Predicting z-pin pull-out behavior via a physically-based traction-separation law incorporating thermo-chemo-mechanical history

Interfacial bridging efficiency of z-pins is traditionally attributed to frictional and geometric locking, often overlooking the complex thermo-mechanical history during manufacturing. This study identifies a critical competition between curing-induced residual stresses and interfacial pre-damage. A meso-scale, thermo-chemo-mechanical finite element framework, integrated with high-fidelity geometric reconstruction, is developed to simulate the complete evolution from resin gelation to cured pull-out. Findings reveal that while curing mismatch generates substantial radial clamping stresses in quasi-isotropic (QI) laminates, these architectures paradoxically exhibit inferior pull-out resistance compared to unidirectional (UD) counterparts. Mechanistically, this discrepancy stems from manufacturing-induced pre-damage: severe fiber distortion and star-shaped resin pockets in QI architectures induce local stress concentration, triggering interfacial debonding prior to any external loading. By incorporating these intrinsic defects into a physically-based traction-separation (T-S) law, the proposed model captures experimental softening and snubbing behaviors, correcting the overestimations inherent in conventional stress-free models. This work demonstrates that the initial state of z-pinned composites is inherently pre-damaged rather than pristine, establishing a process-aware criterion for the high-fidelity design of 3D-reinforced structures.