Dynamic fracture toughness versus crack tip speed relationship at lower than room temperature for high strength 4340VAR structural steel

A transient finite element analysis is carried out to provide insight into the low temperature dynamic material toughness versus crack tip relationship for high strength structural steels under intense stress pulse loading. The problem analyzed here is plane strain fracture of an edge cracked specimen under plane wave loading conditions. The finite element formulation employed accounts for the effects of finite geometry changes, material inertia, and heat conduction. The material is characterized as an isotropically hardening and thermally softening elastic-viscoplastic von-Mises solid. To model crack initiation and crack advance various crack-tip equations of motion based on elastodynamic modelling of crack growth are used. In particular, relatively simple forms for the crack tip equation of motion corresponding to crack growth at either constant energy release rate, or a constant stress intensity factor, or a constant crack tip speed, along with more complicated forms involving a sharp upturn in fracture resistance at a limiting crack tip speed are employed. The results of these models emphasize the existence of a sharp upturn in dynamic fracture toughness in high strength structural steels at a material characteristic limiting crack tip speed at test temperatures as low as -80 degrees C and with crack tip loading rates of the order of (K) over dot approximate to 10(8) MPa root m/s. (C) 1998 Elsevier Science Ltd. All rights reserved.