Strain and Structural Evolution in Carbon Fiber-Reinforced Polyamide-4,10 Composites during Tensile Cycling to Failure

We report residual strain buildup in unidirectional carbon fiber-reinforced polyamide-4,10 composite tapes during low-strain-rate (10–4 s–1) tensile cycling to failure, with fibers oriented 45° relative to the load. Macroscopic strain is attributed to crystal lattice elasticity and unit-cell frustration (one-third) and to amorphous shear deformation (two-thirds), accompanied by fiber reorientation, where the angle decreases below the nominal 45°. The carbon–matrix transcrystalline layer exhibits a higher preferential crystalline orientation than the bulk matrix. Tensile properties were measured simultaneously with microfocused small-angle and wide-angle X-ray scattering (SAXS/WAXS) in situ, enabling direct correlation between mechanical response and structural evolution. In addition, the 100 reflection of the polymer α phase is identified as a fingerprint of composite failure. The material moisture content (1.3%) is lower than in more common polyamides and was assessed by small-angle neutron scattering (SANS). These results provide a structural basis for data-driven models to predict lifetime of the investigated composite system under cyclic loading.