A numerical study of constraint effects on dynamic ductile crack initiation

A recent numerical investigation (J. Mech. Phys. Solids 48 (2000) 1967) has demonstrated that ductile fracture specimens progressively lose constraint (or triaxiality Q) with increase in loading rate. The aim of the present paper is to examine whether the above phenomenon is responsible for the experimentally observed enhancement in dynamic fracture toughness of ductile materials. To this end, boundary layer simulations are first conducted under 2-D plane strain conditions to clearly understand the role of constraint on ductile fracture processes. The Gurson constitutive equations are employed along with a simple fracture initiation model involving a notch and void ahead of it. Next, the elastic-plastic stress fields ahead of the notch tip in three point bend specimens subjected to static and dynamic loading are studied and the evolution histories of the constraint parameter Q with respect to J are obtained. Finally, ductile crack initiation is simulated in the above specimens using the same approach adopted in the boundary layer analyses. The development of ductile fracture processes at various loading rates in specimens with different crack length to width ratios is examined. It is found that the dynamic fracture behaviour of these specimens can be rationalized using the prevailing constraint levels and the results of the boundary layer simulations. (C) 2002 Elsevier Science Ltd. All rights reserved.