eeens

menithcome ae co7%e belam

glue-laminated sandwich beams with one layer of tri-axial glass bres did not prove to be effective. Over-all, it has been demonstrated that the glue-laminated sandwich beams exhibited better performance than

fabricaht bupace, malso bel consd highsandw

commonly used as structural panel for roofs, oors, walls andbridge decks [2]. However, very limited attempt has been madeso far to use these materials for structural beam applicationsalthough engineers have a wide range of composite sandwich pan-els. The main reason could be that most of the currently used corematerials are not appropriate for this type of structural application.

number of sandwich panels together either in the atwise (hori-zontal) or edgewise (vertical) positions. This concept is similar toglue-laminated (glulam) lumber used in timber engineering whereseveral smaller pieces of wood are horizontally or vertically lami-nated (either by nailing or gluing) to produce a single large, struc-tural member to support a greater load [9]. Similarly, severalbridge decks have been constructed by nailing together timberplaced in the edgewise position [10]. Moreover, Lopez-Anido andXu [11] developed a structural system based on the concept ofsandwich construction with strong and stiff FRP composite skins

* Corresponding author. Tel.: +61 7 4631 1385; fax: +61 7 4631 2110.E-mail addresses: manalo@usq.edu.au (A.C. Manalo), aravinthant@usq.edu.au

Composite Structures 92 (2010) 27032711

Contents lists availab

S

sev(T. Aravinthan), karunasa@usq.edu.au (W. Karunasena).innovative structural developments from this material. This com-posite material can also be combined with traditional constructionmaterials or be shaped and formed to carry loads that cannot becarried by the individual sandwich structure. In addition, sandwichstructure can be designed with the desired stiffness and strengthwith no additional weight to suit various structural applications.

Recent applications have demonstrated that composite sand-wich construction can be effectively and economically used inthe civil infrastructure. Composite sandwich materials are now

A new generation bre composite sandwich panel made up ofglass bre-reinforced polymer skins and modied phenolic corematerial has now been developed in Australia [7]. The exuralbehaviour of this innovative composite sandwich material wasinvestigated by Manalo et al. [8]. The results of their study sug-gested that the strength and stiffness of this innovative compositesandwich structure are suitable for structural beam applications.As these composite sandwich panels are produced in limited thick-nesses, a structural beam section could be produced by gluing a1. Introduction

Composite sandwich structuresthin but stiff skins to the lightweigwidely used in the automotive, aerostrial applications. This material hasinteresting alternative to traditionacause of its high bending stiffness antios [1]. The exibility of composite0263-8223/$ - see front matter 2010 Elsevier Ltd. Adoi:10.1016/j.compstruct.2010.03.006the individual composite sandwich beams. 2010 Elsevier Ltd. All rights reserved.

ted by attaching twot thick core have beenarine and other indus-en identied as a verytruction materials be-strength to weight ra-ich construction allows

The commonly used foam core and balsa wood are soft and willcrush under high compressive loads [3,4]. Honeycomb andtrussed-core structure have high compressive strength [5,6] butthe presence of cavities in these core materials reduces their capac-ity to hold mechanical connections. The evolution of a compositesandwich structure with lightweight, high strength core materialand with good holding capacity for mechanical connections pro-vides an opportunity to develop this material for structural beamapplications.Edgewise with greater ductility due to progressive failure of the bre composite skins while the specimens in theatwise position failed in a brittle manner due to debonding between the skin and core. Wrapping theFlexural behaviour of glue-laminated br

A.C. Manalo, T. Aravinthan *, W. KarunasenaCentre of Excellence in Engineered Fibre Composites (CEEFC), University of Southern Qu

a r t i c l e i n f o

Article history:Available online 11 March 2010

Keywords:Composite sandwich beamsGlue-laminated beamsFibre compositesFlexureFlatwise

a b s t r a c t

This study involved experiposite sandwich beams wbeams with 1, 2, 3, and 4bending test in the atwisThe results showed that thbending strength but havehowever indicated that thspecimens with increasing

