Multiscale investigation on tensile behavior of filament-wound composites: Fluctuation effects induced by fiber crossover undulation

The presence of fiber crossover and undulation (FCU) in single-filament winding (SFW) significantly influences the mechanical performance of composite layers. However, owing to the complexity involved in characterizing its influence mechanism, this structural characteristic is frequently neglected in the design of wound composites. In this study, we propose a novel approach that integrates stiffness coefficient analysis with tensile testing. First, a theoretical FCU model is established to analyze the variations in stiffness coefficients. Subsequently, multiscale comparative studies are conducted between SFW and a newly developed multi-filament winding (MFW) technique. The results demonstrate the fluctuating impact of the winding angle on the mechanical properties of composite layers fabricated using the SFW technique. To characterize the microscale failure mechanisms induced by FCU, scanning electron microscopy (SEM) is utilized. Additionally, a mesoscale finite element model is developed, which incorporates fiber bundle undulation patterns. This model is implemented using a user-defined subroutine (VUMAT), allowing for accurate prediction of the evolution of failure modes and strength properties, with an error of less than 7.79%. When compared to MFW, composite layers produced using the SFW technique exhibit higher stiffness, lower damage loads, and shorter damage displacements. Moreover, theoretical analysis reveals that FCU induces pronounced stress concentrations near a winding angle of 22.5° due to strong tensile-shear coupling. Conversely, these defects are mitigated near 67.5°.