On the sensitivity of coherent gradient sensing: Part II—An experimental investigation of accuracy in fracture mechanics applications

Abstract

The sensitivity of the transmission ‘Coherent Gradient Sensing’ (CGS) technique is investigated experimentally in relation to the study of deformations near the tips of cracks in three-point bend specimens. Fringe data from these experiments are interpreted as gradients (geometric interpretation) and finite differences (physical interpretation) of hydrostatic stress fields. These data are used to compare the accuracy of the geometric interpretation of fringes with the physical interpretation of fringes obtained using increased sensitivity in order to confirm the theoretical findings from Part I of this investigation. Also, a least-squares fitting technique was used on the fringe data obtained from the region outside of the near tip 3D zone in order to investigate issues of K-dominance of the stress field in this region. Results from the experimental investigations reported in this paper indicated that increasing the sensitivity of the CGS technique improved the quantity and quality of fringe data. However, the apparent size of the region on the image plane dominated by 3D effects increased and the differences between the geometric and physical interpretations of CGS fringe data were increased as well. In addition, the stress field outside of the 3D zone was determined not to be strictly K-dominant, as was predicted by results from previous experiments.