on a wooden support. The camera was positioned 500mm away from the specimen The heating gun was switched on at low speed and hovered by hand on the front side of the specimen for 5s at an angle approximately 30 0 from the horizontal plane The Thermal camera recorded the change of temperature contrast on the surface of the specimen. Specimen Heating gun Infrared Camera Figure 7. Illustration of equipment used during Long Pulse Thermography 6

7 3.2. Experimental Results Figures 8, 9 and 10 are the thermal images on the near surface of the PVC, Stainless Steel and Mild Steel specimens after excitation. The presence of subsurface defects in all figures is noted by the varying size and intensity of the circular hot spots. Figure 8. PVC specimen s results Figure 9. Stainless Steel specimen s results Figure 10. Mild steel specimen s results 7

8 4. Discussion and Conclusion In this study, Long Pulse Thermography was successfully demonstrated as a non-contacting NDT technique that can be used to detect subsurface defects in PVC, Stainless Steel, and Mild Steel materials. The results obtained from the study were clear and understandable. IRT detected subsurface defects (FBH beneath the surface) as circular hot spots on the surface of the specimen (see Figure 8, 9 and 10). The smallest defect that could be detected in this study was 5mm diameter at 3.5mm from the surface. The appearance of the hot spots after excitation was similar to the results obtained by Stefano et al [12] when performing detection of surface and subsurface defects on impacted composite material, and Reza et al [13] when inspecting glass fiber reinforced plastic (GERP). The PVC specimen s thermal images displayed the greater thermal contrast, followed by the ones from the Stainless Steel and Mild Steel in that order. These results display similarities to the numerical results obtained in Figure 3 and are similar to those obtained by Almond et al [10], while performing an analysis of the delamination detection capability of the pulse stimulated thermographic non-destructive testing technique with five materials (Aluminium alloy, Mild Steel, Stainless Steel, Carbon Fibre and Glass reinforced plastic ). In their study, the carbon fibre specimen produced the greater thermal contrast compared to the other materials because of its lesser value of thermal diffusivity as proof that the thermal contrast is influenced by the thermal diffusivity of the material [10] Table 1. Material properties of the specimens used in this study Material Thermal Conductivity Wm 1 1 K Specific heat 1 1 Jkg K Density kgm 3 Thermal diffusivity PVC Stainless steel Mild steel m 2 1 s The results of this study as observed in Figures 8, 9 and 10 suggest that the thermal contrast is also influenced by both the size and the depth of the defect. Defects close to the surface appeared to have greater thermal contrast than defects in deeper/greater depth, also defects with greater size have greater thermal contrast than smaller defects. This is in accordance with the conclusions by Sheregii [9] and Clemente [14], who conducted studies 8

9 of detecting hidden defects using infrared thermography varying the size and depth of the defects. References [1]. U. Ruben, V. Publo, G. Jon, V. Laura, M. Julio and G. B. Francisco. Infrared Thermography for Temperature Measurement and Non-Destructive Testing. Sensor, 2014, 14, pg [2]. M. Xavier. sa. Applications of Infrared Thermography in Non-destructive Evaluation. Electrical and Computing Engineering Dept. Available from: [Accessed 20 May 2015] [3]. C. L. Clemente, R. T Jose and P. V. M. Xavier. Non-destructive Testing with Thermography. European Journal of Physics, 2013, 34(6), pg. S91 - S109. [4]. S.M. Shepard, Flash thermography of aerospace composites, InIV Conference Panamerican de END Buenos Aires, 2007 Oct 22, Vol. 7. [5]. F.J. Francisco, I.C. Clemente, M.C. Olga, M. P. V. Xavier, M. L. Jose, Enhanced contrast detection of subsurface defects by pulsed infrared thermography based on the fourth order statistic moment, kurtosis, 2009, Vol [6]. T. Suman, A. Aparna, G. Ripul, K. Satish, H.K. Sardana and M. Ravibabu, Detection of subsurface defect using active infrared thermography. [7]. W.B. Larbi, C. Ibarra-Castanedo, M. Klein, A. Bendada and X. Maldague, Experimental comparison of lock-in and pulsed thermography for the nondestructive evaluation of aerospace materials. In 6 th International Workshop, Advances in Signal Processing for Non Destructive Evaluation of Materials (IWASPNDE), Ontario, Canada. Citeseer. 2009, August. [8]. S. P. Grzegorz. Studies on Investigation and Development of Transient Thermography for Detection of Disbonds in Thermal Barrier Coating Systems. Dr Eng Thesis, Imparial College London, December [9]. E.M. Sheregii. Thermography detection of hidden defects. Centre of Microelectronics and Nanotheology Vol 4. pg [10]. D. P. Almond and S. G. Pickering. Analysis detection of the delamination capabilities of pulsed stimulated thermography non-destructive test techniques. Material evaluation, 2014-January, pg [11]. G. Wróbel, Z. Rdzawski, G. Muzia and S. Pawlak, Determination of thermal diffusivity of carbon/epoxy composites with different fiber content using transient thermography. Journal of achievements in materials and manufacturing engineering, 37(2), pp [12]. S. Stefano, P.A. Nicolas, I.C. Clemente, S. Carlo, T. Panagiotis, B. Abdelhakim P. Alfonso, P. Domenica, K Maria and M. Xavier. Surface and Subsurface Defects Detection in Impacted Composite Materials Made by Natural Fibers, Using Nondestructive Testing Methods, International Journal Composite Material, 2014, 4(5A), pg

10 [13]. G. S. Reza, D. Yuxia, P. Kira and M. Xavier. Inspection of Glass Fibre Reinforced Plastic using Near Short Wave Infrared and Ultrasound/Optical Excitation Thermography. Smart Material, Structures and NDT in Aerospace. Montreal, Quebec, Canada, 2-4 November [14]. I.C. Clemente, Studies on Quantitative subsurface defect evaluation by pulsed phase thermography. Ph.D. Thesis: Laval University Quebec,