materials testing teams

Abstract: This work develops a differentiable finite element framework based on an intrinsic cohesive-zone formulation for identifying traction-separation laws from global force-displacement histories and spatially sparse strain measurements. The framework employs physics-constrained neural networks to represent flexible, thermodynamically structured cohesive laws, with sensitivities computed through automatic differentiation and a discrete adjoint that accounts for the history-dependent evolution of interfacial damage. To enable identification of spatially heterogeneous cohesive behavior, we extend learned constitutive priors to irreversible interfacial damage by constructing a shared, low-dimensional family of cohesive laws from representative traction-separation histories. This representation exploits common constitutive structure and parameter sharing to reduce the number of unknowns and regularize the inverse problem, while allowing local cohesive behavior to vary spatially through latent coordinates. Numerical investigations involving double-cantilever beams and fiber-matrix composites demonstrate that accurate reproduction of global force-displacement responses does not necessarily imply accurate recovery of the underlying cohesive laws. Incorporating spatially resolved strain measurements substantially improves constitutive identification, even when the admissible responses are constrained by a learned prior. Comparisons with conventional bilinear cohesive models and Bayesian optimization further illustrate the benefits of the learned constitutive representation and adjoint-based identification for high-dimensional inverse problems.

Abstract: The reuse of metal powders in directed energy deposition using laser powder offers promising sustainability benefits for additive manufacturing, yet its impact on part quality remains unknown. This study investigates the influence of powder reuse on the directed energy deposition process of Inconel 625. Virgin powder was first characterized in terms of flowability, morphology, size and chemical composition, then used to manufacture parts under fixed process parameters. These parts were analysed through two configurations (as-built and hot isostatically pressed) to assess microstructural evolution and mechanical performance. Results show that hot isostatic pressing reduces porosity and promotes grain recrystallization, enhancing ductility while decreasing tensile strength. Recovered powders from one manufacturing cycle, were also characterized. Particle size distribution and morphology remained stable, interstitial contents, particularly oxygen, increased, potentially affecting laser-material interaction and melt pool behaviour in future production. These findings highlight the importance of evaluating powder reuse through a full-process approach, from powder to final part. Future work will focus on the tensile and fatigue performance of parts produced with recovered powders.