Ignace Verpoest

Textile geometry preprocessor for meso-mechanical models of woven composites

A mathematical model of the internal geometry of 2D and 3D woven fabrics is used as a unit-cell geometry preprocessor for meso-mechanical models of composite materials. The model computes a spatial placement of all yarns in a fabric repeat for a given weave structure (a special coding algorithm is employed) and given warp and weft yarns geometrical and mechanical parameters. The source of these data is the manufacturers documentation for a woven fabric and yarn mechanical behaviour in bending and compression, measured on standard textile-laboratory equipment. The principle of minimum energy of a fibrous assembly leads to the prediction of the yarns crimp, the forces in the warp/weft contacts and the yarn compression within a fabric. The input data for use in meso-mechanical models of composite materials is therefore provided in addition to tools for the 3D representation of the woven structure, for the creation of the unit-cell cross-sections and for the computation of the unit cell porosity.

Carbon composites based on multiaxial multiply stitched preforms. Part 1. Geometry of the preform

Experimental investigation of the internal geometry of a multiaxial multiply carbon reinforcement, stitched by warp-knitting, reveals the general features of this promising class of textile reinforcements. The uniform placement of the fibres is disturbed by the stitching, which creates resin-rich zones in the composite. The shape of the stitching loops, change of the stitching yarn thickness along the loop and statistical characteristics of spacing of the stitching has been studied. These features are covered by a descriptive empirical model of the internal geometry of multiaxial multiply fabrics. The model is implemented in the textile modelling software WiseTex, which serves as a preprocessor for meso-mechanics and permeability modelling.

Nesting in textile laminates: geometrical modelling of the laminate

The nesting of reinforcements in textile laminates is studied using a 3D geometrical model of a woven, braided and non-crimp stitched fabrics in the relaxed and sheared state. The following fabric parameters were varied in the numerical experiments: flatness of the yarns, tightness and balance of the fabric, fabric weaving/braiding/knitted pattern, number of layers and degree of shear. Monte-Carlo modelling of the random nesting generated data on the influence these parameters on the average laminate thickness and fibre volume fraction and distributions of these characteristics of the laminate. The results show that the nesting effect becomes more pronounced with the decrease of the fabric tightness and the decrease of the float length in the weave. For non-circular yarns, an average dimension of the cross-section proved to be a valid normalizing parameter to generalize the nesting behaviour of fabrics. Shear deformation of the fabric decreases the nesting. For non-crimp stitched fabrics the nesting is defined by the stitching pattern.

INTERLAMINAR FRACTURE TOUGHNESS OF CFRP INFLUENCED BY FIBRE SURFACE TREATMENT: PART 1. EXPERIMENTAL RESULTS

The bond between a fibre and the matrix is very important for the mechanical behaviour of CFRP. The quality of the fibre/matrix interface will influence the initiation and development of damage. This is certainly true for interlaminar fracture toughness. The present study is focused on the influence of the interface properties on the fracture toughness of CFRP. Carbon fibres with four different wet oxidative surface treatment levels have been investigated. The fibres were embedded in a thermoset resin and in a thermoplastic system. Mode I, mode II and mixed-mode tests were performed to determine critical strain-energy release rates. Results show an important increase in the fracture toughness initiation value with increasing fibre surface treatment level. For tests dominated by mode I loading, however, the propagation value can reach a maximum at intermediate treatment levels as a consequence of fibre bridging effects.

Compression of woven reinforcements: A mathematical model

A mathematical model for prediction of the compressional behaviour of woven reinforcement is described. The model accounts for the compression of yarns and the change of yarn crimp under pressure applied to a fabric layer. Compression curves of glass rovings are studied experimentally and serve as input for simulation of fabric compression. The model is tested against experimental data for glass reinforcement for the lower and the higher pressure range.

Hierarchy of Textile Structures and Architecture of Fabric Geometric Models

Textile materials are characterized by a distinct structural hierarchy, which should be represented by a model of textile geometry and its mechanical behavior. Despite extensive investigations of textile materials and a number of theoretical models existing in the textile literature for different structures, a general model is not available. Hence, the challenge addressed in this work is to take full advantage of the hierarchical principle of textile modeling to create a truly integrated modeling and design tool. The algorithm for extensive simulations of complex yarn interactions uses the minimum energy principle, allowing complex textile structure computations in computer time to be counted in minutes instead of hours or days for FEM implementation of the same nonlinear, nonconservative behavior of yarns under compression and bending. The architecture of the code corresponds to the hierarchical structure of the textile material. General ideas of the hierarchical approach are illustrated by models of the internal geometry of multilayered woven fabrics and weft-knitted fabric topology.

Picture Frame Test of Woven Composite Reinforcements with a Full-Field Strain Registration:

Results of picture frame shear tests with optical registration of the strain fields are presented for glass (plain and twill, three types) and glass/PP woven (plain and twill) fabric reinforcements for composite materials. Four problems were investigated. (1) How does the shear diagram vary with differences in test conditions? The major factor is the sample pretension, which is influenced by its gripping, removing/preserving yarns near the grips and “conditioning” in the shear cycles. (2) Does the shear of the fabric differ from the pure shear prescribed by the frame? The differences are normally negligible. (3) How large are the variations of the local fabric shear? The scatter of the local fabric shear does not exceed 2°. (4) How is shear of the fabric translated into deformations of the yarns on the micro-scale? Different stages of the fabric deformation are identified: rotation of the yarns followed by their lateral compression.

Carbon composites based on multiaxial multiply stitched preforms. Part 2. KES-F characterisation of the deformability of the preforms at low loads

Deformability of multiaxial multiply stitched carbon preforms is studied at low loads using the Kawabata evaluation system. Results of the test in tension, shear, compression, bending and friction are reported. Both bi- and quadriaxial fabrics demonstrated a fairly uniform behaviour. The general features of deformability are formulated and generic values of parameters for evaluation purposes are proposed.

A poly-inclusion approach for the elastic modelling of knitted fabric composites

Abstract Mean field Eshelby based models are proposed for the prediction of the overall properties and the internal stress/strain distribution of knitted fabric composites. The use of a mean field is justified by the partial random character of the textile geometry in multi-layered applications. The position dependent deformation state in the curved yarns is piecewise approximated using the concept of effective ellipsoids replacing each yarn segment, taking into account the local yarn curvature. Conventional averaging methods are applied to the reduced, poly-inclusion composite with non-trivial orientation distribution to predict the internal field variables inside the original yarns. The Mori–Tanaka model and a first order self consistent method are compared and their potential and limitations are briefly discussed. Predicted tensile and shear moduli agree well with experimental data for both mean field methods, although the internal stress distribution for both methods is significantly different.

Modelling of the interaction between matrix cracks and delaminations during impact of composite plates

This paper discusses the investigation of the transverse impact of plate-like composite specimens. It focuses on the initiation of delaminations at matrix cracks and on the interaction of delaminations with matrix cracks. This study shows that an accurate prediction of the development of matrix cracks and delaminations during impact requires the use of energy criteria, taking into account the appropriate fracture toughness values, depending on the interface and the mixed mode loading. Also, the accumulation of damage in the plate has to be taken into account when predicting the damage development.