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Continuous length spread tow +a-b fabrics


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Oxeon's Product Manager Materials, Fredrik Ohlsson, presented the paper titled Continuous-length Spread Tow +α/-β Fabrics on the Composites UK annual conference held 5-6 of May in Manchester, United Kingdom.

The paper is written by Fredrik Ohlsson and Dr. Nandan Khokar and covers the principles of Oxeon's groundbreaking innovation with +α/-β variants of TeXtreme® Spread Tow Fabrics.

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Continuous length spread tow +a-b fabrics

  1. 1. 1/7 CONTINUOUS-LENGTH SPREAD TOW +α /-β FABRICS Fredrik Ohlsson, Product Manager - Materials Dr. Nandan Khokar, R&D Manager Oxeon AB, Borås, SwedenABSTRACTFabrics with +α/-β orientation of spread tows is a recent development. They arecomposed of two sets of Spread Tow Tapes arranged at an angle, either equal orunequal, relative to the fabric’s length direction, such as +45/-45, +30/-60, +50/-25 etc.These +α/-β fabrics are advantageously produced in continuous-lengths and areinnovative solutions for effectively complementing the existing 0/90 Spread Tow wovenFabrics to realize easily and quickly an optimized multidirectional lightweightreinforcement by plying them directly.This paper explains how the indicated +α/-β fabrics directly help replace the conventionalcross-plied UD structures with the added advantages of improved delamination resistanceand inclusion of virtually crimp-less fibers oriented in two angular directions. As aconsequence, production of lightweight multidirectional reinforcements becomes at oncelabour-time-cost saving and efficient while according the benefits of obtaining relativelyhigh performance, thin, easy-to-handle and well draping reinforcement material. Withuse of continuous-length +α/-β fabrics the thickness variation associated withoverlapping joints/splices is eliminated besides wastage of material reduced.Key words: Spread Tow Fabrics (STF), +α/-β STF, Lightweight carbon reinforcementsINTRODUCTIONCarbon fiber Spread Tow Fabric (STF) in 0/90 fiber angles has rapidly grown in popularityamong composite producers with high demand on weight savings, improved mechanicalperformance and surface smoothness. Its usage has spread throughout a variety ofapplication areas for example within sports and leisure industries. Products such as icehockey sticks, golf shafts, rowing boats, wind and kite surfing boards, skis andsnowboards, as also Formula 1 cars, luxury yachts and ultralight aircraft are someproducts where STF is being used currently. At the same time STF materials are beingDoc.No: MARFO07 v1.2 Template v.1.2Oxeon AB | Norrby Långgata 45, SE-504 35, Borås, Sweden | Phone: +46 33 20 59 70 | Fax: +46 33 20 59 79 | |
  2. 2. 2/7evaluated in other industries such as aeronautical/aerospace, automotive andengineering industries.The reason for STF’s success originates from its unique woven structure resulting byinterlacing 20-50 mm wide stabilized Spread Tow Tapes (STT) as warp and weft in plainand twill weave patterns. Use of STT brings a number of advantages which are discussedbelow.With a view to further improve composite materials, use of plied materials has beenconsidered for long to obtain certain mechanical properties in different orientations.However, to ply a conventional woven material in different orientations requires cuttingrelatively smaller pieces from a larger sheet and placing them in relatively differentorientations. This approach evidently creates discontinuities in the fiber structure, whichreduces the reliability of the final composite material, besides creating uneven thicknessdue to overlapping.Therefore, as a natural step to complement the existing range of 0/90 Spread TowReinforcements, continuous-length Spread Tow +α/-β Fabrics with variable fiber anglessuch as +45/-45, +30/-60, +50/-25 etc. have been developed recently. This paperexplains how the indicated +α/-β STF directly helps replace the conventional cross-pliedUD and other plied fabric structures. As a first step to evaluate the +α/-β STF, the+45/-45 variant has been produced.+α/-β FABRIC FEATURESThe need for including fibers in relatively different orientations has so far been realizedby plying either UD sheets or woven fabric sheets. Whereas use of UD sheets ensuresfiber continuity without any structural integrity, the use of woven fabric sheets, thoughhaving structural integrity, creates fiber discontinuity because they have to be cutangularly from a larger sheet and laid. Such ‘patches’ of woven material have to be laidcarefully to match with the edges of the previously laid ‘patches’. Often times the wovenmaterial patches are laid in certain sequences with some overlapping, which causes theproblem of thickness variation at the joints. These fiber discontinuities and thicknessvariations adversely affect the performance of composite materials. In addition to theindicated performance related problems, use of woven patches also has an adverseimpact on the economics of production relating to cutting, plying and wastage ofmaterial.Doc.No: MARFO07 v1.2 Template v.1.2Oxeon AB | Norrby Långgata 45, SE-504 35, Borås, Sweden | Phone: +46 33 20 59 70 | Fax: +46 33 20 59 79 | |
