Determination of fatigue crack tip location and plastic zone dimensions using displacement fields measured by digital image correlation method

Abstract

The advancement of Digital Image Correlation (DIC) has significantly increased interest in its application for fatigue crack tip location and plastic zone dimension assessment. Under cyclic loading, crack tip stress concentration complicates the crack tip field, often leading to significant systematic errors, making accurate crack tip location and plastic zone dimension assessment challenging. This paper introduces a method for identifying the location of crack tips in the course of crack growth based on the gradient of vertical displacement difference on both sides of the crack obtained through DIC, achieving a maximum error of 3% compared to microscopic observations. Furthermore, a methodology for evaluating the dimensions of plastic zones through the utilization of DIC out-of-plane displacement fields is presented. This method more accurately reflects plastic deformation at the crack tip and with the stress intensity factor in an exponential relationship, and the exponent becomes larger with the increase of load ratio, so it is an ideal parameter to characterize the fatigue crack growth under different load ratios. The proposed method indirectly obtains plastic zone dimensions by measuring out-of-plane displacements, by passing material parameters, yield criteria, stress conditions, crack length, and specimen geometry, and no need to calculate high gradient strain at the crack tip, enhancing calculation efficiency and accuracy.