Flexural performance recovery and failure-mode transition in continuous-fibre thermoplastic composites repaired by in-situ additive manufacturing: A combined numerical and experimental study

In-situ continuous-fibre additive repair restores damaged thermoplastic composites without a separate adhesive layer, but how load transfer between the parent laminate and printed patch reconstructs load sharing and governs failure remains unclear. This study combines four-point bending tests with a finite element model that represents the parent laminate, printed patch and parent–patch interface and quantifies the distribution of interfacial load transfer. Circular and square in-situ additive repairs recovered 99.3% and 94.0% of the intact stiffness and 91.0% and 84.1% of the intact peak load. Intact specimens failed near the upper loading rollers by compression-side fibre buckling, whereas in-situ repaired specimens failed in the repaired mid-span region through tensile cracking in the parent laminate and separation within the patch; the parent–patch interface remained macroscopically continuous. Finite element analysis showed that interfacial transfer incorporated the printed patch into the flexural load path. The outer step accounted for approximately three-quarters of the longitudinal interfacial-force magnitude, and its non-uniform transfer pattern preceded damage in the adjacent patch. Geometry-specific references showed that the circular–square difference in transfer distribution arose mainly from boundary orientation, with limited additional traction localisation. These findings show that the parent–patch interface formed by in-situ additive repair enables load sharing and substantial flexural recovery, while the process-induced architecture of the continuous-fibre printed patch provides a competing path for damage development, so that the in-situ repaired specimens recover much of the intact performance without restoring the intact failure mode.