Mechanism-based approach for prediction of ductile fracture

Mechanism-based fracture mechanics attempts to link the micro-structural parameters and continuum properties of the material to the macroscopically measured fracture resistance. Two types of approaches have been proposed in the published literature to implement the mechanism-based concepts for the ductile fracture: explicit void representation and the continuum damage model. This study deals with both approaches.; In the approach using explicit void representation, an array of micro-voids are assumed to pre-exist in the material and are modeled explicitly using refined finite elements in the 3-D small scale yielding (SSY) model to simulate the material failure process and ductile crack growth. A method to determine failure of the ligament between the crack tip and the nearest void is presented. The failure criterion is used to determine the fracture initiation toughness and J-R curve. The effect of void shape on the fracture toughness is examined.; For the approach using a continuum damage model, a precise constitutive model for characterization of the void-containing material behavior during the ductile fracture process is needed. The Gurson-Tvergaard (GT) model is one of the widely used continuum damage models for materials containing spherical voids. A method to calibrate the two micromechanics parameters (q 1,q2) of the GT model is suggested in this study. It was found that these two parameters are functions of the initial void volume and the stress state of the void-containing material volume. The effects of the 3-D stress state, which can be expressed by the triaxiality stress ratio and the lode angle, on the parameters (q 1, q2) are discussed.; A generalized numerical algorithm to calculate the consistent tangent stiffness was derived and applied to the Goludanu-Leblond-Devaux (GLD) continuum damage model for materials containing spheroidal shape voids. The performance comparison between the consistent tangent stiffness and the conventional continuum tangent stiffness demonstrates the significant improvement in convergence characteristics of the overall Newton iterations caused by using the consistent tangent matrix. The GLD model was implemented in ABAQUS via a user material subroutine. A series of parametric studies was conducted to demonstrate the application of the GLD model in fracture analysis.