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Composites: Part B 39 (2008) 271–281

Ductile double-lap joints from brittle GFRP laminatesand ductile adhesives, Part I: Experimental investigation

Julia de Castro, Thomas Keller *

Composite Construction Laboratory CCLab, Swiss Federal Institute of Technology EPFL, BP Ecublens, Station 16, CH-1015 Lausanne, Switzerland

Received 5 September 2005; received in revised form 15 February 2007; accepted 16 February 2007Available online 28 February 2007

Abstract

Quasi-static axial tension experiments were performed in a laboratory environment on adhesively-bonded double-lap joints from pul-truded GFRP laminates. Full-scale specimens were investigated to prevent size effects. Ductile polyurethane and acrylic as well as brittleepoxy adhesives were applied to connect brittle GFRP adherends. The visco-elastoplastic/ductile and visco-elastic/brittle stress–strainbehavior of adhesives was defined. It is shown that joint stiffness depends non-linearly on the ratio of adhesive to adherend modulus,and approaches a threshold value with increasing adhesive modulus. Ductile joints with plasticized adhesives develop uniform load trans-fer over the joint length with increased strength as compared to joints with brittle adhesives. In contrast to joints with brittle adhesives,the joint strength of ductile joints with plasticized adhesive increases almost proportionally with increasing overlap length. Axial strainsare almost uniformly distributed across the joint width and allow for a 2D analysis.� 2007 Elsevier Ltd. All rights reserved.

Keywords: B. Plastic deformation; B. Strength; E. Pultrusion; E. Joints/joining; Adhesive

1. Introduction

Since the early 1990s fiber-reinforced polymer (FRP)composites have been used increasingly in engineering struc-tures. The first applications of these materials were seen inrehabilitation and upgrading for repair and strengtheningpurposes, and have been accepted worldwide [1–3]. Struc-tural FRP components such as profiles and slab structuresare also starting to be used for new constructions. Theyare produced by pultrusion, an automated process usedfor straight profiles with a constant section and high fibercontent. The main applications for structural FRP compo-nents are pedestrian bridges and bridge decks, where theadvantages of these materials such as high specific strength,insensitivity to frost and de-icing salts, and rapid componentinstallation can be exploited [3,4].

1359-8368/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compositesb.2007.02.015

* Corresponding author. Tel.: +41 216 933 226; fax: +41 216 936 240.E-mail address: thomas.keller@epfl.ch (T. Keller).

Despite the great potential of FRP materials, they havenot become widespread due to certain material propertiesthat still hinder their acceptance by structural engineersmore familiar with conventional construction materialssuch as steel or reinforced concrete. One of these materialproperties is the lack of inherent ductility. Ductile materialsallow for the redistribution of internal forces therebyresulting in increased structural safety and the dissipationof energy from impact or seismic actions. They also givewarning of possible structural problems thanks to largeplastic or inelastic deformations before failure. A seconddisadvantage of FRP materials is the difficulty of joiningstructural components due to their brittle, fibrous andanisotropic character. The current practice of bolting isnot material-adapted and in most cases leads to an over-sizing of components [3]. According to [5 and 6], adhesivebonding is far more appropriate for FRP materials.

To overcome these two disadvantages of FRP struc-tures, Keller and de Castro [7] have proposed a new con-cept for structures composed of brittle FRP components

Nomenclature

Ec,e compressive elastic modulusEc,p compressive plastic modulusEt,e tensile elastic modulusEt,p tensile plastic modulusF axial loadGe shear elastic modulusGp shear plastic modulusfc,e compressive elastic stressfc,u compressive ultimate or maximum stressft,e tensile elastic stressft,u tensile ultimate or maximum stress

kjoint joint stiffness (kN/mm)ujoint joint elongationu(i) displacement of point i in axial direction (video-

extensometer)ec,u compressive ultimate or maximum strainet,u tensile ultimate or maximum straincu shear ultimate or maximum strainm Poisson’s ratiose shear elastic stresssu shear ultimate or maximum stress

300

100

900

400

400

100

150

150

260

100

140

10

5

22

5

300

350

5050

5050

50

100

10

50 50

300

100

900

400

400

100

150

150

260

100

140

10

5

22

5

300

350

5050

5050

50

100

10

50 50

Jaws

Fig. 1. Specimen with 100 mm overlap length and external strain gagepositioning.

272 J. de Castro, T. Keller / Composites: Part B 39 (2008) 271–281

providing system ductility through the use of ductile adhe-sive joints and statically indeterminate (redundant) struc-tural systems. The proposed concept envisages tailoredductile adhesives with an initial elastic behavior that is suf-ficiently stiff to meet the short and long-term serviceabilityrequirements (particularly creep). However, when the ser-viceability (SLS) and ultimate (ULS) loads are exceeded,the behavior of the adhesives should change and becomeplastic or at least highly non-linear/inelastic with a muchsmaller stiffness.

