On the analysis of delamination fractures in high-strength steels

The influence of the stress state on the occurrence of delamination during tensile testing has been examined in two high-strength steels. The stress state was varied by either machining notches into cylindrical tensile samples or by conducting the tensile tests under superimposed hydrostatic pressures up to 690 MPa. In addition, the magnitude of the transverse stresses was modified by varying the yield stress of the material due to the tempering treatments. In all cases, fracture occurred either by ductile fracture or by ductile fracture accompanied by extensive delamination along the tangential planes. The mechanism of delamination on the planes involved nucleation of voids at carbides followed by ductile rupture of the surrounding matrix. The results suggest a criterion for delamination which involves both the level of plastic strain and the normal stress acting on the delamination plane. The competition between the failure modes is illustrated using fracture mechanism maps constructed in stress space.