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

The objectives of this paper are to study the applicability of a two-parame ter (J-Q) characterisation of the elastic-plastic crack tip fields in a dyn amically loaded ductile specimen and to ascertain the effect of loading rat e J on the triaxiality parameter Q. To this end, 2D (plane strain) large de formation finite element analyses of a single edge notched specimen (SEN(T) ) subjected to a tensile stress pulse are conducted. The material is assume d to obey the J(2) Row theory of plasticity with small elastic strains and both rate independent and rate dependent behaviour are considered. The resu lts 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 observe d for both rate independent and rate dependent case, it is attributed to in ertial effects. The above phenomenon can satisfactorily explain retarded mi cro-void growth rates near a notch tip, and strong elevation in the dynamic fracture toughness at high loading rates, which have been reported in rece nt computational and experimental studies. (C) 2000 Elsevier Science Ltd. A ll rights reserved.