Research was performed to analyze the influence of duc-tile and brittle adhesives with different adhesive-to-adherendmoduli on the load-carrying behavior of adhesive jointsmade of brittle pultruded glass-fiber-reinforced polymer(GFRP) adherends [8]. Part I of this two-part paper presentsexperimental results regarding the influence of brittle andductile mechanical properties of adhesives on the load-carrying behavior of double-lap joints subjected to quasi-static axial tensile loading. Part II describes joint analysiswith a non-linear finite element model to consider the effectsof adhesive ductility on joint stiffness and strength [9].

2. Specimen description

2.1. Specimen dimensions and parameters

Balanced double-lap joint specimens with different adhe-sives and overlap lengths were fabricated using pultrudedGFRP laminates. The dimensions of specimens with a100 mm overlap length are shown in Fig. 1. In contrastto single-lap joints, the double-lap configuration minimizesbending moments due to load eccentricity. Full-scale spec-imens were used to avoid size effects and enable instrumen-tation of joints with strain gages (see Section 3.2). Thespecimens consisted of three laminates of 500 mm lengthand 100 mm width. The outer laminates were 5 mm thickwhile the inner laminate was twice as thick (10 mm) to pro-vide a constant cross-sectional area and therefore constantnominal axial stresses. Two overlap lengths were studied,100 mm and 200 mm. The laminate length provided suffi-

cient distance between machine grips and joint to not influ-ence the joint area by load introduction effects. The chosenadhesive thickness was 2 mm due to the tolerances of thelaminates and the wires of the strain gages bonded ontothe inner 10 mm thick laminate. The quality of the bond-lines was visually checked before and after testing. Onlyfew air bubbles were found in some joints. However, theywere small and judged to not influence the joint behavior.Three different adhesives were used: a ductile acrylic-basedadhesive (designated ADP), a ductile polyurethane adhe-sive (PU) and a brittle epoxy adhesive (EP). Table 1shows the parameter combinations, the denominationand the number of specimens examined. The specimendenomination is in accordance to [8]. A and B denominatethe 100 mm overlap length (with different surface treat-ment, see Table 1), while D indicates the 200 mm overlaplength.

Table 1Overview of lap joint specimen configurations and experimental results

Series No. ofspecimens

Overlaplength(mm)

Surface treatment Ultimatefailure load(kN)

Ultimateelongation(mm)

Ultimate averageshear stress(MPa)

Jointefficiency(%)

Failuremode

EP.A 12 100 Sanding 141 ± 11 (160a) 4.1 ± 0.4 7.1 ± 0.6 43 ± 3 (48a) Fiber-tearEP.D 3 200 Sanding 182 ± 15 (194a) 4.7 ± 0.6 4.5 ± 0.3 55 ± 4 (58a) Fiber-tearPU.A 3 100 Sanding 160 ± 16 (171a) 4.8 ± 0.6 8.0 ± 0.8 48 ± 5 (52a) Mixedb

PU.B 8 100 Sanding + activator + primer 140 ± 13 (157a) 3.9 ± 0.3 7.0 ± 0.7 42 ± 4 (47a) Light-fibertear/mixedb

ADP.A 17 100 Sanding + activator + primer 131 ± 21 (173a) 6.4 ± 0.8 6.5 ± 1.0 39 ± 6 (52a) Mixedb

ADP.D 3 200 Sanding + activator + primer 253 ± 8 (258a) 8.7 ± 0.6 6.3 ± 0.2 76 ± 3 (78a) Mixedb

a Maximum value of series.b Mixed failure: adhesion promoter-to-substrate failure and light-fiber-tear and/or fiber-tear failure.

Table 2Fiber architecture and fractions by volume of 5 and 10 mm laminates

Reinforcement Laminate100 · 5 mm

Laminate100 · 10 mm

Architecture Vol.% Architecture Vol.%

Rovings (UD) 4:1 straightand blown

37 4:1 straightand blown

32

Combined mats 2 · 1 2 · 2– CSM (g/m2) 300 5 450 6– Woven 0�/90� (g/m2) 150/150 5 300/300 8

Total 47 46

Table 3Tensile properties of 5 and 10 mm laminates (supplier properties)

Laminate Failure stress(MPa)

Failure strain(%)

E-modulus(GPa)

100 · 5 mm 434 ± 18 (240) 1.38 ± 0.19 32.3 ± 2.3 (23)100 · 10 mm 332 ± 14 (240) 1.03 ± 0.07 28.3 ± 2.8 (23)

J. de Castro, T. Keller / Composites: Part B 39 (2008) 271–281 273

2.2. Material properties

2.2.1. GFRP laminates

The laminates were cut from standard structural lami-nates and delivered by the manufacturer (Fiberline Com-posites, Denmark [10]). They consisted of E-glass fibersembedded in an isophtalic polyester resin. The fiber archi-tecture comprised mainly unidirectional rovings towardsthe center and, depending on the laminate thickness, oneor two combined mats towards the outside. A 10 mm thicklaminate with two mats is shown in Fig. 2. The combinedmats consisted of chopped strand mats (CSM) and wovenmats 0�/90� of different weights, both stitched together. Onthe outside, a polyester surface veil (40 g/m2) protectedagainst environmental actions. The fiber fractions deter-mined by a resin burn-off in [11] are listed in Table 2.The failure stress and strain of the laminates were deter-mined through full-scale tensile tests on eight specimens

Fig. 2. Microscopic section through 10 mm GFRP laminate: rovings incenter, two combined mats on each side.

