mechanical behaviour of circular and triangular glass fibres and their composites

Mechanical behaviour of circular and triangular glass fibresand their composites

Ian Bonda,*, Martyn Huckera, Paul Weavera, Stephen Bleayb, Sajad Haqc

aDepartment of Aerospace Engineering, University of Bristol, Queen’s Building, University Walk, Bristol BS8 1TR, UKbDERA Farnborough, Structures and Materials Centre, Farnborough, Hants GU14 0LX, UK

cBAE SYSTEMS Advanced Technology Centre—Sowerby, Bristol BS34 7QW, UK

Received 16 August 2001; received in revised form 14 January 2002; accepted 14 February 2002

Abstract

Single fibre testing of circular (CircGF) and triangular (TriGF) glass fibres of equivalent cross-section has shown the TriGF tohave a 25% higher average tensile strength compared to CircGF. Micro-composite compression testing (using resin bonded tows of12–15 filaments) has revealed the TriGF to have a compression strength 60% greater than CircGF. Some of the increase can be

attributed to an effective increase in second moment of area for the TriGF specimens due to imperfect packing. However, allowingfor this effect there still appears to be an underlying significant improvement in compressive strength performance attributable tothe inherent fibre shape. Mechanical testing under tensile load has shown that the triangular glass fibre reinforced plastic

(TriGFRP) performs marginally better (20%) than that manufactured using circular fibre (CircGFRP) for equivalent fibre volumefractions. Similarly, under compressive loading the TriGFRP outperforms CircGFRP by a significant margin of 40%. Interlaminarshear testing has also indicated that TriGFRP may offer a performance advantage of approximately 5%, although this needs furtherverification to be conclusive. # 2002 Elsevier Science Ltd. All rights reserved.

Keywords: A. Fibres; A. Polymer-matrix composites; B. Mechanical properties; Novel shape

1. Introduction

Composite laminates can easily attain the designrequirements for tensile load with a minimum numberof plies but are then often not sufficiently thick to pro-vide rigidity in flexure or overcome buckling problemsin compression. To solve this problem manufacturersoften design components using relatively thick compo-site laminates or ‘thin’ sandwich panels using a syntacticfoam core, which considerably increases the weight ormanufacturing cost of the component. An alternativesolution is the use of novel shaped glass fibres whichcould increase buckling resistance under compressiveloading without increasing weight and without theinclusion of foam cores. The resulting composite mate-rial could give improved rigidity and stability. On amicroscopic scale there are possible advantages of theincreased rigidity of individual fibres, leading to improved

fibre alignment within the composite and therefore apotential for increased compressive strength. Gains inrigidity are provided by the increase in second momentof area for a given mass of glass reinforcement. Theprimary objective of this study was to compare themechanical behaviour of circular (CircGF) and trian-gular (TriGF) fibres as individual filaments and theireffect on in-plane performance once incorporated intoan epoxy matrix to form a composite material.

In most applications to date (aerospace, military andconsumer items), polymer composites have been used inmodest thicknesses (typically less than 10 mm) to carrywhat are essentially 2-D in-plane loads. More recently,engineers have begun to use polymer composites inthicker sections, where an understanding of thethrough-thickness response is paramount in designing areliable structure, particularly where the strength in thisdirection has a controlling influence on the overallstructural strength of the component [1]. The problemarises due to a reliance upon a relatively weak polymermatrix to resist much of the loading in the through-thickness direction. This is of particular concern when

0266-3538/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PI I : S0266-3538(02 )00035-0

Composites Science and Technology 62 (2002) 1051–1061

www.elsevier.com/locate/compscitech

* Corresponding author. Tel.: +44-117-928-7704; fax: +44-117-

927-2771.

E-mail address: [email protected] (I. Bond).

considering damage arising from impact of objects intoprimary load-bearing composite structure. Much effortis currently being aimed at understanding both thedevelopment of impact damage and the residualstrength characteristics of advanced composite lami-nates [2,3] in order that interlaminar strength can beimproved.

