Beyond the pin-jointed net: maximising the deformability of aligned continuous fibre reinforcements

Beyond the pin-jointed net: maximising the deformability of aligned continuous fibre reinforcements

Composites: Part A 33 (2002) 677±686 www.elsevier.com/locate/compositesa Beyond the pin-jointed net: maximising the deformability of aligned continu...

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Composites: Part A 33 (2002) 677±686

www.elsevier.com/locate/compositesa

Beyond the pin-jointed net: maximising the deformability of aligned continuous ®bre reinforcements Kevin Potter* Department of Aerospace Engineering, University of Bristol, Queen's Building, University Walk, Bristol BS8 1TR, UK Received 15 May 2001; revised 20 December 2001; accepted 7 January 2002

Abstract The ease with which reinforcements can be formed to the shape of the tool has a major in¯uence on the labour required to make composite parts by manual lay-up and autoclave moulding, and thus on the cost of those parts. Woven cloth has been seen as the standard against which the drape and formability of other materials are judged. The emergence of alternative materials and manufacturing approaches, particularly, mechanical rather than manual forming, makes a reappraisal of the bene®ts and limitations of the deformation of woven cloths, a timely issue. This paper seeks to set out such a reappraisal. It compares the manual and mechanical forming processes and considers how the deformation properties of the reinforcement must be made to suit the process. Experimental data is presented to demonstrate that under some circumstances cross-plied unidirectional prepreg gives superior forming characteristics to woven cloth prepreg. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Preform; A. Fibres; B. Defects; E. Autoclave

1. Introduction It has been recognised for very many years that the way in which reinforcements conform to the tool has a major in¯uence on labour costs and on manufacturing quality [1]. There are two separate aspects to the issue of reinforcement conformance. The ®rst is that of the nature of the deformation process(es) and their limiting values, beyond which inhomogeneity of deformation, or some other limiting factor, makes their use unacceptable. The second is that of the loads that are required to generate the deformation and how these loads vary with the level of imposed deformation. These two issues are, of course, intimately connected, but there are advantages in taking them separately. Woven cloth deforms by rotation of the warp and weft tows relative to each other [2]. For the most part woven cloth deforms as if the tows were pinned together at the tow cross-over points, and the deformation can be quite closely modelled by the assumption that the cloth acts as a pin-jointed net (PJN). Assuming a starting point of ^458 any deformation results in a reduction of the area of the sheet of cloth being deformed [3]. For starting angles other than ^458 deformation can initially lead to an increase in area. Components based on such deformations have been * Tel.: 144-117-928-9155; fax: 144-117-927-2771. E-mail address: [email protected] (K. Potter).

reported [4], but are rarely used and only the case of a ^458 baseline will be considered here. Other deformation modes have different characteristics, for example pure shear of unidirectional (UD) prepreg leads to no change in area and transverse stretching of UD prepreg to an increase in area. The scissoring mode for a dry woven cloth initially results in deformation at almost zero load. The load rises very rapidly as the interaction between warp and weft increases and the ®bres are said to `lock' together. Up to the point of `locking', the deformation is essentially totally reversible as minimal slippage of tows past one another will have occurred. Thus in manual lay-up, dry woven cloth is easily formed with only very low loads before the material's deformation limits are reached. The very rapidly rising load curve at the limiting deformation gives excellent feed-back to the operator, such that coverage of the tool without generating wrinkles or other defects does not require extreme levels of skill or training. When cloth is prepregged, the stage of the deformation in which loads are very close to zero disappears and more load is required to generate the deformation at any strain level [5]. However, the very rapid rise in load at ®bre lock is unaffected by the prepregging. In some ways, prepregged woven cloth is more dif®cult to conform to a tool than dry cloth, the tack of the prepreg making slip between the prepreg and the tool (or previously laid-up prepreg) dif®cult or impossible. Even so, a misoriented ply can still be removed from the tool surface

1359-835X/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 1359- 835 X( 02) 00 014-3

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Fig. 1. In¯uence of deformation at tow level. (a) If shear is fully reversed, the length of each ®bre path is identical and buckling may be avoided. (b) If shear is not reversed, an excess ®bre length on the inner radius must lead to ®bre buckling.