[12] and are listed in Table 3. The tensile longitudinalYoung’s moduli (E-modulus) were obtained from mea-sured axial strains on 21 specimens. The conservative prop-erties provided by the manufacturer’s design manual [10]are also listed in Table 3 in brackets.

2.2.2. Adhesives

Two different ductile two-component adhesives wereused: an acrylic adhesive (SikaFast 5221), based on AcrylicDouble Performance Technology (ADP [13]) and a slightlystiffer polyurethane adhesive (S-Force 7851). For compari-son, a brittle two-component epoxy adhesive was also used(SikaDur 330). All adhesives were delivered by Sika AG,Zurich. The adhesive properties were obtained from axialtensile tests according to EN ISO 527-1, axial compressiontests according to ASTM D 695-96 and from napkin-ringshear tests based on EN 11003-1. Average nominal stressesand strains (based on the undeformed geometry) were trans-formed into true stresses and strains (based on deformedgeometry).

Average true stress–strain curves are shown in Fig. 3.Only tensile and compressive curves are given for the epoxy

274 J. de Castro, T. Keller / Composites: Part B 39 (2008) 271–281

adhesive, since shear tests did not provide conclusive resultsdue to a high adhesive-to-adherend modulus ratio. The EPadhesive exhibited almost linear-elastic tensile axial behav-ior and brittle failure. The compression curve from the sameadhesive showed a reduction in stiffness close to failure;compression failure, however, occurred along a surfaceinclined at �40–45� to the vertical axis, representing thetypical failure mode of brittle materials (refer to Fig. 4a).

The PU and ADP adhesives exhibited similar highlynon-linear behavior with large deformations, as shown inFig. 3b and c. The PU, however, was much stiffer thanthe ADP adhesive. All curves showed increased stiffness

0

20

40

60

80

100

0 1 2 3 4

Strain [%]

Str

ess

[MP

a]

tension

compressionEP

0

50

100

150

0 10 20 30 40 50

Strain [%]

Str

ess

[MP

a]

tension

compression

shear

PU

0

10

20

30

0 50 100 150 200 250

Strain [%]

Str

ess

[MP

a]

tension

compression

shear

ADP

b

c

Fig. 3. Average true stress–strain curves for (a) EP, (b) PU, (c) ADPadhesives.

Fig. 4. Compression failure mode of (a) EP specimens, (b) ADPspecimens.

in the low load range with the exception of the ADP com-pression curve. The failure of PU and ADP compressivespecimens was difficult to analyze since the specimens werecompressed up to a nominal contraction of 60%, whichcorresponded to a true compressive maximum strain of47%. The PU compressive specimens exhibited a kind ofbuckling failure. In the ADP compressive specimens, thelarge longitudinal contraction induced a high transverseexpansion, which produced an internal failure, as shownin Fig. 4b.

Average true adhesive property values for tension (from5 specimens) and compression (from 3 or 4 specimens) arereported in Table 4, while Table 5 gives the results for shear(from 2 specimens). The standard deviations varied from1.6% of the mean (ADP, ultimate tensile strain from 5specimens) to 6.8% (EP, ultimate tensile strain from 5specimens) [8]. The values given in Tables 4 and 5 wereobtained as follows: the EP adhesive was modeled with alinear-elastic behavior while the ADP and PU adhesiveswere modeled with a bilinear curve. The lower-load portionof the curve was considered elastic, while the higher-loadportion with much smaller stiffness was denominated byplastic behavior (see discussion in Section 5.1). The elasticmoduli in tension and compression, Et,e and Ec,e, and theshear elastic moduli, Ge, were estimated using the secantof the true stress–strain curves, at the 0.5–1.0% strain inter-vals (see Fig. 3). The tensile and compressive plastic mod-uli, Et,p, and Ec,p, and shear plastic moduli, Gp, of the PUand ADP adhesives were calculated from the secants at the10–20% (PU) and 30–50% (ADP) strain interval, respec-tively [8]. The tensile and compressive elastic stresses, ft,e

and fc,e, and shear elastic stresses, se, designate the elasticto plastic modulus transition. The stresses, ft,u, fc,u andsu, and strains, et,u, ec,u and cu, correspond to the ultimateor maximum values. All adhesives showed significantlyhigher tensile elastic than compressive elastic moduli.

