Ellis Developments Limited

Nottinghamshire, United Kingdom

Fabrication and Performance of Stressed Components with Inserts

A Report on the Academic Part of the Mascet Project to develop embroidered textile preforms for composite materials


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C D Rudd, N A Warrior, V Middleton

Department of Mechanical Engineering

University of Nottingham

Summary

The objective of this work was to investigate the use of embroidery technologies for the manufacture of reinforcement preforms for composites structures produced by liquid moulding. The work formed part of the MASCET project under the DTI/EPSRC LINK Structural Composites programme and involved a consortium of industrial companies including end users in the transport industries. The EPSRC programme described here comprised the science base aspect of the LINK project and included studies of the design, processing and performance of composites produced using embroidered reinforcements.

Two embroidery techniques were assessed the Cornely and Schiffli methods. The study encompassed the materials used for embroidery (fibre, yarn and substrate types), the process variables and the architecture of the reinforcement for both preform elements (patches) and near netshape preforms. The effects of the above variables on inplane permeability and compaction of the preform were studied and the same parameters were used to examine structural properties. In all cases the results were compared with conventional, commercially available reinforcement fabrics.

Finite element analysis (FEA) was used in the design of the reinforcement architecture for several technical demonstrators. These included:

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Generic studies of geometric stress concentrations (hole in a plate) and contact problems (in plane and out of plane fastener pull out).

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An automotive space saver wheel, demonstrating reduced weight over the steel equivalent.

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An aerospace generator drive end frame, demonstrating waste reduction and assembly time reduction, plus weight saving over conventional alloy part.

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Patch reinforcement to strengthen safety belt anchorage points in a prototype automotive floorpan.

In conclusion, a constant flow rate, radial, inplane permeability test was developed, and the processing properties defined for several embroidered structures. These were comparable with conventional fabrics. Structural properties of generic embroidered reinforcement styles were defined, and design guidelines developed for further components. The technical demonstrators showed performance improvements with good potential for applications where interlaminar shear failures tend to dominate, such as out-of-plane bending.

1 Introduction

Preform manufacture remains a major difficulty in the industrialisation of composites manufacturing techniques based upon liquid moulding. Assembly by hand of reinforcement preforms is labour intensive and, for complex parts, tends to be wasteful of raw materials. As an alternative to conventional, fabric based technologies, two existing embroidery techniques, Cornely and Schiffli, were assessed for suitability in laying down high modulus reinforcement fibres in near net shape preforms. The close control over fibre architecture offered by embroidery is potentially attractive for highly loaded structures, enabling fibres to be placed at the positions and orientations necessary to tailor strength and stiffness locally, while reducing labour and waste fibres.

Cornely embroidery uses a single needle head with a substrate material held in a pantograph, which is moved under computer control. One application of this technique in the garment industry is the tacking of heavy cords to the substrate by coiling the cord in a wrapping yarn, then stitching the wrapping yarn to the substrate with a chain stitch. Schiffli embroidery uses rows of needles held on a horizontal rack, with the substrate material mounted in a vertical pantograph with local x and y positioning. The primary yarn is passed through the thickness of the substrate and held in place by a second, interlocking yarn at the rear of the work.

Both processes were used for manufacture of local reinforcement for conventional preforms (patches) and for manufacture of complete, near netshape preforms. The project addressed the design, processing and performance of composites structures using these techniques, covering the following major areas:

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The effects of the embroidery process parameters on preform quality, impregnation characteristics and subsequent laminate physical properties. This work was done predominantly using flat plaques.

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The effects of locally modified fibre architecture around point loads and cutouts for a laminated plate subject to inplane and out-of-plane loads. This was based on a series of flat test specimens and was later applied to automotive structures including a floorpan and a road wheel.

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The design and manufacture of net shape preforms with nearoptimal fibre orientations. This work focused on an aerospace generator drive end frame.

2 Characterisation of the Embroidery Processes

Cornely and Schiffli embroidery processes were assessed using several reinforcement styles, stitching yarns and substrate materials. Glass, carbon and aramid fibres were tested as primary yarns, with polyester, glass and aramid as backing and stitching yarns. Fabrics of non-structural polymer yarn construction and glass fabrics and mats were tested as substrates. In the case of glass fibre reinforcements, results were compared with those from a commercially available, non crimp, quasi-unidirectional fabric.

For the Cornely technique, reinforcement tows of linear density 6002400g/km of intermediate modulus carbon or Eglass were found to be suitable. Polyester, aramid and twisted glass wrapping yarns all performed well, though only the polyester and aramid were acceptable as chain stitching yarn due to the smaller radii involved. Glass fabrics, soluble films, and polyester meshes all performed well as substrates; continuous filament glass fibre mats performed adequately only after pre-consolidation to reduce their loft.