A secondary objective of this study was, therefore, toinvestigate the effect of novel shaped fibres on through-thickness performance; in particular, where the fibremorphology results in a degree of reinforcement in thethrough-thickness or z-direction. It is important to notethat only small amounts of out-of-plane reinforcementare necessary to bring about significant enhancement ofthe interlaminar properties of the laminate[4]. Dick-inson et al. [5] provide a comprehensive review ofmethods by which through-thickness properties can bealtered.

Although no work was found concerning through-thickness property enhancement using novel shapedfibres, Deng et al. [6,7] examined the effect of changingfibre aspect ratio. They compared round [aspect ratio(a.r.)=1], oval (a.r.=4) and peanut (a.r.=2) shapedfibres subjected to tension, flexure, impact and inter-laminar loading. They concluded that during manu-facture significant fibre overlap occurred between thehigher aspect ratio fibres resulting in planes of weaknesswhere little or no matrix was present. This gave rise toreduced delamination resistance and interlaminar shearstrength. The overlapping effect described above couldbe overcome by keeping fibre aspect ratio nearer unityand limiting the shape of fibres to those with a highdegree of symmetry about the fibre axis whilst stillmaximizing fibre surface area.

An experimental glass fibre manufacturing facilitywas already available at Bristol from previous colla-borative work with DERA Farnborough. This facilityhas the capability of drawing precision hollow andnovel shape glass fibres of various diameters with vary-ing degrees of hollowness under precise parameter con-trol and their consolidation into a ‘pre-preg’ for lay-upand subsequent mechanical characterisation. A range ofhollow glass fibre composites have already been manu-factured and studied previously as part of a separateDERA and BAE Systems funded activity [8–11] andthis experience was used as a basis for the activities dis-cussed within this report.

2. Fibre and composite manufacture

2.1. Fibre manufacture

Fibres are fabricated by drawing of glass rod pre-forms using a modified optical fibre manufacturing rig.Fig. 1 schematically portrays the fibre drawing tower

arrangement. The glass preform is held in a chuck at thetop of the tower and slowly fed into a small bore tubefurnace. As the glass softens it is drawn down by thewinding action of the take-up drum to produce a singlefilament. It passes through a size coating and dryingarrangement as it moves downwards. The rate at whichthe glass preform is fed into the furnace, the rotationalspeed of the winding drum and the furnace temperaturedictate the resulting fibre geometry. Once drawn, thefibre is laid down on a drum which is indexed such thatan even and continuous layer of fibre is produced.

The material used for fibre manufacture consisted oftwo widely available alumino-borosilicate glasses, com-mercially known as DURANbhr and SIMAX#. Bothare very close in composition and have virtually iden-tical mechanical properties. The chemical compositionand physical properties of the alumino-borosilicate glassused for all fibre manufacture are given in Table 1.

2.2. Fibre geometry

As stated at the outset, the aim of this project was tocompare the behaviour of triangular (TriGF) and cir-cular (CircGF) fibres and composite materials(TriGFRP and CircGFRP respectively). Thus, for a faircomparison to be made, the cross-sectional-areas of thetwo fibre types must be the same and the resultingcomposites must have the same fibre volume fraction.The fibre dimensions for this study were chosen forpractical reasons. The facility can only manufacturesingle filament. Thus, producing enough fibre for com-posite manufacture is time consuming. Handleability

Fig. 1. Schematic diagram of the fibre drawing tower.

1052 I. Bond et al. / Composites Science and Technology 62 (2002) 1051–1061

was a further issue as larger fibres make mechanicaltesting easier. Note that fibre size was reduced margin-ally for the composite manufacture but it was clear thatinterpretation of results could prove problematic if fibredimensions were significantly different.

A relationship can be derived between the length ofthe side of the triangle and the radius of a circle suchthat they to have the same cross section.

