JEC COMPOSITES MAGAZINE - Issue #58 - June - July 2010 - 58

KNOW-HOW
Reinforcement

Angle-interlock reinforcements:
weaving and the mechanical properties of composites
Composites reinforced with layer-to-layer angle-interlock fabrics are known for their high resistance to delamination and impact damage, but their intricate fibrous architecture makes their mechanical study tedious. A periodic homogenization procedure is proposed here to achieve the equivalent macroscopic elastic constitutive law. This procedure is based on a precise analysis of yarn properties and meticulous observations of the composite architecture. This approach has proved to give accurate effective membrane and bending elastic stiffnesses.
Interlock architecture
Interlock reinforcements are made up of layers of 3D multilayer fabrics linked by Z yarns. The layer-to-layer angleinterlock structure seems to be the most suitable to Fig. 1: X-ray tomography of a dry layer-tofulfil mechanical and layer angle-interlock fabric formability requirements. Since the orthogonal and through-thickness angle-interlock structures have been studied by many authors so far, the analysis of the little-known layer-to-layer architecture is original and more interesting. This analysis is used to determine the effect of binding yarn inclination and waviness on the mechanical behaviour. Such a geometry presents Z-yarns in the warp direction, crossing at least two layers. An 8-layer reinforcement has been retained in the sequel, with Z-yarns linking two plies. Figure 1 is an X-ray tomography of a piece of dry fabric. The composite is made of high-tenacity carbon fibres and epoxy resin.

PATRICK LAPEYRONNIE, PHD STUDENT, ECOLE DES MINES DE DOUAI, PHILIPPE LE GROGNEC, ASSISTANT PROFESSOR, ECOLE DES MINES DE DOUAI, CHRISTOPHE BINÉTRUY, PROFESSOR, ECOLE DES MINES DE DOUAI, FRANÇOIS BOUSSU, ASSOCIATE PROFESSOR, ENSAIT

D reinforcements have been studied for many years as an alternative solution to laminate structures in composite materials. They provide a suitable answer to delamination and impact damage problems. Among them, layer-to-layer angleinterlock structures present the best reliability in relation to these mechanical requirements, together with easy formability. With such composite materials, the classical laminate theory is no longer suitable to estimate the effective mechanical behaviour because of the complex fibrous architecture that results from the presence of yarns in the through-thickness direction. Several specific semianalytical and numerical approaches have already been developed, but they often consider an idealized reinforcement architecture, thus overestimating the mechanical properties of the composite. The objective of this study is to facilitate the mechanical study of 3D interlock composites, using a high-performance tool and incorporating several features inherent to the manufacturing process. On account of numerous existing geometries, a parameterized mesoscopic architecture is developed in a Python program, using Abaqus software in the context of the finite element method. A rigorous periodic homogenization scheme is used to assess the membrane and bending elastic moduli of the equivalent shell-type structure, in the framework of the Love-Kirchhoff plate theory. The composite manufacturing process is also carefully observed to obtain the most realistic numerical modelling. This analysis focuses on the actual geometry of the composite as derived from optical observations and the actual mechanical properties of the damaged yarns. These aspects are discussed and included in the simulation.

Geometry of the reinforced composite
An infusion process is used to manufacture the composite. This technique is preferred in order to reduce the deformation of the reinforcement geometry. Micrographic observations and measurements are achieved on the cross-sections of the interlock composite. The geometrical parameters are defined relying on the yarn paths, the section shape and the spaces between yarns. They include the waviness, crimp and distortion of the yarns due to the weaving and composite processes. A 3D realistic representation of the interlock architecture is then defined on the basis of these parameters. In a first step, some assumptions are made. The weft yarns are modelled vertically aligned and straight, disregarding the

jec composites magazine / No58 June - July 2010

JEC COMPOSITES MAGAZINE - Issue #58 - June - July 2010

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #58 - June - July 2010