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Structural stitching of non-crimp fabric preforms for composites
3 Structural stitching of non-crimp fabric preforms for composites P. MITSCHANG, Institut für Verbundwerkstoffe GmbH, Germany
Abstract: This chapter contains a short introduction on preform technology and answers the question of why stitching technology should be used. Different types of threads are described, followed by a short discussion about the advantages and disadvantages of each, which will help the reader to select the right type for a specific application. Quality aspects of sewn preforms are evaluated. Information is given about stitching technology, based on the requirements of composites. A description of the main technologies and a short overview of different stitching machines and their capabilities for structural stitching are also given. Finally, quality aspects of manufacturing structural stitched non-crimp fabrics (NCFs) are addressed. Special focus is given to machine parameters and their influence on compaction behaviour, permeability and laminate quality. Key words: stitching technology, thread selection, stitch hole ellipse, stitching parameters, stitched preform.
3.1
Introduction
The processing of non-crimp fabric (NCF) composites is generally carried out using a liquid moulding process like Resin Transfer Moulding (RTM) or Vacuumassisted Resin Infusion (VARI). During the manufacturing of structural components, the main focus is on maintaining the necessary fibre orientation in the component. This is the reason why, during the processing of NCFs in liquid moulding processes, a preforming process is established before the actual infusion process can be carried out. In addition to the use of a moulding process with thermoplastic binders, the stitching technique can be offered as preform technology to strengthen the fibres. Due to its development and broad application in the clothing industry, stitching technology offers a considerable range of advantages, such as the variable selection of stitch types and sewing threads. The potential applications of NCF composites, as well as the RTM process, can place demands on their structural seams. Therefore, it is necessary to differentiate between stitching for fixation or positioning, stitching for assembly reasons, and structural stitching (Mitschang et al., 2003). Table 3.1 shows the advantages as well as the challenges of structural stitching. Aside from the quality of the basic textile product (in this case, a non-crimp fabric) and the injection or infusion process (including the hardening of the resin), the quality of a laminate or component is decisively defined by the quality of the preform. Figure 3.1 shows how the quality of the component depends on the materials and processing steps used to produce it. 67 © Woodhead Publishing Limited, 2011
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Table 3.1 Advantages and challenges of structurally stitched non-crimp fabric preforms Advantages
Challenges
Easy handling of complex textile structures Manufacturing of semi-finished products Assembly for 3D geometries Waste reduction Pre-compaction of preforms Complex component Efficient injection or infusion processes Elimination of post-processing (net-shape) Improved 3D properties
3D and surface quality of laminate Thread material as a third component Thread impregnation Prevention of void formation Fibre separation Fibre deflection Prevention of resin-rich zones Reduction of in-plane properties Surface appearance and resin shrinkage
3.1 Factors influencing laminate and part quality.
The following sections describe the selection of sewing threads, an estimation of the available stitching technologies, the applicable machine technology, and the parameters that have to be taken into account in the construction of structural seams for NCFs.
3.2
Threads for structural stitching technology
According to the role of the individual seam, there are different sewing threads available (Ogale et al., 2004). In the first instance, these seams are to be differentiated according to the materials used in their construction. While thermoplastic sewing threads are primarily used for positioning, fixing, and as
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Table 3.2 Classification for yarn types to be used for stitching Thread type
Description
Mechanical properties
Spun yarn
Fibres with twist level, ∼ 400 turns/m 10–50 filaments, twisted ∼150 turns/m Bulky, looped filament orientation Filament core, staple fibre sheath
Medium strength and elongation Good strength and high elongation Inherent high elongation
Twisted multi filaments Textured yarn Core spun yarn
High performance thread
assembly seams, due to their high elongation at break, materials with high strength or stiffness are preferred for structural seams. These materials are mainly glass, aramid and carbon. As a second characteristic, the thread type is crucial in terms of determining the behaviour of the sewing thread. Table 3.2 shows a comparison of different types of sewing thread. The various types of sewing thread are generally all suitable to be used as needle threads as well as bobbin threads. In most cases, textured threads made of very fine polyester filaments (<15 tex) are used as bobbin threads. In the selection of a needle thread, it is important to consider the fact that the thread will have to pass through the needle eye several times and must therefore be able to withstand looping with the bobbin thread (knot formation). It is especially important to take account of this aspect when using carbon sewing threads. Core spun yarns and twisted multifilament yarns are suitable for structural sewing. Some commercially available threads include glass fibre sewing threads from Culimeta (Culimeta, 2010), PPG (PPG Fiber Glass, 2010) and Vetrotex (SaintGobain Vetrotex International, 2010) and carbon fibre sewing threads from Schappe Techniques and Toray. Typical here is the use of relatively thick sewing threads (70–150 tex) with medium to low twist. The low knot strength is a considerable problem in the manufacture of carbon fibre sewing threads, but it can be considerably increased through the use of core spun technology with a hybrid solution. Figure 3.2 shows the different sewing threads based on carbon fibre rovings. Type CF-1, CF-2, and CF-3 are hybrid threads manufactured using a crochet technology (Weimer, 1999), a wrapping technology, or a braiding technology (Schappe, 2004). An advantage of these technologies is that the carbon filaments are placed straight in the core of the sewing thread, which serves to optimise their load-bearing ability. A disadvantage is the cover of the used polyester thread, which is an additional component and an unwanted material in the composite. Alternatively, it also is possible for a carbon fibre roving to be turned or twisted, as shown in Fig. 3.2 by type CF-4, CF-5, and CF-6.
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3.2 Different types of carbon fibre sewing threads (IVW GmbH).
