Microstructure-driven damage mechanics and failure of FFF-sintered 316L and 17-4PH steels

Metal material extrusion based on Fused Filament Fabrication (m-FFF) offers geometrical flexibility but introduces critical challenges regarding the structural integrity of the final sintered components. The multi-stage process inherently generates residual porosity, inter-filament interfaces, and raster-dependent defect architectures. Consequently, understanding the effects of these process-induced discontinuities on stiffness degradation, plastic flow, and damage evolution is of paramount importance. In this work, the damage mechanics and failure behavior of 316 L and 17-4PH stainless steel components produced by m-FFF are investigated across three distinct printing strategies: linear, concentric, and parallel-line patterns. The objective is to determine how alloy microstructure and raster-dependent defect architecture jointly control tensile failure, initial stiffness degradation, progressive damage evolution and residual load-bearing capacity. The experimental framework combines microstructural and fractographic characterizations (optical microscopy, SEM, EDS, XRD) with a comprehensive mechanical testing campaign, including monotonic tensile tests, Digital Image Correlation, and progressive loading–unloading cycles. Microstructural analyses reveal that the filament architecture persists post-sintering as inter-track porosity and track-oriented discontinuities. Notably, the concentric pattern acts as a severe stress raiser, promoting notch-like defects in transition regions that dictate premature failure, whereas the parallel-line pattern ensures a more effective load-transfer capability. To quantify the marked pattern dependence of the mechanical response, an initial damage variable is introduced based on the elastic modulus reduction relative to the fully dense reference material. This initial damage is modest for the optimal 316 L configuration but remains significant across all 17-4PH specimens, highlighting the pronounced sensitivity of the martensitic alloy to defect morphology. The progressive loading–unloading response is subsequently formalized within a Lemaitre-type continuum damage framework, initialized by the process-induced damage variable. The reconstructed damage trajectories exhibit a nonlinear, two-stage evolution accurately captured by the identified damage law. Finally, post-damage testing demonstrates that prior loading histories can paradoxically enhance ultimate strength—and ductility for linearly deposited 316 L—driven by the competing mechanisms of plastic hardening and local stress redistribution around defects, despite concurrent stiffness degradation. These findings demonstrate that assessing the defect tolerance and structural reliability of m-FFF sintered steels requires a coupled mechanics approach, integrating raster architecture, initial defect-induced damage, and plasticity-driven damage evolution.