Cure-induced residual stresses in a notched laminate

Abstract

The cure-induced residual stresses in the vicinity of the free edge of an open hole in IM7/977-2 carbon/epoxy laminates have been investigated. Holes in laminated structures are intended for fastener joints, access ports and as an equivalent damage size for damage tolerance evaluations. The hole in the laminate is commonly produced post cure by either drilling or grinding operations. Due to the residual thermo-mechanical and chemical strains in the laminate, a self-equilibrating free edge and in-plane stress field is generated upon drilling the hole. Use of strain gages is limited to measurement relative to the fully cured state and cannot accurately capture the strain gradients. Thus, this study was carried out using finite element model of an open-hole tension configuration. The finite element models were developed for 16- ply unidirectional tape IM7/977-2 [04/904]S and [0/90/45/-45]2S laminates. The simulations were used to highlight the in-plane and through- thickness stresses along critical sections resulting from cure induced mechanisms. The results indicated significant interlaminar stresses in the vicinity of the hole due to curing. Based on rudimentary analysis, it is shown that the failure criteria must include the residual stresses for improved predictions of failure initiation in multidirectional laminates.