Boisse Advances in Composites Manufacturing and Process Design

1. Auflage 2015
ISBN: 978-1-78242-320-1
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 492 Seiten

The manufacturing processes of composite materials are numerous and often complex. Continuous research into the subject area has made it hugely relevant with new advances enriching our understanding and helping us overcome design and manufacturing challenges. Advances in Composites Manufacturing and Process Design provides comprehensive coverage of all processing techniques in the field with a strong emphasis on recent advances, modeling and simulation of the design process. Part One reviews the advances in composite manufacturing processes and includes detailed coverage of braiding, knitting, weaving, fibre placement, draping, machining and drilling, and 3D composite processes. There are also highly informative chapters on thermoplastic and ceramic composite manufacturing processes, and repairing composites. The mechanical behaviour of reinforcements and the numerical simulation of composite manufacturing processes are examined in Part Two. Chapters examine the properties and behaviour of textile reinforcements and resins. The final chapters of the book investigate finite element analysis of composite forming, numerical simulation of flow processes, pultrusion processes and modeling of chemical vapour infiltration processes. - Outlines the advances in the different methods of composite manufacturing processes - Provides extensive information on the thermo-mechanical behavior of reinforcements and composite prepregs - Reviews numerical simulations of forming and flow processes, as well as pultrusion processes and modeling chemical vapor infiltration

Autoren/Hrsg.

