Effect of microstructure on the high cycle fatigue behaviour of DP 600 and SPFH 590 steels with and without a hole

The performance of ferrite-martensite (DP 600) and ferrite-pearlite (SPFH 590) steels with a similar tensile strength of similar to 650 MPa under cyclic loading was investigated. This work presents the high cycle fatigue properties of DP 600 and SPFH 590 steel in the presence and absence of a hole. In presence of the hole, the fatigue strength for both the steels drops significantly, with drop being more for SPFH 590 steel. While the fatigue strength in absence of a hole is a strong function of yield strength of material, the behaviour in presence of the hole depends on the steel microstructure. The presence of martensite in case of DP 600 steel delayed the crack propagation by branching and crack tip blunting. Additionally, martensite also increases the local strain, low angle grain boundary and dislocation density ahead of the crack tip, resulting in an increased amount of plastic deformation and blunting of the crack tip.