Developments in Visual and Other NDE Methods II Defect Detection using Dual-Beam Shearography and Lock-in Infrared Thermography S-W. La, K-S. Kim, H-C. Jung, H-S.Chang, S-O. Jang, K-S. Kim, Chosun University, Republic of Korea; Y-J. Kang, Chonbuck National University, Republic of Korea ABSTRACT The wall thinning defect of nuclear power pipe is mainly occurred by the affect of the flow accelerated corrosion of fluid. This type of defect becomes the cause of damage or destruction of in carbon steel pipes. Therefore, it is very important to measure defect of welding part and whole field of pipe. In this paper, defect of nuclear power pipe were measured by using dual-beam shearography and infrared thermography. INTRODUCTION Shearography can measure the slope of the deformation by appropriately organizing the opticalinterferometer so that it does not require the optical table, and is very excellent to measure the defect of an object. This technique has introduced to the aerospace and nuclear industry for the nondestructive testing. This study proposes new modified Shearography called dual-beam Shearography, which can measure the out-of-plane deformation and the in-plane deformation by using another illuminated laser beam. Infrared thermography (IR) is a non-contact and non-destructive technique which can measure temperature variations of surface. Defect detection of infrared thermography by Lock-in method is possible for measuring a temperature evolution of object by thermoelastic effect accept for the external factors. Thus, those technique can be a kind of non-contact nondestructive evaluation to apply to industrial structures (automobile and airplane etc). In this paper, defect of nuclear energy pipe can be measured by using dual-beam shearography and infrared thermography, and quantitatively evaluated PRINCIPLE Dual-beam Shearography Fig. 1 is coordinate composition of dual-beam shearography to measure x-axis deformation. For measuring deformation object is alternately illuminated with laser beam by identical angle about z-axis. When right beam and left beam is illuminated the object surface, phase change is caused by the deformation of object. If the illuminating right and left beams lie in the x, z- plan, and the shearing is in x-direction, the phase change R with respect to the right beam is given by 2π ω u R = [1 + cos( + θ )] + sin( + θ ) δx (1) λ x x Similarly, the phase change L with respect to the left beam is given by 2π ω u L = [1 + cos( θ )] + sin( θ ) δx (2) λ x x Where λ is the wavelength of laser, δx is the shearing distance, u/ x is the parameter to be measured.
The four speckle image corresponding to the 4-frame phase shifting with the illumination of right beam is continuously stored. Similarly, the four speckle phase shifting image corresponding to left beam the illuminating is also stored. In-plane deformation in x-direction can be calculated by subtracting L from R.[5] 2π u R L = 2sinθ δx (3) λ x Also gradient of out-of-plane displacement in x- direction can be calculated by addition of two phase change. 2π u R + L = 2(1 + cosθ ) δx (4) λ x Figure 1 - Dual-beam interferometer geometry Lock-in Thermography The principle of lock-in IRT was first described by Carlomagno and Berardi [6]. Basically, in lock-in analysis the heat source induced from the outside has to be calibrated (for each frequency) to make the temperature waveform of the heat source truly sinusoidal. The IR camera picks up a series of thermal images and compares temperatures by extracting a sinusoidal wave pattern at each pixel of the image as shown in Fig. 2. The resultant sinusoidal temperature T(x,y) at the surface (z = 0) in one dimension is given by i[ ωt φ ( x, y )] T y) = Ae (5) A is the thermal wave amplitude and φ y) is the phase in the image plane. The surface temperature field can be reconstructed based on three or four thermal images with a phase step of T/4. These four images S n are used to derive the magnitude image A(x, y) and the phase image φ y). Calculation of the phase and the amplitude of the temperature field are quite simple: 1 S 1 ( x, y ) S 3 y ) φ y ) = tan (6) S 2 y ) S 4 y ) A ( x, y ) y 2 2 = ( S 1 ( x, y ) S 3 ( x, y )) + ( S 2 ( x, y ) S 4 ( x, )) (7) Compared with common thermal images, the phase image is more useful for quantitative evaluation of metal materials because contrast changes at defect spots are clearer. The defect in lock-in IRT is
defined as a phase difference, which is the difference between the phase of the defect area and that of the healthy area [7]. The phase difference φ y) at each pixel of the thermal image between the defect area and the healthy area is defined by Eq. (8), φ y ) = φ d y ) φ y ) (8) s Figure 2 - Principle of Lock-in thermography where φ d y ) is the phase of the pixel (x, y) of the defect area and φ s y ) is the phase of the pixel (x, y) of the healthy area. The phase differences at the sample surface contain sufficient information about the shape and location of the subsurface defect, and the boundary of the defect results in maximum and minimum phase changes that help determine its size and location. The slope can easily be obtained by the shearing-phase technique, where the shearing phase, φ SP in Eq. (9), is calculated by subtraction of adjacent phases with shearing magnitude, φ SP = φ ( x + δs, y ) φ ( x, y ) (9) This equation results from subtraction of the original image from the shifted image by the shearing magnitude, a certain number of pixels along the x or y axis. The shearing-phase distribution provides maximum, minimum, and zero phases. The distance between two characteristic points (maximum and minimum phase) is used for defect determination and zero phase point is used for the identification of defect center. EXPERIMENT Specimen Fig. 3 shows the shape and dimension of specimen. ASTM A106GrB, pressure specimen with yield strength 240MPa, tensile strength 415MPa, minimum thickness 1.8mm, length of pipe(l) 342.00mm, external diameter(do) 113.40mm, inside diameter(di) 99.00mm, defect length(l) 113.40mm, the elongation of length direction 30% was used. Table 1 shows type of specimens.