The process of turning and twisting carries a risk of causing pre-damages and leads to the suboptimal placement of fibres in the composite, as wells as inadequate load-carrying ability, due to the twists it creates in the filaments. Another important parameter for the selection of fibres is fibre sizing. The impregnation properties of the thread and the chemical compatibility between the sewing thread and the resin during the curing process is defined by the fibre design and the fibre sizing used. In addition to the requirement to be compatible with the resin system, a further function of fibre sizing is to reduce the sliding friction between the thread and the needle, as well as between the thread and the stitched textile structure. If there is too much friction, abrasive thread wear can occur and lead to thread breakage. Thus, not only will the efficiency and economic performance of the process be endangered, but the structural properties of the stitching will also be considerably reduced. The properties of strength, stiffness, elongation, and friction are all important for the complete performance of a chosen thread.
3.3
Stitching technology and sewing machines
Almost all stitching technologies are applicable in the manufacturing of preforms for NCFs (Poe et al., 2002). The specific selection has to be carried out according to the demands of its respective application. The modified lock stitch, chain stitch, blind stitch, one-side stitch and tufting are particularly suitable for structural stitched laminates (Ogale and Mitschang, 2004; Herkt et al., 2006; Drechsler,
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3.3 Schematic diagram of stitch formation using lock stitch and micrograph of a non-crimp fabric laminate stitched with a modified lock stitch.
2000). Figure 3.3 shows the lock stitch formation, as well as a NCF laminate stitched with a modified lock stitch using a glass fibre sewing thread. The modified lock stitch differs from the deployed standard lock stitch used in the apparel industry in terms of the interlace position of the needle and bobbin thread. While in the apparel industry, the interlace is positioned within the work piece, for structural stitching the interlace must be positioned on the rear side of the laminate due to there being minimal fibre undulation inside the laminate. This can be achieved through the adjustment of the tensile forces between the needle and bobbin thread during stitch formation. Aside from the lock stitch, which is well known and used in the apparel industry, there have been other seams established for structural stitching. These include one-side stitching techniques (such as KSL blind stitch RS 510, KSL double needle RS 530, and ITA sewing head) and tufting (KSL Tufting RS 522). Table 3.3 summarises the most commonly used stitch types in relation to their influence on the quality and properties of the preform. The one-side stitching heads (Fig. 3.4 and Fig. 3.5) are characterised by the fact that they form a chain stitch with one or two threads and that access to the textile (NCF) is granted only from one side. The blind stitch RS 510 can be performed while the textile is held in a tool because the curved blind stitch needle need not necessarily penetrate the textile material. Compared to stitch types such as the lock stitch, one-side stitch and blind stitch, no interlacing of the needle thread occurs during tufting (Fig. 3.6). The needle thread is inserted into the textile material by use of a hollow needle and forms a loop in the reinforcing material due to the difference in friction forces between the needle and thread, and the textile and thread. Typical of this stitch are the visible loops on the rear side of the laminates. Theoretically, it would also be possible for the loops to end in the material without penetrating it, but this cannot be realised in practice because the interior friction forces between the reinforcing textile and the sewing thread cannot be reproduced. Automation is a major aspect of structural stitching during the preforming process. Two different types can be distinguished. The first type consists of 2D
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Table 3.3 Stitch type and related influence on preform characteristics Stitch type (IAW DIN 61400)
Thread consumption
Handling
Lock stitch (Kl. 301)
– Hybridisation and high efficiency possible – High thread tensile forces
– Good shear – Different fibre – Exactly strength of displacement on adjustable layer package the upper or – Low fibre – Low tendency lower surface of material to warp the material content in the – High thread laminate tensile forces
Blind stitch – No bobbin – Possible (Kl. 103) thread stitching in a necessary solid tool – Large thread – Lower layers demand are not fixed – Low efficiency
One side stitch
– No bobbin thread necessary – High fibre consumption
Tufting
– Very high – No efficiency with interlacing, optimal thus no placement in joining the laminate between the (no loops) single layers
Fibre disorientation
– Lower single layers are not influenced by the stitching process – Local thread concentration, therefore, high shear
Compression
– Compaction of the layers not adjustable – Due to precompaction slight deviation of needle possible
– Good shear – Material – Seam width strength of penetration with influences the layer package 2 needles material in a – Different larger area orientation of 3D-reinforcement – Low thread strength – Thick needle necessary, thus movement in the laminate
– Thread strength (joining) insufficient for insertion of compaction force – Low fibre material content in the laminate
automatic sewing machines, which are generally equipped with the lock stitch. Figure 3.7 shows a two-dimensional (2D) plant for the manufacture of plane preforms, so-called textile reinforcements. The second type consists of the so-called sewing robots, which can be flexibly equipped with different stitching heads. These sewing robots have been developed especially for structural stitching in the spatial (3D) direction. Figure 3.8 shows the use of a flexible robot system for structural stitching as a gantry system with a suspended sewing robot.
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3.4 Stitching heads and schematic diagram of stitch formation using blind stitch (KSL, Lorsch; left: blind stitch RS 510, right: double needle RS 530).
3.5 Schematic diagram of stitch formation using one-side stitch and micrograph (ITA, RWTH Aachen).
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3.6 Tufting head and schematic diagram of stitch formation during tufting (KSL, Lorsch; tufting RS 522).
Table 3.4 shows more clearly the selection of the particular stitch types and sewing threads which have to be used, depending on the required seam type and task (Ogale, 2007).
3.4
Quality aspects for structural stitching
The sewing thread is inserted into the laminate oriented widthways, so positioned vertically to the in-plane reinforcement fibres. In order for the sewing thread to
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3.7 2D sewing plant for manufacturing of plane preforms (IVW, Kaiserslautern).