Area of the triangle;

A ¼1

2bh ð1Þ

where b is the base and h is the height. For an equi-lateral triangle;

h ¼ b:sin�

3

� �ð2Þ

So the area A becomes;

A ¼

b2:sin�

3

� �

2ð3Þ

Area A of a circle in terms of external diameter D;

A ¼�D 2

4ð4Þ

For an equilateral triangular section of base b, thediameter D of a circular section with equal cross sec-tional area is;

D ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2b2:sin

�

3

� �

�

vuutð5Þ

To compare the behaviour of the two fibres for abending mode we have to compare the second momentof area, I. For an equilateral triangle through the cen-troid and parallel with the base;

It ¼b:h3

36ð6Þ

Substituting the result for h in terms of b gives;

It ¼ b4:sin

�

3

� �h i3

36ð7Þ

The second moment of area Ic of a circular section interms of D is;

Ic ¼�D 4

64ð8Þ

Thus Ic equal to that of an equilateral triangle of baselength b is;

Ic ¼

�

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2b2:sin

�

3

� �

�

vuut2664

3775

64

4

ð9Þ

Which simplifies as;

Ic ¼b4 sin

�

3

� �h i2

16�ð10Þ

As rigidity R is;

R ¼ E:I ð11Þ

where E is the Young’s modulus for the material. Therelative rigidity ratio for circular cross-section versus tri-angular cross-section with equal cross sectional area is;

IR ¼IcIt

ð12Þ

IR ¼2:25

�:sin�

3

� � ¼ 0:827 ð13Þ

Table 1

Composition and physical properties of borosilicate glass used for fibre manufacture

Composition wt.% Physical properties

Silica (SiO2) 80.5 Coefficient of expansion 3.3 � 10�6 C�1 (20–300 �C)

Boric Oxide (B2O3) 12.5 Density 2.23 � 103 kg m�3

Sodium Oxide (Na2O) 4.5 Young’s modulus 63 Gpa

Alumina (Al2O3) 2.0 Strain point 510 �C (�=1014.7 dPa.s)

Calcium/Magnesium Oxides (CaO/MgO) 0.5 Tg 540C (Z=1013.2 dPa.s)

Annealing point 560 �C (�=1013 dPa.s)

Softening point 825 �C (�=107.6 dPa.s)

Working temperature 1270 �C (�=104 dPa.s)

I. Bond et al. / Composites Science and Technology 62 (2002) 1051–1061 1053

So under these conditions a triangular cross-sectionfibre is 20.9% stiffer than one of circular cross-sectionfor equal mass per unit length. Resistance to bucklingfor a fibre depends on the second moment of area so wecan expect higher buckling resistance for the TriGF thanfor CircGF under compressive loading.

2.3. Fibre manufacturing in practice

For a number of practical manufacturing reasons, thefibre dimensions were slightly different for each of thedifferent methods of mechanical characterizationundertaken, as indicated in Table 2. This was because itwas found to be difficult to manufacture large quantitiesof triangular fibre, a prerequisite for pre-preg manu-facture, at the smaller dimensions. Thus fibre dimen-sions were increased for the circular and triangular fibreused to manufacture the composite laminate.

Fig. 2 is an SEM micrograph showing a cluster ofTriGF with a dimension of 50 mm and drawn from a tri-angular glass preform of 29 mm (apex to apex) at anindicated temperature of 900 �C. Fig. 3 is an opticalmicrograph which shows a cross-section through an uni-directional composite of 60 mm triangular fibres incorpo-rated in a Hexcel 914 epoxy matrix. The fibre volumefraction is approximately 50%. As can be clearly seen inFig. 2, the fibres have lost some of their triangularityduring processing, resulting in a rounding of whatshould have been sharp corners. This is unavoidable

during fibre drawing as surface energy effects will notallow sharp edges to be maintained. In practice, this lossof sharpness is expected to be quite beneficial in termsof reducing stress concentrations between fibre andmatrix in the resulting composite.

2.4. Fibre size coating

The sizing mixture used was based upon a generalpurpose silane based coupling agent Z6224 (made byOSI). This was chosen for its compatibility with polye-ster and epoxy resins and also because a supply wasreadily available. The size mixture comprised the fol-lowing (per litre of distilled water):

� 16 ml 1M acetic acid (6%/vol)� 1–2 ml TAA (titanium acetyl acetonate, Du Pont

TYZOR), provides a scratch resistant coating� 10 ml Z6224 Silane, coupling agent (OSI), low

chloride epoxy and polyester compatible.