The tensile tests were performed at a displacement rateof 0.5 mm/min up to 0.25% strain. The rate was thenincreased to 5 mm/min until failure. Additional tensile testswere performed on ADP specimens with varying loadingrates (10, 50, and 100 mm/min in the second phase) to

Table 4Average true tensile and compressive mechanical properties of adhesives EP, PU and ADP

Adhesive Et,e (MPa) Et,p (MPa) ft,e (MPa) ft,u (MPa) et,u (%) Ec,e (MPa) Ec,p (MPa) fc,e (MPa) fc,u (MPa) ec,u (%) m (�)

EP 4563 – – 39 1 3064 – – 84 3 0.37PU 586 31 15 25 32 433 79 13 135 47 0.42ADP 101 14 3 29 97 15 10 1 16 47 0.40

Table 5Average true shear mechanical properties of adhesives PU and ADP

Adhesive Ge (MPa) Gp (MPa) se (MPa) su (MPa) cu (%)

PU 355 42 10 21 40ADP 33 3 3 8 230

0

5

10

0 20 40 60

Tensile strain [%]

Ten

sile

str

ess

[MP

a]

t=30 min

t=0 min

t=12 h

ADP

Δ

Δ

Δ

Fig. 6. Nominal tensile load-unload-reload stress–strain curves for ADPadhesive, reloading after 0 and 30 min and 12 h.

0

20

40

60

80

100

0 100 200 300 400 500 600 700 800

Time [min]

Res

idua

l-to-

tota

l str

ain

ratio

[%

]

Fig. 7. Time-dependent ratio of residual-to-total strain (total = recov-ered + residual) of ADP adhesive.

J. de Castro, T. Keller / Composites: Part B 39 (2008) 271–281 275

investigate the sensitivity to the loading rate. According toHart-Smith [14], higher ultimate strength and lowerultimate strain could be expected when testing at higherdisplacement rates. This behavior, however, was notobserved, as shown in Fig. 5. Both ultimate strength andstrain did not vary significantly with increasing loadingrate, but the elastic stress (indicating the transition fromelastic to plastic behavior) increased with increasing load-ing rate and the plastic portion of the curve was shiftedupwards. The elastic and plastic moduli remained almostconstant.

In view of ductile adhesive behavior, load-unload-reloadtensile experiments were also performed on ADP specimensto study the loading, unloading and reloading paths and theresidual deformation after unloading. The specimens werefirst loaded up to �20% of the specimen strength, followedby a complete unloading. The specimens were keptunloaded during increasing periods of time Dt (Dt = 0, 5,30 min and 12 h). The experiments finished with a reloadinguntil failure. Loading, unloading and reloading were per-formed at the same displacement rate of 5 mm/min. Result-ing nominal loading-unloading-reloading stress–straincurves are shown in Fig. 6. All specimens showed a highlynon-linear loading curve and a reloading curve thatapproached the original loading curve with increasing load.For the unloading period Dt = 0 min, the unloading and

0

5

10

15

20

25

30

35

0 20 40 60 80 100

Tensile strain [%]

Ten

sile

str

ess

[MP

a]

v=5 mm/min

v=100 mm/min

v=10 mm/min

v= 50 mm/min

ADP

Fig. 5. Average true tensile stress–strain curves for ADP adhesive atdifferent loading rates (5, 10, 50, 100 mm/min).

reloading path were quite similar showing a residual strainof �35%. The residual strain decreased with increasingunloading period Dt due to an increase of time-dependentrecovered strain (recovered strain according to [15], some-times referred to as delayed elastic strain [16]). Fig. 7 showsthe resulting time-dependent ratio of residual to total strain(total = recovered + residual). The ratio rapidly decreasedfor unloading periods Dt up to 30 min to �40%, but thenstabilized and approached a threshold of �20% after 12 h.

2.3. Specimen manufacturing

The surface treatment of the GFRP laminates consistedof degreasing, sanding (until removal of the polyester sur-face veil) and cleaning of the bonded area. The EP.A,EP.D and PU.A specimens were then ready for adhesiveapplication, while on PU.B, ADP.A, and ADP.D specimens

276 J. de Castro, T. Keller / Composites: Part B 39 (2008) 271–281

an activator and a primer was applied first, as stipulated inthe specifications of the adhesive supplier. Glass balls of2 mm diameter were placed on the bonding area to guaran-tee a constant and accurate adhesive-layer thickness. Thecuring time was at least one week under ambient laboratoryconditions (23 ± 2�C). After one week of curing, theresponses did not significantly change any more.