Composite

journal homepage: www.elll rights reserved.composite sandwich beams

land, Toowoomba 4350, Australia

tal investigation onto the exural behaviour of glue-laminated bre com-a view of using this material for structural beams. Composite sandwichposite sandwich panels glued together were subjected to 4-point static

nd edgewise positions to evaluate their stiffness and strength properties.mposite sandwich beams in the edgewise position failed with 25% higherlower bending stiffness than beams in the atwise position. The resultsnding stiffness of atwise specimens converges to that of the edgewiseinations. More importantly, the specimens in the edgewise position failed

le at ScienceDirect

tructures

ier .com/locate /compstruct

bonded to an inner glulam panel. The glulam panels were fabri-cated with bonded eastern hemlock laminations. In addition, mostcurrently available commercial FRP decks are constructed usingassemblies of adhesively bonded bre composite pultruded shapes[2,12,13]. These examples show that the concept of gluing a num-ber of composite sandwich panels to form a structural beam ishighly practical.

This paper presents the results of an experimental investigationon the behaviour of glue-laminated composite sandwich beams in

an increase in deection even without an increase in the applied

2704 A.C. Manalo et al. / Composite Struthe atwise and edgewise positions to determine their applicationfor structural beams. A number of composite sandwich beamswere glued together and subjected to static 4-point bending testto determine their exural behaviour. The loaddeection behav-iour, strength and failure mechanisms of the glue-laminated com-posite sandwich beams are reported. The effects of the number andthe orientation of laminations as well as the glass bre wrappingon the strength and stiffness of glue-laminated composite sand-wich beams are also discussed.

2. Experimental program

2.1. Material properties

The structural composite sandwich beams tested in this studyare made up of glass bre composite skins co-cured onto the mod-ied phenolic core material using a toughened phenol formalde-hyde resin [7]. The bre composite skin is made up of two pliesof stitched bi-axial (0/90) E-CR glass bre fabrics manufacturedby Fiberex Corporation. The 0 glass bre weighs 400 g/m2 andhas a thickness of 0.512 mm while the 90 glass bre weighs300 g/m2 and has a thickness of 0.384 mm. The modied phenolicfoam core is a proprietary formulation by LOC Composites Pty. Ltd.,Australia. The composite sandwich panel has a nominal thicknessof 20.0 mm and has an overall density of approximately 1000 kg/m3. The mechanical properties of the bre composite skin andthe modied phenolic core material of this innovative compositesandwich panel were determined in earlier study by Manaloet al. [8] and are listed in Table 1.

2.2. Test specimen and preparation

Fig. 1 shows the preparation of glue-laminated composite sand-wich beam specimens. The specimens for the characterisation ofindividual sandwich beam behaviour were cut directly from thecomposite sandwich panels provided by the manufacturer. A num-ber of composite sandwich panels were assembled and glued to-gether in 2, 3 and 4 layers using Techniglue-HP R5 structuralepoxy resin supplied by ATL Composites (Fig. 1a). The glued sand-wich panels were then clamped for 24 h to initially cure the epoxyand were removed from clamping to post-cure at 90 C for 8 h toattain good bonding between the composite sandwich laminations.After curing, the glue-laminated sandwich panels were cut into therequired specimen width (Fig. 1b). The glue-laminated compositesandwich beams with 3 and 4 laminations were prepared with

Table 1Effective mechanical properties of bre composite skin and core material.

Property Skin Core

Modulus of elasticity (MPa) 14,280 1350Maximum tensile stress (MPa) 246 4.25Maximum tensile strain (%) 1.60 0.57Maximum compressive stress (MPa) 201 21.35

Maximum compressive strain (%) 1.24 1.94Thickness (mm) 1.8 16.4load due to progressive failure of the non-horizontal skins. The g-ure also shows that the load of specimen 2LSW-F increased linearlywith deection. This linear behaviour was observed until a load of6000 N and a deection of 7 mm. After this load, a non-linear re-sponse was observed until failure. When the tensile cracks oc-curred in specimen 2LSW-E at an applied load of 11,000 N, aslight drop in the load was observed. As the loading continued,there is a gradual decrease in bending stiffness due to the progres-sive failure of the bre composite skins. After compressive failureof the skins, the beams were still able to carry load but showedlarge deection until failure.