To activate the coupling agent, distilled water is acid-ified by addition of acetic acid. The remaining parts arethen added. The mixture is allowed to stand for twentyminutes to allow the silane to be fully hydrolysed. Themixture remains useable for approximately 24 h.

2.5. Pre-preg manufacture

Fibres were drawn from both DURAN# andSIMAX1 borosilicate glasses, both of which are veryclose in composition and have virtually identicalmechanical properties, Table 1. Table 2 shows the rangeof fibre types produced. CircGF were made from 10 mmdiameter glass preform rod and TriGF from 29 mmapex-to-apex triangular preform.

An HDPE drum (1.088 m circumference, 330 mmwidth) is covered with a layer of peel-ply. A quantity of

Table 2

Manufactured fibre geometry

Mechanical characterization Circular fibre

diameter

(mm)

Triangular fibre

base dimension

(mm)

Single fibre tension and compression 45 60

Composite in-plane tension

and compression

35 50

Interlaminar shear strength 45 60

Fig. 2. SEM micrograph of 50 mm triangular glass fibres.

Fig. 3. Optical micrograph cross-section through unidirectional 60 mm

triangular glass fibre composite (Vf�50%).


fibre calculated to give the desired volume fraction iswound in layers onto the surface of the drum. A 0.1 mm(nominal) thickness strip of Hexcel 914 epoxy resin filmis applied to the outside of the fibre coated drum alongwith a 3 mm thick sheet of neoprene bonded cork formechanical protection. Breather fabric is applied to thelaid-up film and the whole assembly is vacuum bagged,heated to 80 �C and held for 1 h to allow the resin toinfiltrate the fibres. The pre-preg is then slit from thedrum, trimmed and is ready for laminate preparation.

2.6. Composite manufacture

Material for testing was laid up into plates accordingto the details in Table 3. These stacks were built up onan aluminium sheet covered with release film. Thestacks were surrounded with a neoprene-bonded corkdam of approximately 3 mm thickness and 20 mmwidth. The uncured stacks were overlaid with releasefilm and aluminium caul plates were centred on eachstack. Each caul plate was cut to fit loosely within thecork dam providing a gap of approximately 1–2 mm.Breather fabric and bagging film were finally overlaidand the whole assembly cured in an autoclave.

The autoclave cure cycle used was as follows:

� full vacuum plus 100 psi (5 bar) external pres-sure;

� temperature raised at 2 �C/min. to 100 �C;� dwell at 100 �C for 30 min;� vent vacuum to atmosphere keeping 100 psi (5

bar) applied;� temperature raised at 2 �C/min to 175 �C;� dwell at 175 �C for 60 min;� slow cool to ambient (autoclave natural cooling

rate approx. 0.25 �C/min);� 100 psi (5 bar) removed when below 45 �C.

3. Mechanical characterization of circular and trian-

gular fibres

3.1. Tensile testing

A small sample of single unsized fibres were sub-jected to tensile testing. Results from these tests areshown in Table 4. Fibres were tested using a Houns-field Test Machine fitted with a 5 N load cell at a rate of

0.01 mm s�1 according to the ASTM D3379–75 testmethod.

Individual fibres were mounted on a test card usingcyanoacrylate adhesive. The standard test card ensureda constant gauge length of 50 mm for each test speci-men. The test card was then inserted into the grips ofthe Hounsfield apparatus. At mid-gauge the sides of thetest card were removed using a glowing taper. Thismethod reduces the chance of accidental damage to thefibre. The fibre was then allowed to settle. The appara-tus was set at an appropriate scale to give a reading tothree decimal places and was reset to zero before eachtest. The peak reading at which the fibre broke wasrecorded.

Although the diameters of the fibres were kept asconsistent as possible during the manufacturing process,variations of up to 2 mm existed. Fibres were measuredto obtain more accurate fibre diameters to be used indetermination of failure stresses. For each type of fibre,six sample fibres were selected and their outer diameterswere measured using an optical microscope-mountedCCD camera and image analysis software. Measure-ments across each fibre diameter were made using apixel counter. Eight measurements were taken along thelength of each sample. These measurements were cali-brated with a graticule. An average value was calculatedfor each fibre and then the average for the six samples.This final value was then used for strength derivation.

