A SIMPLE METHOD FOR MEASURING SURFACE STRAINS AROUND CRACKS

A simple system has been developed to measure surface strains that occ ur during in situ deformation of mechanical test specimens. The system uses photolithographically deposited displacement markers and compute r image recognition routines to determine in-plane displacements and s trains from digital images. The strain calculating routines are integr ated into a simple mouse-driven software package that facilitates the transformation from digital images to useful strain field information. Additional routines have been developed to determine crack tip stress fields and J integrals. Crack tip stress intensities have been calcul ated from strain maps obtained for traction-free cracks in stainless s teel. The J integrals were found to be independent of contour and cons istent with applied stress intensities. Crack tip stress intensities w ere calculated for bridged cracks in lamellar TiAl. The toughening eff ect of the bridging zones was determined by including the bridged regi on in the contours, Resistance curves generated from strain maps were consistent with those measured during mechanical testing.