3.8 Flexible robot systems for structural stitching (KSL, Lorsch).
achieve the required effect of structural stitching, it is necessary to insert into the textile structure an adequate filament diameter in the form of bundled single filaments, which should be reinforced. Thicker sewing threads (filament bundles, as shown in Fig. 3.2) are normally used to keep the number of stitches per unit area low. When the needle penetrates the textile (NCF) the in-plane filaments are moved aside and the sewing thread is inserted. An ellipse is formed around the stitch hole due to the movement of the in-plane fibres. The dimensions of the ellipse depend upon the set-up of the layer orientation, the sewing thread, the stitching direction, and the machine parameters. It is possible to characterise the ellipse by measuring its half-axis. The real ellipse area has approximately 65%
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Table 3.4 Seam type related applicability of stitch types and threads Seam type
Stitch type
Thread applicable
Fixing and positioning
Modified lock stitch
Thin, flexible, high elongation, polyester Medium thick, less elongation, polyester Medium thicker, polyester Thicker polyester, Nomex®, or Kevlar® Thick, less elongation, polyester, Kevlar® Thick, polyester Polyester, Nomex®, or Kevlar® Polyester, Kevlar®, glass Carbon, glass, Kevlar® Carbon, glass, Kevlar® Carbon, glass, Kevlar®
Chain stitch
Assembling
Blind stitch Modified lock stitch Chain stitch One-side stitch Blind stitch
Structural
Tufting Modified lock stitch One-side stitch Tufting
of the area of an ideal ellipse, as calculated from the minor and major ellipse axis values measured from the laminate (Ogale, 2007). For the purpose of analysis, the stitch hole can also be modelled as a diamond shape of which the longer axis is oriented in parallel with the fibre direction of the individual layer (Roth, 2005; Heß et al., 2007). Figure 3.9 shows the top view of a stitched structure with the specification of the minor and major half axis. The set-up of the laminate has a major influence on the formation of the ellipse. The longer (major) half-axis of the ellipse is always positioned in the direction of the fibre orientation. If the stitching direction is running vertically to the fibre direction, a maximum expansion can be observed. The closer the stitching direction is to the fibre orientation, the lower the in-plane fibre undulation will be. This means that, in the case of a real laminate, each layer has a different orientation in fibre undulation, due to the variable laminate set-up. Figure 3.10 shows a fourply set-up, as well as a micrograph of the respective laminate. The fact that the fibre undulation is different in each laminate layer results in there being a loss of the in-plane mechanical properties, which should not be neglected. The formation of the stitch hole expansion depends directly on the type of sewing thread used. Figure 3.11 shows the ellipse formation (top view) and the stitch hole expansion in the cross-section of the laminate. A comparison between a flexible polyester thread and a very rigid carbon fibre sewing thread is shown in Fig. 3.11. The larger stitch expansion and, thus, the ellipse enlargement for the rigid carbon fibre thread is clearly visible. The optimised adjustment of the sewing machine parameters, especially the balance
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3.9 Ellipse formation in the laminate (top view) with half axis.
3.10 Influence of the layer set-up on ellipse formation in the laminate.
between needle and bobbin thread tension, also have a major influence on the quality of the seam. Figure 3.12 shows the impact of thread tension being too high, and compares this to an optimised machine parameter. The stitch hole expansion increases and the knot placement of the needle and bobbin thread moves into the laminate and generates additional fibre undulations in the intermediate layers. A further aspect that is directly influenced by ellipse formation is the surface quality of the laminate. A local compaction of the reinforcement fibres and visible fibre undulations in the upper laminate layer (Figure 3.13) occurs due to the penetration of the needle and the insertion of the needle threads. This leads to matrix-rich areas at the surface and the formation of sink marks due to the hardening of the used matrix macroscopic. After the preform has been placed in
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3.11 Influence of the thread material on ellipse formation.
3.12 Influence of the machine parameters on ellipse formation.
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3.13 Surface characteristic due to fibre concentration (left) and matrix shrinkage (right).
3.14 Reduction in ellipse size by preform compaction.
the injection mould, the preform is normally compacted to the final height required to reach the necessary fibre volume content. Figure 3.14 demonstrates the influence of preform compaction on ellipse formation, based on textured and twisted multifilament threads. It is possible to reduce the ellipse area by a factor of three to five depending on the sewing thread used. The compression force that is necessary in order to reach the required fibre volume content depends on the sewing thread used and the sewing machine parameters selected. Figure 3.15 clarifies this by using a
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3.15 Influence of stitch density on compaction pressure to reach a defined fibre volume content.
four-layer NCF [0/90]4. The layer set-up was stitched at three different stitch densities (13.33 and 3.33 stitches per cm2 and unstitched) with a twisted multifilament thread and high thread tension. Up to a fibre volume content of approximately 55%, no significant influence was recognised. A reduction in the compaction force necessary to reach a high-fibre compaction can be seen when using a very high stitch density and a high thread tension. An increase in the compaction force must be accepted when using lower stitch densities or low thread tension. This results from the fibre nesting being hindered by the inserted needle thread during compaction. This behaviour is unique to an NCF under the conditions described, and cannot be directly transferred to other lay-ups or other textiles, such as woven fabrics. A very high pre-compaction of the layer set-up during the stitching of structural seams can create flow channels at the surfaces of the preform due to insufficient compaction behaviour in the mould. These influence the injection flow in such a manner that the injected resin runs first of all along the seams and from there impregnates the reinforcing fibre structure. Figure 3.16 shows the change in flow front geometry in a biaxial NCF lay-up influenced by a stitch density of 3.33 stitches per cm2. Due to the fact that structural stitching is only used locally within a complete structure, this effect must be classified as negative. Flow channels are hardly used in the form of desired flow assistance due to their inadequate reproducibility.