The values in Table 4 indicate that the TriGF out-performed the CircGF under tensile loading by a marginof approximately 25%. The scatter was also found to begreater for the CircGF and hence a significantly lowerWeibull modulus was derived. These findings are sur-prising since it could be argued that the higher surfacearea of the TriGF (approximately 28%) could result in ahigher probability of surface flaws and defects leadingto lower average strength. However, this testing exam-ined virgin fibre immediately after manufacture wherethere would be limited opportunity for surface damageto occur. Theoretical analysis, considering shape, sug-gests that there should be negligible difference in beha-viour as shape has little effect under tensile loadingunless significant stress concentrations are present.

3.2. Compressive testing

The bespoke test method adopted for fibre compres-sion testing was to use a small tow of fibre to provide a

Table 3

Manufactured composite specification

35 mm Circular

FRP (in-plane)

45 mm Circular

FRP (ILSS)

50 mm Triangular

FRP (in-plane)

60 mm Triangular

FRP (ILSS)

Plate dimensions 250�110�1.9 220�100�2.6 120�110�2.2 220�100�3.6

No. of ply layers 15 16 15 19


microcomposite (Fig. 4). A short length (2 mm) of anunsupported resin-bonded fibre bundle (approximately15 individual filaments) was mounted between accu-rately machined, rigid, low density, polymer blocks (20� 20 � 20 mm3) by threading through fine brass tubing(1mm external diameter, 300 mm internal diameter)which was inserted into pre-drilled holes in the polymerblocks. The arrangement is constrained within an alu-minium sleeve and the polymer blocks are supportedprior to testing by the insertion of a key through a cut-out in the sleeve. This key prevents premature loadingof the fibre bundle, only being removed immediatelyprior to testing. Care must be taken to ensure that thepolymer blocks slide freely within the aluminium sleevebut do not allow rotation in any orientation. Themicrocomposite was produced by hand drawing of fila-ments, coated in epoxy resin, through a piece of taperedbrass tubing (down to 300mm internal diameter) whichresulted in a tightly packed, approximately circularspecimen. Voidage between filaments was unavoidablefor both fibre types and was more noticeable in the tri-angular microcomposite specimens.

The jig was mounted within a compression cage on anelectro-mechanical tensile test machine using a 50 Nload cell at a speed of 1 mm min�1 with peak loadrecorded. Cut-outs in the aluminium sleeve allowedobservation of the fibre bundle during testing to verifythat buckling did not take place.

This test methodology does not correspond exactlywith single fibre testing and thus results are indicative offibre compressive performance rather than absolute.Two groups of TriGF tows were tested. The first incor-porated 15 fibres and the second only 12 fibres. Thereason for this reduction was to overcome the practicaldifficulties involved in threading triangular fibresthrough the brass tubing when mounting in the com-pression jig. A consequence of the reduction was thatthe cross-sections of glass under test were not main-tained at the same value.

The average values in Table 5 indicate that the TriGFoutperformed the CircGF under compressive loadingby a margin of greater than 60% with 12 fibres in a tow.The corresponding scatter from the TriGF data wassignificantly higher than for the CircGF. Some ofthis higher strength can probably be attributed to thegreater ‘effective’ second moment of area of the tow as a

result of the lower packing efficiency of the triangularfibres. This explanation may seem somewhat counter-intuitive to what would be expected when stacking tri-angular and circular shapes into a constricted section.However, it was evident during the specimen manu-facture that the TriGF tows were much more difficult toprepare and would not easily tolerate the interweavingof fibres within the tow. Disregarding for a momentthe effect of fibre distribution, there does seem to bean underlying, significant, increase in compressivestrength performance of triangular fibres compared withcircular.

Observation during testing and examination of frac-ture surfaces after failure indicated that gross bucklinghad not occurred but that some form of micro or shearbuckling was probably the most likely cause of failure.This is not surprising with an aspect ratio (unsupportedlength/fibre bundle diameter) of between 6 and 11 forthe specimen gauge length. Thus, the compressive‘strength’ values derived can be considered valid as arelative measure between the different fibre shapes butcare must be taken when extrapolating to a real fibrecompressive strength.

