Stress state dependent fracture behaviour of porous PM steels

Ductile fracture of metals is a result of void nucleation, growth and coalescence. Various criteria have been proposed to model the ductile fracture strain as a function of the stress triaxiality that greatly influence the fracture process. In the present investigation, the wellknown Rice and Tracey approach (with a re-evaluation conducted by Huang) was used to model the ductile fracture behaviour of two porous steels, produced by Powder Metallurgy (PM): a ferritic–pearlitic Fe–0.4%C PM steel and a high-strength steel produced by using diffusion-alloyed Fe–4%Ni–1.5%Cu–0.5%Mo–0.5%C powder. Tensile, compressive and bending tests were carried on un-notched and notched specimens. The experimental curves were used as a reference for the Finite Element (FE) modeling of the tests aimed at evaluating the equivalent fracture strain at fracture and the correspondent stress triaxiality for each geometry. The results obtained for the Fe–0.4%C PM steel proved the suitability of the modified Rice and Tracey relationship to successfully obtain a simple fracture criterion. However, in the case of high-strength steel, a mixed ductile/brittle fracture behaviour was observed because of the microstructural heterogeneity of the alloy. Because of this, the Rice and Tracey model overestimates the experimental equivalent fracture strains and has to be accordingly corrected.