and another attempt can be made to position the ply without it having to be scrapped. By contrast, a ply of UD prepreg is far more dif®cult to conform to a tool surface than a ply of woven cloth. Two deformation modes are available: these are transverse stretching and shear between the tows. In addition, if the tow trajectories require in-plane curvature then some shear within the tow is also required. Unless this shear is fully reversed (see Fig. 1) local buckling/crimping of the tows must occur. This is, of course, also true for tows in woven cloth, but the inherent crimp tends to disguise the effect in that case. The shear deformation mode is many times stiffer than the transverse stretching deformation mode [6] and neither deformation mode can support more than a few percent strain (for continuous ®bre prepreg) without substantial strain inhomogeneity becoming apparent. When deformation is applied to single plies of UD prepreg, in directions not controlled by ®bres, the load rises to a maximum and then peaks and falls away as strain localisation occurs, so that deformation is irreversible. Handling materials with this characteristic requires far more skill from operators than the use of woven cloth. 2. Deformation properties The study of the deformation of woven cloths and other reinforcement forms has been dominated by considerations of how the materials deform either in ¯at format or over simple shapes. The great bulk of the latter work has looked at deformation over volumes of revolution such as hemispheres, hemispherically capped cones, discs and oblate spheroids, etc. [7±10]. Only a relatively small amount of work has been reported on shapes of practical signi®cance such as real component geometries [11]. As it happens, the most studied class of forming geometries is an excellent match to the deformation characteristics of woven cloth. It is simple enough to de®ne a geometry that it would be more dif®cult or even impossible to drape a piece of woven cloth over (see Fig. 2), but such a deliberate challenging of the limits of woven cloth deformations appears not to have been reported. The dominance of woven cloth as the most widely used deformable reinforcement is therefore a combination of some extremely useful properties; such as reversibility of deformation and good tactile feedback when used in manual

Fig. 2. Geometries such as this will be more challenging to cloth drape than volumes of revolution.

forming operations, and some essentially adventitious reasons such as the excellent performance of woven cloths when used to drape surfaces of revolution. When considering mechanical forming rather than manual forming, some of the bene®ts of woven cloth become much less important and some of the bene®ts become liabilities. For example, the reversibility of deformation has no advantage when forming is carried out mechanically. Equally, the sudden rise in load as the tows begin to interact is a mechanical effect; any tows that are severely distorted will provide a driving force for the relaxation of the deformed shape. In addition, in manual forming, the rise in load may indicate an initial imperfection of alignment or positioning which can be corrected. In mechanical forming, if the tows become locked, either the forming load will be suf®cient to damage tows in forcing the reinforcement into the desired shape, the cloth will become folded or wrinkled, or the forming will not achieve the speci®ed geometry. Lastly, when carrying out manual forming it is a simple, although time-consuming, matter to cut out of the reinforcement pattern any areas that are beyond the forming limits and to add local reinforcement to recover the strength in that area. When mechanical forming is being carried out, the target may be to take a stack of plies of prepreg and form them into the required shape; the option of cutting out small, hard-to-form areas is not then realistic. In this case, reinforcement forms that have a sudden cut-off in their formability are not as attractive as forms in which additional formability might be traded for some local reduction in mechanical performance. When mechanical forming of multiple plies is being used, the intent is to deform a stack of plies either into the ®nal component geometry or into a sub-set of that geometry. In the latter case, the formed ply stack might be stored for some time prior to being assembled into a lay-up kit that will be used for the assembly of the ®nal component geometry. The ®nal component geometry may be made up of multiple mechanically formed ply stacks and may also require the manual forming of some additional plies. This approach is quite common in the manufacture of RTM parts [12] and may make a substantial contribution to the overall positive economics of that process. The requirement to build up a component from a multiplicity of formed ply stacks places some additional constraints on the desired deformation behaviour of the reinforcements. The two principal constraints are that the formed ply stack should not be

K. Potter / Composites: Part A 33 (2002) 677±686

Fig. 3. Springback of cloth prepreg formed to a hemisphere at 50 8C.

subject to relaxation of the geometry during storage and that the out of plane shear required to accommodate forming around a radius should not lead to buckling of the ®bres on the inside surface. If the ply stack is being formed to a double curvature then additional in-plane shear between the layers of reinforcement may be required, depending on the lay-up of the stack. Any woven cloth that is deformed such that the warp and weft begin to interact will have a tendency to relieve that deformation on the removal of forming loads. For dry reinforcements the relief is essentially instantaneous, although not complete as irreversible yarn compaction can occur. It has been shown that the use of a heat activated powder binder can `set' the deformation of dry cloths and such binders are widely used in the manufacture of RTM preforms. Conventional preimpregnated woven cloths exhibit a lower tendency to relax than dry cloths when deformed at room temperature (RT). Even so, the level of near instantaneous springback on removal from the former may be suf®cient to cause severe problems of ®t between multiple formed ply stacks (see Fig. 3). With time, additional springback occurs, further degrading the desired geometry. Part of this additional springback arises from the nature of the matrix resins usually used for the manufacture of composites. In order to optimise a prepreg for manual lay-up it is necessary to ensure that the material can be tacked to the tool or previously laid up reinforcement. Tack has been shown to be a bulk property, controlled by the viscoelastic response of the matrix, rather than being simply a surface property [13]. This viscoelasticity of the matrix provides an additional restoring force on the deformed geometry in the short term. It might be assumed that the matrix lubricates shear between the tows, then helps to resist their return to the original state. However, in practice, the resin's viscoelasticity provides a `memory' of the undeformed state and leads to a partial recovery of this state, especially when the formed ply is free-standing rather than tacked to a tool surface. When the prepreg is heated there is a great reduction in the resin viscosity and a corresponding reduction in the forming loads to impose a required shape. At least as importantly the elevated temperature permits a more rapid relaxation of the stresses within the matrix, resulting in much less springback from the formed shape. The level of springback shown in Fig. 3 can be substantially reduced by the use of higher

