Long Design and Manufacture of Textile Composites

1. Auflage 2005
ISBN: 978-1-84569-082-3
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 492 Seiten

Reihe: Woodhead Publishing Series in Textiles

The term 'textile composites' is often used to describe a rather narrow range of materials, based on three-dimensional reinforcements produced using specialist equipment. In Design and manufacture of textile composites, however, the term is used to describe the broad range of polymer composite materials with textile reinforcements, from woven and non-crimp commodity fabrics to three dimensional textiles.Whilst attention is given to modelling of textile structures, composites manufacturing methods and subsequent component performance, it is substantially a practical book, intended to help all those developing new products with textile composites. Chapters on modelling include material models and data of use to both researchers and manufacturers, along with case studies for real components. Chapters on manufacturing describe both current processing technologies and emerging areas, and give practical processing guidelines. The last section contains a number of chapters covering applications from a broad range of areas, including transportation, sporting construction and medical applications. As well as illustrating typical components in each area, associated design methodologies and interactions between processing and performance are covered.Design and manufacture of textile composites is an invaluable guide for manufacturers of polymer composite components, end-users and designers, researchers in the fields of structural materials and technical textiles and textile manufacturers. It will also provide manufacturers of traditional textiles with new areas to investigate and potential markets. - Covers a broad range of polymer composite materials with textile reinforcements - Edited by a leading authority in the field with contributions from a worldwide team of authors

Autoren/Hrsg.