3. Experimental set-up and instrumentation

3.1. Experimental set-up and procedure

The experimental set-up is shown in Fig. 8. Specimenswere subjected to an axial tensile quasi-static load by aSchenk Hydropuls-Zylinder Typ PL testing machine witha static capacity of 1000 kN and a displacement range upto ±250 mm. The diameter of the horizontal hydraulicallycontrolled circular jaws was 150 mm. Grip lengths of140 mm and 100 mm for the 10 mm and 5 mm laminate,respectively, were used (see Fig. 1), making the specimenlengths between grips 660 mm and 560 mm for the 100and 200 mm overlap lengths, respectively. The tests wereconducted in a laboratory environment at room tempera-ture. The load was applied at a constant displacement rateof 0.6 mm/min until specimen failure.

3.2. Instrumentation and measurements

The load and total specimen elongation were measuredby transducers within the testing machine. Specimens were

Fig. 8. Schenk testing machine with clamped specimen.

instrumented with up to ten external strain gages (see Fig. 1)to measure axial strain distributions along the length andthrough the width of the specimen, and to detect load eccen-tricities in the transverse or through-thickness directionsdue to possible misalignments of the laminates.

In addition, internal strain gages were applied inside thebonded joints on the 10 mm laminate according to twoconfigurations shown in Fig. 9. Configuration 1 consistedof eight gages placed in three sections to measure the straindistributions across the joint width. Configuration 2 con-sisted of ten gages staggered in nine sections to providethe strain distributions along the overlap length. As strainsnormally greatly increase towards the joint edges comparedto the middle portion of the joint, the gages were concen-trated between 5 and 20 mm from the joint edges. Straingages of the type 1,5/120LY18, manufactured by HBM,were used. A small gage size of 5 mm · 6 mm was chosento prevent an effect on the adhesive bond. The less than0.5 mm thick wires were embedded in the 2 mm thick adhe-sive layer and taken out of the joint at the edges on theshortest ways. Identical behavior and ultimate failure loadsof specimens with and without internal gages proved thatgages did not affect load transfer.

Furthermore, a video-extensometer (Messphysik MTV-1362CA) was used to record the position of fifteen mea-surement points on the joint area, that is, five points oneach axial line at distances of 22 or 25 mm (see Fig. 10).Axial joint elongations and stiffnesses were estimated fromthe measured relative elongations of the laminates betweenthese points.

100

10 104040

100

1010

4040

100

15

5050

1027

20

2627

10

20

5

10

5

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10 104040

100

1010

4040

100

15

5050

1027

20

2627

10

20

5

10

5

15

10

Fig. 9. Internal strain gage positioning: (a) configuration 1 and (b)configuration 2.

1 2 3 4 5

6 7 8 9 10

11 12 13 14 15

u (1), u (11) u (10)

FF

100

22 2225 253 3

1 2 3 4 5

6 7 8 9 10

11 12 13 14 15

u (1), u (11) u (10)

FF

100

22 2225 253 3

Fig. 10. Positioning of video-extensometer measurement points 1–15 anddefinition of point displacement u(i).

0

50

100

150

200

250

300

0 2 4 6 8 10

Elongation [mm]

Load

[kN

] EP.D1 ADP.D1

Fig. 12. Load-elongation curves for specimens EP.D1 and ADP.D1(200 mm overlap).

J. de Castro, T. Keller / Composites: Part B 39 (2008) 271–281 277

4. Experimental results

4.1. Specimen load-elongation response

Table 1 summarizes the following main experimentalresults: ultimate failure load, ultimate elongation, ultimateaverage shear stress, joint efficiency (each with average val-ues and standard deviations) as well as failure modes. Theaverage shear stress was calculated dividing the load by thetwo bonded areas. The joint efficiency is defined as the ratioof the ultimate joint failure load to the ultimate adherendfailure load [17]. The ultimate failure loads in brackets cor-respond to the maximum values obtained from each series.

Fig. 11 shows measured load-elongation curves of repre-sentative specimens of series EP.A, PU.B and ADP.A(100 mm overlap length). The responses of specimensEP.A and PU.B were almost identical and linear-elasticup to �70 kN (approx. 50% of ultimate failure load forboth series). At this load level, a slight decrease in specimenstiffness was observed. The joint efficiencies were between40% and 50%. Specimens ADP.A exhibited bilinear behav-ior as found for the ADP adhesive, with a stiffness reduc-tion at �30 kN (�25% of ultimate failure load). Theaverage joint efficiencies were lower than for the EP andPU joints, but they showed a high scatter with comparablyhigh maximum values of 52%, see Table 1.

Fig. 12 shows measured load-elongation curves of repre-sentative specimens of series EP.D and ADP.D (200 mmoverlap length). Specimens EP.D were linear-elastic againup to �70 kN (�40% of ultimate failure load). SpecimensADP.D exhibited bilinear behavior with a stiffness reduc-tion at �70 kN (28% of ultimate failure load). The averagejoint efficiencies were 55% for EP. D and 76% for ADP.Djoints with a maximum of 78%.