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Fig. 4. Cross-plied UD prepreg formed to a hemisphere at 50 8C shows a reduced springback compared to woven cloth prepreg (cf. Fig. 3).

temperature forming. Whilst tack can be eliminated, if required, by treatment of the prepreg surface the basic viscoelasticity of the matrix would remain unless the matrix formulation were to be modi®ed. When cross-plied lay-ups of UD prepreg are deformed it is seen that their deformation is globally well modelled by the same PJN assumption that is used for woven cloth. Locally, more lateral slip between tows is seen in regions of rapidly changing ®bre angle for UD prepreg than for cloth, as would be expected since stacks of UD prepreg lack any ®xed kinematic link between the ®bre directions. Thus it might be expected that stacks of UD prepreg at 0.90 orientations would show a lower level of springback than that seen with woven cloth prepreg and this is indeed the case, see Fig. 4. Thus, the ®xed kinematic linkage in woven cloths tends to increase the level of springback from the formed state, and may also limit the ability of the reinforcement to form over some geometries without damaging bridging or ®bre wrinkling. Taken together these factors indicate that, in the context of mechanical forming of plies or ply stacks, the presence of a ®xed kinematic link appears, on balance, not to be ideal. This statement is at variance with the general experience of woven cloth as a very deformable material and UD prepreg as being very dif®cult to make conform to a doubly curved geometry. For this reason, a short series of experiments was carried out to compare the formability of cloth prepreg with that of 0/90 pairs of UD prepreg. 3. Forming trials The forming trials utilised vacuum forming to provide the driving force, and a forming tool as shown in Fig. 5. The shape of the vacuum form tool was selected for various reasons: 1. The combination of internal and external corners is a much more challenging forming regime than a volume of revolution. 2. Localised pan-down features are commonplace in composite panels, e.g. for fastening points, etc. so that this geometry is relevant to practical applications of composites.

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Fig. 5. Geometry of tool for vacuum forming trials.

3. The shape allows the sensitivity of forming to be established for 0/90 and ^45 lay-ups. 4. The ¯at upper surface will tend to constrain some potential tow paths in a similar way to that which would be seen in many real components which were to be mechanically draped. A vacuum form tool was fabricated from medium density ®breboard, to the geometry shown in Fig. 5. The in-plane dimensions were chosen to ®t within the available vacuum forming equipment and the height was selected as representative of many composite parts. The cutout sizes were selected to give a range of aspect ratios within one form tool. The woven prepreg had a tow width of about 0.5 mm, whereas the UD prepreg had a tow width of about 4 mm, although the tow boundaries are not very pronounced in the UD material. It was considered that the cutout sizes should be large enough that the structure of the reinforcements, in terms of tow dimensions, would not be a major in¯uence on the forming. The form tool was sealed, varnished and coated with release wax. However, in initial trials it became clear that the tacky prepreg would stick to the waxed form tool, both preventing free movement of the preform and making it impossible to remove the formed prepreg without damage. To overcome these limitations, the prepreg charge was covered in a layer of thin (0.125 mm) polyethylene ®lm, which was, in its turn, dusted with (30 mm) nylon 6 powder to eliminate all tack and provide some lubrication. The surface of the vacuum former was also dusted with nylon powder. These modi®cations permitted the formed samples to be removed from the vacuum form tool without damage. Under the forming conditions used, the polyethylene ®lm formed eaxactly to the form tool immediately on application of the vacuum. It is therefore assumed that the polyethylene ®lm was not having a major in¯uence on the formability of the reinforce-

