Unsymmetrical stacking sequences as a novel approach to tailor damage resistance under out-of-plane impact loading

In current composite design, stacking sequence symmetry around the laminate mid-plane is an unarguable constraint to avoid warpage during manufacturing. However, several load cases induce unevenly distributed stresses through the laminate thickness, such that symmetric laminates may not be the optimal solution. In this paper, we explore the damage resistance to out-of-plane low velocity impact loading of an unsymmetrical laminate with zero extension-bending coupling matrix ([B]), thereby assuring no undesired coupling deformations during mechanical or thermal loads. Using impact and quasi-static indentation tests, C-scan inspection and numerical modelling, we compare the damage pattern between an unsymmetrical laminate with ply clustering at the impacted face and a laminate with ply clustering at the non-impacted face (produced by flipping the former laminate upside down). The laminate with clusters at the impacted side exhibits better damage resistance for lower impact energies. More importantly, the location of the damage events obeys the predictions assumed when the laminate was designed, demonstrating the room for improvement by tailoring unsymmetrical laminates to particular load cases