On how unsymmetrical laminate designs with tailored ply clusters affect compression after impact strength compared to symmetric baseline

Abstract

Out-of-plane loads induce unsymmetrical damage modes in the laminate thickness direction. Consequently, the authors have recently proposed overcoming the conventional laminate symmetry constraint by designing unsymmetrical laminates with zero coupling responses. While impact damage is able to be tailored with unsymmetrical laminates, comparing them to symmetric laminates and assessing their impact damage tolerances had yet to be addressed. In this paper, we study three unsymmetrical laminates with localized ply clusters positioned at different locations (at the impacted, at the middle and at the non-impacted sides), along with a standard symmetric laminate as a baseline. Using low-velocity impact, X-ray micro-computed tomography and compression after impact (CAI), we compared the impact and post-impact responses to understand the effect local ply clusters and the delamination location have on the CAI strength. Results revealed that the unsymmetrical laminate with the ply clusters in the middle, where the dominant delaminations also occured, improved the CAI strength by a maximum of 10% when compared to the symmetric baseline. Laminates with delaminations at the outer surfaces offered lesser resistance to buckling. While our study demonstrates that symmetric laminates are not the optimal damage tolerant solution for impact load cases, it also evidences the feasibility of unsymmetrical laminates.