In-situ characterization of fatigue crack closure and propagation behavior in 1Cr18Ni9Ti steel under a single tensile overload

The fatigue crack closure and propagation behavior of 1Cr18Ni9Ti stainless steel under cyclic loading with single tensile overloads at different crack growth stages were investigated using in-situ digital image correlation. The evolution of the crack tip opening displacement and stress intensity factors was analyzed by extracting displacement fields from high-resolution optical microscopy images. The results indicate that under constant amplitude loading, crack closure intensifies with crack growth, with the crack opening stress intensity factor (K_op) increasing from 9.36 MPam at a crack propagation length of 0.3 mm to 28.83 MPam at 1.3 mm. The application of a single overload caused significant crack tip blunting, temporarily eliminating crack closure and leading to a sharp reduction in the effective stress intensity factor. Overload retardation is more pronounced in the short crack stage, leading to a 70% reduction in crack propagation rate and requiring approximately 25,000 cycles for recovery, whereas in the long crack stage, recovery occurs within 8,000 cycles. SEM fracture surface analysis confirmed that crack tip blunting played a key role in post-overload retardation, with crack re-initiation occurring at the blunted tip, governing delayed propagation. These findings provide new insights into the fatigue behavior of 1Cr18Ni9Ti steel under variable amplitude loading, contributing to improved fatigue life predictions for aerospace structures.