A new mechanism for the formation of ply wrinkles due to shear between plies

The formation of out-of-plane ply deformation causes significant reductions in the mechanical properties of composites. In-plane fibre misalignments also cause reductions in the compressive strength, yet the origins of these defects are misunderstood. This paper presents a new mechanism for the formation of wrinkles, which is based upon the shear forces generated as a result of mismatches in the coefficient of thermal expansion of composite and tool, as well as the process of ply slippage that occurs during consolidation into radii. Using a U-shaped tool, defects in composite spars have been characterised using light microscopy, showing that the tool geometry and prepreg bridging leads to “instability sites,” which lead to wrinkles up to 750 μm in height as well as in-plane misalignment of 0° plies of up to 50°. Increasing the frictional shear stress through omission of release film prevents the formation of wrinkles, supporting the mechanism presented in this paper.