An Elastic-Plastic Finite Element Analysis of Crack Initiation, Stable Crack Growth, and Instability

An elastic-plastic (incremental and small strain) finite element analysis was used with a crack growth criterion to study crack initiation, stable crack growth, and instability under monotonic loading to failure of metallic materials. The crack growth criterion was a critical crack-tip-opening displacement (CTOD) at a specified distance from the crack tip, or equivalently, a critical crack-tip-opening angle (CTOA). Whenever the CTOD (or CTOA) equaled or exceeded a critical value, the crack was assumed to grow. Single values of critical CTOD were found in the analysis to model crack initiation, stable crack growth, and instability for 7075-T651 and 2024-T351 aluminum alloy compact specimens. Calculated and experimentally measured (from the literature) CTOD values at initiation agreed well for both aluminum alloys. These critical CTOD values from compact specimens were also used to predict failure loads on center-crack tension specimens and a specially designed three-hole-crack tension specimen made of the two aluminum alloys and of 304 stainless steel. All specimens were 12.7 mm thick. Predicted failure loads for 7075-T651 aluminum alloy and 304 stainless steel specimens were generally within ±15% of experimental failure loads, while the predicted failure loads for 2024-T351 aluminum alloy specimens were generally within ±6% of the experimental loads. The technique presented here can be used as an engineering tool to predict crack initiation, stable crack growth, and instability for cracked structural components from laboratory specimens such as compact specimens.