A similar load deection behaviour was observed in specimenswith 3 and 4 laminations (Fig. 4). The load deection curves forspecimen 3LSW-F and 4LSW-F are almost linear until the develop-ment of exural tensile cracks in the core material. A decrease instiffness was then observed until failure of the specimen. Adecrease in stiffness was also observed in specimen 3LSW-E and3. Experimental results

3.1. Loaddeection behaviour of composite sandwich beams

The load and midspan deection behaviour of individual com-posite sandwiches under 4-point static bending is shown inFig. 3. The gure shows that the deection of specimen 1LSW-F in-creased almost linearly with load up to nal failure. The specimenfailed at an applied load of 4550 N with a midspan deection of24 mm. The load of specimen 1LSW-E increased linearly withdeection but showed a reduction in stiffness at a load of around5000 N due to tensile cracking of the core. The specimen then con-tinued to carry load until 5500 N. Before the nal failure, there wassimilar width and depth to eliminate the geometrical effects. Thedescriptions of the test specimens are listed in Table 2.

The last four test conguration shown in Table 2 were preparedby wrapping the specimens with one layer of 750 g/m2 tri-axialglass bre composite (0/+45/45). In the preparation of thesespecimens, the corners of the glue-laminated composite sandwichbeams were rounded to a 15 mm radius in order to bend and wrapthe bres without causing damage. The bre wraps were providedthrough hand lay-up process in two different stages covering thetop and bottom surfaces with one layer of tri-axial glass bresand the sides with two layers of bres. Hyrex 201 epoxy resin[14] was used to impregnate and bond the bre wraps to theglue-laminated composite sandwich beam specimens. The mix ra-tio, by weight, of the epoxy and hardener was 100:20. After apply-ing the bre wraps, the specimen were pre-cured for 24 h atambient temperature and were post-cured at 90 C for 8 h beforethey are tested.

2.3. Test set-up and procedure

The 4-point static bending test on composite sandwich beamswas performed in accordance with the ASTM C393-00 standard[15]. The load was applied at 0.4 and at 0.6 of the span through a100 kN universal testing machine with a loading rate of 3 mm/min. The actual test set-up and instrumentation for the static ex-ural test of composite sandwiches are shown in Fig. 2. Before eachtest, the loading pins were set to almost touch the top surface ofthe composite sandwich specimen. The applied load, displacementand strains were recorded using a data logger. The failure mecha-nisms were also monitored and recorded.

ctures 92 (2010) 270327114LSW-E when tensile cracks of the core developed. As the loadingcontinues, there is a gradual decrease in the bending stiffness dueto progressive failure of the bre composite skins. Specimen

Fig. 1. Preparation of glue-laminated composite sandwich beams.

Table 2Description of specimen for exural test of composite sandwich beams.

Specimen Illustration Number of specimens D (mm) B (mm) Length, LT (mm) Support span (L) Orientation of testing

1LSW-F D

B

5 20 50 500 400 Flatwise

1LSW-ED

B

5 50 20 500 400 Edgewise

2LSW-FD

B

2 40 50 500 400 Flatwise

2LSW-ED

B

2 50 40 500 400 Edgewise

3LSW-F

B

D2 60 60 1400 1200 Flatwise

3LSW-ED

B

2 60 60 1400 1200 Edgewise

4LSW-F

D

B

2 80 80 1400 1200 Flatwise

4LSW-E

D

B

2 80 80 1400 1200 Edgewise

3LSW-WF

B

D1 60 60 1400 1200 Flatwise

3LSW-WE

D

B

1 60 60 1400 1200 Edgewise

4LSW-WF

D

B

1 80 80 1400 1200 Flatwise

4LSW-WE

D

B

1 80 80 1400 1200 Edgewise

A.C. Manalo et al. / Composite Structures 92 (2010) 27032711 2705

Stru2706 A.C. Manalo et al. / Composite3LSW-E and 4LSW-E continued to carry load even after compres-sive failure of the outer bre composite skins. The specimen thenbehaved non-linearly with a reduced stiffness up to failure.

In both composite sandwich beams with 3 and 4 laminations,the specimens in the atwise positions behaved slightly stifferthan specimens in the edgewise position. However, the compositesandwich beams in the edgewise position failed at a higher loadthan the specimens in the atwise position. Finally, the loaddeection curve indicated that the composite sandwich beamstested in the atwise position failed in a brittle manner while thecomposite sandwiches in the edgewise beams failed in a ductileexural mode. This could be due to the difference in the failuremode which is discussed in the next section.