4. Mechanical characterization of composite material

4.1. Specimen preparation

The specimens for the in-plane characterization oftensile and compressive strength were prepared accord-ing to Figs. 5 and 6. End tab material was applied priorto cut-up and consisted of 0�/90� glass/epoxy and Ciba-Geigy two-part low-temperature-cure epoxy adhesive.Chamfers were formed to reduce stress concentrationand avoid failure of the specimen at or near the endtabs. The samples were cut to size using a diamondwheel. Special attention was given to producing flat andparallel edges for the compression test samples.

4.2. Fibre volume fraction

Two methods were used to determine the fibre volumefraction of each plate. The first method used image analy-sis of optical images to measure fibre area and fibre dis-tribution (Table 6). Localised areas were studied at severalregions in the plate to check for inhomogeneities.

The second method used was the removal of thematrix by incineration. A section of composite wasaccurately weighed then subjected to 600 �C for 1 h toincinerate the matrix. The remaining glass debris wasthen re-weighed, Table 7. In practice this provided aweight fraction but since the density of each componentis known and voidage was found to be negligible, thenthe weight fraction allows derivation of an approximatevolume fraction.

Table 4

Average failure stress and standard deviation for tensile tested single

unsized fibres tested immediately after manufacture

Fibre

geometry

No. of

tests

Failure stress

(MPa)

Std.Dev.

(MPa)

Weibull modulus,

m (2 parameter)

Circular 45 mm 30 351.06 54.57 4.86

Triangular 60 mm 30 441.11 33.46 14.14


4.3. Mechanical testing of composite specimens

A 25 kN servohydraulic test machine within the Aero-space Engineering laboratory was used for testing. Thismachine has a data acquisition system for the recordingof displacement and load data in ASCII format. Themachine was operated under displacement control at aspeed of 1 mm/min in tension and 2 mm/min in com-pression and data was recorded at a rate of 5 Hz.

4.3.1. Tensile testingStrain gauges were applied to two tensile specimens of

each type, Fig. 5. They were aligned in the fibre direc-tion to provide longitudinal strain data. As only a singlegauge was used, the Poisson’s ratio has not beenobtained. Table 8 provides a summary of the compositetensile performance for each fibre geometry.

The average tensile strength for the triangular speci-mens is slightly higher than the value for the circularspecimens. Normalising assuming a simple ’Rule ofMixtures’, with respect to the fibre volume fraction forthe CircGFRP (Vf=22.3%), the TriGFRP (Vf=21.4%)has an approximate average tensile strength of 291 MPa(c.f. CircGFRP 241 MPa). This discrepancy can prob-ably be explained by improved manufacturing techni-ques with the TriGFRP, in particular better fibrealignment and distribution. This is supported by a lowercoefficient of variation for strength and higher Weibullmodulus and also an apparently higher tensile strengthfor the individual TriGF filaments, as discussed above(Table 4). However, fewer samples of TriGFRP weretested (11 c.f. 20) due to a limited supply of material.Fibre shape (i.e. second moment of area, I) would be

Fig. 4. Schematic of compression test rig for ‘‘fibre’’ testing.

Table 5

Average failure stress and standard deviation for compression tested

fibre tows tested immediately after manufacture

Fibre

geometry

Fibres

in

tow

No.

of

tests

Failure

stress

(MPa)

Std.Dev.

(MPa)

Co-efficient

of variation

(%)

Circular 45 mm 15 8 503.58 62.72 12.46

Triangular 60 mm 12 6 813.46 132.27 16.26

Table 6

Fibre diameter and volume fraction measurements for tensile and

compressive composite specimens

Circular fibre

composite

Triangular

fibre composite

Fibre diameter

(mm)

Laser measurement 35 50

Image analysis 34 47

Volume fraction

(%)

Microscopy method 22.3 27.0

Burning method 22.3 21.4


expected to have negligible effect on tensile performancein the fibre direction but the increased fibre surface areaof the TriGF could result in poorer tensile performancedue to a higher probability of surface defect sites.Degradation of the fibres during composite manu-facture may possibly explain why the apparent increasein TriGF tensile strength is not fully discerned in theTriGFRP.

