Low cycle fatigue behaviour of dual-phase steel with different volume fractions of martensite

Abstract

Completely reversed low cycle fatigue tests were carried out at a constant strain rate of 10⁻²s⁻¹ on 3 mm thick sheet specimens of a dual-phase steel treated to give 1.5 to 28% martensite without changing the carbon content. Hysteresis loop shape, stress/strain response and plastic strain energy as a function of applied cycles are analysed for different microstructures. Strain/life and plastic strain energy per cycle (ΔW̅_P)/life (2N_f) plots are discussed in terms of microstructure. It is shown that during cycling the shape of the hysteresis loop continuously changes at lower volume fractions of martensite whereas it remains more or less constant for microstructures with a higher percentage of martensite. A Coffin-Manson type of plot between log (ΔW̅_P) and log (2N_f) is found to be applicable to test results of dual-phase steel with a wide range of martensite contents and is thus more versatile than the plot between log (Δε_p/2) and log (2N_f) for predicting the fatigue life.