3-D forming of continuous fibre reinforcements for composites 251 been made because this type of simulation requires modest computer effort and is therefore especially suited for optimization processes.In addition to the simulation of the forming processes,the evaluation of design sensitivi- ties is considered as well.Simulation of the mechanical behaviour of CFRTP products is discussed in Section 8.4,although the discussion is restricted to thin-walled structures.As descriptions of the finite element method can be found in many textbooks,this discussion is restricted. However,the evaluation of the required laminate stiffnesses is non-stan- dard and is therefore included in the present chapter.Automated opti- mization is addressed in Section 8.5.Here,optimization is carried out on the basis of a so-called approximation concept.This approach replaces the actual optimization problem by a sequence of simpler approximate opti- mization problems.The main advantages of this approximation concept are: (1)it is applicable without information on design sensitivities and(2)noisy response evaluations can be dealt with. 49 8.2 Forming of continuous fibre reinforced polymers 8.2.1 Introduction With the development of high-performance continuous fibre reinforced polymers,the need for new production processes became clear.Hand lay. up,the most important production process for continuous fibre reinforced thermoset structures,is not suitable for most thermoplastic composites.This and the fact that large numbers produced with the hand lay-up process ur时 cannot lead to cost-competitive products,are the main reasons for the development of new industrial production techniques for composite mate- rials.Some pressing processes have the potential of becoming cheap and fast,and are therefore suitable for mass production. In this section only the rubber forming process will be discussed,since this low-pressure form pressing process seems among the most promising of its kind. 8.2.2 Rubber forming The rubber forming process is a matched die press forming process.One of the dies,male or female,consists of rubber.Figure 8.7 gives a general outline of the rubber forming process.The important stages in the forming process are: preconsolidation (depending on the prepreg type); heating stage (can be included in the preconsolidation stage); forming stage (draping); (re)consolidation stage
been made because this type of simulation requires modest computer effort and is therefore especially suited for optimization processes. In addition to the simulation of the forming processes, the evaluation of design sensitivities is considered as well. Simulation of the mechanical behaviour of CFRTP products is discussed in Section 8.4, although the discussion is restricted to thin-walled structures. As descriptions of the finite element method can be found in many textbooks, this discussion is restricted. However, the evaluation of the required laminate stiffnesses is non-standard and is therefore included in the present chapter. Automated optimization is addressed in Section 8.5. Here, optimization is carried out on the basis of a so-called approximation concept. This approach replaces the actual optimization problem by a sequence of simpler approximate optimization problems.The main advantages of this approximation concept are: (1) it is applicable without information on design sensitivities and (2) noisy response evaluations can be dealt with. 8.2 Forming of continuous fibre reinforced polymers 8.2.1 Introduction With the development of high-performance continuous fibre reinforced polymers, the need for new production processes became clear. Hand layup, the most important production process for continuous fibre reinforced thermoset structures, is not suitable for most thermoplastic composites.This and the fact that large numbers produced with the hand lay-up process cannot lead to cost-competitive products, are the main reasons for the development of new industrial production techniques for composite materials. Some pressing processes have the potential of becoming cheap and fast, and are therefore suitable for mass production. In this section only the rubber forming process will be discussed, since this low-pressure form pressing process seems among the most promising of its kind. 8.2.2 Rubber forming The rubber forming process is a matched die press forming process. One of the dies, male or female, consists of rubber. Figure 8.7 gives a general outline of the rubber forming process. The important stages in the forming process are: • preconsolidation (depending on the prepreg type); • heating stage (can be included in the preconsolidation stage); • forming stage (draping); • (re)consolidation stage. 3-D forming of continuous fibre reinforcements for composites 251 RIC8 7/10/99 8:26 PM Page 251 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:28 AM IP Address: 158.132.122.9
