A numerical investigation of loss of crack tip constraint in a dynamically loaded ductile specimen

The objectives of this paper are to study the applicability of a two-parameter (J-Q) characterisation of the elastic-plastic crack tip fields in a dynamically loaded ductile specimen and to ascertain the effect of loading rate J on the triaxiality parameter Q. To this end, 2D (plane strain) large deformation finite element analyses of a single edge notched specimen (SEN(T)) subjected to a tensile stress pulse are conducted. The material is assumed to obey the J(2) Row theory of plasticity with small elastic strains and both rate independent and rate dependent behaviour are considered. The results demonstrate that even a deeply cracked SEN(T) specimen which maintains high triaxiality under quasi-static loading can exhibit progressive loss of triaxiality as the loading rate increases. Since this behaviour is observed for both rate independent and rate dependent case, it is attributed to inertial effects. The above phenomenon can satisfactorily explain retarded micro-void growth rates near a notch tip, and strong elevation in the dynamic fracture toughness at high loading rates, which have been reported in recent computational and experimental studies. (C) 2000 Elsevier Science Ltd. All rights reserved.