Non-destructive Evaluation of Defects Depth by Lock-in Shearography

Digital shearography is a laser interferometry-based method that is used to detect defects non-destructively by obtaining the surface response of the specimen to the external loading. Despite the possibility of estimating the size of defect in this method, access to quantitative information is very difficult. In this study, shearography with lock-in technique was investigated in order to obtain quantitative depth information. For this purpose, planar defects were modeled in different depths and phase images were simulated in different modulation frequencies using finite element method. The results showed that the ability to detect defects decreases by increasing the depth of defects. Also, defects detectability is improved in lower frequencies. By thermal loading withdifferent modulation frequencies, it is possible to control the thermal wave penetration depth, which makes it possible to measure the depth of defect. Linear regression showed that there is a correlation between defect depth and the penetration depth related to blind frequency.

Keywords:

Lock-in Shearography , Modulation Frequency , NDE , FEM