Non-destructive evaluation of internal cracks in glass fiber reinforced composites using the laser shearing interferometry method

Abstract

Laser shearing interferometry is one of the new methods of non-destructive testing, which is based on the interference of monochromatic waves reflected from the surface of the sample. This method can evaluate the entire surface of the sample with high accuracy and speed, by directly measuring the out-of-plane displacement gradient. In this paper, the possibility of detecting subsurface cracks with various lengths and angles in composite samples has been investigated using the shearing method and thermal stimulation system. To this aim, artificial and controlled cracks of different lengths and angles were created in the manufactured composites. After validating the performance of the shearing arrangement, two radiation heat sources were used to apply the load on the samples. The effect of loading size variables, shearing size and direction, crack length, and their angle on the quality of the results was investigated. The results show that the change in loading size plays a more important role than the change in shearing size in correct crack detection. To achieve the best results in crack detection on the selected samples, optimal loading in thermal mode equal to 12 and 15 seconds from the front of the sample was obtained. Moreover, the optimal size of the shearing in the examined composite samples was estimated to be about 10% of the width of the image recorded by the camera. Using the optimized values, all subsurface cracks were identified.