ments. The vacuum forming machine used was a commercial model (C.R. Clarke (UK) Ltd, Thermoforming Centre 911). Heating was achieved using ceramic radiant elements of the same dimensions as the samples to ensure a uniform temperature. 4. Forming procedure The forming procedure was the same in all cases. 1. Place form tool and reinforcement blank in vacuumforming machine, taping a thermocouple to the underside of the prepreg in one corner. 2. Bring in heating element and heat to the required temperature. 3. Remove heating element and put in place 0.125 mm thick LDPE ®lm as a vacuum barrier. 4. Bring temperature of prepreg back to the required temperature (this procedure was adopted to avoid overheating of the LDPE ®lm). 5. Pull vacuum as soon as the temperature has reached the required level, and observe deformation of prepreg blank. 6. Allow to cool with the vacuum released and remove from the former. 7. Examine the quality of forming. The quality of the forming was considered at the following positions for each sample. 1. The outer 20 mm radii (B). 2. The 40 mm deep 40 mm internal radius cutout (40/40) (C). 3. The 20 mm deep 40 mm internal radius cutout (20/40) (D). 4. The 30 mm deep 30 mm internal radius cutout (30/30) (E).

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5. The 20 mm deep 30 mm internal radius cutout (20/30) (F). 6. The 5 mm base radius. The prepreg blank size was the same in all cases at 270 mm £ 270 mm, positioned centrally on the vacuum form tool. Several trials were carried out for most of the sample types to ensure that the results were consistently observed. 5. Materials tested 1. Five harness satin glass cloth/916 prepreg (5HS/916) of 493 gsm, approximate single ply cured thickness 0.23 mm. 2. UD glass ®bre/913 prepreg (UDG/913) of 274 gsm, approximate single ply cured thickness 0.13 mm (tested as 2-ply ˆ 0.26 mm). 5.1. Sample type 1 5HS/916, tested as a single ply, aligned at ^45 to the sides of the former, formed at 55 8C. Overall, forming incomplete with none of the cutout regions fully formed, nor fully forming into the base radius. The worst forming was in the 30/30 cutout, followed by the 40/40, the 20/30 and the 20/40. In no case did the forming come close to giving a vertical wall in the cutout regions or to forming into the base radius in the cutout region. Each of the outer 20 mm radii showed some wrinkling of the cloth. 5.2. Sample type 2 5HS/916, tested as a single ply, aligned at ^45 to the sides of the former, formed at 75 8C. Overall, the forming was better than for sample 1. However, the forming was incomplete for the 30/30 and 40/40 cutouts. As before, the 20/40 cutout was the best and this was close to being completely formed, with the 20/30 cutout being slightly worse. In this sample, the base radius was well formed in most areas (outside the cutout regions); however, the improvement in forming this radius was associated with an increase in the level of wrinkling at the external 20 mm corners. 5.3. Sample type 3 UDG/913 tested as a 2-ply stack at ^45 (cold preconsolidated 1 h under vacuum), formed at 55 8C. Overall, this sample formed better than either samples 1 or 2, showing only slightly incomplete forming into the cutout regions, however, the level of wrinkling at the external 20 mm corners was slightly worse than for samples 1 and 2.

Fig. 6. Sections, at the locations noted, through formed cloth prepreg aligned at ^45 to the tool edges.

5.4. Sample type 4 UDG/913 tested as a 2-ply stack at ^45 (cold preconsolidated 1 h under vacuum), formed at 75 8C. This sample was fully formed into all the detailed geometry of the internal cutout regions and base radii. However, wrinkling was still evident at the 20 mm external corners. Fig. 6 shows the forming quality of woven cloth sample type 2 by a consideration of sections on the centreline of each cutout. For comparison, the UD prepreg cross-ply at ^45 was formed very close to the tool geometry. The cross-sections were measured as soon as possible after forming to avoid the in¯uence of springback. 5.5. Sample type 5 5HS/916, tested as a single ply, aligned at 0.90 to the sides of the former, formed at 75 8C. This sample formed well only on the external 20 mm radii where extensive deformation occurred and wrinkling was largely eliminated. All the internal cutout regions were incompletely formed. Only in the 40/40 case did the prepreg come in contact with the form tool surface within the con®nes of the cutout region. Even in this case the prepreg did not approach a complete forming to the base radius. The 20/40 cutout showed more bridging, with the 30/30 worse and the 20/30 worst of all. It should be noted that this order of the forming quality in the various cutouts is completely different to that for the ^45 case (samples 1 and 2). 5.6. Sample type 6 UDG/913 tested as a 2-ply stack at 0.90 (cold preconsolidated 1 h under vacuum), formed at 75 8C. The forming into the cutout regions was tighter to the form tool than for the woven cloth, the 20/40 and 40/40 cutouts were much better formed than in sample type 5. However, the 20/30 and 30/30 cutouts were still bridged by ®bre. The external 20 mm corner radii were not as well formed as in sample type 5 and some wrinkling could be seen. It was noted that the outer edges of the two layers of prepreg in the UD prepreg forming cases (sample types 3,

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Fig. 7. Formed cloth prepreg and cross-plied UD prepreg, in unmodi®ed and modi®ed (with inter-ply resin layer) conditions, all samples at 0.90 to the tool edges.