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3.16 Influence of stitching on the flow front of a biaxial NCF (left: unstitched; right: stitch density 3.33 stitches per cm2; fibre volume content 50%).
3.5
Applications and future trends
At present, the use of structural sewing of NCFs is not widespread. The first applications of this technology can be found in aviation (Weimer, 2003), e.g. in the Airbus A380 pressure bulkhead or very fast rotating fan wheels. Structural sewing is used in these applications to improve the assembly properties of preforms, as well as the material properties of the component. From the component perspective, the essential motivation is the improvement of the impact behaviour and, consequently, the reduction of a delamination risk in the laminates. Therefore, it is the structural components with impact hazard in particular, such as the bottom of fuselages, rail vehicles and automobiles, or structures in airflows, such as turbine blades, the wing-leading edges of aeroplanes, or the blades of wind energy plants, that are predestined for future areas of application. The further development of stitching technology for the structural stitching of NCFs depends greatly on the complete development of stitching technology as the preform technology for liquid composite moulding processes. The machine technology is sufficiently developed. Specific adjustments are expected in the area of sewing thread development. The development of carbon fibre sewing threads, especially if achieved in combination with CFRPC compatible sizings, will serve to increase the effectiveness of NCFs. A further area of minor investigation is the influence of stitching on the actual impregnation process. Until now, there have been only publications on the influence of stitching on the flow of resin in fabrics (Rieber and Mitschang, 2010) but in principle these results should be transferable. By discovering new knowledge in the area of the structural effects of stitched NCFs, conclusions can be drawn about the optimal specifications such as stitch density, fibre diameter and others. General information for the use of the stitching technology in the preforming and structural application of NCFs is published in Long (2005), Beier et al.
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(2006) and Weimer and Mitschang (2001). Direct textile technologies for the manufacturing of 3D reinforcement structures are investigated by Tong et al. (2002). An excellent summary of the state of the art of structural stitching can be found in Ogale (2007).
3.6
References
Beier U, Fischer F, Sandler J K W, Altstadt V, Weimer C and Buchs W (2007), Mechanical Performance of Carbon Fibre-Reinforced Composites Based on Stitched Preforms, Composites Part A, 38, 1655–1663. Culimeta (2010), Technical yarns. Available from http://www.culimeta.de/technical-yarns. php (Accessed 26 January 2010). Drechsler K (2000), Advanced textile structural composite – needs and current developments, The Fifth International Conference on Textile Composites. 18–20 September, 2000, Leuven, Belgium. Herkt M, Middendorf P, Less C, Riedel W, Schouten M and Drechsler K (2006), 3-D reinforcement of composite T-joints by means of robot-assisted stitching technology, SAMPE April 30 May 2006, Long Beach, CA, USA. Heß H, Roth Y C and Himmel N (2007), Elastic Constants Estimation of Stitched NCF CFRP Laminates Based on a Finite Element Unit-Cell Model, Composites Science and Technology, 67, 1081–1095. Long A C (2005), Design and manufacture of textile composites, Cambridge, Woodhead Publishing Limited. Mitschang P, Ogale A, Schlimbach J, Weyrauch F and Weimer C (2003), Preform technology: a necessary requirement for quality controlled LCM processes, Polymer and Polymer Composites, 8, 605–622. Ogale A, Weimer C and Mitschang P (2004), Selection of sewing threads for preform manufacturing, Advanced Composite Letters, 13, 145–153. Ogale A, and Mitschang P (2004), Tailoring of Textile Preforms for Fiber-reinforced Polymer Composites, Journal of Industrial Textiles, 34, 77–96. Ogale A (2007), Investigations of sewn preform characteristics and quality aspects for the manufacturing of fiber reinforced polymer composites, IVW-Schriftenreihe, 70. Poe Jr C C, Dexter H B and Raju I S (2002), A review of the NASA textile composite research. Available from http://techreports.larc.nasa.gov/ltrs/PDF/1997/aiaa/NASAaiaa-97-1321.pdf (Accessed 7 October 2002). PPG Fiber Glass (2010), Bobin and texturized yarns. Available from http://www.ppg.com/ glass/fiberglass/products/pages/default.aspx (Accessed 26 January 2010). Rieber G, and Mitschang P (2010), 2D Permeability changes due to stitching seams, Composites Part A, 41, 2–7. Roth Y C (2005), Beitrag zur rechnerischen Abschätzung des Scheibenelastizitätsverhaltens in Dickenrichtung vernähter Faser-Kunststoff-Verbund-Laminate, IVW-Schriftenreihe, 55. Saint-Gobain Vetrotex International (2010), Glass filament yarns. Available from http:// www.vetrotexeurope.com/yarns.html (Accessed 26 January 2010). Schappe (2004), Carbon thread for composite industry by Schappe Techniques. Available from http://www.schappe.com (Accessed 12 December 2009). Tong L, Mouritz A P and Bannister M K (2002), 3D fibre reinforced polymer composites, Oxford, Elsevier.
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Weimer C (1999), Carbon fiber sewing thread for compound fiber and plastics materials, German Patent DE19932842. Weimer C and Mitschang P (2001), Aspects of the Stitch Formation Process on the Quality of Sewn Multi-Textile-Preforms, Composites Part A, 32, 1477–1484. Weimer C (2003), Preform-Engineering: Applied Technologies to Incorporate Part and Process Functions into Dry Textile Reinforcements, Composites Science and Technology, 63, 2089–2098.
