research article effect of temperature variation on bond characteristics between...

Research ArticleEffect of Temperature Variation on Bond Characteristicsbetween CFRP and Steel Plate

Shan Li Tao Zhu Yiyan Lu and Xiaojin Li

School of Civil Engineering Wuhan University Wuhan 430072 China

Correspondence should be addressed to Yiyan Lu yylu901163com

Received 20 September 2016 Accepted 9 November 2016

Academic Editor Jun Deng

Copyright copy 2016 Shan Li et al This is an open access article distributed under the Creative Commons Attribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

In recent years application of carbon fiber reinforced polymer (CFRP) composite materials in the strengthening of existingreinforced concrete structures has gained widespread attention but the retrofitting of metallic buildings and bridges with CFRPis still in its early stages In real life these structures are possibly subjected to dry and hot climate Therefore it is necessary tounderstand the bond behavior between CFRP and steel at different temperatures To examine the bond between CFRP and steelunder hot climate a total of twenty-one double strap joints divided into 7 groups were tested to failure at constant temperaturesfrom 27∘C to 120∘C in this paper The results showed that the joint failure mode changed from debonding along between steel andadhesive interface failure to debonding along between CFRP and adhesive interface failure as the temperature increased beyond theglass transition temperature (119879119892) of the adhesive The load carrying capacity decreased significantly at temperatures approachingor exceeding 119879119892 The interfacial fracture energy showed a similar degradation trend Analytical models of the ultimate bearingcapacity interfacial fracture energy and bond-slip relationship of CFRP-steel interface at elevated temperatures were presented

1 Introduction

Carbon fiber reinforced polymer (CFRP) has been widelyimplemented in rehabilitating or strengthening deterioratingstructures such as buildings and bridges in civil engineeringdue to its preferable mechanical properties including highstrength light weight corrosion resistance and formabil-ity [1ndash3] This has been particularly the case for concretestructures However the use of CFRP for strengthening orretrofitting metallic buildings and bridges is still in its earlystages [4ndash6]

In the repair of steel structures with CFRP method theCFRP is bonded to the steel surface using epoxy adhesivesThe load is transferred to the CFRP material by epoxy adhe-sivesThe performance of the CFRP-strengthened steel struc-tures is governed by the effectiveness of interfacial bondingTherefore it is necessary to study the bond performance ofCFRP-steel interface through experimental studies on simpleCFRP-steel bonded joints According to it some researcheshave been conducted at ambient temperature and revealedthat the adhesive layer was the weak link in this composite

system and one of the main failure modes observed was thedebonding of CFRP from the steel substrate [7ndash12]

In some districts where the temperaturemay exceed 50∘Cor even highermechanical properties of epoxy adhesivesmaydecrease considerably since their 119879119892 are usually in the rangefrom 55 to 120∘C [13] Beyond the 119879119892 value the adhesivetransforms from a glassy state to a rubbery state accompa-nied with the degradation of the ability to transfer stressefficiently [14ndash16] As a result the mechanical performanceof CFRP-strengthened steel structures subjected to elevatedtemperatures primarily depends on that of the thermosetadhesive In order to investigate the bond behavior of CFRP-steel systems subjected to elevated-temperature exposure afew researchers in recent years have undertaken relevantinvestigations Nguyen et al [17] studied the mechanicalperformance between normal modulus CFRP and steel plateat temperatures around the glass transition temperature (119879119892)of the adhesive A mechanism-based mode to describe theeffective bond length the strength and the joint stiffnessat elevated temperature was presented and verified Al-Shawaf et al [18] investigated the bond characteristics of

Hindawi Publishing CorporationInternational Journal of Polymer ScienceVolume 2016 Article ID 5674572 8 pageshttpdxdoiorg10115520165674572

