Low-cycle fatigue properties and unified fatigue life prediction equation of hot-rolled twin-roll-cast AZ31 sheets with different thicknesses

Highlights: • Low-cycle fatigue properties of rolled AZ31 sheets of different thickness are analyzed. • Twinning in compression and detwinning + slip in tension occur during cyclic deformation. • Fatigue lives are similar regardless of the sheet thickness and loading direction. • Total strain energy density is a suitable fatigue damage parameter. • A total strain energy density–based fatigue life prediction model is established. This study investigates the low-cycle fatigue properties of AZ31 sheets with different thicknesses of 1, 1.5, 2, and 3 mm—which are fabricated by twin-roll casting and subsequent hot rolling—through fully reversed strain-controlled fatigue tests. As the thickness of the sheets decreases, their average grain size decreases, texture intensity increases, and tensile yield strength and elongation gradually increase. At strain amplitudes of greater than or equal to 0.6%, {10–12} twinning in compression and detwinning and subsequent slip in tension occur repeatedly, which forms asymmetric hysteresis loops. The different grain sizes of the sheets result in different compressive peak stresses during the fatigue tests. However, the overall cyclic deformation behavior is similar in all the sheets, and consequently, their fatigue lives exhibit an insignificant difference. The loading direction also has a negligible influence on both the cyclic deformation behavior and the fatigue life, which implies that the sheets exhibit in-plane isotropic fatigue properties. The stress amplitude and plastic strain amplitude vary considerably during the fatigue test. In contrast, the variation in total strain energy density is insignificant over the entire fatigue life, and therefore, it is a proper fatigue damage parameter for predicting fatigue life. A unified fatigue life prediction equation using the total strain energy density is established, and the fatigue lives predicted using the equation are found to be in good agreement with the experimentally determined values, regardless of the strain amplitude, sheet thickness, and loading direction.