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

A transient finite element analysis is carried out to provide a perspective on dynamic fracture models incorporating the decohesion of fracture surfaces, with a focus on improved modeling and understanding quantitative features of dynamically propagating cracks under intense stress pulse loading. The problem analyzed here is plane-strain fracture of an edge cracked specimen under plane wave loading conditions. In order to ascertain the validity of the various cohesive surface fracture models, the results of the FEM simulations are compared with experimental observations made during the low temperature, plate-impact fracture experiments on 4340VAR steel (200 degrees C temper, R-c = 55). The finite element analysis is carried out within a framework where the continuum is characterized by two constitutive relations; one that relates stress and strain in the bulk material, the other relates the traction and separation across a specified set of cohesive surfaces. The bulk material is characterized as an isotropically hardening and thermally softening elastic-viscoplastic von Mises solid. The finite element formulation employed, accounts for the effects of finite geometry changes, material inertia, and heat conduction. Crack initiation and crack growth emerge naturally as outcomes of the imposed loading, and are calculated directly in terms of the material's constitutive parameters and the parameters characterizing the cohesive surface separation law. From the results of these simulations it is observed that the cohesive surface model, which incudes a cohesive surface strength and a characteristic length is not capable of predicting the dynamic crack growth observed in the experiments. However, the computed results are observed to be in good agreement with the experimental results when the work of separation per unit area appearing in the cohesive surface separating law, includes a cohesive-surface separation rate dependent cohesive strength. Moreover, the computational results emphasize the existence of a sharp upturn in dynamic fracture toughness in high strength structural steels at a material characteristic limiting crack tip speed even at test temperatures as low as -80 degrees C and under ultra high crack tip loading rates ((K) over dot(I) approximate to 10(8) MPa root m/s). (C) 1999 Elsevier Science Ltd. All rights reserved.