Dynamic brittle fracture of high strength structuralsteels underconditions of plane strain

Abstract

A transient finite element analysis is carried out to provide a perspective on dynamicfracture models incorporatingthe decohesion of fracture surfaces, with a focus on improvedmodeling and understanding quantitative features ofdynamically propagating cracks under intensestress pulse loading. The problem analyzed here is plane-strainfracture of an edge crackedspecimen under plane wave loading conditions. In order to ascertain the validity of thevariouscohesive surface fracture models, the results of the FEM simulations are compared withexperimentalobservations made during the low temperature, plate-impact fracture experiments on4340VAR steel (200°Ctemper, R c = 55) . The finite element analysis iscarried out within a framework where thecontinuum is characterized by two constitutiverelations; one that relates stress and strain in the bulk material, theother relates the traction andseparation across a specified set of cohesive surfaces. The bulk material ischaracterized as anisotropically hardening and thermally softening elastic–viscoplastic von Mises solid. Thefiniteelement formulation employed, accounts for the effects of finite geometry changes, materialinertia, and heatconduction. Crack initiation and crack growth emerge naturally as outcomes ofthe imposed loading, and arecalculated directly in terms of the materials constitutive parametersand the parameters characterizing the cohesivesurface separation law. From the results of thesesimulations it is observed that the cohesive surface model, whichincudes a cohesive surfacestrength and a characteristic length is not capable of predicting the dynamic crackgrowthobserved in the experiments. However, the computed results are observed to be in goodagreement with theexperimental results when the work of separation per unit area appearing inthe cohesive surface separating law,includes a cohesive-surface separation rate dependentcohesive strength. Moreover, the computational resultsemphasize the existence of a sharp upturnin dynamic fracture toughness in high strength structural steels at amaterial characteristic limitingcrack tip speed even at test temperatures as low as −80°C and under ultra highcrack tip loadingrates ( K ̇ I ≈ 10 8 MPa m / s) .