A mechanics-informed framework for fiber length selection in discontinuous CFRTP: Integrating impact performance and cascade recycling

Carbon fiber reinforced thermoplastic (CFRTP) architectures span a reinforcement continuity spectrum from continuous unidirectional (UD) laminates through sheet molding compound (SMC) to carded non-woven fabric (NWF), reflecting a cascade pathway from virgin to recycled fiber. This study investigates how reinforcement length, orientation, and localized continuity govern impact performance, processability, and downstream recycling compatibility, focusing on thin-ply SMC with 44 μm tapes. Previous studies established static property saturation near 20 mm, but the length required for dynamic energy absorption remained unclear. Three-point bending impact tests showed that impact strength and specific energy absorption continued to increase from 20 to 30 mm. This condition also matches the input length required for NWF carding, indicating convergence between improved impact performance and downstream processing compatibility. NWF with controlled anisotropic ratios represented the next recycled stage. UD represented the cascade origin and upper performance bound and was locally over-molded onto SMC, increasing energy absorption without added mass when further gains from tape length or anisotropic ratio became impractical, a key consideration in automotive design. Tape length, orientation, and hybridization therefore provided complementary, architecture-dependent contributions to impact response. Because idealized morphology did not accurately predict SMC response, measured tape geometry and orientation were incorporated into multiscale representative volume element (RVE) modeling, reducing prediction error from 10-16% to below 3% and enabling finite element (FE) simulations with deviations below 15%. Integrating performance, manufacturability, and recycling constraints identified a system-specific design window and established a mechanics-informed framework for fiber length selection in discontinuous CFRTP systems.