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Abstract: This chapter contains a short introduction on preform technology and answers the question of why stitching technology should be used. Different types of threads are described, followed by a short discussion about the advantages and disadvantages of each, which will help the reader to select the right type for a specific application. Quality aspects of sewn preforms are evaluated. Information is given about stitching technology, based on the requirements of composites. A description of the main technologies and a short overview of different stitching machines and their capabilities for structural stitching are also given. Finally, quality aspects of manufacturing structural stitched non-crimp fabrics (NCFs) are addressed. Special focus is given to machine parameters and their influence on compaction behaviour, permeability and laminate quality. Key words: stitching technology, thread selection, stitch hole ellipse, stitching parameters, stitched preform.
3.1
Introduction
The processing of non-crimp fabric (NCF) composites is generally carried out using a liquid moulding process like Resin Transfer Moulding (RTM) or Vacuumassisted Resin Infusion (VARI). During the manufacturing of structural components, the main focus is on maintaining the necessary fibre orientation in the component. This is the reason why, during the processing of NCFs in liquid moulding processes, a preforming process is established before the actual infusion process can be carried out. In addition to the use of a moulding process with thermoplastic binders, the stitching technique can be offered as preform technology to strengthen the fibres. Due to its development and broad application in the clothing industry, stitching technology offers a considerable range of advantages, such as the variable selection of stitch types and sewing threads. The potential applications of NCF composites, as well as the RTM process, can place demands on their structural seams. Therefore, it is necessary to differentiate between stitching for fixation or positioning, stitching for assembly reasons, and structural stitching (Mitschang et al., 2003). Table 3.1 shows the advantages as well as the challenges of structural stitching. Aside from the quality of the basic textile product (in this case, a non-crimp fabric) and the injection or infusion process (including the hardening of the resin), the quality of a laminate or component is decisively defined by the quality of the preform. Figure 3.1 shows how the quality of the component depends on the materials and processing steps used to produce it. 67 © Woodhead Publishing Limited, 2011
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Table 3.1 Advantages and challenges of structurally stitched non-crimp fabric preforms Advantages
Challenges
Easy handling of complex textile structures Manufacturing of semi-finished products Assembly for 3D geometries Waste reduction Pre-compaction of preforms Complex component Efficient injection or infusion processes Elimination of post-processing (net-shape) Improved 3D properties
3D and surface quality of laminate Thread material as a third component Thread impregnation Prevention of void formation Fibre separation Fibre deflection Prevention of resin-rich zones Reduction of in-plane properties Surface appearance and resin shrinkage
3.1 Factors influencing laminate and part quality.
The following sections describe the selection of sewing threads, an estimation of the available stitching technologies, the applicable machine technology, and the parameters that have to be taken into account in the construction of structural seams for NCFs.
3.2
Threads for structural stitching technology
According to the role of the individual seam, there are different sewing threads available (Ogale et al., 2004). In the first instance, these seams are to be differentiated according to the materials used in their construction. While thermoplastic sewing threads are primarily used for positioning, fixing, and as
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Table 3.2 Classification for yarn types to be used for stitching Thread type
Description
Mechanical properties
Spun yarn
Fibres with twist level, ∼ 400 turns/m 10–50 filaments, twisted ∼150 turns/m Bulky, looped filament orientation Filament core, staple fibre sheath
Medium strength and elongation Good strength and high elongation Inherent high elongation
Twisted multi filaments Textured yarn Core spun yarn
High performance thread
assembly seams, due to their high elongation at break, materials with high strength or stiffness are preferred for structural seams. These materials are mainly glass, aramid and carbon. As a second characteristic, the thread type is crucial in terms of determining the behaviour of the sewing thread. Table 3.2 shows a comparison of different types of sewing thread. The various types of sewing thread are generally all suitable to be used as needle threads as well as bobbin threads. In most cases, textured threads made of very fine polyester filaments (<15 tex) are used as bobbin threads. In the selection of a needle thread, it is important to consider the fact that the thread will have to pass through the needle eye several times and must therefore be able to withstand looping with the bobbin thread (knot formation). It is especially important to take account of this aspect when using carbon sewing threads. Core spun yarns and twisted multifilament yarns are suitable for structural sewing. Some commercially available threads include glass fibre sewing threads from Culimeta (Culimeta, 2010), PPG (PPG Fiber Glass, 2010) and Vetrotex (SaintGobain Vetrotex International, 2010) and carbon fibre sewing threads from Schappe Techniques and Toray. Typical here is the use of relatively thick sewing threads (70–150 tex) with medium to low twist. The low knot strength is a considerable problem in the manufacture of carbon fibre sewing threads, but it can be considerably increased through the use of core spun technology with a hybrid solution. Figure 3.2 shows the different sewing threads based on carbon fibre rovings. Type CF-1, CF-2, and CF-3 are hybrid threads manufactured using a crochet technology (Weimer, 1999), a wrapping technology, or a braiding technology (Schappe, 2004). An advantage of these technologies is that the carbon filaments are placed straight in the core of the sewing thread, which serves to optimise their load-bearing ability. A disadvantage is the cover of the used polyester thread, which is an additional component and an unwanted material in the composite. Alternatively, it also is possible for a carbon fibre roving to be turned or twisted, as shown in Fig. 3.2 by type CF-4, CF-5, and CF-6.
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3.2 Different types of carbon fibre sewing threads (IVW GmbH).
