Dynamic brittle fracture of high strength structural steels under conditions of plane strain (Q1302814)

This paper is devoted to the study of the validity of cohesive surface models in predicting fast fracture dynamics in high-strength structural steels. For solution of this problem, the authors apply the low temperature, plate-impact fracture experiments on 4340VAR steel and perform finite element analysis. The considered dynamic fracture models incorporating the decohesion of fracture surfaces provide qualitative understanding of dynamic crack behavior under intense stress pulse loading. The analysis includes a plane-strain fracture of an edge cracked steel specimen under plane wave loading conditions, and at temperature lower than room temperature. The experimental results are achieved impacting a disk-shaped specimen containing a mid-plane crack by a thin flyer plate. For better evaluation of various fracture models, the motion of four different points on the specimen surface have been measured simultaneously during the experiment by interferometer. In computational analysis the continuum is characterized by two constitutive relations: a volumetric law of an isotropically hardening and thermally softening elastic-viscoplastic von Mises solid, and a cohesive surface constitutive relation involving the tractions and displacement jumps across the cohesive surface. The finite element formulation accounts for the effects of finite geometry changes, material inertia, material rate sensitivity and heat conduction. The finite element discretization is based on the linear triangular elements that are arranged in a ``crossed-triangle'' quadrilateral pattern, and in which the displacements and temperature vary linearly, accomodating isochoric deformation. The computational analysis shows that the cohesive surface model, which includes a cohesive surface strength and a characteristic length, is not capable of predicting the dynamic crack growth observed in the experiments. The computed free surface particle velocity profiles agree with experimental ones either during the initial time window or the latter part of the time window, but not for both at any selection of the cohesive surface parameters. Therefore the authors have to consider the rate-dependent cohesive surface model. The results of the simulations agree good with the experimental ones when the cohesive surface model includes a work of separation which depends on the cohesive-surface separation rate. Then, based on the \(J\)-integral estimation, the authors are able to compute the dynamic material toughness. The results emphasize the existence of a sharp upturn in dynamic fracture toughness at a material characteristic limiting crack tip speed, even at test temperatures as low as \(-80^\circ\)C, and under ultra-high crack-tip loading rates. Finally, the energy partitioning during the dynamic fracture process is studied, and contours of von Mises effective stress, effective plastic strain and local temperature in the vicinity of the growing crack are presented.