Effect of ply waviness defects on the viscoelastic properties of carbon/epoxy composites

Ply waviness is a common manufacturing defect in carbon fiber-reinforced polymer (CFRP) composites that is known to reduce quasi-static mechanical properties. However, its influence on viscoelastic properties remains largely unexplored. This study investigates the effect of embedded ply waviness defects on the viscoelastic properties of carbon/epoxy laminates, focusing on storage modulus, loss modulus, tan δ, and the glass transition temperature ( T g ). Dynamic mechanical analysis was performed on single-clamped cantilever specimens using both temperature and frequency sweeps. Two laminate types were evaluated: (1) predominantly unidirectional composites with embedded ply waviness, and (2) cross-ply laminates without defects. Temperature sweep results show that the average T g of specimens with ply waviness is 3.8 °C higher than that of cross-ply laminates, indicating constrained polymer chain mobility and delayed α -relaxation. Reduced molecular mobility also leads to a smaller change in storage modulus in the rubbery state, as well as lower loss modulus and tan δ . This suggests that less energy is dissipated as heat during the polymer relaxation process and that the damping capability of the ply waviness specimens is reduced. The frequency sweep results revealed that modulus hardening of ply waviness specimens with increasing frequency is lower than in cross-ply laminates, indicating a greater contribution of viscous deformation and reduced elastic stiffness due to fiber misalignments. These findings demonstrate that ply waviness defects affect the time- and temperature-dependent behavior of CFRPs, underscoring the importance of incorporating the effect of this manufacturing defect into predictive models for composite structures subjected to dynamic or thermal loads.