Mechanics-Based Analysis of Interfacial Stress Transfer and Stiffness Stability in Thermoplastic Composites Reinforced with Multi-Source Regenerated Carbon Fibers

Abstract Background The stochastic nature and surface variability of regenerated carbon fibers (rCF) present significant challenges for their reliable application in high-performance structural thermoplastics. Understanding the interplay between fiber architecture and interfacial mechanics is essential for optimizing reinforcement efficiency. Objective This study investigates the reinforcing mechanics of rCF with distinct intermediate architectures: a 3D non-woven mat (hereafter referred to as NrCF) and a 2D recycled paper (PrCF) bound by an inert PET binder. The goal is to decouple the intrinsic fiber variability from the macroscopic composite performance using a mechanics-based framework. Methods An impregnated fiber bundle tensile test was adapted to isolate the collective strength of the discontinuous fibers. The interfacial stress transfer efficiency and stiffness characteristics of the resulting Polyamide 66 composites were analyzed using the Kelly-Tyson and Cox shear-lag models, respectively. Furthermore, the interfacial confinement effect on thermal stability was quantified via thermogravimetric analysis (TGA). Results The NrCF source fibers exhibited a six-fold strength advantage over the PrCF source. Mechanistically, the chemically reactive interface in NrCF significantly enhanced the interfacial shear strength ( τ ) and reduced the critical fiber length ( l c ), enabling efficient stress transfer and superior flexural strength (up to 334.9 MPa). In contrast, the PrCF system exhibited premature failure and matrix-sensitive stiffness due to the weak physical interface governed by the inert binder. Additionally, the robust interfacial bonding in NrCF restricted molecular mobility, significantly delaying thermal degradation compared to the unconstrained PrCF system. Conclusions This work demonstrates that for discontinuous regenerated fibers, the restoration of interfacial chemical reactivity is a critical design parameter that outweighs fiber volume fraction in determining structural integrity. The proposed testing protocols and mechanical interpretations provide a rigorous pathway for certifying recycled carbon fiber composites.