4.2. Failure modes

Failure of specimens EP.A and EP.D occurred in the 5and 10 mm laminates between the two outer fiber layers(between the combined mat and the UD-rovings in the

0

50

100

150

200

0 1 2 3 4 5 6 7

Elongation [mm]

Load

[kN

]

PU.B1EP.A4 ADP.A3

Fig. 11. Load-elongation curves for 100 mm overlap specimens EP.A4,PU.B1, ADP.A3.

5 mm laminates and between the two combined mats inthe 10 mm laminates) or inside the first combined mat, seeFig. 13(a–b). According to ASTM D 5573-99 the failuremode was classified as a ‘‘fiber-tear’’ failure (also referredto as interlaminar adherend failure). This typical type offailure is caused by the relatively low through-thicknessshear and tensile strength of the laminates (see Part II, Sec-tion 5) [6,12]. Failures occurred in a brittle manner withoutprevious appearance of cracks in the joint area. In order toinvestigate the crack initiation and failure process, Kellerand Vallee [12] performed similar experiments on similarEP specimens involving a high-speed camera. They con-cluded that failure was initiated in the inner 10 mm laminatewith smaller through-thickness strength and demonstratedthat the cracks in the 5 mm laminate occurred due todynamic effects. The failure surfaces inside the 10 mm lam-inates were at a depth of�0.5 mm. The depth varied slightlydue to the variability of the fiber-layer position (see Fig. 2).

Specimens PU.A and PU.B exhibited two kinds of fail-ure modes, a ‘‘light-fiber-tear’’ failure or a mixed failurecombining ‘‘adhesion promoter-to-substrate’’ and ‘‘light-fiber-tear’’ failure according to ASTM D 5573-99. Light-fiber-tear failure occurred in the laminates near the surfaceand was characterized by a remaining thin layer of matrixon the adhesive with few or no fibers, see Fig. 13c. Adhe-sion promoter-to-substrate failure (also denominated asadhesion failure) occurred in the interface between the pri-mer and activator layers and revealed physical or chemicalincompatibility.

Specimens ADP.A and ADP.D exhibited a mixed failurecombining fiber-tear and/or light-fiber-tear and adhesion-promoter-to substrate failure, see Fig. 13d. The mixed fail-ure mode explained the large scatter of the ultimate failureloads of the ADP.A specimens.

4.3. Joint elongation and joint stiffness

Load-joint elongation curves for representative speci-mens of series EP.A, PU.B and ADP.A, with a 100 mm over-lap length, are illustrated in Fig. 14. Axial joint elongation,

Fig. 13. Failure modes (a) and (b) EP specimens: fiber-tear, (c) PU: light-fiber-tear, (d) ADP: adhesion promoter-to-substrate failure and light-fiber-tearfailure (mixed).

0

50

100

150

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Joint elongation [mm]

Load

[kN

] PU.B7 ADP.A9

EP.A8

Fig. 14. Load-joint elongation of specimens EP.A8, PU.B7 and ADP.A9.

Table 6Joint stiffness and joint-to-specimen elongation ratio of series EP.A, PU.Band ADP.A at different load levels (three specimens per series)

Jointcharacteristics

EP.A(50 kN)

PU.B(50 kN)

ADP.A(20 kN)

ADP.A(100 kN)

Joint stiffness(kN/mm)

474 ± 4 385 ± 22 135 ± 31 33 ± 6

Joint/specimenelongation (%)

8 ± 1 9 ± 1 24 ± 5 51 ± 2

0.00

0.05

0.10

0.15

0.20

0.25

0 20 40 60 80 100

Position across width [mm]

Axi

al s

trai

n [%

]

right section

left section

linear fit

Fig. 15. Axial strain distribution across joint width of series EP.A at50 kN, measurements and linear fit.

278 J. de Castro, T. Keller / Composites: Part B 39 (2008) 271–281

ujoint, was estimated from video-extensometer measure-ments as follows:

ujoint ¼ uð10Þ � 0:5 � ðuð1Þ þ uð11ÞÞ ð1Þ

where u(i) is the displacement of points i = 1,10,11 as indi-cated in Fig. 10. Load-joint elongation curves for EP andPU joints were linear and demonstrated that specimen stiff-ness reduction, described in Section 4.1, was not due to adecrease in joint stiffness. ADP joints exhibited bilinearbehavior, similar to what was seen for the ADP adhesive.From the joint elongation, the joint-to-specimen elonga-tion ratio was calculated, which indicates the joint’s por-tion on specimen stiffness. Furthermore, a joint stiffness,kjoint, was defined and calculated by dividing axial loadincrements, DF, by joint elongation increments, Dujoint:

kjoint ¼DF

Dujoint

ð2Þ

Table 6 lists the resulting joint stiffnesses and joint-to-spec-imen elongation ratios (average values and standard devia-tions) of series EP.A, PU.B and APD.A (three specimens

per series). The EP and PU joints stiffnesses remained al-most constant while those of the ADP joints changed signif-icantly with increasing load. The ADP joint stiffnesses weretherefore defined at two load levels, 20 kN (elastic range)and 100 kN (plastic range), while for the EP and PU jointsstiffnesses were calculated at 50 kN (also see Part II).