2 International Journal of Polymer Science

high modulus CFRP-steel bonded joints using three differentepoxy resins at temperatures between 20∘C and 60∘C Attemperatures near and greater than119879119892 as a result of increasedadhesiversquos softening and degradation in properties the pre-vailing failure mode was debonding failure a significantdecrease in the ultimate joint load was observed and thestrain level along the CFRP surface almost totally declinedBut beyond these studies on the bond behavior betweenCFRP and steel at elevated temperatures remain lacking andfurther investigation is needed to better understand the per-formance of CFRP-steel interface at elevated temperatures

This paper presents results concluded from a series ofCFRP-steel double strap joints tested in tension at temper-atures ranging from 27∘C up to 120∘C Comparisons arepresented on failure modes joint ultimate bearing capacitiesbetween specimens at different temperatures Combiningthe Chowdhury model [14] and Hart Smith model [19] ananalytical model was developed for predicting the ultimatebearing capacity of CFRP-steel joints Moreover a model wasproposed based on the literature [9 20 21] to characterizethe temperature dependence of CFRP-steel joint behaviorsubjected to elevated temperatures

2 Experimental Program

21 Test Specimens A total of 21 double strap joints dividedinto 7 groups were tested Three identical samples wereprepared for each specific temperature For each compositejoint the temperature is the only variable that is changedthroughout the experiment (ie 119879 = 27 40 50 60 80 100and 120∘C) All double strap bond specimens were named inthe form of S-T119909 For example specimen S-T50 strengthenedwith CFRP composite was tested at 50∘C (T50)

22 Preparation of Specimens Figure 1 shows a schematicview of the CFRP-steel double strap joints investigated inthis study including steel substrate CFRP composite andadhesive layer For each specimen two CFRP sheets werebonded symmetrically to the 3mm thickness steel surfaceover a length of 150mm In the preparation of the specimenthe surface of the steel plate was uniformly roughened byemery wheel and cleaned with acetone to remove grease oiland rust to ensure proper bonding between the compositesystem and steel Two CFRP sheets were first cut at 30mmwidth and fully infiltrated After the steel plate was placed inposition and the adhesive was applied on the steel surfacethe epoxy-bonded CFRP sheets were attached to the desiredregion and rolled on the adhesive until a uniform bondinglayer was formed All specimens were then cured in thelaboratory at ambient temperature for a week

23 Material Properties Mechanical property of steel platewas obtained through tensile coupon test as per ISO 6892-1 [22] CFRP sheet was a unidirectional material and itsproperties were obtained in accordance with ASTM D3039[23] A two-part epoxy adhesivewith the glass transition tem-perature (119879119892) being 50∘C was applied for all test specimensThe saturant resin (Part A) and hardener (Part B) were mixed

Adhesive

CFRP sheet Steel plate

30mm

150mm 3mm

70mm

Figure 1 CFRP composite double strap joint specimen

with a ratio of four to one by weight Material properties ofthe two adherends and adhesive are summarized in Table 1

24 Test Setup and Instrumentation For control specimenseight strain gaugeswere attached on the top of theCFRP sheetwith spacing of 20mm in the bonded zone and one straingauge in the unbonded zone was used to record the tensileload For specimens at elevated temperatures strain measurewith adhesively bonded strain gauges is difficult due to thesusceptibility of glue and strain gauge to heat Neverthelessthe displacement values calculated from strain readingsbasically agree well with the LVDT readings reported byothers [10] Therefore two L-shaped copper plates whichhad a low coefficient of thermal expansion (165 times 10minus5∘C)were attached symmetrically for measuring displacementat the loaded end A device using electric coil for heatingwas designed for elevated temperature tests Specimens werepreheated in the device to their target temperatures for aperiod of 20 minutes and kept inside throughout the testprocess The arrangements of strain gauges and L-shapedcopper plates are shown in Figure 2All specimenswere testedindividually in direct tension to failure under displacementcontrol (05mmmin) using a universal hydraulic testingmachine (capacity of 100 kN) The schematic view of testsetup is shown in Figure 3