The process of turning and twisting carries a risk of causing pre-damages and leads to the suboptimal placement of fibres in the composite, as wells as inadequate load-carrying ability, due to the twists it creates in the filaments. Another important parameter for the selection of fibres is fibre sizing. The impregnation properties of the thread and the chemical compatibility between the sewing thread and the resin during the curing process is defined by the fibre design and the fibre sizing used. In addition to the requirement to be compatible with the resin system, a further function of fibre sizing is to reduce the sliding friction between the thread and the needle, as well as between the thread and the stitched textile structure. If there is too much friction, abrasive thread wear can occur and lead to thread breakage. Thus, not only will the efficiency and economic performance of the process be endangered, but the structural properties of the stitching will also be considerably reduced. The properties of strength, stiffness, elongation, and friction are all important for the complete performance of a chosen thread.
3.3
Stitching technology and sewing machines
Almost all stitching technologies are applicable in the manufacturing of preforms for NCFs (Poe et al., 2002). The specific selection has to be carried out according to the demands of its respective application. The modified lock stitch, chain stitch, blind stitch, one-side stitch and tufting are particularly suitable for structural stitched laminates (Ogale and Mitschang, 2004; Herkt et al., 2006; Drechsler,
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3.3 Schematic diagram of stitch formation using lock stitch and micrograph of a non-crimp fabric laminate stitched with a modified lock stitch.
2000). Figure 3.3 shows the lock stitch formation, as well as a NCF laminate stitched with a modified lock stitch using a glass fibre sewing thread. The modified lock stitch differs from the deployed standard lock stitch used in the apparel industry in terms of the interlace position of the needle and bobbin thread. While in the apparel industry, the interlace is positioned within the work piece, for structural stitching the interlace must be positioned on the rear side of the laminate due to there being minimal fibre undulation inside the laminate. This can be achieved through the adjustment of the tensile forces between the needle and bobbin thread during stitch formation. Aside from the lock stitch, which is well known and used in the apparel industry, there have been other seams established for structural stitching. These include one-side stitching techniques (such as KSL blind stitch RS 510, KSL double needle RS 530, and ITA sewing head) and tufting (KSL Tufting RS 522). Table 3.3 summarises the most commonly used stitch types in relation to their influence on the quality and properties of the preform. The one-side stitching heads (Fig. 3.4 and Fig. 3.5) are characterised by the fact that they form a chain stitch with one or two threads and that access to the textile (NCF) is granted only from one side. The blind stitch RS 510 can be performed while the textile is held in a tool because the curved blind stitch needle need not necessarily penetrate the textile material. Compared to stitch types such as the lock stitch, one-side stitch and blind stitch, no interlacing of the needle thread occurs during tufting (Fig. 3.6). The needle thread is inserted into the textile material by use of a hollow needle and forms a loop in the reinforcing material due to the difference in friction forces between the needle and thread, and the textile and thread. Typical of this stitch are the visible loops on the rear side of the laminates. Theoretically, it would also be possible for the loops to end in the material without penetrating it, but this cannot be realised in practice because the interior friction forces between the reinforcing textile and the sewing thread cannot be reproduced. Automation is a major aspect of structural stitching during the preforming process. Two different types can be distinguished. The first type consists of 2D
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Table 3.3 Stitch type and related influence on preform characteristics Stitch type (IAW DIN 61400)
Thread consumption
Handling
Lock stitch (Kl. 301)
– Hybridisation and high efficiency possible – High thread tensile forces
– Good shear – Different fibre – Exactly strength of displacement on adjustable layer package the upper or – Low fibre – Low tendency lower surface of material to warp the material content in the – High thread laminate tensile forces
Blind stitch – No bobbin – Possible (Kl. 103) thread stitching in a necessary solid tool – Large thread – Lower layers demand are not fixed – Low efficiency
One side stitch
– No bobbin thread necessary – High fibre consumption
Tufting
– Very high – No efficiency with interlacing, optimal thus no placement in joining the laminate between the (no loops) single layers
Fibre disorientation
– Lower single layers are not influenced by the stitching process – Local thread concentration, therefore, high shear
Compression
– Compaction of the layers not adjustable – Due to precompaction slight deviation of needle possible
– Good shear – Material – Seam width strength of penetration with influences the layer package 2 needles material in a – Different larger area orientation of 3D-reinforcement – Low thread strength – Thick needle necessary, thus movement in the laminate
– Thread strength (joining) insufficient for insertion of compaction force – Low fibre material content in the laminate
automatic sewing machines, which are generally equipped with the lock stitch. Figure 3.7 shows a two-dimensional (2D) plant for the manufacture of plane preforms, so-called textile reinforcements. The second type consists of the so-called sewing robots, which can be flexibly equipped with different stitching heads. These sewing robots have been developed especially for structural stitching in the spatial (3D) direction. Figure 3.8 shows the use of a flexible robot system for structural stitching as a gantry system with a suspended sewing robot.
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3.4 Stitching heads and schematic diagram of stitch formation using blind stitch (KSL, Lorsch; left: blind stitch RS 510, right: double needle RS 530).
3.5 Schematic diagram of stitch formation using one-side stitch and micrograph (ITA, RWTH Aachen).
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3.6 Tufting head and schematic diagram of stitch formation during tufting (KSL, Lorsch; tufting RS 522).
Table 3.4 shows more clearly the selection of the particular stitch types and sewing threads which have to be used, depending on the required seam type and task (Ogale, 2007).
3.4
Quality aspects for structural stitching
The sewing thread is inserted into the laminate oriented widthways, so positioned vertically to the in-plane reinforcement fibres. In order for the sewing thread to
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3.7 2D sewing plant for manufacturing of plane preforms (IVW, Kaiserslautern).