4 and 6) remained essentially coincident after the forming. That is to say that very little slip occurred between the plies. Three further sample types were made up to increase the potential for slippage between the plies and examine the effect of this on the worst forming case (0.90 to form tool edges). 5.7. Sample type 7 UDG/913 tested as a 2-ply stack at 0.90 without any preconsolidation, formed at 75 8C. The forming into the cutout regions was generally similar, although somewhat better, than in sample 6, but the level of wrinkling on the external corners was very much lower. Some slight slippage between the plies could be seen in the corners of the formed prepreg, over a distance of 1 mm or less. 5.8. Sample type 8 UDG/913 tested as a 2-ply stack at 0.90 with a layer of 914 resin ®lm between the plies to act as a lubricant (no preconsolidation), formed at 75 8C. This sample was essentially fully formed into the 40/40 and 20/40 cutouts, very close to being fully formed into the 20/30 cutout and much better formed into the 30/30 cutout than any other sample. In addition, no wrinkling was evident on the external corners. Some slippage between the plies could be seen in the corners of the formed prepreg, over a distance of up to about 5 mm. 5.9. Sample type 9 UDG/913 tested as a 2-ply stack at 0.90 with a dusting of 30 mm nylon 6 powder between the plies to eliminate any tack at RT and decouple the two plies (no preconsolidation), formed at 75 8C. Overall, the forming was similar to that seen in sample 7; however, the forming into the 20/30 and 30/30 cutouts was rather poor, the plies separated in these cutouts, with the top ply being heavily creased by the LDPE vacuum ®lm. Better

forming might have been obtained using a thicker, rubber, face sheet than the thin and easily wrinkled LDPE sheet. A greater level of slip was seen between the plies than in sample 7. Whilst the prepreg tack was eliminated at RT by the powder dusting, the two plies were seen to be well bonded together after forming, in the regions where the vacuum pressure was reacted by the tool. Lastly, two plies of UD prepreg with a single ®bre direction were tested in the same way (0 direction along the edge of the form tool) and gave a deformation pattern with similar characteristics to sample 7 overall, in so far as the forming of the cutouts was superior to that of the woven cloth. However, there was clearly more localisation of the forming, leading to a more defective geometry. It seems clear from this that the two ®bre directions in the 0/90 samples did provide some mutual support, assisting in providing a good forming quality. Fig. 7 compares the forming quality of woven cloth prepreg and unmodi®ed and modi®ed (with the addition of an unreinforced resin layer) UD prepreg cross-plies at 0±90 to the tool edges. All results are compared in Table 1. There is a very clear difference in formability when the reinforcements are tested with the tows at ^45 and 0.90 to the edges of the tool, and the order of forming quality with regard to cutout dimensions is quite different. An attempt can be made to understand this by a consideration of some of the geometrical features of the cutouts. Table 2 lists various features of the cutouts. These are the ratio of height to the depth of the cutout; the ratio of the height to the width of the cutout; the ratio of the depth to the width of the cutout and the mean increase in surface area required to form the cutout region if no material were to be drawn in from surrounding areas. The order of quality at ^45 (best to worst) is 20/40, 20/30, 40/40 and 30/30. From Table 2, it can be seen that the greater the required increase in area the worse is the forming behaviour for the woven cloth. This is hardly surprising, as the PJN deformation mode results in a decrease in the surface area of the formed region. An examination of grid lines drawn on the sample surface shows that the deformation is restricted

Complete Complete Close to complete Incomplete very poor Close to complete Incomplete very poor Complete Close to complete Complete Complete 2 7

Complete Incomplete

Incomplete Complete Complete Complete Complete Complete Complete Incomplete Incomplete Slightly incomplete Complete Incomplete very poor Incomplete poor Incomplete poor Incomplete very poor Incomplete Slightly incomplete Complete Incomplete poor Incomplete poor Incomplete poor Incomplete Close to complete Slightly incomplete Complete Incomplete Close to complete Close to complete Incomplete poor Incomplete Slightly incomplete Complete Incomplete poor Incomplete Incomplete Wrinkled Wrinkled badly Wrinkled badly Wrinkled badly Complete unwrinkled Complete slight wrinkling Complete 5 4 3 1 9 8 6

