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

The dynamic drop-weight test is taken as a convenient basis for assessing t he fidelity and predictive ability of cohesive models of fracture in applic ations involving dynamic crack growth. In the experimental phase of the stu dy, coherent gradient sensing (CGS) has been used to study dynamic fracture in C300 maraging steel. The specimens were subjected to three-point bend i mpact loading under a drop weight tower. High-speed photographs of the CGS interferograms were analyzed to determine the crack tip location, the veloc ity and the dynamic fracture toughness as a function of time. Post-mortem e xamination of the specimens revealed the fractography of the fracture surfa ces, including the development of shear lips. In a parallel numerical phase of the study, fracture has been modeled by recourse to an irreversible coh esive law embedded into cohesive elements. These cohesive elements govern a ll aspects of the separation and closure of the incipient cracks. The cohes ive behavior of the material is assumed to be rate independent. The finite element model is three dimensional and consists of quadratic ten-noded tetr ahedra. The numerical simulations have proven highly predictive of a number of observed features, including: the crack growth initiation time; the tra jectory of the propagating crack tip; and the formation of shear lips near the lateral surfaces. The simulations therefore establish the feasibility o f using cohesive models of fracture and cohesive elements to predict dynami c crack-growth initiation and propagation in three dimensions. (C) 2000 Els evier Science Ltd. All rights reserved.