3865 - OBSERVATIONS OF DUCTILE FRACTURE PROCESSES UNDER VERY LOW TRIAXIALITY IN QUENCHED AND TEMPERED VAR STEEL AND PRELIMINARY INTERPRETATION

This paper presents preliminary results of an investigation aimed at understanding and modeling ductile fracture of clean heat-treatable high strength steels, particularly under low triaxiality loading conditions. Results of torsion and tension/torsion experiments are presented and complemented by detailed fractographic observations. We find that ductile fracture under low (0 to 0.15) as well as higher triaxiality loading is controlled by void nucleation at tempering carbides. Once nucleated voids grow and coalesce without significant additional macroscopic deformation. We apply the dislocation model proposed by Kwon and Asaro to calculate the carbide-matrix interface stress as a function of plastic strain and mean stress. Using our experimental torsion and tension/torsion data, we obtain estimates of the critical nucleation stress ranging from 1880 to 2140 MPa for the range of triaxialities investigated. These values are consistent with values reported in the literature for other steels and their relatively narrow range suggests that the Kwon-Asaro model may be used to predict void nucleation at carbides for low as well as for high triaxialities.