Comparison of similarly normalised tensile modulifor CircGFRP and TriGFRP (17.08 and 16.55 Gpa,respectively) indicates a reasonably good correlation.However, it must be noted that these values werederived from only two samples for each condition withstrain gauges attached to the gauge length. Figs. 8 and 9show examples of the stress–strain curves obtained for acircular and triangular fibre composite respectively. Thestrain-to-failure is surprisingly low in both cases, espe-cially when compared with conventional GRP. The datawas checked and it was found that all the gauges wereoperating until the failure of the material. One possibi-lity is the low volume fraction of the specimens, whichwould give lower strain at failure and also the relativelylarge fibre diameters (35 and 50 mm, respectively)resulting in failure at lower strain levels due to a volumeeffect.

4.3.2. Compressive testingThe end tabs for the compression samples are smaller

(Fig. 6) than for the tensile, due to a need to maximizethe number of samples available from the original plateand minimize waste of material. A 10 mm gauge lengthwas chosen such that the buckling effect was minimized.For compressive testing, the end tab length is not asimportant as for tensile. As a consequence, a 20 mm endtab length was considered suitable, giving an overallspecimen length of 50 mm. However, the compressiontest grip assembly requires end tabs of 40 mm (Fig. 7).Thus 20 mm lengths of silver steel were used as blocksupports in each grip assembly. This solution overcomesthe problem of sample length, but does require care inhaving perfect parallel faces on the supports in order toavoid initiating buckling of the composite specimen.

The results are shown in Table 9. It is clear from thesethat the triangular fibre composite has better compres-sive performance. This outcome can be justified evenallowing for both the lower (�1%) TriGFRP volumefraction and the greater uniformity of the material as

Fig. 5. Schematic and photograph of in-plane tensile test specimen.

Fig. 6. Schematic and photograph of in-plane compressive test speci-

men.

Table 7

Measurements for volume fraction determinationa

Before

incineration (g)

After

incineration (g)

Fibre volume

fraction (%)

Tens. and compr. specimens

CircGFRP 1.704 0.600 22.3

TriGFRP 1.736 0.590 21.4

ILSS specimens

CircGFRP 0.849 0.430 53.3

TriGFRP 2.039 0.955 55.3

a Density of borosilicate glass=2.23 g cm�3 and Hexcel 914

resin=1.29 g cm�3.

Table 8

Results of tensile testing for circular and triangular fibre composites

Composite Type

(No. of samples)

Tensile modulus

(GPa)

Strain to

failure (%)

Tensile strength

(MPa)

Co-efficient

of variation (%)

Weibull modulus,

m (2 parameter)

Fibre Vf

(resin burn-off) (%)

Circular (20) 17.08 1.72 251.47 7.69 15.07 22.3

Triangular (11) 16.55 1.49 291.32 5.73 19.21 21.4


demonstrated by a lower co-efficient of variation andhigher Weibull modulus.

Again normalising to the same lowest Vf, theCircGFRP has a compressive strength of 401 MPa,compared to 672 MPa for the TriGFRP, a difference of40%. Again, fewer TriGFRP samples (9 c.f. 18) wereavailable for testing due to limited availability, but thereis an undisputable difference in compressive perfor-mance.

In an earlier section, a numeric value for secondmoment of area, I, was derived to illustrate the likelyincrease (16.3%) in Euler buckling performance for tri-angular single fibres compared to circular. However, theassumption of simple Euler buckling is somewhat sim-pler than the loading experienced by the fibres within amatrix where the buckling mode is likely to be sig-nificantly different. It would appear from these resultsthat despite differences in fibre volume fraction andplate quality, TriGFRP’s do seem to offer a substantialimprovement in compressive performance.