3 Experimental Results

31 Failure Mode Two predominant failure modes couldbe observed and described as failure Mode I and Mode IIMode I failure refers to debonding between steel substrateand the adhesive Mode II failure was debonding betweenCFRP sheet and the adhesive The failure modes of allspecimens were summarized in Table 2 with selective imagesof failed specimens presented in Figure 4 Specimens tested attemperatures between 27∘C and 50∘C were representative ofMode I failure The debonding initiated from the loaded endand headed to the free end along the steel surfaceThe resultssuggest that at temperatures below 119879119892 the adhesive used isbrittle and glassy and has microcracking propensity flawsand imperfections This will weaken the interfacial adhesivelayer and lead to the debonding failure between steel and theadhesive [18] When heated to tested temperatures from 60to 120∘C the failure was characteristic of Mode II failurewith a little island-like adhesive left on the steel plate Inaddition more residual epoxy was attached on the surfaceof the steel plate as the temperature increased A similarfailure mode was observed in previous researches [9 18 24]The epoxy remnant on the steel plate is indicative of poor

International Journal of Polymer Science 3

Table 1 Material properties

Material Thickness(mm)

Youngrsquosmodulus(GPa)

Yieldstrength(MPa)

Ultimatestrength(MPa)

Failurestrain()

Shearstrength(MPa)

Steel plate 3 2050 3500 4510 200 mdashCFRP 0111 2520 mdash 35530 14 mdashEpoxy resina mdash 24 330 14 21aManufacturer data

Strain gauge

820mm

(a)

LVDT

(b)

Figure 2 Instrumentation (a) distribution of strain gauges (b) layout of LVDTs

Load cellHeatingdevice Specimen

Clamp

Figure 3 Test setup

bonding betweenCFRP sheet and the adhesiveThe steel plateand CFRP fibers can maintain their strengths at temperatureabove 120∘C (maximum temperature in this study) Thisfailure shift is believed to be dominated by the temperaturedependence of the adhesive layer rather than the adherendsIt is also suggested that the adhesive layer may degrade at afaster rate than the resin matrix in CFRP composites Giventhat the adhesive and resin matrix were made of the sameepoxy polymer this different ratio in degradation may bebecause of the composite action between CFRP fibers andresin and different stress levels at adhesive layer and resinmatrix [17]

32 Load-Displacement Behavior Figure 5 shows the load-displacement curves for all the specimens The displacementvalue for control specimen was calculated from readingsof the strain gauges while those for specimens at elevatedtemperatures were obtained from the LVDT readings Allthe curves behaved linearly in the initial loading followedby a softening stage and ended with a long plateau untilfinal failure It is evident from Figure 5 that the initialstiffness of specimens dropped with higher temperatureThisdegradation was pronounced at temperatures above 119879119892 of theadhesive

The term ldquobond strengthrdquo refers to the ultimate load thatcan be resisted by the CFRP sheet before the CFRP sheetdebonds from the steel substrate The bond strengths of allthe specimens tabulated in Table 2 are the average values ofeach series In the range from 27 to 50∘C the results showeda progressive decrease in bond strength as the temperatureincreased The average strengths of S-T40 (10000 kN) andS-T50 (8950 kN) are 579 and 1568 lower than thatof S-T27 (10614 kN) respectively When the temperatureexceeded 119879119892 (50∘C) a significant reduction of ultimate loadwas observedThemost drastic dropwas seen on specimen S-T120 (0360 kN) of which the remaining joint bond strengthis only 339 of the initial strength at 27∘C (10614 kN) Thismeans that there is hardly bond strength between the CFRPsheets and steel plate This is in line with results reported byothers [25] The internal stress created within the interfacedue to the volatilization or decomposition as a result ofadhesive volume decrease together with the thermal stresscaused by differences of the coefficients of linear thermalexpansion of the adhesive and the adherends would increasethe porosity and flaws within the adhesive which in turnhave a detrimental effect on adhesive-joint capacity

4 Analysis on Test Results

41 Bond Strength Prediction Based on the failure modementioned in Section 31 the bond strength of compositejoint at ambient temperature can be predicted by the HartSmithmodel [19] whichwas developed based on the adhesivefailure of double strap joints Because 119864119904119905119904 gt 2119864119891119905119891 in thisstudy the ultimate load of double strap joint can be writtenas

