A Novel Phase Shifting Technique of Coherent Gradient Sensing Method for Measuring Crack-tip K-Dominance

A novel phase shifting method for coherent gradient sensing (CGS) is proposed to automate fringe processing. The phase shifting of CGS interferograms can be simply realized by placing three plane-parallel plates sequentially between the two gratings in the traditional CGS method. The phase shifting method is deduced theoretically and the full-field wrapped phase value can be calculated accurately from four or more interferograms. A calibration test that determines the elasto-optic constant for a polymethyl methacrylate (PMMA) sheet is implemented as a verification experiment. Results show the proposed phase shifting method is valid for transmission CGS measurement. As an application, the proposed phase-shifted CGS method is applied to measure the crack tip K-dominance in a PMMA sheet. In addition, the phase shifting method is shown to be valid for reflective CGS measurement.