Effect of ply thickness and ply level hybridization on the compression after impact strength of thin laminates

Abstract

There is a lack of research available on how thin laminates respond to impact and post impact loads, even though thin structures are used in present-day aircrafts. This experimental paper employs thick, standard and thin uni-directional plies to investigate the effect ply thickness has on thin laminates on their impact and compression after impact (CAI) response. Further, we propose two hybrid laminates where thick or standard plies are mixed with thin plies, respectively, in an effort to improve the CAI strength of thin laminates. Results reveal that, contrary to thick laminates, thin laminates made of only thin plies exhibit extensive fibre failure, leading to a considerably reduced CAI strength. Moreover, the hybrid laminate where thick 0° plies are mixed with thin plies improves the CAI strength by 40% over the thin ply baseline laminate. Thus, hybridization with thin laminates appears to be an economic prospective in terms of improving damage tolerance.

Introduction

One of the aeronautic industry’s main concerns is the impact behaviour of thin structures ($<$2 mm), such as fuselages and wing skins, because low velocity impact can drastically reduce their residual structural strength [1]. However, many studies are devoted to investigating the impact and compression after impact (CAI) response of “standard” thickness laminates ($\approx$4 mm, as recommended in ASTM D7136/D7136M-15 [2]), along with the effect ply thickness has on CAI strength [3], [4], [5].

Previous studies have shown the improved performance of thin plies over standard and thick plies in terms of first ply failure and delay of damage onset [6], [7], [8]. In the framework of impact and post impact response, research investigations [3], [5] reported an average improvement of 20% in CAI strength with thin plies for laminates with thicknesses ranging between 3.6 and 4.4 mm. Thin-ply laminates showcased quasi-brittle failure instead of extended cracking and delaminations as observed in thicker plies. Reviewing thinner laminates, Garcia et al. [9] recently studied the effect of ply thickness on CAI strength with 2.15 mm thick non-crimp fabric laminates. Standard plies showed similar and 27% higher CAI strength when compared to thin plies at 10 and 14 J impact energies, respectively. Hardly any work related to CAI response is reported for laminates with thicknesses less than 2 mm, albeit with the exception of Sanchez et al. [10], who compared CAI strength between quasi isotropic, cross ply, and woven fabrics made out of thick plies for laminates ranging from 1.6 to 2.2 mm in thickness.

Laminate design [11], [12], [13], [14] and material system reinforcement [15], [16], [17] are two approaches used to improve impact damage resistance and more specifically CAI strength. Laminate design, understood as tailoring the stacking sequence, is considered more economically feasible than material reinforcement. For instance, ply level hybridization consists of mixing plies of different thicknesses in an attempt to enhance a targeted response. Sihn et al. [4] suggested this approach for future work to improve the impact damage resistance of composite structures without increasing the layup costs. Furtado et al. [18] performed selective ply hybridization with different types of fabric architectures by mixing thin and intermediate fabric layers, which eventually resulted in an improved notched response. Arteiro et al. [19] demonstrated that blocking 0° fabric layers improves the structural behaviour of aerospace graded thin ply laminates. Sebaey et al. [20] studied the effect mixing thin and thick fabric layers has on damage tolerance using thick laminates, and when compared to the baseline thin ply, reported an increase of 15% in CAI strength for a configuration of thick plies surrounded by thin plies.

This study is the result of an industrial investigation headed by Airbus, in collaboration with the AMADE research group (University of Girona), INEGI (University of Porto) and the University of Dayton Research Institute, USA. The study investigates the effect of ply thickness on impact damage resistance and CAI strength through impact and CAI experimental tests for thin laminates ranging between 1.5 and 1.8 mm. Thick (268 gsm), standard (134 gsm), and thin (75 gsm) uni-directional (UD) plies are selected for the ply thickness study. Additionally, we study two hybrid laminates, the first of which is a mix of standard 0° plies and thin plies, while the second contains thick 0° plies along with thin plies. Results show that thin plies within thin laminates exhibit extensive fibre breakage and lead to the lowest CAI strength. Ply thickness hybridization alleviated the amount of fibre failure with increased delamination damage, and improved the CAI strength remarkably over the baseline thin ply.