For the Schiffli technique, only aramid (Kevlar 29) primary stitching yarns were used successfully. Glass and carbon yarns broke easily due to the small turn radii required. Polymer fabric substrates performed well, glass fabric and pre-consolidated mats also performed adequately, although the latter tended to reduce needle life due to the abrasive nature of the fibres.

A systematic study of the effect of several of the Cornely embroidery parameters upon processing and mechanical properties was made using Taguchi methods. Four parameters were studied, each at two levels and the results of permeability and mechanical property tests were used to optimise the embroidery process. The findings are summarised in the following sections.

In plane permeability

In view of the relatively large number of tests to be made during the project, a novel inplane permeability test method was developed. This was necessary to overcome the poor reproducibility and long lead time associated with measurements of this type. The new method, which offers several important advantages, relies upon the solution of Darcy's law for a constant flow rate, radial flow process. Pressure histories are logged along two orthogonal axes within a cavity, then plotted on a logarithmic time base, yielding a straight line graph from which the permeability tensor may be calculated. This technique may be applied using conventional laboratory benchtop equipment or in situ using a resin metering pump. Such tests were made using a Universal testing machine to drive a variety of surrogate fluids through a free standing test rig and a reaction injection moulding machine was also used to make online measurements during test plaque manufacture. The latter method proved remarkably convenient for rapid characterisation of materials and was in good agreement with other test methods.

The results of the Taguchi study showed the most influential parameter to be the linear density of the reinforcement roving. Higher linear densities produced the highest axial and transverse permeabilities due to the smaller proportion of stitching yarn present. The stitching yarn reduced the porosity for a given reinforcement volume fraction, with a smaller area available for fluid flow. Surprisingly, higher stitch frequencies were also found to increase permeabilities, despite the attendant reduction in porosity. Microscopic examination showed that the coiling stitch introduced crimp channels which were found to assist flow and this effect proved to dominate over that of the porosity.

The interaction between roving filament diameter and linear density was the third significant effect, increasing permeability when the linear density was 2400 g/km, and filament diameter was 17 mm. This effect was due to the packing geometry of the filaments within the reinforcement. In regions where the filaments were closely packed, the larger filaments had larger channels between them, marginally increasing the overall permeability of the reinforcement.

In general, while the measured permeabilities of the Cornely embroidered specimens were of similar magnitude to those of commercial reinforcement fabrics, the embroidered reinforcements exhibited less anisotropy than comparable quasi-unidirectional fabrics. This was attributed to the blocking of longitudinal flow channels by the stitching yarns. Typical (axial) permeabilities for the embroidered samples were around 25% those of the unidirectional fabrics while the transverse permeabilities were similar.

Transverse Compressibility

Compaction tests were made using a compression cage mounted in an Instron testing machine. The embroidered reinforcements performed similarly to conventional fabrics at low pressures ( less than 1 bar). However, at higher pressures the embroidered reinforcements exhibited greater compliance due to the lower stitch tension and absence of powder binder, enabling the tows to spread, providing a higher fibre volume fraction than the fabric at the same compaction pressure.

The evaluation of the effect of embroidery parameters on compaction showed the most significant parameters were linear density and filament diameter, with a higher linear density and larger filament diameter leading to a more compliant reinforcement. The linear density of the roving is important since, as in the case of permeability, this is accompanied by a reduction in the proportion of stitching yarn and a higher porosity for a given reinforcement fraction. Thus the stitching yarns play an important role in determining the achievable reinforcement content in a laminate. Larger filament diameters increased compliance which may be due to the reduced filament count (and lower internal friction) for a given linear density.

Structural Properties

Embroidered materials and control specimens from conventional fabrics were vacuum impregnated using unsaturated polyester resin to provide tensile test pieces. The results showed roving linear density to have the largest influence upon the tensile modulus of quasi-unidirectional embroidered plaques with higher linear density yielding higher modulus. Again, this was attributed to the lower proportion of (non-reinforcing) stitching yarns. For ultimate tensile strength, filament diameter appeared to be the most influential factor, with larger filaments yielding a higher UTS which reverses the usual trend. This effect was attributed to the superior processing characteristics of the preforms made from 17µm fibres.

3     Effects of Fibre Architecture on Structural Performance

Design Analysis

The fibre orientations for the flat test specimens and the demonstrator components were designed with the aid of FEA. The constantly changing fibre path, and therefore the orthotropic material properties, cannot be defined using conventional FE modelling techniques. In the present work, the structure was discretised by first aligning orthotropic elements with the local fibres direction, and then by consideration of the overall geometry. Typically, this resulted in much finer meshes than would be required for isotropic analyses. Obtaining a satisfactory structure therefore became an iterative process:

(i)    Align reinforcement fibres, as far as physically possible, with maximum principal stress directions determined from isotropic FEA.

