Measurement of crack-tip and punch-tip transient deformations and stress intensity factors using Digital Gradient Sensing technique

The optical method of Digital Gradient Sensing (DGS) is extended to the study of fracture mechanics and impact mechanics problems. DGS is based on the elasto-optic effect exhibited by transparent materials subjected to non-uniform state of stress causing the angular deflection of light rays propagating through the material. Under plane stress conditions, the deflections of light rays can be related to two orthogonal in-plane stress gradients. In this paper, the principle of the method is presented first, followed by crack and punch experiments on PMMA plates for quantifying stress gradients near stress risers. The crack-tip stress intensity factors under both quasi-static and dynamic loading conditions are extracted from DGS measurements and are in good agreement with analytical and finite element results. The problem of a square-punch impacting the edge of a PMMA sheet is also studied using the new method by exploiting punch-tip–crack-tip analogy. The dynamic punch-tip stress intensity factors are extracted from the optical measurements and are again in good agreement with the ones from a complementary finite element analysis of the problem.

► A method to measure stress gradients in transparent materials is proposed.
► It is based on elasto-optic effect and digital image correlation technique.
► The method is demonstrated for measuring SIFs near static and dynamic cracks.
► Using punch tip–crack tip analogy, square punch impact problem is studied.
► Elasto-dynamic computations are carried out to complement measurements.