Fig. 2. Test set-up and instrumentation of ex

Fig. 3. Load and midspan deection relationship of specimen 1LSW and 2LSW.

Fig. 4. Load and midspan deection relationship of specimens 3LSW and 4LSW.3.2. Failure behaviour of composite sandwich beams

Experimental investigation showed that the bre compositesandwich beams exhibited different failure behaviours in the at-wise and in the edgewise positions. These different failure modesof the composite sandwich beams are shown in Fig. 5. Tensilecracks of the core were observed at the bottom of specimen1LSW-F but these cracks did not cause immediate failure. The brecomposite skins on the tensile face of the specimen bridged thecracked core together to prevent failure. The specimen 1LSW-Ffailed due debonding between the core and the skins followed bycompressive failure of the bre composite skins as shown inFig. 5a. Tensile cracks of the core were also observed in specimen1LSW-E. However, the presence of the non-horizontal skins inthe edgewise position prevented the premature failure and madethe composite sandwich beams fail in a ductile failure mode. Thespecimen 1LSW-E failed due to progressive compressive failureof the bre composite skin followed by tensile failure of the skin(Fig. 5b).

Flexural cracks were observed on the core of the bottommostsandwich layers for specimens with 2, 3 and 4 laminations. Thesecracks originated at the top of the tensile skin and progressed withthe application of load. The bre composite skins, however,bridged the cracked core together to prevent the immediate failureof the specimen. When the depth of the exural cracks on the core

ural test for composite sandwich beams.

ctures 92 (2010) 27032711reached the level of the next bre composite skin, the crack widthincreased and a signicant drop in the load was observed. Thepresence of the bre composite skins, however, prevented theextension of the cracking of the core to the core of the next com-posite sandwich laminations. The glue-laminated sandwich beamsin the atwise positions failed due to compressive buckling of thebre composite skins followed by the debonding between the bot-tom skin and the core material as shown in Fig. 5c and e. Tensilecracks of the core were also observed in the edgewise specimenat the early application of load. The non-horizontal skins, however,prevented the tensile cracks in the core from widening to causeimmediate failure. As more cracks developed on the core, the dam-age on the specimen increased, thereby, decreasing the stiffness,and subsequently increasing the deection. The specimen contin-ued to carry load even after compressive failure of the outermostbre composite skin as shown by the small cracks which devel-oped near the loading point. The continuous application of loadcaused the outermost compressive skins to delaminate from thecore material and caused the cracks to propagate horizontally atthe region of constant moment. This failure resulted in decreasein lateral stability and eventually compression buckling of the de-tached bre composite skins. Splitting of the tensile bre compos-

StruA.C. Manalo et al. / Compositeite skins were also then observed. Final failure of the glue-lami-nated sandwich beams in the edgewise positions was due to simul-taneous compressive failure of the bre composite skins andcrushing of the core material followed by tensile failure of the skins(Fig. 5d and f). These different failure modes have affected thestrength capacity of the composite sandwich beams. Furthermore,the results of the experimental investigation showed neitherdelamination nor slipping occurred on the glue lines. These resultssuggest that the structural epoxy adhesives used in this study hasprovided a highly efcient glue joint between the composite sand-wich laminations and the full capacity of the glue-laminated com-posite sandwich beams was attained.

3.3. Load and longitudinal strain relationship

The load and longitudinal strain relationship of specimens3LSW and 4LSW are shown in Fig. 6. In these gures, the longitu-dinal tensile strain is designated with (T) and the longitudinal

Fig. 5. Failure of composctures 92 (2010) 27032711 2707compressive strain with (C). The results showed that the strainsin both tension and compression increased linearly with load atthe early stage of load application for all the tested specimens.However, a stiffer loadstrain relation curve can be noticed forspecimen 3LSW-F than 3LSW-E but became almost similar forspecimen 4LSW.

