Resin treatment of free edges to aid certification of through thickness laminate strength

Abstract

Large aerospace parts are typically certified by testing narrow specimens, such as curved laminates, which have exposed free edges. These edges (not present in the production part) have been found to reduce the 3D strength of curved laminates by over 20%, showing this certification method is unreasonably conservative. The free edges also create a singularity, such that Finite Element (FE) modelling is challenging, which is typically approximated using non-linear analysis of cohesive interlaminar zones. A new treatment process is developed whereby a layer of resin is applied to the free edges of curved laminates. This significantly reduces the edge effect and delays failure. The resin edge treatment increases the strength of the curved laminate test specimens by 16%. The treatment also simplifies FE modelling by allowing for non-zero stresses normal to the laminate edge, removing the singularity. This enables use of linear FE models, which converge at the laminate edge. A linear FE method developed in this paper is conservative and predicts the strength of treated curved laminates to within 5% of the average test value. Hence it is shown that the resin edge treatment can be used to improve reliability of both certification tests and FE models.

A Bayesian framework for assessing the strength distribution of composite structures with random defects

“…al. have shown that the Camanho failure criterion predicts failure to within 5% of average experimental test values [16] with treated edges to mitigate premature failure. Here, for simplicity, we discount the edge effects by not evaluating the failure criterion close to the boundary to isolate the effects of wrinkles.…”

“…The plies were assumed to have a thickness of 0.24 mm, with a 0.015 mm interface layer of pure resin between each ply. This is based upon measurements taken from micrograph images of the curved laminates as described by Fletcher et al [16]. The assumed mechanical properties for both the fibrous ply material and the resin rich interface material are given in Section 3.4.…”

“…The assumed mechanical properties for both the fibrous ply material and the resin rich interface material are given in Section 3.4. A discussion on how they have been chosen from various sources is given by Fletcher et al [16].…”

“…These wrinkle measurements have supported a growing research interest in the mechanical knockdown of wrinkle defects. The formation of wrinkles not only disrupts the even distribution of fibre and resin, but can significantly increase interply shear stresses triggering failure at significantly reduced loads [5,[15][16][17]. Numerical studies have used wrinkle measurements, to perform parametric studies using finite elements to explore the deterministic effect of variations of wrinkle shape on structural integrity [5,15,18].…”

“…Numerical studies have used wrinkle measurements, to perform parametric studies using finite elements to explore the deterministic effect of variations of wrinkle shape on structural integrity [5,15,18]. These studies include prediction of both, failure initiation [16,17] and mix-mode propagation [5]. Notably, in a recent study, Xie et.…”