4.4. Axial strain distributions across width and along overlap

Fig. 15 illustrates measured axial strain distributions onthe 10 mm laminates across the joint width for EP.A joints

J. de Castro, T. Keller / Composites: Part B 39 (2008) 271–281 279

at a load of 50 kN. Sections at a distance of 10 mm fromeach joint edge, right and left sections, are shown (seeFig. 9a, configuration 1). From these measurements itcould be concluded that the strain distribution across thejoint width remained almost constant. Fig. 15 also showsa corresponding approximation of the axial strains by a lin-ear curve fit. ADP joints showed similar curves.

Fig. 16a–c show the axial strain distribution on the10 mm laminates along the overlap length for EP.A,PU.B and ADP.A joints at a load of 50 kN. Measuredaxial strains (dots) were approximated by a linear regres-sion in the 5–95 mm portion of the 100 mm overlap. Theresulting slopes of the EP and PU lines were significantly

0.00

0.05

0.10

0.15

0.20

0 20 40 60 80 100

Position along overlap [mm]

Position along overlap [mm]

Position along overlap [mm]

Axi

al s

rtai

n [%

]A

xial

srt

ain

[%]

Axi

al s

rtai

n [%

]

measurements

linear regression

EP.A

0.00

0.05

0.10

0.15

0.20

0 20 40 60 80 100

measurements

linear regression

PU.B

0.00

0.05

0.10

0.15

0.20

0 20 40 60 80 100

measurements

linear regression

ADP.A

b

c

Fig. 16. Axial strain distributions along overlap length at 50 kN for series(a) EP.A, (b) PU.B, (c) ADP.A, measurements and linear regression.

smaller than the slope of the ADP line. The ADP linetended towards zero strain, while the EP and PU curvesremain clearly above the zero point in the 0–5 mm portionof the overlap.

5. Discussion

5.1. Ductility of adhesives

Ductility of elastoplastic materials is characterized bythe area between the loading and unloading path that cor-responds to the dissipated inelastic energy [18]. For viscousmaterials, however, the situation is more complicated dueto the time dependence of residual and recovered strainafter unloading. In contrast to nonviscous elastic and elas-toplastic materials, the reloading path deviates from theunloading path with increasing time between unloadingand reloading (unloading period), as shown schematicallyin Fig. 17. With ideal visco-elastic materials, no residualstrains remain and the reloading path matches the loadingpath. In this case, although the unloading path deviatesfrom the loading path and an area between the two pathsremains similar to that of elastoplastic materials, thebehavior cannot be designated as ductile, since the effectis purely caused by viscosity. If however, the time-depen-dent recovered strain does not fully compensate the resid-ual strain after unloading, the area between loading andreloading path can be considered as dissipated inelasticenergy and therefore be used as a measure of ductility.The stress–strain behavior of adhesives, such as that ofthe ADP described in Section 2.2.2 and shown in Fig. 7,can therefore be considered visco-elastoplastic and thusductile. In this study, the PU and ADP adhesives were con-sidered to be ductile adhesives in contrast to the EP adhe-sive, which was considered to be brittle.

5.2. Joint stiffness

The joint-to-specimen elongation ratio of EP and PUjoints was only 8–9% (see Table 6). Considering the joint-to-specimen length ratio, 100 mm/660 mm = 15% (see

0

0

0

0

Strain [%]

Str

ess

[MP

a]

unlo

adin

g

load

ing

relo

adin

g

inelastic energy

elastic energy

residual

time-dependent recovered

instantaneous recovered

Fig. 17. Visco-elastoplastic and ductile behavior.

280 J. de Castro, T. Keller / Composites: Part B 39 (2008) 271–281

Fig. 1), and the double adherend thickness in the jointoverlap, it was concluded that the EP and PU adhesivedeformation contributed 11% at most to the joint deforma-tion and, therefore, that most of the elongations came fromthe adherends. The corresponding contribution of theadhesive to joint deformation in ADP joints, however,was considerably higher: 67% in the elastic and 84% inthe plastic range.

EP and PU joints exhibited almost linear behavior withconstant stiffness up to brittle failure. The ductile PU adhe-sive therefore remained in the elastic range and did notshow plastic deformations. The ADP adhesive, however,exceeded the elastic strain and the ADP joints exhibitedductile behavior. Ductile adhesive could therefore be saidto not automatically provide ductile joint behavior, sincethe elastic strain must be exceeded before joint strength isreached. On the other hand, the elastic strain must not beexceeded at the serviceability limit state and the elasticmodulus must be sufficiently high to keep creep deforma-tions in an admissible range.