At a tensile strain of around 6000 micro strains, the strain gaugeat the tension side of the specimen broke, indicating the develop-ment of cracks in the core. It is noteworthy that this level of strainis comparable with the failure strain of the core in tension estab-lished from the test of coupons (see Table 1). The bre compositeskins prevented the sudden failure of the composite sandwichbeams. Non-linearity in the compressive strain was then observedindicating the further development of cracks in the core material.The strain gauge on top of the specimen broke at a compressivestrain of around 12,500 microstrains which indicated the compres-sive failure of the bre composite skins. Again, this value of strainrepresents the strain at which the bre composite skins failed in

ite sandwich beams.

hip

2708 A.C. Manalo et al. / Composite Structures 92 (2010) 27032711compression determined from the coupon tests. In this level ofstrain, the specimen tested in the atwise position failed instantlywhile the specimen in the edgewise position continued to carryload and failed at a higher load than expected. Thus, the maximumlevel of strain at which the specimen in the edgewise positionfailed was not captured by the strain gauges. Final failure of theedgewise specimen occurred only when the core crushed in com-pression followed by buckling of the inner bre composite skins.

4. Discussion

4.1. Effect of glue laminations on the stiffness of composite sandwichbeams

Evaluation of the effect of gluing on the stiffness of the compos-ite sandwich beams was conducted. The exural stiffness, EI of theglue-laminated sandwich beams was calculated using the elasticproperties of the bre composite skins and core material in Table1 and simple sandwich beam theory. Calculations were madeassuming that no interlayer slips occurred and the laminated sand-wiches acted as a solid section with perfect bonding. The contribu-tion of the epoxy adhesives in the exural stiffness is alsoneglected. The exural stiffness in the atwise position was esti-mated using Eq. (1) and in the edgewise position using Eq. (2).

EIflat Xn Bt3s Btsd2s

!Es Bt

3c Btcd2c

!Ec

" #1

Fig. 6. Load and strain relationsi1 12 12

EIedge nD3

6tsEs tc2 Ec

2

where B is the width of the sandwich beam, ts is the thickness of theskin, tc is the thickness of the core, ds and dc are the distances fromthe centre of the skins and the core to the neutral axis of the gluedsection, respectively, D is the depth or thickness of the sandwichbeam while Es and Ec are the modulus of elasticity of the skin and

Table 3Predicted and calculated EI, Eapp and bending strength of the sandwich beams.

Specimen EI (106) (N mm2) (EI)eff (106) (N mm2)1LSW-F 256 2471LSW-E 844 8192LSW-F 1287 12072LSW-E 1411 13733LSW-F 4753 48513LSW-E 4107 42704LSW-F 13,997 14,8114LSW-E 12,581 13,196core, respectively, and n is the number of glue-laminated compositesandwiches.

Eq. (3) was obtained based on the deection formula of a uni-form, static composite sandwich beam with loading congurationshown in Fig. 2a using classical beam theory. The effective bendingstiffness, (EI)eff of the composite sandwich beams (which considersthe combined effect of bending and shear deformations) was deter-mined from the results of the experimental investigation. Using theinitial linear elastic portion of the loadmidspan deection curve(Figs. 3 and 4), (EI)eff was calculated using the relation:

EIeff 59

3000L3

DPDv

3

where (DP/Dv) is the slope of the loaddeection curve. The appar-ent bending modulus of elasticity, Eapp of the composite sandwicheswas then computed by dividing (EI)eff by the second moment of areaof the homogenised cross-section of the composite sandwich beam.The predicted EI and the calculated stiffness (EI)eff, apparent bend-ing modulus and the maximum load, Pmax and bending strength,rb,max of the composite sandwich beams obtained from the experi-ment are reported in Table 3.

The results show that for individual composite sandwiches inthe atwise position, shear deformation has no signicant effecton the bending stiffness as the predicted EI is almost equal to theresult of the experimental investigation. On the other hand, the(EI)eff for the two sandwich beams bonded together is 37% lower

of specimens 3LSW and 4LSW.than the predicted values. This suggests that shear deformationcould have contributed to the total deformation of the compositesandwich in the edgewise position due to the decreased span todepth ratio. For longer beams with 3 and 4 laminations, the (EI)effis slightly higher than the predicted EI. The increased bending stiff-ness observed in the test is attributed to the friction effects be-tween the laminations provided by the epoxy adhesives. Thedifference between the predicted EI and (EI)eff is higher in speci-mens with four laminations as these specimens have lower span

Eapp (N/mm2) Pmax (N) rb,max (MPa)

8073 4554 2023957 5589 1944188 9472 1863723 13,788 2594253 9318 1953969 11,247 2594047 20,869 2003988 26,086 257

to depth ratio than the specimens with three laminations. In gen-eral, both EI and (EI)eff in the atwise position is higher than that inthe edgewise position for specimens with 3 and 4 laminations.