Dual-Beam Shearography system The Dual-Beam Shearography system is shown in Fig. 4 The Nd:YAG Laser was used in this system, and the laser was irradiated to each direction of the specimen using shutter after that the data were acquired in accordance with each direction. Shearography was used to measure defect of straight pipe by internal pressure change. At first, the non-defect specimen was measured to compare measurement result according to existence of defect. And, the specimen with defect was measured. The wall thinning defect of straight pipe inside measured out-of-plane deformation to each pressure by constant pressure. At this time, shearing amount was 5 mm in y axis direction. Figure 3 - Shape of pipe specimen 1. Specimen with no defect ID No. Defect Length (mm) Width Thickness tp (mm) (2θ) SSP-0A 0.00 0 7.2 2. Defect thickness related type ID No. Defect Length (mm) Width Thickness tp (mm) (2θ) SSP-2G 113.40 90 3.6 SSP-2H 113.40 90 1.8 Table 1 - Type of Specimens Figure 4 - Dual-Beam Shearography system
Infrared Thermography system Figure 5 - lock-in photo-infrared thermography The schematic of lock-in photo-infrared thermography using Silver 480 made by the Cedip company in France is shown in Fig. 5 In the system, the function generator controlled the frequency of the halogen lamp. After the devises were synchronized, the phase map was offered from the infrared light detector to the user. RESULTS AND DISCUSSION Measurement using Dual-Beam Shearography The wall thinning defects were made in the pipes of ASTM A106 Gr.B and the circumferential length of the defects was predicted according to the sort of the defects. The total kinds of defects are 2. The experiments focused on the data of the result by the various pressure differences in the normal straight pipe without defect process were performed. The gap of pressure is 0.02MPa from 0.02MPa to 0.1MPa to measure the defect of the straight pipe specimen with the wall thinning defects. The wall thinning defect of specimen was measured by the change of pressure using the Dual-Beam shearography with 5mm shearing of y axis direction. Fig. 6. shows the phase map, which is the result of non-defect specimen. Figure 6 - Phase map of SSP-OA (a) pressure: 0.02MPa (b) pressure: 0.04MPa (c) pressure: 0.06MPa (d) pressure: 0.08MPa (e) pressure: 0.1MPa (f) pressure: 0.14MPa Figure 7 - Phase map of SSP-2G as the change of pressure amount
The measuring results of all specimens were acquired using the profiles of y axis direction. The phase map and measuring result of SSP-2G is shown in Fig. 7 and Fig, 8. Fig. 9 and Fig. 10 show the phaser map and measuring result of SSP-2H. The more increased the gap of the pressure, the shape of the wall thinning defects appeared wall. Moreover, as the minimum thickness was thinner, the shape of the wall thinning defect was clear in a small pressure gap. Through the value of deformations acquired from the line profile of the two specimens, as the minimum thickness was thinner, the value of deformation was bigger about three times as much as at the same pressure gap.. Figure 8 - Deformation of SSP-2G as the change of pressure amount (a) pressure : 0.02MPa (b) pressure : 0.04MPa (c) pressure : 0.06MPa (d) pressure : 0.08MPa (e) pressure : 0.1MPa (f) pressure : 0.14MPa Figure 9 - Phase map of SSP-2H as the change of pressure amount Figure 10 - Deformation of SSP-2H as the change of pressure amount
Measurement using Infrared Thermography The emissivity of specimen kept 0.95 by coating the surface of specimen with paint. The heat source and the detector were synchronized using lock-in method under optimum frequency(50 mhz). In this study, the experiment with a specimen that has no defect was performed to compare with the result by the existence and non-defect specimen. After that experiment, using the result of previous experiment as the base line, experiments were performed by thickness of the wall thinning defects. The surrounding temperature was kept to 20±0.5 to detect the defect of the specimens, and then the heat of the radiated energy was measured by using infrared camera. The images of infrared thermography and the result of experiments in accordance with each condition are shown in Fig. 11 and Fig. 12. (a) non-defect (b) SSP-2H (c) SSP-2G Figure 11 - Infrared thermography images according to the defect thickness Figure 12 - Data comparison by defect thickness Fig. 12. shows the temperature changes in compliance with the thickness of defects acquired from Infrared thermography and measured length of defects and the real length of defects are similar. The defect length of the specimens was about 120mm. The range of error was within 5% in the result. Through this research, the possibility of having application to the processed defect artificially using the lock-in infrared thermography was verified. From now on, the research for compensating the errors to get the high accuracy might be needed. CONCLUSION In this paper, the inspection technique was proposed using Shearography and Infrared thermography to complement the technical and economical losses of the existing non-destructive technique. From the result, it was accurately measured that length of wall thinning defect which is located on the pipe inside. From the Shearography result, As the minimum thickness was thinner, the shape of the wall thinning defect was clear in a small pressure gap. Also, as the minimum thickness was thinner, the value of deformation was bigger about three times as much as at the same time. The result of Infrared thermography experiments was that the temperature rise linearly in compliance with the minimum thickness.
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