Optical mapping of crack tip deformations using the methods of transmission and reflection coherent gradient sensing: a study of crack tip K-dominance

A new full field optical technique-'Coherent Gradient Sensing' (CGS)-is developed and used to study crack tip deformations in transparent as well as opaque solids. A first order diffraction analysis is provided for the technique and its feasibility is demonstrated both in transmission and reflection modes. Preliminary results from the dynamic crack growth experiment clearly demonstrate the capability of CGS to be an effective experimental alternative to other optical methods used in dynamic fracture studies. Notably, it is a full field technique which works with optically isotropic materials. The static fringe patterns obtained from the experiments are analyzed in regions outside the 3-D zone. For geometries where the region outside the 3-D zone is K-dominant, the fringes provide an accurate value of 2-D stress intensity factor. For geometries where the region outside the 3-D zone is not K-dominant, Williams' expansion is used in conjunction with a least squares procedure to obtain the stress intensity factors.