Ductile fracture behavior of BCC and FCC metals at a wide range of strain rates

This paper deals with ductile fracture behaviors of a typical BCC metal with the variation of the strain rate. Ductile fracture behaviors of AISI 4130 steel for BCC metals are investigated to identify their rate-dependency by tracing several stress paths based on the Lou-Huh ductile fracture criterion. Tensile tests are conducted to obtain fracture strains at a wide range of strain rates ranging from 0.001 s-1 to 1000 s-1 with three different shapes of specimens: dog-bone specimens for the uniaxial tension test; diagonally notched specimens for the in-plane shear test; and grooved-in-thickness specimens for the plane-strain tension test. Equivalent strains to fracture are measured by a 2-D digital image correlation (DIC) method on the surface of the specimens just before the onset of fracture. The damage function of the Lou-Huh fracture criterion is calibrated to obtain the fracture coefficients at each strain rate so that the fracture loci of AISI 4130 steel are constructed with the variation of the strain rate. The equivalent strain to fracture decreases noticeably as the strain rate increases beyond the strain rate of 1 s-1, which is due to the formation of the shear band under the in-plane shear condition. The fracture loci of OFHC copper for FCC metals are also obtained for comparison to the AISI 4130 steel for BCC metals.