252 3-D textile reinforcements in composite materials press direction Blankholder Prepreg Flat plate with a hole in the middle 8.7 A sketch of the rubber forming process. Heating stage A The laminate can be heated by contact heat(conduction heating)between two hot plates.When this is done with sufficient pressure,pre-consolidation can become unnecessary [5].A disadvantage of the conduction heating 、专sng method is the fact that the laminate actually touches the heating equipment. Consequently,good release agents must be used to prevent sticking of the material to the plates. Convection heating in an oven is also possible,but will usually be time consuming,and the use of inert gas is sometimes necessary to prevent oxidation of the polymer at high temperatures.Inert gas environments are also recommended when using the very fast infrared heating methods.A disadvantage is that for thick sheets temperature gradients develop through the thickness and this will sometimes restrain formability during the forming stage.Radiation heating.,however,is a clean and quick heating method in general.It provides a flexible and a well-manageable heating device. Forming stage When a fabric reinforced thermoplastic laminate is forced in a specific shape by the rubber forming process,the fibre reinforcement has to adjust to that same shape.The continuity of the fibres plays an essential role.In
Heating stage The laminate can be heated by contact heat (conduction heating) between two hot plates. When this is done with sufficient pressure, pre-consolidation can become unnecessary [5]. A disadvantage of the conduction heating method is the fact that the laminate actually touches the heating equipment. Consequently, good release agents must be used to prevent sticking of the material to the plates. Convection heating in an oven is also possible, but will usually be time consuming, and the use of inert gas is sometimes necessary to prevent oxidation of the polymer at high temperatures. Inert gas environments are also recommended when using the very fast infrared heating methods. A disadvantage is that for thick sheets temperature gradients develop through the thickness and this will sometimes restrain formability during the forming stage. Radiation heating, however, is a clean and quick heating method in general. It provides a flexible and a well-manageable heating device. Forming stage When a fabric reinforced thermoplastic laminate is forced in a specific shape by the rubber forming process, the fibre reinforcement has to adjust to that same shape. The continuity of the fibres plays an essential role. In 252 3-D textile reinforcements in composite materials 8.7 A sketch of the rubber forming process. RIC8 7/10/99 8:26 PM Page 252 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:28 AM IP Address: 158.132.122.9
3-D forming of continuous fibre reinforcements for composites 253 Gap between yarns 、 8.8 Schematic drawing of the intraply shear deformation mode. contrast to metals or nonreinforced thermoplastics,continuous fibre rein- forced plastics cannot undergo a deformation with only local adjustment of the material.Local bending or curving of the laminate material will influ- ence the entire laminate.There are several ways in which the adjustments of the fibre reinforcement can take place.These adjustments are often called deformation modes. Although composite products generally are composed of laminates,that is to say of more than one layer,the deformation capacity of one individ- ual layer plays a dominant role in the forming process.These deformations are called intraply deformations.In general,five different deformation modes of a single,flat layer of fabric can be identified,as was shown by Mack and Taylor [6],Robertson et al.[7],Heisey and Haller [8]and Robroek [4]: fibre stretching (elongation of the fibres); fibre straightening (undulation of the woven fibres); 8 intraply shearing (trellis effect of the fibres); intraply slip (sliding of the fibres); bucking (in-plane and out-of-plane buckling). As has already been shown by Robertson et al.[7]and confirmed by Potter [9]and Van West [10],the shear deformation mode(Fig.8.8)is the most important mode for deforming fabrics into 3-D products.Simulations as discussed in the next section must therefore incorporate this deforma- tion mode. Since the shearing dominates the deformation,it is important to know which forces are needed for the shearing.It appears [4]that the forming forces of these fabric reinforced plastics are small.Normally,a laminate con- sists of more than one layer.Such a laminate can be represented as a stack of fibre-rich layers alternated with thermoplastic resin-rich layers.In a ther- moforming process (such as rubber forming),the resin-rich layers are soft- ened by heating and will have a certain viscosity.They will allow the fibre-rich layers to slip with respect to each other when the laminate is bent
contrast to metals or nonreinforced thermoplastics, continuous fibre reinforced plastics cannot undergo a deformation with only local adjustment of the material. Local bending or curving of the laminate material will influence the entire laminate. There are several ways in which the adjustments of the fibre reinforcement can take place. These adjustments are often called deformation modes. Although composite products generally are composed of laminates, that is to say of more than one layer, the deformation capacity of one individual layer plays a dominant role in the forming process. These deformations are called intraply deformations. In general, five different deformation modes of a single, flat layer of fabric can be identified, as was shown by Mack and Taylor [6], Robertson et al. [7], Heisey and Haller [8] and Robroek [4]: • fibre stretching (elongation of the