Fig. 7 shows the relationship between the apparent bendingmodulus of the glue-laminated composite sandwich beams andthe number of laminations. The results show that the Eapp of theglue-laminated composite sandwich beams in the atwise positiondecreases with increasing number of laminations. This decrease inEapp when the composite sandwich beams were laminated in theatwise positions is expected as the bre composite skins nearthe neutral axis of the section did not contribute as much stiffnessas the outermost skins. For both 3 and 4 laminations, the bendingstiffness in the atwise position is 7% higher than in the edgewiseposition. Moreover, the Eapp of the individual composite sandwichbeams in the edgewise position is almost equal to that of the glued

the composite sandwich beams together. This load sharing mecha-nisms led to the increased performance of the glue-laminatedsandwich beams. Furthermore, the loaddeection behaviour ofthe composite sandwiches tested in the edgewise positions sug-gests that, in this position, the specimen will fail in a ductile man-ner due to progressive failure of the skin. Finally, the resultssuggest that in multiple composite sandwich beams in the edge-wise position, the defect in individual sandwiches is compensatedby the stronger, adjacent sandwich beams.

4.3. Effect of bre wrapping on stiffness and strength

The comparison of the loaddeection relation of glue-lami-nated composite sandwich beams with and without bre wrapsare shown in Figs. 9 and 10. The gures show that the initialloaddeection behaviour of all specimens was linear and became

A.C. Manalo et al. / Composite Strucomposite sandwich beams. This clearly shows that the modulus ofelasticity in the edgewise position is not affected by the number oflaminations as the shear stresses induced by the exure are notcarried across the glue lines. Most importantly, the results showedthat the Eapp of the composite sandwich beams in the atwise andedgewise positions converges with increasing laminations. Thisinformation suggested that in the construction of beams with high-er depth of the same width, it is better to glue together compositesandwich structures in the edgewise position as these beams willresult in higher strength and with fewer glue lines.

4.2. Effect of glue laminations on bending strength of compositesandwich beams

The relative performance of the glue-laminated compositesandwich beams tested in the atwise and edgewise positionswas determined by calculating the apparent bending strengthbased on the results of the experimental investigation. Similarassumptions that no interlayer slips occurred and the laminatedsandwiches acted as a solid section with perfect bonding weremade. The bending strength of the composite sandwich beamswas determined from the maximum load measured in the experi-ment and was calculated using Eq. (4).

rb;exp PLD10EIeffEs 4

Fig. 8 shows the maximum bending strength of the glue-lami-nated composite sandwich beams for the different number of lam-inations. The results show that for individual composite sandwichbeams, the bending strength of specimens in the atwise positionis similar to that of the specimen in the edgewise position. In thesesandwich beams, the specimens failed due to compressive failureFig. 7. Apparent bending modulus of glue-laminated sandwich beams.of the bre composite skins at a stress of around 200 MPa. It isnoteworthy that this stress value represents the level at whichthe bre composite skins failed in compression as determined fromthe coupon test.

It is seen from Fig. 8 that, in the glue-laminated compositesandwich beams tested in the atwise position, the bendingstrength capacity is not affected by the presence of glue lines.There was no difference on the bending strength observed forthe different number of laminations. For all the composite sand-wich beams tested in the atwise position, failure occurred in abrittle manner due to compressive failure of the topmost skin fol-lowed immediately by debonding between the core and the brecomposite skins. This result suggests that the bending strength ofglue-laminated sandwich beams in the atwise position dependslargely on the compressive properties of the bre composite skins.