Boisse, Philippe

Leseproben

2 Knitting processes for composites manufacture
G. Dusserre; G. Bernhart    Université de Toulouse, Toulouse, France Abstract
This chapter depicts the advantages and drawbacks of knits as composite reinforcements and gives a nonexhaustive overview of some properties and applications. First, the knitting process capabilities are described, focusing on the opportunity given to knit net-shape 3D preforms with holes and various local properties. Then, the main characteristics of knit permeability are discussed. Finally, the mechanical behavior of dry knits and knit-reinforced composites are investigated. The improvement of the composite mechanical properties by adding inlaid yarns is particularly emphasized, from both experimental and modeling points of view. Keywords Knit-reinforced composites Anisotropic behavior Permeability Elasticity Analytical modeling Acknowledgments
The authors gratefully acknowledge the financial support of part of these works by DRIRE and the French Region Midi-Pyrénées, in the framework of the collaborative projects RT2I and CORTEX. Special thanks are expressed to Laura Balea, Bernard Cabanes, Nicolas Dumont, Francis Planel, Naoufel Ben Salem, and Adrien Touzé for their great contribution. 2.1 Introduction
The main asset of knits as reinforcements of composite materials lies in the knitting process more than in the knit itself. Indeed knitting processes are efficient ways to manufacture automatically net-shape preforms of technical fibers. 3D preforms can be knitted with one or several types of fiber, with various local knitting patterns and, thus, various local thicknesses and properties. It is also possible to knit holed preforms without cutting the yarn. All these specificities are of special interest for the composite manufacturing industry to reduce the time spent to layup the preform in the mold before consolidation, for instance by resin transfer molding or resin infusion. The literature details many examples of polymeric matrix composites reinforced with a knitted fabric. Thermoplastics (Rozant et al., 2001; Abounaim et al., 2011) and elastomers (Huang and Ramakrishna, 2000) have been used to take advantage of the capability of the knit to be stretched, either by allowing the composite part to be shaped once consolidated (e.g., by thermoforming) or in order to obtain a deformable reinforced material. This chapter focuses on composites with thermosetting resins, among which epoxy resins are often used (Balea et al., 2014; Ramakrishna, 1997; Aktas et al., 2013). In this particular case, the very low rigidity of the fabric is an obstacle to reach high mechanical properties with knit-reinforced composites. Textile strategies are thus involved to improve these properties. This chapter aims to depict the advantages and drawbacks of knits as composite reinforcements, as well as to give a nonexhaustive overview of some properties and applications. The improvement of the composite mechanical properties by adding inlaid yarns will be particularly emphasized, from both experimental and modeling points of view. 2.2 Knitting process description
Knitting processes are usually classified into two categories: warp and weft knitting, referring to the direction in which two consecutive loops of the same yarn are knitted. In the specific vocabulary related to weft knitting processes, a set of stitches made by a single needle, aligned in the warp direction, is called a wale. A course is a set of stitches aligned in the weft direction and each knitted successively by a different needle. The warp knitting process requires as many yarns as the number of wales, and each yarn is knitted in the wale (or warp) direction, by two adjacent needles alternatively. The weft knitting process involves one single yarn or more, knitted in the course (or weft) direction by the successive needles. A wale is comprised of the stitches knitted by a single needle during the knitting of the consecutive courses. This chapter focuses on weft-knitted fabrics, but a significant literature also exists on the use of warp knits as composite reinforcements. The knitting of technical yarns for composite applications may not bring specific issues for most of the fibers. Many authors use glass fibers (Balea et al., 2014; Ramakrishna, 1997; Aktas et al., 2013; Rudd et al., 1990; Leong et al., 1998; Chou et al., 1992; Abounaim et al., 2010) but also basalt (Balea et al., 2014) or aramid (Khondker et al., 2004). Carbon (Balea et al., 2014; Chou et al., 1992; Qi et al., 2014) and silicon carbide (Heiss et al., 2012) fibers have also been knitted, but some troubles may arise necessitating adapting the process in order to avoid the rupture of a large amount of fibers in the needle hook during knitting. 2.2.1 Standard weft knitting processes
Both flat and circular weft knitting machines possess one or two needle beds. Circular knitting machines are dedicated to the manufacturing of tubular preforms, and flat knitting machines are suitable in general for flat or 3D preforms, including tubular ones. The knitted fabrics are thus classified into two groups depending on the use of one or two needle beds. If all the needles of one needle bed work, the basic single bed fabric obtained, called plain knit, only comprises front stitches (Figure 2.1a). The basic double beds fabric, called 1 × 1 rib knit, obtained when all the needles of both needle beds work (Figure 2.1b), comprises alternate wales of front and back stitches. From these two basic fabrics, many variants can be derived by selecting the working needles according to a regular pattern. Ramakrishna (1997) took an inventory of the different fabrics available by knitting. About 14 single needle bed fabrics and 20 double needle bed ones were listed. Among them, the most used are both basic knits, plain (Balea et al., 2014; Dusserre et al., 2010) and 1 × 1 rib (Aktas et al., 2013; Chou et al., 1992; Dusserre et al., 2010) knits, but also interlock (Chou et al., 1992; Sun et al., 2009) and full cardigan fabrics (Chou et al., 1992). One of the most popular is the double needle bed Milano knit (Aktas et al., 2013; Leong et al., 1998; Chou et al., 1992; Figure 2.1c). These knitting patterns can be juxtaposed in the same fabric to manufacture a preform with various thicknesses and fiber volume fractions in selected areas and thus to get local tailor-made properties. Figure 2.1 Basic knits used as composite reinforcements: (a) plain knit, (b) 1 × 1 rib knit, and (c) Milano-rib knit. The stitch transfer technique, using specific needles, allows transferring a stitch from a needle to the opposite needle on the other needle bed. Thanks to a relative displacement of the needle beds in the course-wise direction, a second transfer makes possible to merge two wales, or on the contrary to add a new wale in the width of the fabric. This technique allows knitting holed fabrics (Figure 2.2a) without necessitating cutting the yarn (Kameo et al., 1999). This technique is also employed to knit 3D preforms (see for instance a knitted beanie) by changing the number of wales at each course (Duhovic, 2004; Figure 2.2b). Figure 2.2 Examples of stitch transfer leading (a) to a holed fabric and (b) to merged wales allowing to knit 3D preforms. 2.2.2 Plain knitted fabrics including inlaid yarns
Other knitting techniques can be used to enlarge the possibility to change the local fibrous construction. A float stitch (Khondker et al., 2004; Leong et al., 2000; Figure 2.3a) is obtained on a plain knit when a needle remains at rest during the knitting of a course. A straight yarn segment is thus introduced instead of a stitch, and the stitch knitted by the same needle in the previous course is held on the needle until the stitch knitted by the needle in the next course passes through it. This technique can be employed to increase the amount of straight yarns in the fabric and, thus, improve the mechanical behavior of the composite in the course-wise direction. Figure 2.3 Float (a) and tuck (b) stitches and inlaid yarn (c) linked to the plain knit by a tuck. A tuck stitch (Leong et al., 2000; Figure 2.3b) is obtained when a stitch is held on the needle during the next course knitting. By this way, the first stitch is not knitted with the second one, which does not form a loop, but with the third one. The yarn of the second course is thus hooked to the third one, without being knitted, allowing to decrease its curvature. To be more efficient and still increase the proportion of straight yarns in the preform, floated yarns can be combined with tuck stitches. Balea et al. (2014) showed the effect of adding inlaid yarns to a plain knit (Figure 2.3c) on the mechanical behavior of the resulting composite. 2.3 Permeability of knitted reinforcements
Liquid composite molding (LCM) processes involve an impregnation of the reinforcement by a resin flow through the fabric. Knits are suitable for such processes thanks to their high in-plane permeability (Table...