Three dimensional cohesive-element analysis and experiments of dynamic fracture in C300 steel

Abstract

The dynamic drop-weight test is taken as a convenient basis for assessing the fidelity and predictive ability of cohesive models of fracture in applications involving dynamic crack growth. In the experimental phase of the study, coherent gradient sensing (CGS) has been used to study dynamic fracture in C300 maraging steel. The specimens were subjected to three-point bend impact loading under a drop weight tower. High-speed photographs of the CGS interferograms were analyzed to determine the crack tip location, the velocity and the dynamic fracture toughness as a function of time. Post-mortem examination of the specimens revealed the fractography of the fracture surfaces, including the development of shear lips. In a parallel numerical phase of the study, fracture has been modeled by recourse to an irreversible cohesive law embedded into cohesive elements. These cohesive elements govern all aspects of the separation and closure of the incipient cracks. The cohesive behavior of the material is assumed to be rate independent. The finite element model is three dimensional and consists of quadratic ten-noded tetrahedra. The numerical simulations have proven highly predictive of a number of observed features, including: the crack growth initiation time; the trajectory of the propagating crack tip; and the formation of shear lips near the lateral surfaces. The simulations therefore establish the feasibility of using cohesive models of fracture and cohesive elements to predict dynamic crack-growth initiation and propagation in three dimensions.