Influence of Ply Waviness on Stiffness and Strength Reduction of Composite Laminates

An analytic model is developed to investigate the influence of ply waviness on the stiffness and strength of composite laminates. The model predicts elastic properties and thermal expansion coefficients of sublaminates containing wavy layers ply stress for prescribed mechanical and thermal load cases strength reduction associated with ply waviness. Parametric studies are conducted for sublaminates of AS4 GraphitePEKK and S2 GlassPEKK which exhibit layer waviness. Results show that stiffness and strength reduction are significant in the direction of ply undulation only. Mechanisms of stiffness reduction are attributed to out-of-plane rotation of wavy layers. This waviness may also create significant interlaminar shear stress that reduces strength. The magnitude of the property reduction increases as the amplitude of the undulation increases and the half-wavelength of the undulation decreases. Ply waviness represents one mechanism that may partially explain why the design and failure analyses of composite structures based on flat, autoclave cured coupon property data minimal waviness, do not correlate well with test results.