Study of three-dimensional mixed-mode fracture in ductile materials

Mixed-mode fracture in ductile materials is often a critical safety concern for load-bearing structures. It involves the growth of cracks in metallic structures under general geometric and loading conditions. Due to its complex nature, this phenomenon remains an important subject of current research. The present study investigates three-dimensional mixed-mode fracture in ductile materials through careful analyses of several sets of stable tearing crack growth experiments on aluminum alloy, specimens. In particular, mixed-mode I/II experiments on Arcan specimens and mixed-mode I/III experiments on tension-torsion specimens have been analyzed using the finite element method under three-dimensional conditions. Stable tearing crack growth in these experiments have been modeled and simulated using both general-purpose and custom finite element codes.; The focus of the study is on two issues. The first is to explore the application of a mixed-mode fracture criterion based on the crack-tip-opening-displacement (CTOD or COD) under three-dimensional and large deformation conditions. The second is to investigate three-dimensional aspects of mixed-mode fracture failure, such as crack slanting, tunneling and turning, and on developing a predictive methodology for such three-dimensional events. The behavior of crack-tip field parameters, such as CTOD, effective plastic strain (epsilon p), stress constraint (ratio of the mean stress and the effective stress, sigma ms/sigmae), have been studied in detail, and possible correlations between crack-tip field parameters and crack growth directions have been investigated.; The results of this study indicate that (a) the mixed-mode CTOD fracture criterion is able to predict experimentally measured load-crack-extension curves and in-plane crack paths under three-dimensional and mixed-mode loading conditions; (b) for local shearing type fracture failure, the direction of the maximum effective plastic strain (or the direction of the maximum extent of the effective plastic strain contours) correlates strongly with the direction of the crack growth path on section planes through the thickness of the specimen, and hence with the overall orientation of the slant fracture surface; (c) crack tunneling has a significant influence on the distribution of deformation fields around the crack front. In particular, it increases the CTOD value behind the crack front and reduces the stress constraint level ahead the crack front.