  3. 3. 3/7An approach to overcome some of the discussed problems has been through use ofNon-Crimp-Fabrics (NCF) materials. However, use of NCF materials has certaindrawbacks such as their non-symmetrical arrangement of UD plies, relatively higherthickness and areal weight of material, in-plane delamination, use of epoxy incompatiblepolyester stitching yarns with its corresponding knock-down factor due to stitching crimp.In an attempt to overcome the problems associated with the use of UDs, NCFs andconventional woven materials, the +α/-β STF has been developed. In Figure 1 are shownthree different +α/-β constructions. As can be inferred, they represent the constituentSTT arranged in (a) obtuse angle, (b) right angle and (c) acute angle. Theseconstructions of course have the STT oriented in equal but opposite angles. Should therebe a need for having an +α/-β construction wherein the STTs are incorporated unequally,then such an +α/-β STF can be also produced as shown in Figure 1(d).The fiber angle flexibility in the material provides, for example, the possibility to tailorproduce an +α/-β STF with STT in about +55/-55 degrees to uniquely enable directproduction of pressure vessels. Such a fabric could be wound over itself to as manylayers as needed to meet the required performance demand.Figure 1. Different variants of +α/-β STF constructions: (a) obtuse angled, (b) right angled and (c) acuteangled. While these variants have the STT in equal and opposite orientations, the variant (d) has unequalorientations of the STT.Doc.No: MARFO07 v1.2 Template v.1.2Oxeon AB | Norrby Långgata 45, SE-504 35, Borås, Sweden | Phone: +46 33 20 59 70 | Fax: +46 33 20 59 79 | |
  4. 4. 4/7At present +α/-β STF can be produced using STT of different width from high strength(HS), Intermediate modulus (IM) and high modulus (HM) carbon fiber yarns of high towcount (12k and above) in fabric areal weights starting at 80 gsm, 76 gsm and 130 gsmrespectively. STTs of lower areal weight will most likely be available shortly hence, theareal weight of +α/-β STF can also be reduced.All these variants of +α/-β STF constructions are producible in continuous lengths, whichbenefits in eliminating cutting, laying and splicing. Further, such a continuous +α/-β STFmaterial can be directly preimpregnated as the tapes are well integrated to resist fabricdeformation in both length and width directions. An example of +α/-β STF produced in+45/-45 STT orientation is shown in Figure 2. Figure 2. An +α/-β STF with +45/-45 STT orientationDoc.No: MARFO07 v1.2 Template v.1.2Oxeon AB | Norrby Långgata 45, SE-504 35, Borås, Sweden | Phone: +46 33 20 59 70 | Fax: +46 33 20 59 79 | |
  5. 5. 5/7MECHANICAL PERFORMANCEGiven the arrangement of STT, a composite made from an +α/-β STF is not optimal totake loads in its longitudinal and lateral directions without getting deformed just as acomposite made from 0/90 STF will shear when loaded angularly. Notwithstanding thisaspect, +α/-β STF is designed to be used along with conventional 0/90 STFs to meet therequired laminate performance demands. Needless to state, individually +α/-β STFs isconsidered beneficial for torsional applications.The mechanical performance of +α/-β STF comes from virtually no in and out of planefiber crimp (i.e. crimp angle), fewer interlacing points (i.e. crimp frequency) andrelatively higher fiber volume fraction (i.e. cover factor). Further, because STT has itsfibers well distributed, its wetting by the matrix can be achieved easily and quickly toensure improved load transfer in a composite material.Because the constituent STT are integrated throughout the +α/-β STF the in-planedelamination risk is substantially reduced. In principle this implies that each +α/-β STFsheet comprising fibers in two equally opposite angles is symmetrically balanced.Therefore, use of such +α/-β STF enables to obtain a symmetrically balanced pliedconstruction directly using relatively fewer sheets. Tables 1 and 2 below together withFigure 3 and Figure 4 illustrate how symmetrical quasi- and bi-directional balancedconstructions are obtained using relatively fewer STF sheets compared to the existingNCFs and UDs. Table 1. Symmetrical Quasi-Isotropic construction Material Ply sequence No. of fiber layers to achieve symmetryUD [0/90/+45/-45/-45/+45/90/0] 8NCF [NCF(0/90/+45/-45)/NCF(-45/+45/90/0)] 8STF [STF(0/90)/STF(+45/-45)/STF(0/90)] 6Doc.No: MARFO07 v1.2 Template v.1.2Oxeon AB | Norrby Långgata 45, SE-504 35, Borås, Sweden | Phone: +46 33 20 59 70 | Fax: +46 33 20 59 79 | |
  6. 6. 6/7 Figure 3. Symmetrical Quasi-Isotropic construction of UD/NCF (left) and +α/-β STF (right) Table 2. Symmetrical bi-directional constructionMaterial Ply sequence No. of fiber layers to achieve symmetryUD [+45/-45/-45/+45] 4NCF [NCF(+45/-45)/NCF(-45/+45)] 4STF [STF(+45/-45)] 2 Figure 4. Symmetrical bi-directional construction of UD/NCF (left) and +α/-β STF (right)Doc.No: MARFO07 v1.2 Template v.1.2Oxeon AB | Norrby Långgata 45, SE-504 35, Borås, Sweden | Phone: +46 33 20 59 70 | Fax: +46 33 20 59 79 | |
  7. 7. 7/7METHOD OF PRODUCTIONThe method of producing +α/-β STF is completely new in that it is unlike the principles ofweaving and braiding. The production is virtually endless as there are no STTs oriented inthe fabric length direction. Accordingly, the fabric can be produced in any desired lengththat may be required. The angles of STT can be varied as desired on the same machine.CONCLUSIONSThe new +α/-β STF offers new opportunities in composite material production both interms of performance and economics. It can be produced using STT of different widths,areal weights and fiber types (HS, IM and HM). Its lowest areal weight at present is 76gsm. The availability of +α/-β STF in different angular variants allows to ply them directlyfor obtaining multidirectional fiber orientations. The availability of such fabric in continueslength enables waste reduction and splicing/joints whereby a composite of uniformthickness can be obtained.ACKNOWLEDGEMENTSSincere thanks are expressed to Mr. Henrik Blycker, CEO, Oxeon AB, Sweden, for hisencouragement and support at all times.Doc.No: MARFO07 v1.2 Template v.1.2Oxeon AB | Norrby Långgata 45, SE-504 35, Borås, Sweden | Phone: +46 33 20 59 70 | Fax: +46 33 20 59 79 | |