4.3.3. Interlaminar shear strength testingAs part of a separate investigation into improving the

through-thickness performance of composite materials,interlaminar shear strength testing was undertaken onboth CircGFRP and TriGFRP with approximately thesame fibre volume fraction (Table 10). The methodchosen was a CRAG test method for ILSS based on theuse of short beam shear (3 point bending) [12]. Speci-men dimensions were 2.5 mm�12.5 mm�22.5 mm(thickness, T�width, W�length, L) with a roller sup-port span, S, of 10 mm for the circular specimens and3.5 mm�17.5 mm�27.5 mm (thickness, T�width, W�

length, L) with a roller support span, S, of 14 mm forthe triangular specimens. Loading was applied at a ratesufficient to cause failure in approximately 30 s.

Interlaminar shear strength (ILSS) was derived usingEq. (14);

ILSS ¼3P

4W:Tð14Þ

Fibre volume fractions were determined using a resinburn-off test of 1 h at 600 �C as discussed above(Table 7). The results in Table 10 indicate that theTriGFRP offers a small (approximately 5.5%) improve-ment in mean ILSS over the CircGFRP. The scatter ofresults in the latter case is somewhat greater with a co-efficient of variation of 10% (c.f. 3.5%); thus it could beargued that the apparent increase in ILSS is a littleuncertain. However, the fact that the larger triangularspecimens (240% increase in volume) do not to show alower ILSS due to a size effect, suggests that TriGFRPoutperforms CircGFRP in this instance. The fibre volumefractions (53% for CircGFRP and 55% for TriGFRP)also favour an improved performance from the TriGFRP.

Fig. 7. Schematic of jig for static compressive testing.

Fig. 8. Stress–strain curve for circular fibre composite under tensile

loading.

Fig. 9. Stress–strain curve for triangular fibre composite under tensile

loading.


Thus, at this stage, it is difficult to draw a definite con-clusion as to the extent of increase in interlaminar shearstrength due to the triangular fibre shape but it doesappear that some improvement has occurred.

5. Conclusions

Single fibre tensile testing has revealed the TriGF tohave a 25% higher average tensile strength compared toCircGF with a considerably lower degree of scatter andconsequently higher Weibull modulus. Fibre tow(micro-composite) compression testing has revealed theTriGF to have greater than 60% higher average com-pression strength compared to CircGF albeit with a lar-ger degree of scatter. Some of the increase can beattributed to an effective increase in second moment ofarea due to a lower fibre packing efficiency within thefibre tow. However, disregarding this effect, thereappears to be an underlying significant improvement incompressive strength performance.

It is fair to state that a triangular shape is by nomeans the optimum with regard to maximising com-pressive performance. Further studies into the develop-ment and use of lobed star shaped fibres couldpotentially yield even greater gains than those suggestedwithin this investigation.

Pre-impregnated tape has been manufactured incor-porating solid circular and triangular glass fibre geome-tries using a bespoke technique. This tape has beensuccessfully laid-up and composite laminate plates pro-duced.

Mechanical testing under tensile load has shown thatthe TriGFRP performs marginally better (20%) thanthat manufactured using circular fibre for equivalentfibre volume fractions. Similarly, under compressiveloading the TriGFRP outperforms the CircGFRP by asignificant margin of 40%. Interlaminar shear testing

has also indicated that the TriGFRP offers a perfor-mance advantage of approximately 5%, although thisneeds further verification to be conclusive.

Acknowledgements

This investigation was funded by the MoD underCorporate Research Package TG04. The authors wouldlike to thank DERA Farnborough and BAE Systemsfor their support and funding of this work.

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Table 9

Results of compressive testing for circular and triangular fibre composites

Composite type

(No. of samples)

Compressive strength

(MPa)

Co-efficient of variation

(%)

Weibull modulus,

m (2 parameter)

Fibre Vf (resin burn-off) (%)

Circular (18) 418.24 6.67 17.57 22.3

Triangular (9) 672.13 3.09 37.26 21.4

Table 10

Results of ILSS testing for circular and triangular fibre composites

Composite type

(No. of samples)

Inter-laminar

shear strength (MPa)

Co-efficient of

variation (%)

Fibre volume fraction,

Vf (%)

Circular (20) 51.92 10.12 53

Triangular (18) 54.83 3.56 55


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