Effect of fatigue-mediated secondary microcrack on crack resistance of twinned materials

•Combined effects of fatigue-mediated microcrack and twin lamella on FCGR are studied.•Fatigue-mediated microcrack can transform the mechanical responses of twin lamella.•The angle between the loading axial and the twin lamella plays an important role.•Competitive mechanism of shielding and accelerating effects of microcrack is revealed.