Measurement of three-dimensional Mode-I stress intensity factor using multiple embedded grid moire technique

Multiple embedded grid moire and strain gage techniques are used to calculate the variation in Mode-I stress intensity factor throughout the thickness of ASTM E-399 standard compact tension specimens of Merlon polycarbonate. The specimen grids near the crack tip on the surface and in the interior were recorded for the unloaded condition and for various loaded states using high resolution photographic techniques. Optical processing produced moire patterns from which the change in the crack opening displacement could be determined. From the information about the crack opening displacement in the region of the crack tip, the Mode-I stress intensity factor was calculated for various interior planes and on the surface of the specimen. The stress intensity factor was found to be higher in the midplane than on the surface, and it causes crack initiation to start at the midplane.