Manufacturing process effects on multiaxial mechanical behavior and anisotropy evolution of fiber-reinforced polymer composites for automotive applications

Processing-induced variations play a crucial role in defining the mechanical anisotropy and structural efficiency of fiber-reinforced polymer (FRP) composites. This study investigates the multiaxial mechanical behavior and anisotropy of unidirectional carbon (CFRP), glass (GFRP), and basalt (BFRP) epoxy composites fabricated using two controlled processing techniques: hand lay-up (HLU) and vacuum-assisted resin transfer molding (VARTM). Process-based density, fiber volume fraction, and void content are quantitatively correlated with tensile, compression, in-plane shear, flexural, and interlaminar shear properties. Across all fiber types, VARTM produces higher fiber volume fractions and lower void contents than HLU, resulting in improved stiffness and strength in most loading modes. However, the magnitude and direction of improvement depend on fiber type and loading mode. The stiffness- and strength-based anisotropy indices reveal fiber-dependent changes in directional mechanical response with the manufacturing process. Tensile anisotropy increases for CFRP and BFRP but decreases for GFRP, whereas compression anisotropy decreases for CFRP and GFRP and increases for BFRP. Flexural stiffness anisotropy remains comparatively stable across the three reinforcement systems. Interlaminar shear strength (ILSS) index indicates that VARTM improves shear-transfer efficiency in CFRP and BFRP. ANOVA demonstrates that the influence of manufacturing process and its interaction with fiber type is property-dependent, with stronger interaction effects observed in transverse, flexural, and interlaminar responses. These findings establish a clear process-structure-anisotropy relationship for understanding the effect of manufacturing process on the directional mechanical behavior of FRP laminates, guiding the design and optimization of lightweight automotive composite structures.