The joint stiffness was not linearly correlated to theadhesive stiffness. The average joint stiffness of the EPjoints was only 23% higher than that of the PU joints, eventhough the average tensile elastic modulus of the EP adhe-sive was 7.8 times higher than that of the PU adhesive.Similarly, the ADP joint stiffness decreased only by a factorof 4.1 from the elastic range (at 20 kN) to the plastic range(at 100 kN), even though the tensile elastic modulus of theadhesive decreased 7.2 times from the elastic to the plasticrange. Fig. 18 shows, however, that a logarithmic correla-tion exists between joint stiffness and the adhesive-to-adherend modulus ratio. The logarithmic curve fit showsa steep increase in joint stiffness in the low adhesive modu-lus range and an almost constant value for higher adhesivemoduli, approaching a threshold value of �500 kN/mm forjoints with 100 mm overlap length.

5.3. Load transfer in the joint

From the constant strain measurement results across thejoint width (Fig. 15) it was concluded that the joint behav-

0

100

200

300

400

500

600

0 2 4 6 8 10 12 14 16

Adhesive-to-adherend modulus ratio [%]

Join

t stif

fnes

s [k

N/m

m]

PU

ADP (elastic range)

EP

ADP (plastic range)

Fig. 18. Joint stiffness as a function of adhesive-to-adherend modulusratio.

ior could be simulated with a 2D finite element model (seePart II), simplifying considerably the analysis.

The linear regression curves for the axial strains along thejoint length of the EP and PU joints, shown in Fig. 16, indi-cated that a considerable portion of the load was transferredin the joint edge regions (jump of strains between 0 and5 mm), while the middle joint portion was less loaded (smal-ler slope of regression curves). In the ADP joints, however,the load was transferred almost uniformly on the whole jointlength, since the curve almost crossed the point of zerostrain.

5.4. Joint strength

The EP joint failure occurred in the laminates by fiber-tear failure, while ADP and PU joints exhibited a mixedfailure including adhesion failure and fiber-tear or light-fiber-tear failure. The mixed failure mode led to a lowerstrength, which was directly related to the dimensions ofthe areas of bad adhesion (areas showing adhesion pro-moter-to-substrate failure, see Fig. 13d). ADP joints withthe smallest areas of bad adhesion showed higher strengththan EP and PU joints as was expected due to adhesiveplastification and associated uniformly distributed loadtransfer along the whole joint length. The full potentialof ductile joints, however, could not be shown with the100 mm overlap joints due to the adhesion problems.

Doubling the overlap length (from 100 to 200 mm),however, almost doubled the joint strength of the ADPjoints due to a uniform load transfer involving the wholejoint length (93% strength increase on average for a 100%overlap increase). In contrast to ADP joints, EP jointsshowed only a 29% strength increase (on average). In theEP joints, mainly the middle portion with low load transferwas enlarged, thus showing a much smaller increase instrength. Despite the problems of adhesion, the advantagesof ductile joints were clearly demonstrated for the 200 mmoverlap joints.

6. Conclusions

Quasi-static tensile experiments on adhesively-bondeddouble-lap joints made of brittle GFRP adherendswith ductile and brittle adhesives were performed to inves-tigate the influence of the adhesive modulus and ductilityon joint stiffness and strength. The conclusions from thisexperimental investigation on full-scale specimens are thefollowing:

(1) Adhesives exhibiting a visco-elastoplastic stress–strain behavior with significant residual deformationafter the recovery process are considered to beductile.

(2) To provide ductile joint behavior, the elastic strain ofa ductile adhesive must be exceeded before jointfailure.

J. de Castro, T. Keller / Composites: Part B 39 (2008) 271–281 281

(3) The joint stiffness depends non-linearly on the ratioof adherend-to-adhesive modulus and approaches athreshold value with increasing adhesive modulus.

(4) Ductile joints with plasticized adhesives show uniformload transfer over the joint length with increasedstrength compared to joints with brittle adhesives.The increased joint strength, however, could be dem-onstrated only partially (for 200 mm overlap joints)due to partial imperfect bond quality.

(5) The joint strength of ductile joints with plasticizedadhesive increases almost linearly with increasingoverlap length in contrast to joints with brittle adhe-sives that exhibit a limited increase in strength.

(6) Axial strains are nearly uniformly distributed acrossthe joint width and allow for a 2D analysis.

The study was conducted within a laboratory environ-ment without the consideration of temperature and mois-ture effects. Furthermore, the creep behavior of ductileadhesives was not addressed. The elastic strain of tailoredductile adhesives must not be exceeded at the serviceabilitylimit state and the elastic modulus must be sufficiently highto keep creep deformations in an admissible range.

Acknowledgements

The authors wish to acknowledge the support of theSwiss Innovation Promotion Agency CTI (Contract number4676.1 KTS), Sika AG, Zurich (supplier of the adhesives),Fiberline Composites A/S, Denmark, and MaagtechnicAG, Zurich (supplier of the pultruded laminates).

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