Novel enhancement of fatigue life in steels through extrinsic mechanisms enabled by additive manufacturing

This study presents a novel additive manufacturing-enabled strategy for enhancing fatigue life in monolithic metals through location-specific extrinsic toughening. Compact tension specimens of 18Ni(300) Maraging steel were fabricated using laser powder bed fusion, incorporating unbonded, dog-bone-shaped ligament features strategically placed along the anticipated crack path. These ligaments remain mechanically inactive until the advancing crack bypasses them, at which point they engage and act as crack-bridging elements, reducing the effective stress intensity at the crack tip. Three ligament types—Rounded-End, Flat-End and Integrated ligaments—were investigated across various diameters to evaluate the influence of geometry on fatigue performance. Experimental fatigue crack growth tests revealed up to 57% local reduction in crack growth rate and fatigue life extension of up to 13%, primarily in rounded-end specimens. Crack closure and bridging were identified as the dominant toughening mechanisms, with early-positioned ligaments proving most effective. Surface roughness of fabricated ligaments emerged as a critical limitation, leading to premature fatigue failure in some designs. The findings demonstrate, for the first time, that geometry-driven location-specific extrinsic toughening can be systematically implemented in a single-phase metal via additive manufacturing, offering a new design tool for localized, architecture-based damage tolerance in structural alloys that cannot be realized through conventional methods.