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Large-strain, three-dimensional finite element analyses with incremental plasticity were performed for a variety of crack geometries to study local crack-tip stress-strain fields and associated global fracture parameters under conditions of large-scale yielding. The geometries analyzed include thin, single-edge crack tension, single-edge crack bending, and center-crack tension fracture specimens with varying crack depth (a/W) ratios. Two materials were investigated: a high-hardening, low-strength steel and a moderate-hardening, high-strength steel. Mesh refinement studies were performed to ensure convergence of the finite element predictions. The studies examine the effects of in-plane crack-tip element size, initial crack-tip radius size, and number of through-thickness layers on predicted distributions of crack-tip stress and plastic strain and predicted values of the J-integral and CTOD. In addition, the finite element predictions of specimen behavior were verified experimentally by direct measurements, namely load displacement, load longitudinal strain, and load CTOD, made during and following testing of the fracture specimens.
Representative results of the finite element analyses are presented and compared to previously published data where pertinent. Results from the mesh refinement studies and the verification testing are shown. Predicted trends among the specimens and materials in local distributions of crack-tip plastic strain, triaxiality, and opening stress as well as in global parameters, J-integral and m-factor, are discussed.
The finite element analysis results show that the deformation at the crack tip remains strongly three dimensional, with crack-tip stresses demonstrating near plane strain behavior on the midplane and near plane stress on the free surface, even in relatively thin specimens of finite size. Micromechanical models of ductile fracture based on plastic strain and triaxiality would predict that fracture begins straight ahead of the crack tip on the midplane and that the ductile fracture process occurs relatively close to the crack tip (e.g., within one CTOD). The J-integral was not found to decline monotonically from midplane to free surface but to increase near the free surface in most specimens.
finite element analysis, fracture, crack-tip stress strains, large deformation, experimental verification
Leader, Materials and Failure Analysis Group, NASA Ames Research Center, Moffett Field, CA
Associate professor, Stanford University, Stanford, CA