J-Q Characterization of Constraint Effects of a Three-Dimensional Cracked Specimen

Failure assessment of the integrity of a ductile flawed structural component is done currently by a one-parameter fracture mechanics approach. The J -integral is the one-parameter used; it has proven to be useful in order to predict ductile crack initiation. However, when tension loading dominates and/or a fully plastic condition develops around the crack, the J -integral alone does not describe completely the crack-tip stress field and a second parameter is needed. In this work, an accurate modeling of the elastic-plastic stress field around a deep crack in a three-dimensional three-point bend specimen is carried out. Numerical results for the crack-tip stress field are used to evaluate a crack-tip constraint parameter Q , in terms of applied loading, from contained plasticity to large-scale yielding. The parameter Q , measures the degree of stress triaxiality and constraint around the crack-tip. In order to obtain the stresses in the near-crack-tip field with high accuracy, a detailed mesh with higher order three-dimensional finite elements is located around the crack front. The modeling of crack-tip blunting deformation is performed by using a small notch radius in the crack-tip. Large-strain and finite-rotation nonlinear behavior effects around the crack-tip are included. The material, an ASTM A 516 steel, is modeled with incremental theory of plasticity. Numerical results of the Q triaxiality parameter are presented for increasing level loads to obtain an extended yield condition. Additional results of J -integral parameter and crack-tip opening displacement, for different load ratios and for different position across the specimen thickness are shown.