Long, A C

Leseproben

2 Mechanical analysis of textiles
A.C. Long    University of Nottingham, UK P. Boisse    INSA Lyon, France F. Robitaille    University of Ottawa, Canada 2.1 Introduction
This chapter describes the mechanical behaviour of textile reinforcements, with the primary aim of understanding their behaviour during forming and consolidation processes. A number of deformation mechanisms are available. However, during typical composites manufacturing processes, it is generally agreed that the most important of these are in-plane shear and tensile behaviour and through-thickness compaction. Of these mechanisms, the ability of fabrics to shear in-plane is their most important feature during forming, although given their low shear stiffness, in-plane tensile behaviour represents the largest source of energy dissipation. Compaction behaviour defines the fibre volume fraction that can be obtained after manufacturing. Other properties such as fabric bending and ply/tool friction are not considered here, primarily because these have received relatively little attention elsewhere and little data are available. This chapter introduces a number of experimental methods for characterising the deformation of textiles. These methods have been developed within research studies, usually to obtain material data for manufacturing simulation (see Chapter 4). One important consideration here is that none of these tests is standardised – and in fact nearly all published studies use slightly different test methods and specimen dimensions. This issue is being addressed at present as part of a round-robin exercise1. Several standard tests are used in the wider textiles community (e.g. BS ISO 4606:1995, BS 3356:1990, BS 3524-10:1987). Of particular relevance here is the ‘Kawabata Evaluation System for Fabrics (KES-F)’, a series of test methods and associated testing equipment for textile mechanical behaviour including tensile, shear, bending, compression and friction2. However, while this system has been used widely for clothing textiles, its application to reinforcement fabrics has been limited3. This is probably because KES-F provides single point data at relatively low levels of deformation, coupled with the limited availability of the (expensive) testing equipment. As discussed in Chapter 1, the geometry of textile reinforcements can be described at a number of length scales. Individual fibres represent the microscopic scale, with large numbers of fibres (typically several thousand) making up the tow or yarn. The scale of the yarns and of the fabric repeating unit cell is the mesoscopic scale. Finally, the fabric structure constitutes the macroscopic scale. The macroscopic mechanical behaviour of fabrics depends on phenomena at smaller scales, and in particular it is dependent on geometric and contact non-linearities. Such considerations will be used throughout this chapter to develop predictive models for textile mechanical behaviour. 2.2 In-plane shear
2.2.1 Characterisation techniques
As mentioned above, in-plane (or intra-ply) shear is generally considered to be the primary deformation mechanism during forming of textile reinforcements to three-dimensional geometries. Characterisation of this mechanism has therefore received a great deal of attention. The objectives are usually twofold: to measure the non-linear mechanical response of the material during shear, and to characterise the limit of deformation. At the simplest level, mechanical behaviour is of interest for ranking of materials in terms of ease of forming. More recently such data have been required for mechanical forming simulation software based on finite element analysis (see Chapter 4). Here shear force should be normalised by a representative length to eliminate sample size effects. The limit of forming is often characterised by measurement of the ‘locking angle’, which represents the maximum level of shear deformation that can be achieved before fabric wrinkling occurs. In practice this limit varies widely and is highly dependent on the test method employed. The primary use of locking angle data is to identify areas of wrinkling within kinematic draping codes for fabric forming. The majority of published studies have characterised resistance to intra-ply shear using two approaches (Fig. 2.1). Bias extension tests, involving uniaxial extension of relatively wide samples in the bias direction, are favoured by a number of researchers4–6, as the testing procedure is relatively simple. However the deformation field within the sample is non-uniform, with maximum shear observed in the central region and a combination of shear and inter-yarn slip observed adjacent to the clamped edges. In addition the shear angle cannot be obtained directly from the crosshead displacement, so that the test must be monitored visually to measure deformation. Nevertheless this test can provide a useful measure of the locking angle, which in this case represents the maximum shear angle achieved during the test. Above this angle deformation occurs entirely by inter-yarn slip, indicating that the energy required to achieve shear deformation has reached a practical limit. Although this may not indicate the exact angle at which wrinkling would occur, its measurement is repeatable as the bias extension test is not affected significantly by variability in boundary conditions. Bias extension testing is discussed in more detail in Chapter 3, in the context of forming of pre-impregnated composites. 2.1 Characterisation tests for in-plane shear of biaxial fabrics – bias extension (left) and picture frame shear (right). The other popular test method for shear resistance of textile reinforcements is the picture frame test7–10. Here the fabric is clamped within a frame hinged at each corner, with the two diagonally opposite corners displaced using a mechanical testing machine. Cruciform specimens are used typically, with the corners of specimens removed adjacent to the bearings in the corners. Samples must be mounted such that the fibres are parallel to the sides of the picture frame prior to testing. Any small misalignment will lead to tensile or compressive forces in the fibre directions, resulting in large scatter in measured force readings. Nevertheless the picture frame test has proved popular as it produces uniform shear deformation (if performed with care). A number of picture frame test results obtained at Nottingham are reported here as examples of typical behaviour. The picture frame shearing equipment used is illustrated in Fig. 2.1, where the distance between the clamps (l) is 145 mm. Crimped clamps are used typically to ensure that the fabric does not slip from the grips during testing. The apparatus is operated using a Hounsfield mechanical testing machine, which monitors axial load versus crosshead displacement. Here the results are converted into shear force versus shear angle using the following relationships. The shear force (Fs) can be obtained from the measured force in the direction of extension (Fx) using: s=Fx2cosF   2.1 where the frame angle F is determined from the crosshead displacement Dx and the side length of the shear frame (l) using: =cos-1[ 12+Dx2l ]   2.2 The shear angle (reduction in inter-yarn angle) is given by: =p2-2F   2.3 For tests reported here, a pre-tensioning rig was used to position dry fabrics within the picture frame. This consists of a frame within which fabric samples are clamped prior to mounting in the picture frame. Two adjacent sides of the pre-tensioning frame are hinged, to which a measured force is applied to impart tension to the fabric. This device serves two purposes, to align the material within the rig and to enhance repeatability. For the results reported here, a tension of 200 N was applied to fibres in each direction. All tests were conducted at a crosshead displacement rate of 100 mm/min, although experiments at other rates for dry fabric have produced almost identical results11. A minimum of six tests were conducted for each fabric, with error bars produced using the t-distribution at 90% confidence limit. Initial yarn width and pitch (centreline spacing) values were measured using image analysis from digital images oriented normal to the fabric. Video images were also used to estimate the locking angle, although here the procedure was more effective with the camera placed at an oblique angle to the plane of the fabric. Samples were marked with horizontal lines, which buckled when wrinkling occurred. A range of glass fibre reinforcements were tested, including woven fabrics and non-crimp fabrics (NCFs). Descriptions of the fabrics tested are given in Table 2.1, which also includes locking angles averaged for at least four samples. Table 2.1 Fibre architecture descriptions and measured locking angles for woven and non-crimp glass fabrics characterised in shear. In each case, tow properties and spacings were identical in warp and weft P150 Plain...