Elastic-Plastic Fracture Mechanics for Two-Dimensional Stable Crack Growth and Instability Problems

Abstract

An elastic-plastic fracture mechanics methodology for treating two-dimensional stable crack growth and instability problems is described. The paper draws on “generation-phase” analyses in which the experimentally observed applied-load (or displacement) stable crack growth behavior is reproduced in a finite-element model. In these calculations a number of candidate stable crack growth parameters are calculated for the material tested. The quality of the predictions that can be made with these parameters is tested with “application-phase” analyses. Here, the finite-element model is used to predict stable crack growth and instability for a different geometry, with a previously evaluated parameter serving as the criterion for stable growth. These analyses are applied to and compared with measurements of crack growth and instability in center-cracked panels and compact tension specimens of the 2219-T87 aluminum alloy and the A533-B grade of steel. The work shows that the crack growth parameters (COA)c, Jc, dJc/da, and the linear elastic fracture mechanics (LEFM)-R, which sample large portions of the elastic-plastic strain field, vary monotonically with stable crack extension. However, the parameters (CTOA)c, R, Go, and Fc, which reflect the state of the crack tip process zone, are essentially independent of the amount of stable growth when the mode of fracture does not change. Useful, stable growth criteria can therefore be evaluated from the crack tip state at the onset of crack extension and do not have to be continuously measured during stable crack growth. The possibility of making accurate predictions for the extent of stable crack growth and the load level at instability is demonstrated using only the value of J c at the onset of crack extension.