Gluing the composite sandwich beams together in the edgewiseposition resulted in an increase of at least 25% in bending strength.The results also show that the bending strength of sandwich beamsin the edgewise position increases with increasing number of lam-inations. The bending strength of glue-laminated specimens ishigher than the individual laminations. In the edgewise position,the non-horizontal skins prevented the widening of the tensilecracks in the core, thus preventing premature failure. Even aftercompressive failure of the bre composite skins at the outermostsandwich laminations, the beam continued to carry load until fail-ure as the load was shared to the inner bonded sandwich lamina-tions. The structural epoxy adhesives has also provided somereinforcing effect which prevented the compressive failure andbuckling of the bonded non-horizontal bre composite skins,delaying its failure, thereby, increasing its strength. This showsthat the glue lines acted as a load-distributing element and hold

Fig. 8. Bending strength of glue-laminated sandwich beams.

ctures 92 (2010) 27032711 2709non-linear with a reduced stiffness up to failure. As expected, thespecimen with bre wraps behaved slightly stiffer and failed at ahigher load compared to specimen without wraps. The higher

Strustiffness of the wrapped specimen became more apparent when

Fig. 10. Load and midspan deection relationship of specimens 4LSW and 4LSW-W.

Fig. 9. Load and midspan deection relationship of specimens 3LSW and 3LSW-W.

2710 A.C. Manalo et al. / Compositecracking of the core occurs and until nal failure. This could bedue to the bridging effect provided by the bre wraps on the localdefects in the specimen. Moreover, the composite sandwich beamswith bre wraps tested in atwise position behaved slightly stifferthan the specimens in edgewise position.

The behaviour of specimen with bre wraps tested in the at-wise position is similar to the specimen without wraps beforecracking of the core. In specimens 3LSW-F and 4LSW-F, a big dropin the load is observed when compressive failure of the skin oc-curred. On the other hand, compressive failure of the skins andcracking of the core in specimens 3LSW-WF and 4LSW-WF are rep-resented by smaller load drops. Interestingly, the rst load drop oc-curred at almost the same level of applied load and deection forboth wrapped and unwrapped specimens. As loading continues,the load starts to rise again but with a reduced stiffness as shownin the loaddeection curve. The wrapped specimen failed at 1835% higher load and a higher deection than the unwrapped spec-imens, thus showing more ductile behaviour. However, the resultsalso show that the bre wraps could not prevent or delay the com-pressive failure of the bre composite skin. The bre wraps willonly held the composite sandwich laminations together therebypreventing the separation between the skins and the core andincreasing the failure strength.

In the edgewise position, the bre composite wraps acted as aload-distributing element which resulted in a more ductile loaddeection behaviour. For specimens 3LSW-E and 4LSW-E, the pro-gressive failure of the bre composite skin is represented by smallload drops similar to a saw-tooth pattern. As a consequence ofwrapping, the progressive failure of the bre composite skins inspecimens 3LSW-WE and 4LSW-WE is characterised by a decreas-ing capacity but with a smoother non-linear loaddeection curve.Similarly, the progressive failure of the bre wraps did not createvisible load drops due to small percentage of additional bres (sin-gle wrap). The non-linear loaddeection response was terminatedby a sudden drop in the load as a result of the composite sandwichbeam failure. The increase in failure load of specimen with brewraps is in the order of 1015% compared to specimens withoutwraps. Noticeably, both the specimen with and without bre wrapsfailed at almost the same amount of deection. This result showsthat the strength and ductility of the glue-laminated compositesandwich beams are controlled primarily by the composite sand-wich beams and not that of the bre wraps. Nevertheless, the brewraps provided additional load sharing mechanism amongst thebonded composite sandwich beams. The presence of bre wrapsprevented the compressive buckling and debonding of the outer-most bre composite skins. This resulted in a wrapped glue-lami-nated composite sandwich beams tested in the edgewise positionto fail in a more ductile manner than the unwrapped specimens.

In general, the maximum load recorded for specimens with -bre wraps is almost the same in the atwise and edgewise posi-tions. However, if the rst initiation of damage (represented by adrop in the load) is considered as the failure of the specimen, thecomposite sandwich beams in the atwise position has a 2025%lower capacity than the specimen in the edgewise position. Thisdifference in strength between wrapped specimens in the edge-wise and atwise positions is similar to the unwrapped specimens.Finally, the increase in stiffness and strength is due to additionalreinforcement provided by the bres and not the conning effectof bre wrapping.