(ii)    Complete orthotropic FEA using elements aligned with fibre directions, with discrete layers defined by laminate theory.

(iii)     Refine model, by realigning fibres with principal stress planes from orthotropic analysis and recalculating.

Strength was estimated by a factor of safety on first ply failure indicated using the Tsai Wu failure criterion. This required some manual remeshing every time the fibre paths were altered, although there is obviously scope for automating this potentially laborious task. The design methods, along with the embroidery methods were assessed using the generic problems and demonstrator parts described below.

Generic Studies

Three characteristic problems were considered, each involving a flat test specimens with a circular cut-out.

(i)    A narrow plate, containing a circular, central hole, under uniaxial tension (hole in a plate under tension).

(ii)    A similar narrow plate loaded in tension via a pin joint (inplane fastener pullout).

(iii)     A plate subject to out-of-plane bending via a central bolt pullout load (out-of-plane faster pullout).

The materials tested included quasi-isotropic and quasi-unidirectional specimens produced using Cornely and Schiffli embroidered fabrics, embroidered local inserts and through thickness stitching. The results were compared in all cases with conventional quasi-isotropic and quasi-unidirectional zero crimp fabrics. The objective was to assess the effect of modifications to the inplane and out of plane fibre architecture on resistance to concentrated loads. Thus the potential of embroidery could be examined for each particular load case with a view to applying the results for more complex engineering components and structures.

In general, the use of embroidered preform elements (patches) on the surface of conventional preforms was disappointing and produced little improvement over the base fabric due to early patch delamination under loading. However, modest improvements in performance was noted where the dominant failure mode was intra laminar shear. Cornely embroidered preforms with in plane fibre architectures modified to follow the geometric path surrounding stress concentrations generally showed an improvement over conventional preforms since the fibres could be readily aligned with the principal stress directions. This technique was particularly effective for inplane fastener pullout loadings. Through thickness stitching showed improvements in interlaminar properties which were generally proportional to the stitch density. This suppressed delamination effectively under out of plane loadings. Some fibre breakage was evident at high stitch densities which resulted in property reductions and further work is necessary to ascertain optimal conditions in this respect.

Automotive space saver wheel (430 mm diameter)

Space saver wheels are fitted as standard by an increasing number of manufacturers in order to reduce mass and costs in addition to the stated purpose of increasing luggage capacity. Despite the use of a reduced width, the pressed steel space saver supplied with vehicles such as Jaguars remains relatively heavy at 11.2kg. The wheel represents an interesting challenge for composites manufacture and performance since, although exempt from normal braking requirements which impose a high operating temperature capability, manufacture by pressed steel or cast alloy is a well established and relatively low cost manufacturing route. The space saver was adopted as a technology demonstrator to verify the design method and to assess the potential of embroidery for reducing the labour and waste involved in manufacturing part of a relatively complex preform. The main body of the wheel was designed based on a quasi-isotropic laminate produced from zero crimp, carbon fibre fabric. A static design based on the bending fatigue requirement resulted in a hub of simple disc form and a weight saving of 56% compared with the steel version. Cornely embroidered carbon patches were stitched (using aramid yarn) to the main body of the wheel, the patches being located on the two outer surfaces in vicinity of the hub, with the fibres aligned to inhibit crack propagation and to increase local stiffness at the highly stressed attachment points. The resulting assembly was impregnated by vacuum infusion in a single sided composite mould using a textured bagging film to form the outer face. Due to time constraints only one such component was produced in this way, although a second wheel was produced using fabric alone (i.e. without the local reinforcement) for comparison purposes. Both wheels were then passed to Ford Motor Company for fatigue testing under the LINK collaboration. Results were awaited as this report was prepared. A second design iteration was performed for which the regions of low stress in the hub were cut out to provide a spoked form, with local reinforcements surrounding the cut outs and, as before, at the bolt holes. The projected weight saving for this version (in carbon) was 63%.

Generator drive end frame (250 mm diameter)

The objectives of this prototype study (in collaboration with Lucas Applied Technology) were to reduce the waste fabric generated during preform manufacture, to reduce the perform assembly time while matching or improving the structural performance and reducing the weight compared to the conventional aluminium alloy part. The design criteria included a radial stiffness requirement and the maintenance of an interference fit for a central bearing over a range of loading conditions and operating temperatures. The design analysis was done using PATRAN/PFEA. The first design iteration was developed using a quasi-isotropic laminate which met all of the performance requirements with a 30% weight saving and a predicted factor of safety of 2.2. A second iteration, in which the hub was designed with predominantly circumferential fibres with radially reinforced spokes and a quasi-isotropic outer flange, was analysed with a predicted safety factor of 5.9. This was based upon the same space envelope as the aluminium and quasi-isotropic version.