3.8 Flexible robot systems for structural stitching (KSL, Lorsch).
achieve the required effect of structural stitching, it is necessary to insert into the textile structure an adequate filament diameter in the form of bundled single filaments, which should be reinforced. Thicker sewing threads (filament bundles, as shown in Fig. 3.2) are normally used to keep the number of stitches per unit area low. When the needle penetrates the textile (NCF) the in-plane filaments are moved aside and the sewing thread is inserted. An ellipse is formed around the stitch hole due to the movement of the in-plane fibres. The dimensions of the ellipse depend upon the set-up of the layer orientation, the sewing thread, the stitching direction, and the machine parameters. It is possible to characterise the ellipse by measuring its half-axis. The real ellipse area has approximately 65%
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Table 3.4 Seam type related applicability of stitch types and threads Seam type
Stitch type
Thread applicable
Fixing and positioning
Modified lock stitch
Thin, flexible, high elongation, polyester Medium thick, less elongation, polyester Medium thicker, polyester Thicker polyester, Nomex®, or Kevlar® Thick, less elongation, polyester, Kevlar® Thick, polyester Polyester, Nomex®, or Kevlar® Polyester, Kevlar®, glass Carbon, glass, Kevlar® Carbon, glass, Kevlar® Carbon, glass, Kevlar®
Chain stitch
Assembling
Blind stitch Modified lock stitch Chain stitch One-side stitch Blind stitch
Structural
Tufting Modified lock stitch One-side stitch Tufting
of the area of an ideal ellipse, as calculated from the minor and major ellipse axis values measured from the laminate (Ogale, 2007). For the purpose of analysis, the stitch hole can also be modelled as a diamond shape of which the longer axis is oriented in parallel with the fibre direction of the individual layer (Roth, 2005; Heß et al., 2007). Figure 3.9 shows the top view of a stitched structure with the specification of the minor and major half axis. The set-up of the laminate has a major influence on the formation of the ellipse. The longer (major) half-axis of the ellipse is always positioned in the direction of the fibre orientation. If the stitching direction is running vertically to the fibre direction, a maximum expansion can be observed. The closer the stitching direction is to the fibre orientation, the lower the in-plane fibre undulation will be. This means that, in the case of a real laminate, each layer has a different orientation in fibre undulation, due to the variable laminate set-up. Figure 3.10 shows a fourply set-up, as well as a micrograph of the respective laminate. The fact that the fibre undulation is different in each laminate layer results in there being a loss of the in-plane mechanical properties, which should not be neglected. The formation of the stitch hole expansion depends directly on the type of sewing thread used. Figure 3.11 shows the ellipse formation (top view) and the stitch hole expansion in the cross-section of the laminate. A comparison between a flexible polyester thread and a very rigid carbon fibre sewing thread is shown in Fig. 3.11. The larger stitch expansion and, thus, the ellipse enlargement for the rigid carbon fibre thread is clearly visible. The optimised adjustment of the sewing machine parameters, especially the balance
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3.9 Ellipse formation in the laminate (top view) with half axis.
3.10 Influence of the layer set-up on ellipse formation in the laminate.
between needle and bobbin thread tension, also have a major influence on the quality of the seam. Figure 3.12 shows the impact of thread tension being too high, and compares this to an optimised machine parameter. The stitch hole expansion increases and the knot placement of the needle and bobbin thread moves into the laminate and generates additional fibre undulations in the intermediate layers. A further aspect that is directly influenced by ellipse formation is the surface quality of the laminate. A local compaction of the reinforcement fibres and visible fibre undulations in the upper laminate layer (Figure 3.13) occurs due to the penetration of the needle and the insertion of the needle threads. This leads to matrix-rich areas at the surface and the formation of sink marks due to the hardening of the used matrix macroscopic. After the preform has been placed in
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3.11 Influence of the thread material on ellipse formation.
3.12 Influence of the machine parameters on ellipse formation.
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3.13 Surface characteristic due to fibre concentration (left) and matrix shrinkage (right).
3.14 Reduction in ellipse size by preform compaction.
the injection mould, the preform is normally compacted to the final height required to reach the necessary fibre volume content. Figure 3.14 demonstrates the influence of preform compaction on ellipse formation, based on textured and twisted multifilament threads. It is possible to reduce the ellipse area by a factor of three to five depending on the sewing thread used. The compression force that is necessary in order to reach the required fibre volume content depends on the sewing thread used and the sewing machine parameters selected. Figure 3.15 clarifies this by using a
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3.15 Influence of stitch density on compaction pressure to reach a defined fibre volume content.
four-layer NCF [0/90]4. The layer set-up was stitched at three different stitch densities (13.33 and 3.33 stitches per cm2 and unstitched) with a twisted multifilament thread and high thread tension. Up to a fibre volume content of approximately 55%, no significant influence was recognised. A reduction in the compaction force necessary to reach a high-fibre compaction can be seen when using a very high stitch density and a high thread tension. An increase in the compaction force must be accepted when using lower stitch densities or low thread tension. This results from the fibre nesting being hindered by the inserted needle thread during compaction. This behaviour is unique to an NCF under the conditions described, and cannot be directly transferred to other lay-ups or other textiles, such as woven fabrics. A very high pre-compaction of the layer set-up during the stitching of structural seams can create flow channels at the surfaces of the preform due to insufficient compaction behaviour in the mould. These influence the injection flow in such a manner that the injected resin runs first of all along the seams and from there impregnates the reinforcing fibre structure. Figure 3.16 shows the change in flow front geometry in a biaxial NCF lay-up influenced by a stitch density of 3.33 stitches per cm2. Due to the fact that structural stitching is only used locally within a complete structure, this effect must be classified as negative. Flow channels are hardly used in the form of desired flow assistance due to their inadequate reproducibility.
