AbstractIn some ductile amorphous polymers, the fatigue lifetime behavior can exhibit two distinct crack propagation modes: a craze dominated sub‐critical discontinuous crack growth mode, which leads to a low stress, short fatigue lifetime branch and a shear fracture mode, which produces a higher stress, long lifetime branch. When both branches coexist, the fatigue lifetimes at higher stress amplitudes are often longer than those at lower stress amplitudes! This unusual lifetime inversion has been explored for a polyestercarbonate at various temperatures, frequencies, degrees of physical aging, and fatigue overload historie