The preforms for the second design iteration were embroidered using a Cornely machine with 1200g/km glass tows on 200g/m² plain woven substrate. Manufacture involved producing multiple layers with fibres laid according to the FEA predictions. The near net shape potential of embroidery permitted the elimination of 55% waste fibre compared with conventional fabric.

4 Conclusions

The initial feasibility studies on manufacturing capabilities demonstrated the practicality of embroidering high modulus fibres for low waste production of preforms for liquid composite moulding. The aggressive nature of the stitching process limits the choice of substrate, reinforcement and stitching materials, though suitable materials were identified and utilised successfully. Minimising the volume of stitching material within the preform improved permeability and compliance, which in turn led to improved structural performance under tensile loading.

Finite Element analysis proved to be a useful design tool for investigating the performance of modified fibre architectures, though the iterative process of optimising fibre architecture was time consuming. Experimental results demonstrated that structural performance under inplane geometric stress conditions and in and out-of-plane contact situations can be improved by the correct application of embroidery techniques. This has excellent potential for improving interlaminar properties in structures which are subject to out-of-plane bending or impact loads. Manufacture of complex 2 dimensional preforms using embroidery can substantially reduce waste, and also reduce the assembly time of multi layer preforms over using conventional fabrics.

Journal Papers

C.D. Rudd, L.J. Bulmer, D.J. Morris, and K.N. Kendall "Compaction and in-plane permeability characteristics of quasi unidirectional and continuous filament random reinforcements", Materials Science and Technology, Vol. 12, No 5, P436-444, May 1996

C D Rudd, N A Warrior and J Ellis ~Embroidered Reinforcements for Structural Composites" Materials Technology (in press 22.10.96)

Published Conference Papers

C.D. Rudd, J.P Chick D.J. Morris and N.A. Warrior, In-plane Permeability Characterisation of Mats and Fabrics Using SRIM. Proceedings ICCM-10, Whistler, BC (1995), Vol 3, P181-188, Woodhead Publishing.

C.D. Rudd, D.J. Morris, J.P. Chick and N.A. Warrior, "Material Characterisation for SRIM" Proceedings ICAC-95, Institute of Materials, Nottingham, UK (1995), Vol. 1, P21 1-218

D.J. Morris, C.D. Rudd, S.P. Gardner and N.A. Warrior, "The Effects of Embroidery Parameters upon Processing and Mechanical Properties of Cornely Embroidered Quasi-Unidirectional Reinforcement." Proceedings FPCM-96, Aberystwyth UK (1996)

Conference Presentations

D.J. Morris "In-plane Permeability Measurement Techniques", Symposium on Resin Transfer Moulding, University of Nottingham, March 15 1996.

N A Warrior "Performance Enhancement in Composite Laminates Using Embroidered Preforms" ICAC-95, Institute of Materials, Nottingham, UK (6-7 Sept 1995)

J Ellis and N A Warrior "Design and Manufacture of Stressed Components using Embroidery" LINK Structural Composites Open Day, University of Nottingham, 8 July 1996

N A Warrior and J Ellis " Manufacture of Structural Components using Embroidery Techniques" LINK Structural Composites Awareness Workshop, DRA Farnborough, 23 Oct 1996

D J Morris "Processing and Performance of Structural Composites using Embroidery Techniques" INTERPLAS Technical Sessions - 13 Nov 1996

S.P. Gardner "The Use of Embroidery Techniques in Structural Composites" Ph. D. Thesis 1997 The University of Nottingham

D.J. Morris "The Effects of Textile Variables on the Processing Properties of Reinforcement Fabrics" Ph.D. Thesis 1997 The University of Nottingham

S.P. Gardner, D.J. Morris, N.A. Warrior and C.D. Rudd "Embroidery Techniques Applied to A Composite Plate with A Central Hole, Under Uniaxial Tension"

S.P. Gardner, D.J. Morris, N.A. Warrior and C.D. Rudd "Embroidery Techniques Applied to A Hole in A Composite Plate Under In-plane Fastener Pull-out Loading"

N A Warrior, C Brown, K N Kendall and C D Rudd "Improving Interlaminar Fracture Toughness of Composite Laminates Using the Embroidery Technique"

N A Warrior, G Casey, K N Kendall and C D Rudd "Improving Fastener Pullout Properties Using Hybrid Preforms Produced by Embroidery"

Technical Reports

12 Progress Reports issued to LINK partners at 3 monthly intervals during project life.

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Julian Ellis will be delighted to hear from you. Telephone on +44 (0) 7976  425899