Investigation of lock-in infrared thermography for evaluation of subsurface defects size and depth

In this study, the investigation on lock-in infrared thermography was done for the detection and estimation of artificial subsurface defects size and depth in stainless steel sample. The experimental and the finite element analysis were performed at several excitation frequencies to interrogate the sample ranging from 0.182 down to 0.021 Hz. A finite element model using ‘ANSYS 14.0’ was used to completely simulate the lock-in thermography. The four point method was used in post processing of every pixel of thermal images using the MATLAB programming language. A signal to noise ratio analysis was performed on both phase and amplitude images in each excitation frequency to determine the optimum frequency. The relationship of the phase value with respect to excitation frequency and defect depths was examined. Amplitude image was quantitatively analyzed using Vision Assistant, a special tool in LABVIEW program to acquire the defects size. The phase image was used to calculate the defects depth considering the thermal diffusivity of the material and the excitation frequency for which the defects become visible. A finite element analysis result was found to have good correlation with experimental result and thus demonstrated potentiality in quantification of subsurface defects.

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