fibres); • fibre straightening (undulation of the woven fibres); • intraply shearing (trellis effect of the fibres); • intraply slip (sliding of the fibres); • bucking (in-plane and out-of-plane buckling). As has already been shown by Robertson et al. [7] and confirmed by Potter [9] and Van West [10], the shear deformation mode (Fig. 8.8) is the most important mode for deforming fabrics into 3-D products. Simulations as discussed in the next section must therefore incorporate this deformation mode. Since the shearing dominates the deformation, it is important to know which forces are needed for the shearing. It appears [4] that the forming forces of these fabric reinforced plastics are small. Normally, a laminate consists of more than one layer. Such a laminate can be represented as a stack of fibre-rich layers alternated with thermoplastic resin-rich layers. In a thermoforming process (such as rubber forming), the resin-rich layers are softened by heating and will have a certain viscosity. They will allow the fibre-rich layers to slip with respect to each other when the laminate is bent 3-D forming of continuous fibre reinforcements for composites 253 8.8 Schematic drawing of the intraply shear deformation mode. RIC8 7/10/99 8:26 PM Page 253 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:28 AM IP Address: 158.132.122.9
254 3-D textile reinforcements in composite materials interply slip 8.9 Schematic drawing of the interply shear deformation mode. by forming forces.This deformation mode is called interply slipping(Fig. 8.9).During this slipping the resin-rich layer must act as a lubricant,other- wise the generated tangential stresses become too high and can cause failure of the laminate bend [11]. Since the pressures needed for the forming are relatively low (<1 bar during forming,<40 bar in the final consolidation stage),tooling materials other than the ones used in a metal,matched die,forming process become feasible.The thickness of the fabric after deforming is mainly determined by the local amount of shear [5].When this thickness variation is obstructed, pooM high pressures on the fabric will occur,which will obstruct the shearing of the fabric.Therefore,the gap between the male and female dies should not be constant in general,and a variation in the thickness must be allowed.A relatively simple way to accomplish this is to use a die made from a rela- 2-0 tively soft material.This material should deform at places where the thick- ness varies.Another way of avoiding high local pressures is by adapting the 周 shape of one of the moulds to the thickness variation.Since the increase in thickness is directly related to the deformation of the laminate,a detailed simulation of the fabric deformation is in that case essential for the design 、 豆 of the dies. Suitable materials for the relatively soft moulds are silicon rubbers,since they can withstand high temperatures for short periods (temperatures higher than 400C).The hardness of the silicon rubber is variable between 55 Shore A and 73 Shore A.A disadvantage of silicon rubber is that it is notch-sensitive.For lower temperature rubber forming processes(temper- ature lower than 250C)it is therefore preferable to use a tougher mater- ial,for instance PU(polyurethane)rubber [5]. The choice for a rubber male of female die depends on the product.The quality of the surface that is in contact with the metal die ranges from tex- tured to super glossy.The surface that touches the rubber die is rough,in general.In most cases this determines which die should be made of rubber. At present most products are made with a metal female and a rubber male die. A disadvantage,related to the use of rubber as male die material,is that the shape of the die itself can change before the consolidation stage of the thermoplastic composite.This easily leads to folds and wrinkles.It can also lead to the unwanted effect of fibre bridging,as shown in Fig.8.10.At places
by forming forces. This deformation mode is called interply slipping (Fig. 8.9). During this slipping the resin-rich layer must act as a lubricant, otherwise the generated tangential stresses become too high and can cause failure of the laminate bend [11]. Since the pressures needed for the forming are relatively low (<1 bar during forming, <40 bar in the final consolidation stage), tooling materials other than the ones used in a metal, matched die, forming process become feasible. The thickness of the fabric after deforming is mainly determined by the local amount of shear [5].When this thickness variation is obstructed, high pressures on the fabric will occur, which will obstruct the shearing of the fabric. Therefore, the gap between the male and female dies should not be constant in general, and a variation in the thickness must be allowed. A relatively simple way to accomplish this is to use a die made from a relatively soft material. This material should deform at places where the thickness varies. Another way of avoiding high local pressures is by adapting the shape of one of the moulds to the thickness variation. Since the increase in thickness is directly related to the deformation of the laminate, a detailed simulation of the fabric deformation is in that case essential for the design of the dies. Suitable materials for the relatively soft moulds are silicon rubbers, since they