The mechanical performance of 3D woven sandwich composites

M.K. Bannister a,*, R. Braemar b, P.J. Crothers c

a Cooperative Research Centre for Advanced Composite Structures Ltd. (CRC-ACS), 506 Lorimer Street, Fishermens Bend, Vic. 3207, Australiab Department of Aerospace Engineering, RMIT University, 226 Lorimer Street, Fishermens Bend, Vic. 3207, Australia

c ASTA Components, 226 Lorimer Street, Fishermens Bend, Vic. 3207, Australia

Abstract

Composite sandwich structures were manufactured from a 3D woven fabric consisting of two face fabrics interconnected by pile

yarns (Distance Fabric). Specimens were produced from Distance Fabric (DF) consolidated with vinyl ester resin with and without a

polyurethane foam core and compared to specimens produced from a precast polyurethane foam core with composite skins added

separately. Flatwise compression, edgewise compression, climbing drum peel and ¯exure tests were conducted and all demonstrated

a dramatic improvement in properties from the combination of DF and foam core. These improvements are postulated to arise from

the mutual reinforcement of the pile yarns and foam core. Ó 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Sandwich structure; Distance Fabric; Textiles; Flexure; Compression; Foam core

1. Introduction

Sandwich construction, producing two structuralcomposite faces separated by a light-weight honeycombor foam core, has been one of the most traditional andsuccessful structural designs using composite materialsdue to the advantage of a dramatically increased sti�-ness to weight ratio when compared to other materials.However, the manufacture of conventional sandwichstructures usually consists of the strengthening skinsbeing adhesively bonded to the foam or honeycombcore. This can be a costly process due to the requirementfor the manufacture to take place in several stages;machining of the core followed by the lay-up andbonding of the skins. There also is the di�culty of ob-taining a strong bond line between the skin and the core,which can often lead to reductions in performance orfailure of the component when subjected to impactconditions.

The development of advanced textile technology hasmade it possible to manufacture 3D woven structureswhich can be used to reinforce polymer matrices [1].Integrally woven sandwich structures, or DistanceFabrics (DF), in which the core is directly interwovenwith the skins through vertical pile yarns (Fig. 1), hasbeen produced commercially for a number of years and

has been shown to give improved mechanical perfor-mance [2±7]. However, the commercial use of thesematerials has been limited due to di�culties in devel-oping a cost-e�ective, automated, manufacturing pro-cess utilising these fabrics. Recent work at theCooperative Research Centre for Advanced CompositeStructures (CRC-ACS) has lead to a signi®cant break-through in the development of a manufacturing tech-nique that can produce good quality sandwichstructures from DF in a highly automated process(Fig. 2). This technique also allows the incorporation ofextra structural skins as well as a foam core in a one-stepconsolidation process that could lead to signi®cant costbene®ts.

This paper presents the results of a number of me-chanical tests that were performed on composite sand-wich structures manufactured using this technique. Thespecimens were manufactured from E-glass DF with andwithout a polyurethane foam core and compared tosandwich specimens made from precast polyurethanecores and E-glass fabric skins.

2. Materials and experiments

The test specimens were manufactured from DF ob-tained from Vorwerk GmbH and was designatedTechnoTex 13822. This was an E-glass DF and hada fabric weight of 1400 g/m2 and a fully extended

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Composite Structures 47 (1999) 687±690

* Corresponding author. Fax: +61-3-9646-0583.

E-mail address: mbcrcas@ozemail.com.au (M.K. Bannister).

0263-8223/99/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved.

PII: S 0 2 6 3 - 8 2 2 3 ( 0 0 ) 0 0 0 3 5 - 0

sandwich height of approximately 25 mm. The fabric wasconsolidated in a liquid moulding process with Dera-kane 411-350 vinyl ester resin, with Andonox catalyst,using the proprietary CRC-ACS technique. The sampleswere postcured for 4 h at 40°C to fully cure the vinylester.

Specimens consisting only of the consolidated DFwere manufactured in this way to examine the perfor-mance of the DF alone (specimens referred to as DF).To investigate the performance of the DF material witha foam core some of the cured specimens were ®lled witha Type G rigid polyurethane foam (specimens referredto as DF + PU). Foam densities obtained were on anaverage 100 kg/m3. Specimens were also manufacturedfrom precast blocks of polyurethane foam of similardensity to which was added E-glass fabric skins com-parable to the areal weight and weave architecture of theDF skins (specimens referred to as SW). The skins of thespecimens were consolidated directly onto the corethrough a resin infusion process with the Derakane 411-350 vinyl ester resin. These specimens were produced tocompare the performance of the two types of DF spec-imens against a similar, traditional sandwich structure.

A number of tests relevant to sandwich structureswere performed in order to characterise the mechanicalperformance of the DF sandwich specimens (all sampleswere, on an average, 25 mm high). These tests wereFlatwise Compression (ASTM C 365-94), Flexure

(3-point bend) (ASTM 393-62), Climbing Drum SkinPeel (ASTM D 1781-93) and Edgewise Compression(ASTM C 643-94).

3. Experimental results and discussion

3.1. Flatwise compression

The test samples for ¯atwise compression were100 mm � 100 mm and the results of the tests are shownin Fig. 3. The DF and SW samples were found to havecomparable compression strengths, however, the com-bination of the DF and the polyurethane core producedstrengths almost 4.5 times higher. This result was muchgreater than the simple addition of the two separatestrengths and showed that there is a synergistic e�ect ofthe material combination.