5HS ^45 (55 8C) 5HS ^45 (75 8C) UDG ^45 (55 8C) UDG ^45 (75 8C) 5HS 0.90 (75 8C) UDG 0.90 (75 8C) UDG 0.90 (75 8C) unconsolidated UDG 0.90 (75 8C) plus resin ®lm UDG 0.90 (75 8C) tack eliminated

Base radius F (20/30) E (30/30) D (20/40) C (40/40) B (20) Rating

Forming quality at various points Sample type

Table 1 Collected results from forming trials (The rating for forming quality is a subjective assessment of the overall ®t between form tool and formed material. Sharp discontinuities and wrinkles within the cutout regions are rated more severely than smooth but incomplete ®lling of the cutout regions)

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to the cutout regions with little or no deformation on the top surface of the form tool. Material can be drawn in from the free edge, but not to a suf®cient extent to overcome the required increase in overall area. The order of quality at 0/ 90 (best to worst) is 40/40, 20/40, 30/30 and 20/30. Again, from Table 2, it can be seen that the greater the ratio of height to width the worse is the forming behaviour. In this case forming must be associated with a drawing in of material from the top surface of the form tool into the cutouts. It is unsurprising, therefore, that the cutouts requiring the greatest drawing in should show the worst forming. This drawing in can be seen quite clearly from the grid lines drawn on the sample surfaces. It is clear from these tests that the maximum formability is achieved when the kinematic link between the two ®bre directions is severed. Cross-plied UD prepreg forms better in all cases than the 5 harness satin glass cloth prepreg (5 harness satin is generally considered to set the standard for formability of cloth reinforcements). The level of interaction between the layers in the UD cross-ply can be modi®ed to further improve the formability. However, having a minimal level of interaction between the plies is not bene®cial to formability, under the forming conditions reported here. It should be noted that all the experiments carried out and reported earlier imposed a very low level of constraint on the forming from the LDPE vacuum ®lm and no guidance of the deformation (e.g. via the use of blankholders) was attempted. The intent was to consider the worst-case scenario, to maximise the in¯uence of the material deformability rather than the design of forming equipment. It may be possible to achieve improvements to the formability reported here by the use of guided forming, and/or a thicker and less easily wrinkled face sheet. Equally, the focus here has been on `pure' formability and no consideration has been given to such questions as ®nal ®bre angles, changes in thickness, localisation of forming strains or loss of consolidation, etc. Lastly, the forming tool was relatively small so that ®bre could be drawn across the tool surface to permit it to conform to the cutout regions. This may well not be possible for a larger tool or for other geometries than those considered here. For comparison purposes an attempt was made to lay up UD prepreg onto the form tool in a conventional manner. This proved, as expected, to be very dif®cult with a great deal of tailoring and patching with small pieces of prepreg being required. Laying down a single ply of UD prepreg Table 2 Geometrical features of the cutouts (based on nominal dimensions, i.e. ignoring radii) Cutout

Height/depth

Height/width Depth/width

Area increase (%)

20/30 30/30 20/40 40/40

1 0.67 1 0.5

0.35 0.33 0.29 0.25

18 26 12 22

0.35 0.5 0.29 0.5

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took 20 min, compared to a vacuum forming cycle of less than 5 min for the 2-ply 0.90 cross-ply. Equally, the ®nal lay-up was of rather poor quality and locally very variable in thickness due to the patching. 5. 6. Ply stack forming, requirements for ideal reinforcements Having established that woven cloth prepreg is not necessarily the optimum reinforcement form for the production of complex geometries via mechanical forming it is worth looking in a little more detail at the preferred properties of a reinforcement material intended for use in mechanical forming, especially of ply stacks. Each property requirement is stated and followed by a short discussion of the current status. 6.1. Preferred properties The given list is not necessarily in order of importance, as different properties will be more important in some processes or with some geometries. A ranking order could be produced for a speci®c set of circumstances, but would not be universally valid. 1. Rapid forming around radii without out of plane wrinkling. To avoid out-of-plane wrinkling in rapid forming at RT, dry reinforcement must be used to permit unlimited inter-ply shear. At elevated temperatures, conventional prepreg gives adequate results although inter-ply lubrication permits higher forming rates or lower temperatures to be used. 2. Zero springback on forming around a radius. Springback on radius is minimised when forming is carried out hot, with very slow RT forming the next best thing. Tack eliminated or lubricated UD prepreg stacks can be formed rapidly at RT but suffer from high springback (unless formed shape is held post-forming). 3. No ®xed kinematic link. Woven cloths have a ®xed kinematic link that is of great bene®t in manual layup. In mechanical forming the ®bre locking can impact very negatively on forming. Unmodi®ed UD prepreg stacks act largely as though they are kinematically linked (i.e. ®t fairly well to PJN model). Non-crimped fabrics (NCF) can have a lower level of kinematic link, this depends on NCF geometry and level of prepregging. Tack reduced or lubricated UD prepreg stacks would be expected to act as though they have a much lower level of kinematic linkage, the level could be controlled by the level of tack reduction or lubrication. 4. No bridging, no tow level wrinkling. In the extreme, bridging could only be eliminated by the use of a reinforcement form capable of extension in the ®bre direction. These can be made from aligned discontinu-

6.