can withstand high temperatures for short periods (temperatures higher than 400 °C). The hardness of the silicon rubber is variable between 55 Shore A and 73 Shore A. A disadvantage of silicon rubber is that it is notch-sensitive. For lower temperature rubber forming processes (temperature lower than 250°C) it is therefore preferable to use a tougher material, for instance PU (polyurethane) rubber [5]. The choice for a rubber male of female die depends on the product. The quality of the surface that is in contact with the metal die ranges from textured to super glossy. The surface that touches the rubber die is rough, in general. In most cases this determines which die should be made of rubber. At present most products are made with a metal female and a rubber male die. A disadvantage, related to the use of rubber as male die material, is that the shape of the die itself can change before the consolidation stage of the thermoplastic composite. This easily leads to folds and wrinkles. It can also lead to the unwanted effect of fibre bridging, as shown in Fig. 8.10.At places 254 3-D textile reinforcements in composite materials 8.9 Schematic drawing of the interply shear deformation mode. RIC8 7/10/99 8:26 PM Page 254 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:28 AM IP Address: 158.132.122.9
3-D forming of continuous fibre reinforcements for composites 255 Rubber mould Fibre Fibre bridging 7个个个 >F1 Steel mould 8.10 The yarns of the fabric deform the rubber mould. where a large curvature of the surface is found,the fibres of the fabric will try to bridge the corner,resulting in an undefined shape of the product.To overcome this problem,extra rubber can be added locally. If preference is given to a metal male die and a rubber female die,the shape of the product cannot become undefined.Obtaining the proper 5 pressure distribution for consolidation is often the main problem in this 周A case.For mass production the only matching die candidate materials are metals. The function of the clamping device,also referred to as the buckling guide or blankholder,is different from similar metal forming processes. Although it is meant to prevent out-of-plane deformations of the laminate, the primary function of the guide is to make sure that the fibres are under tension during the forming process in order to avoid in-plane,rather than out-of-plane,buckling.Hence the buckling guide should act as a steering device. The forming of the product is governed by the pressure distribution on the laminate.Large local deformations,for instance near corners,can be stimulated by increasing the pressure locally.By using a clamping device the friction forces can be controlled.If the friction forces are not capable of deforming the fabric sufficiently,pins,locking the fabric at certain places, can also be used.Unfortunately,the forces exerted by these pins also result in a tearing of the fabric in the vicinity of the pins.It is therefore inevitable that the pins are placed at positions that remain outside the product,result- ing in extra scrap material. Usually the fabric is placed on a supporting plate(Fig.8.7).The fabric is held at the edges by the blankholder,i.e.the fabric is not supported in the middle.When the fabric is heated by conduction,the hole in the support-
where a large curvature of the surface is found, the fibres of the fabric will try to bridge the corner, resulting in an undefined shape of the product. To overcome this problem, extra rubber can be added locally. If preference is given to a metal male die and a rubber female die, the shape of the product cannot become undefined. Obtaining the proper pressure distribution for consolidation is often the main problem in this case. For mass production the only matching die candidate materials are metals. The function of the clamping device, also referred to as the buckling guide or blankholder, is different from similar metal forming processes. Although it is meant to prevent out-of-plane deformations of the laminate, the primary function of the guide is to make sure that the fibres are under tension during the forming process in order to avoid in-plane, rather than out-of-plane, buckling. Hence the buckling guide should act as a steering device. The forming of the product is governed by the pressure distribution on the laminate. Large local deformations, for instance near corners, can be stimulated by increasing the pressure locally. By using a clamping device the friction forces can be controlled. If the friction forces are not capable of deforming the fabric sufficiently, pins, locking the fabric at certain places, can also be used. Unfortunately, the forces exerted by these pins also result in a tearing of the fabric in the vicinity of the pins. It is therefore inevitable that the pins are placed at positions that remain outside the product, resulting in extra scrap material. Usually the fabric is placed on a supporting plate (Fig. 8.7). The fabric is held at the edges by the blankholder, i.e. the fabric is not supported in the middle. When the fabric is heated by conduction, the hole in the support- 3-D forming of continuous fibre reinforcements for composites 255 8.10 The yarns of the fabric deform the rubber mould. RIC8 7/10/99 8:26 PM Page 255 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:28 AM IP Address: 158.132.122.9