The DF samples failed through buckling of the pileyarns. These yarns ¯exed signi®cantly before matrixcracking was heard, following which the yarns began tofail catastrophically leading to complete sample failure.The foam samples failed through a crushing of the foamcells with ¯ecks of foam falling from the specimen.Failure of the DF + PU samples was ®rst observed in thefoam with small cracks forming and ¯ecks of foamfalling from the specimen. Buckling of the pile yarns wasobserved but at a much higher load than the DF sam-ples, leading to the conclusion that the presence of thefoam was supporting the pile yarns against buckling.Following buckling, matrix cracking occurred in the pileyarns, followed by yarn failure and ®nal specimenfailure.

3.2. Flexure

The 3-point bend tests were conducted to determinethe shear characteristics of the various samples and theresults are presented in Fig. 4. The specimen dimensions(180 mm long � 50 mm wide � 25 mm high) werechosen to try and ensure that the specimens failed byshear failure of the core. This type of failure occurred in

Fig. 1. Illustration of DF showing fabric skins connected by vertical

yarns.

Fig. 2. Sandwich structure manufactured with DF and noncrimp

skins.

Fig. 3. Flatwise compressive strength.

688 M.K. Bannister et al. / Composite Structures 47 (1999) 687±690

the DF and SW specimens but the DF + PU specimenswere observed to fail in the compressive surface skin,therefore, the true core shear failure strength for thisspecimen would be higher than the measurement givenhere.

It is clear from the data in Fig. 4 that the combinationof the DF and the polyurethane core again leads to adramatic improvement in the ¯exural properties of thesandwich specimens, far greater than the performance ofthe individual components. Again it is thought that thepresence of the foam supports the pile yarns and pre-vents them from collapsing under the action of a shearforce. Similarly, the presence of the pile yarns sti�ensand strengthens the foam, preventing failure from oc-curring at low shear loads. The shear performance dueto the DF component of the specimens is also thought tobe helped by the fact that the pile yarns are not com-pletely vertical. These yarns tend to have an equal dis-tribution of ``S'' & ``Z'' shapes oriented in the lengthdirection of the test specimens which is a result of theweaving technique used to manufacture the raw DF it-self. These curvatures in the pile yarns would tend toprevent shear failure to a larger degree than specimenswith completely vertical pile yarns.

3.3. Climbing drum peel

The climbing drum peel test specimens had overalldimensions of 76 mm width and 300 mm length with25 mm long grip areas machined into the specimens (asper ASTM speci®cations). The results of these tests weremixed. The SW specimens all failed in the appropriatemanner with the skin being cleanly peeled o� the poly-urethane core at an average peel load of 21 kg. The DFspecimens resisted any attempts to peel the skin away,instead of peeling the samples bent and folded aroundthe test drum. In order to obtain relevant results itwould be necessary to sti�en the back face in order toresist the specimen bending. The DF + PU specimensproduced varied results. Of the four specimens tested,two resisted peeling and failed in a similar manner to theDF samples. The remaining two specimens peeled over asmall length (approximately 10 mm) at average loads of

250 and 175 kg, respectively, but this load increased asthe peel continued until the specimen was eventuallypulled out of the rig.

Although it is not possible to make absolute com-parisons between the di�erent types of specimens basedupon these results, it is obvious that the presence of thepile yarns in the DF and DF + PU samples has dra-matically increased the peel resistance of the sandwichstructures compared to a conventional structure. Fur-ther testing with improved specimens are needed beforestrict comparisons can be made.

3.4. Edgewise compression

The specimens for edgewise compression were iden-tical to those used for ¯atwise compression and the re-sults are shown in Fig. 5. Both the DF and SW samplesfailed at similar stresses and both failed by cracking ofthe faces in the centre of the gauge section with the facesof the SW specimen also delaminating slightly from thecore. These data suggest that the presence of the pileyarns in the DF specimens provides a similar level ofsupport against buckling of the faces as the foam core inthe SW specimens. The DF + PU samples failed atstresses 40% greater than that carried by the DF sam-ples, again with the faces cracking in the centre of thegauge section. It is not clear why this increase in failurestress was observed but the results again demonstratethe bene®ts of combining a foam core with DF toachieve a sandwich structure with improved mechanicalperformance.

4. Conclusions

A proprietary manufacturing process has been de-veloped at the CRC-ACS that allows high qualitycomposite sandwich structures to be produced using DFmaterials. This manufacturing technique also allows theincorporation of extra structural skins as well as a foamcore in a one-step consolidation process that could lead

Fig. 5. Edgewise compressive strength.

Fig. 4. Shear strength measured in 3-point bend.

M.K. Bannister et al. / Composite Structures 47 (1999) 687±690 689

to signi®cant cost bene®ts when compared to traditionalmethods of forming sandwich structures.

Specimens of 25 mm high DF were consolidated withvinyl ester resin using this process, following which, halfof the specimens were ®lled with a polyurethane foamcore. Specimens were also manufactured from precastpolyurethane cores that had composite skins added tothe face through a liquid moulding process. These skinshad comparable properties to the DF skins.

Flatwise compression, edgewise compression, climb-ing drum peel and ¯exure tests were carried out on allthree specimen types and the results clearly showed sig-ni®cant improvements in performance through the com-bination of the DF and the foam core. This improvementis thought to be due to the mutual reinforcement of thecore and pile yarns, where the core is supporting the pileyarns from buckling or shearing and the pile yarns gen-erally sti�en and strengthen the foam core.

Further work is needed to evaluate the mechanicalperformance of these materials, in particular, the impactperformance and peel resistance, but these initial resultsclearly demonstrate the potential of these materials toproduce composite sandwich structures with dramati-cally improved performance.

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