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Investigation of Loading Parameters in Detection of Internal Cracks of Composite Material with Digital Shearography

Laser shearography is a speckle interferometric technique for full-field measurement of surface displacement derivatives. This technique has proved highly effective for nondestructive testing (NDT) of many kinds of material, especially composites. The key to the success of shearography as a NDT method is to impose an appropriate stress status on the object under test. In this paper mechanical loading for detection of artificial cracks in glass fiber reinforced composites is investigated. In this regard, a dimensionless parameter named "load ratio" or "LR" is defined to present the loading status in the elastic region. Crack-type defects were simulated by cutting several grooves with different sizes and orientations in an 8layer fiber reinforced composite plate. The tests have been performed with different "LR"s to evaluate the capability of shearography in detection of the cracks and to extract the proper ranges of load ratio from the test results. It has been showed that the load ratio is an important parameter in the inspection procedure beside other parameters like shearing distance and direction. The effects of the shearing and crack direction in the defect detection capability of the shearography system have also been investigated.