7.

8.

9.

ous ®bres, but the excellent deformation properties are coupled with a reduction in mechanical performance [14,15]. Bridging associated with ®bre locking can be reduced by using a reinforcement without a ®xed kinematic link. No loss of consolidation, minimal further consolidation required during subsequent processing. Severe loss of consolidation is seen in ^45 stacks of UD prepreg tested in bias extension at RT [16]. This loss of consolidation is associated with tow level wrinkling and is worst for rapidly formed samples. Although this wrinkling is very severe in bias extension testing at RT it has not been observed in the hot forming of hemispheres or the more complex shapes considered earlier. This may be associated with the higher temperatures used, and may also be due to the existence of a biaxial stress ®eld in the 3D forming experiments. Woven cloth seems not to suffer from loss of consolidation and the level of the effect with NCF depends on the level of impregnation, the test rate and probably with the stitch geometry. Substantial consolidation during further processing would be needed to overcome any loss of consolidation, increasing the likelihood of consolidation-induced moulding defects. Compatible with all subsequent processes from RTM to vac bag, etc. Woven material and NCF are compatible with a wide range of processes, depending on their impregnation state. Whilst dry forms of UD material are available their deformation characteristics appear not to have been investigated. UD prepreg is only usable in a narrower range of processes. No limitations of layup/orientations of ply stack. Ideally, it should be possible to form stacks of arbitrarily oriented reinforcement (e.g. quasi-isotropic or 0 ^F ). For this to happen, slip must be possible between the plies. This sort of slip will have to be encouraged in some way, by tack reduction/elimination, by lubrication or by high temperatures. For well-tacked together stacks of conventional prepreg only a small amount of misalignment [16] is suf®cient to cause severe wrinkling in test specimens tested under uniaxial conditions. Predictable ®nal ®bre geometry, highly reproducible forming. Simple predictive approaches are only available for materials that behave as PJNs [17,18]. UD prepreg can be modelled but only by complex and time-consuming approaches [19]. Reproducibility of forming operations is largely unknown. Final ®bre angles close match to design requirements. To get ®nal ®bre angles exactly where they should be requires a very good predictive capacity and reproducible forming. In addition for arbitrary shapes it is likely that initial ®bre trajectories would have to be curved. Whilst this could be achieved, it would not be a low cost option and some compromise between as-formed and ideal ®bre trajectories is likely to be needed for the foreseeable future.

K. Potter / Composites: Part A 33 (2002) 677±686

10. No loss of Vf% or other performance factors. Woven cloths will give a slight reduction in Vf% and in mechanical properties compared to UD prepreg in ¯at panels. Some NCFs give rather poor performance (paradoxically, due to the crimp imparted by the stitches). Aligned discontinuous materials give a reduction in Vf% and a few % additional reduction in properties as a result of the short ®bres. The reduction in performance for UD prepreg that has been formed is unknown. No currently available material forms seem to be ideal with regard to the effect of forming on cured properties. 11. No forming-speci®c quality issues (i.e. new defect generation mechanisms). The tow-wrinkling phenomenon seen in RT bias extension forming would have to be thoroughly understood so that inspection and acceptance criteria could be developed. There may be other similar issues with other reinforcement forms or forming processes. 12. No major changes needed in the formulation of prepreg, i.e. no major material requali®cation costs. Some modi®cations to materials and processes may be inevitable, the one-off cost of requali®cation needs to be assessed against long-term gains. 13. No major changes needed in design approach or methodology. Ply stack forming generally means replacing a ply by ply design approach with one based on the use of blocks of plies. It may be possible to develop forming/ materials approaches where ply dropping is possible whilst retaining the advantages of mechanical forming. No current quali®ed material forms appear to meet the ideal preferred property requirements (indeed some of the ideal requirements are probably mutually incompatible). If a ®xed locking angle is to be avoided then woven cloths are excluded. The level of apparent kinematic linkage (i.e. meeting PJN predictions) can be varied from rather high for stacks of conventional UD prepreg, to low levels for some NCFs and lubricated UD prepreg, to none for tack eliminated UD prepreg. The ideal single ply would probably be an essentially UD ply capable of great lateral and shear deformations, but without any ®xed kinematics and with a load/deformation curve that never rises to extreme loads, perhaps having a constant load plateau. Multiple plies of such a material should have a controlled level of bonding with each other, such that the ply stack's forming behaviour can take advantage of the PJN mechanism when this gives bene®cial behaviour, but avoids the limitations of this mechanism to allow improved local forming. A stack of lubricated or tack-limited cross-plies of UD prepreg might represent a suitable starting point for developments. A last series of experiments was carried out in an attempt to validate this conclusion. In this case a four-ply stack of UD glass/913 prepreg was made up at a stacking sequence of ^45, 0, 90 (from the toolface taking one edge of the forming tool as the 08 datum). 913 resin ®lm was inserted between each layer of