4.4. Effect of wrapping on failure behaviour

The failure modes of glue-laminated composite sandwichbeams with bre wraps are shown in Fig. 11. The results show thatbre wrapping has some signicant effects on the failure behav-iour of the composite sandwich beams. The presence of bre wrapsprevented the immediate failure of the specimen as it held up thelaminated sandwich beams together which resulted in a higherstrength before failure.

At the early stage of loading, the noise related to micro-crackingof the core was evident. Prior to failure, the cracking noise are morefrequently heard. In all specimens, the failure of the glue-lami-nated composite sandwich beams with bre wraps was initiatedat the compressive part at the constant moment region. This failuremechanism is similar to what was observed in the specimen with-out bre wraps. Also, the failure mechanisms of wrapped speci-mens indicated that very little conning effect was provided bythe bre wrap. After compressive failure of the skin in the compos-ite sandwich beams was detected, progressive failure of the brewraps immediately followed. Several points of debonding failurewere observed between the bre wrap and the specimen at thecompressive side followed by splitting of the bre wraps in ten-sion. Thus, it was concluded that the failure of glue-laminatedcomposite sandwich beams is governed by the strength of thecomposite sandwich beams and not that of the bre wraps. How-ever, the wrapped specimen failed with more ductility than thespecimen without wrap.

The test results have shown some positive effects of bre wrap-ping on the exural behaviour of glue-laminated composite sand-wich beams. In all of the experimental cases, there wasconsiderable amount of increase in the strength and stiffness ofthe wrapped specimens compared to specimens without brewraps. However, its overall effect on the behaviour of glue-lami-nated composite sandwich beams cannot be justied because ofthe additional cost of preparation and bres for wrapping. The fail-ure initiation of glue-laminated composite sandwich beams withbre wraps is almost the same as that for specimen without wraps.

ctures 92 (2010) 27032711More effective results could have been obtained with specimenswith more layers of bre wrapping but increasing the number ofbre wraps would denitely entail higher costs.

pos

A.C. Manalo et al. / Composite Structures 92 (2010) 27032711 27115. Conclusions

The exural behaviour of glue-laminated composite sandwichbeams was determined through experimental investigation. Theresults showed that gluing the composite sandwich beams to-gether resulted in a stronger and more stable section than individ-ual sandwich beams alone. The results also suggest that using thesame amount of material, glue-laminated composite sandwichbeams in the edgewise position could offer up to 25% increase instrength compared to beams in atwise position but with a slightlylower bending stiffness. The presence of non-horizontal skins inthe edgewise position increases the load carrying capacity and re-sult in a more ductile failure behaviour. The nal failure of thespecimen in the edgewise position is due to simultaneous com-pressive failure of the inner skins, crushing of the core and tensilefailure of the bre composite skins. In the atwise position, thefailure is governed by the compressive failure of the skin followedby debonding between the skin and the core, thus resulting to brit-tle failure. Furthermore, the overall effect of one layer of tri-axialglass wrapping on the exural behaviour of glue-laminated com-posite sandwich beams cannot be justied with the additional costof bres and preparation. Even with bre wraps, the failure initia-tion of glue-laminated composite sandwich beams is almost thesame as that for specimen without bre wraps. Finally, the resultsof this study demonstrated the high possibility of developing a

Fig. 11. Failure of glue-laminated comstructural beam from glue-laminated composite sandwich struc-tures. Currently, research is being conducted to optimise theglue-laminated composite sandwich beams with the objective ofexploring the practical application of this for composite structures.

Acknowledgements

The authors gratefully acknowledge the technical and materialssupport provided by Dr. Gerard Van Erp and the staff of LOC Com-posites Ltd., Pty., Australia. The support of Mr. Atul Sakhiya and Mr.Christopher Pickford in the preparation and testing of the compos-ite sandwich beams are greatly acknowledged.

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Flexural behaviour of glue-laminated fibre composite sandwich beamsIntroductionExperimental programMaterial propertiesTest specimen and preparationTest set-up and procedure

Experimental resultsLoaddeflection behaviour of composite sandwich beamsFailure behaviour of composite sandwich beamsLoad and longitudinal strain relationship

DiscussionEffect of glue laminations on the stiffness of composite sandwich beamsEffect of glue laminations on bending strength of composite sandwich beamsEffect of fibre wrapping on stiffness and strengthEffect of wrapping on failure behaviour

ConclusionsAcknowledgementsReferences