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3.16 Influence of stitching on the flow front of a biaxial NCF (left: unstitched; right: stitch density 3.33 stitches per cm2; fibre volume content 50%).
3.5
Applications and future trends
At present, the use of structural sewing of NCFs is not widespread. The first applications of this technology can be found in aviation (Weimer, 2003), e.g. in the Airbus A380 pressure bulkhead or very fast rotating fan wheels. Structural sewing is used in these applications to improve the assembly properties of preforms, as well as the material properties of the component. From the component perspective, the essential motivation is the improvement of the impact behaviour and, consequently, the reduction of a delamination risk in the laminates. Therefore, it is the structural components with impact hazard in particular, such as the bottom of fuselages, rail vehicles and automobiles, or structures in airflows, such as turbine blades, the wing-leading edges of aeroplanes, or the blades of wind energy plants, that are predestined for future areas of application. The further development of stitching technology for the structural stitching of NCFs depends greatly on the complete development of stitching technology as the preform technology for liquid composite moulding processes. The machine technology is sufficiently developed. Specific adjustments are expected in the area of sewing thread development. The development of carbon fibre sewing threads, especially if achieved in combination with CFRPC compatible sizings, will serve to increase the effectiveness of NCFs. A further area of minor investigation is the influence of stitching on the actual impregnation process. Until now, there have been only publications on the influence of stitching on the flow of resin in fabrics (Rieber and Mitschang, 2010) but in principle these results should be transferable. By discovering new knowledge in the area of the structural effects of stitched NCFs, conclusions can be drawn about the optimal specifications such as stitch density, fibre diameter and others. General information for the use of the stitching technology in the preforming and structural application of NCFs is published in Long (2005), Beier et al.
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(2006) and Weimer and Mitschang (2001). Direct textile technologies for the manufacturing of 3D reinforcement structures are investigated by Tong et al. (2002). An excellent summary of the state of the art of structural stitching can be found in Ogale (2007).
3.6
References
Beier U, Fischer F, Sandler J K W, Altstadt V, Weimer C and Buchs W (2007), Mechanical Performance of Carbon Fibre-Reinforced Composites Based on Stitched Preforms, Composites Part A, 38, 1655–1663. Culimeta (2010), Technical yarns. Available from http://www.culimeta.de/technical-yarns. php (Accessed 26 January 2010). Drechsler K (2000), Advanced textile structural composite – needs and current developments, The Fifth International Conference on Textile Composites. 18–20 September, 2000, Leuven, Belgium. Herkt M, Middendorf P, Less C, Riedel W, Schouten M and Drechsler K (2006), 3-D reinforcement of composite T-joints by means of robot-assisted stitching technology, SAMPE April 30 May 2006, Long Beach, CA, USA. Heß H, Roth Y C and Himmel N (2007), Elastic Constants Estimation of Stitched NCF CFRP Laminates Based on a Finite Element Unit-Cell Model, Composites Science and Technology, 67, 1081–1095. Long A C (2005), Design and manufacture of textile composites, Cambridge, Woodhead Publishing Limited. Mitschang P, Ogale A, Schlimbach J, Weyrauch F and Weimer C (2003), Preform technology: a necessary requirement for quality controlled LCM processes, Polymer and Polymer Composites, 8, 605–622. Ogale A, Weimer C and Mitschang P (2004), Selection of sewing threads for preform manufacturing, Advanced Composite Letters, 13, 145–153. Ogale A, and Mitschang P (2004), Tailoring of Textile Preforms for Fiber-reinforced Polymer Composites, Journal of Industrial Textiles, 34, 77–96. Ogale A (2007), Investigations of sewn preform characteristics and quality aspects for the manufacturing of fiber reinforced polymer composites, IVW-Schriftenreihe, 70. Poe Jr C C, Dexter H B and Raju I S (2002), A review of the NASA textile composite research. Available from http://techreports.larc.nasa.gov/ltrs/PDF/1997/aiaa/NASAaiaa-97-1321.pdf (Accessed 7 October 2002). PPG Fiber Glass (2010), Bobin and texturized yarns. Available from http://www.ppg.com/ glass/fiberglass/products/pages/default.aspx (Accessed 26 January 2010). Rieber G, and Mitschang P (2010), 2D Permeability changes due to stitching seams, Composites Part A, 41, 2–7. Roth Y C (2005), Beitrag zur rechnerischen Abschätzung des Scheibenelastizitätsverhaltens in Dickenrichtung vernähter Faser-Kunststoff-Verbund-Laminate, IVW-Schriftenreihe, 55. Saint-Gobain Vetrotex International (2010), Glass filament yarns. Available from http:// www.vetrotexeurope.com/yarns.html (Accessed 26 January 2010). Schappe (2004), Carbon thread for composite industry by Schappe Techniques. Available from http://www.schappe.com (Accessed 12 December 2009). Tong L, Mouritz A P and Bannister M K (2002), 3D fibre reinforced polymer composites, Oxford, Elsevier.
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Weimer C (1999), Carbon fiber sewing thread for compound fiber and plastics materials, German Patent DE19932842. Weimer C and Mitschang P (2001), Aspects of the Stitch Formation Process on the Quality of Sewn Multi-Textile-Preforms, Composites Part A, 32, 1477–1484. Weimer C (2003), Preform-Engineering: Applied Technologies to Incorporate Part and Process Functions into Dry Textile Reinforcements, Composites Science and Technology, 63, 2089–2098.
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