685

prepreg, with a thicker layer between the 245 and 0 layers to decouple the ^45 and 0±90 layers. As before, the prepreg was coated with a thin membrane to prevent it sticking to the tool. The sample was mounted on the form tool and a rubber sheet was sealed down to a tool plate on which the form tool was mounted, so that a vacuum could be drawn beneath the rubber sheet. The assembly was mounted in an oven at 80 8C and allowed to heat to that temperature, whereupon a vacuum was drawn under the sheet. In these experiments, more time would be available for forming than in the previous series. The formed assembly was then cured under vacuum pressure only and then inspected for the quality of the forming. For comparison purposes equivalent mouldings were made with the same 5HS glass/916 woven cloth prepreg used earlier, set at 0.90 to the forming tool. The 5HS glass/916 woven cloth prepreg gave exactly the same formed geometry as it did in the trials using a vacuum thermo-former. Despite having four ®bre directions the stack of UD prepreg gave excellent results, with the two 40 mm radius cutouts being essentially perfectly formed and the two 30 mm radius cutouts being very close to fully formed (see Fig. 8). 7. Conclusions The deformability of woven cloth has for many years set the standard against which other reinforcements are measured. These materials largely deform as if they were a PJN and this behaviour has many advantages, especially in manual forming operations. However, the woven structure leads to a sudden cut-off in the deformability that has a deleterious in¯uence on absolute levels of formability and may cause problems in mechanical, rather than manual, forming. Cross-plied stacks of UD prepreg have been shown to give superior forming capabilities to woven cloth in forming over shapes that are more challenging than surfaces of revolution. This superior capability has been ascribed to the lack of a ®xed kinematic link between the plies. It has been shown experimentally that the level of linkage between the UD prepreg plies can be controlled to give an improved forming capability. A consideration of the requirements for ply stack forming con®rms that the approach of modifying the properties of UD continuous ®bre prepreg represents a good starting point for the

Fig. 8. Geometry of cured vacuum formed shapes (viewed from the tool face). Left-hand side: 4-ply 45, 135, 0, 90, modi®ed UD prepreg. Righthand side: single ply of 5HS glass/916 prepreg.

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development of materials with improved performance in mechanical forming. [10]

References [1] Pagliuso S. Composite drapabilityÐa too often ignored impacting cost characteristic. National SAMPE Symposium and Exhibition, 27th, San Diego, CA, May 4±6, 1982, Proceedings. p. 133±40. [2] Mack C, Taylor HM. The ®tting of woven cloth to surfaces. J Textile Inst 1956;47:477±88. [3] Potter K. The in¯uence of accurate stretch data for reinforcements on the production of complex mouldings. Part 1. Deformation of aligned sheets and fabrics. Composites 1979;July. [4] Potter K, Robertson F. Bismaleimide formulations for resin transfer moulding. Proceedings of the 32nd International SAMPE Symposium, Anaheim, 1987. [5] Simon ER, Canavan RA, Murray PJ, Geraghty EO, Bradaigh CM. Hot drape forming of thermoset matrix composites, characterisation and simulation. Proceedings of the Fifth International Conference on Flow Processes in Composite Materials, 1999. [6] Potter K. Deformation mechanisms of ®bre reinforcements and their in¯uence on the fabrication of structural parts. Proceedings of ICCM3, Paris; 1980. [7] Long AC, Rudd CD, Blagdon M, Smith P. Characterising the processing and performance of aligned reinforcements during preform manufacture. Composites, Part A 1996;27A:247±53. [8] Dong L, Blachut J. Investigation of progressive and ultimate failure for woven-fabric composite structures. Proceedings of ICCM12, Paris, 1999. [9] Mohammed U, Lekakou C, Bader MG. Experimental studies and

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