119875119906 = 120593119887119891radic2120591119886119905119886 (12120574119890 + 120574119901) 4119864119891119905119891 (1 + 2119864119891119905119891119864119904119905119904 ) (1)


Table 2 Test results of all specimens

Specimens Composite Temperature(∘C)

Ultimate load(kN)

Predicted load(kN)

Failuremode

S-T27 CFRP 27 10614 10746 IS-T40 CFRP 40 10000 10149 IS-T50 CFRP 50 8950 9309 IS-T60 CFRP 60 8056 7661 IIS-T80 CFRP 80 2792 3030 IIS-T100 CFRP 100 1162 0901 IIS-T120 CFRP 120 0360 0494 II

(a) (b)

Figure 4 Typical failure modes (a) Mode I (b) Mode II

01 02 03 04 05 0600Loaded end displacement (mm)

0

1

2

3

4

5

6

Load

(kN

)

S-T27S-T40S-T50S-T60

S-T80S-T100S-T120

Figure 5 Load-displacement curves

in which

120593 = 1 119871 ge 119871119890119871119871119890 119871 lt 119871119890

119871119890 = 1198751199062120591119886119887119891 + 2120582120582 = radic119866119886119905119886 ( 1119864119891119905119891 + 2119864119904119905119904)

(2)

where 120593 is the bond length factor 119887119891 is the width of CFRPcomposite 120591119886 119905119886 120574119890 120574119901 and 119866119886 are the shear strengththickness elastic shear strain plastic shear strain and shearmodulus of adhesive respectively 119864119891 119905119891 and 119864119904 119905119904 are theelastic modulus and thickness of CFRP and steel adherendsrespectively 119871 and 119871119890 are the bond length and effective bondlength

On the other hand the ultimate loads of specimens atelevated temperatures can be calculated by the Chowdhurymodel [14] which is given by

119875119906 (119879) = 119875119906 + 1198751198772 minus 119875119906 minus 1198751198772 tanh [119896 (119879 minus 1198791015840)] (3)

where 119875119906(119879) is the ultimate load of specimen at specifictemperature 119879 119875119906 is the ultimate load at room temperatureobtained by (1)119875119877 is the residual load capacity 119896 is a constant1198791015840 is the temperature when a 50 reduction is observedThe values of parameters 119875119877 119896 and 1198791015840 achieved throughregression analysis are equal to 0422 0048 and 68806respectively

All predicted ultimate loads are listed in Table 2 Itshould be noted that the value of 120574119901 is taken as 5120574119890 in (1)because this assumption gives much better bond strengthprediction in previous research [11] The good agreementbetween prediction and test data shown in Figure 6 can beattributed to the fact that the relationship in (3) is totallyempirical

42 Discussion on the Interfacial Fracture Energy The bondstrength of a joint well established for CFRP-concrete jointis also applicable to CFRP-steel joint which is in line withresults reported by Xia and Teng [9] And the double-lapshear test can be treated as two single-lap shear tests being


ExpTheo

40 60 80 100 12020Temperature (∘C)

0

2

4

6

8

10

12

Ulti

mat

e loa

d (k

N)

Figure 6 Ultimate load of specimens at elevated temperatures

simultaneously conducted and thus the ultimate load ofdouble strap joint is given by

12119875119906 = 119887119891radic2119866119891119864119891119905119891 (4)

Taking the effect of thermal exposure into considerationthus the interfacial fracture energy of specimens at elevatedtemperatures can be calculated by

119866119891 (119879) = [(12) 119875119906 (119879)]221198872119891119864119891 (119879) 119905119891 (5)

The effect of elevated temperature is to degrade the mechani-cal properties of epoxy-impregnated CFRP composite espe-cially when the temperature exceeds the glass transmissiontemperature 119879119892 According to the degradation model byBisby [20] the temperature-dependent elastic modulus ofCFRP can be calculated by the following equation

119864119891 (119879)119864119891 = 1 minus 11988612 sdot tanh [minus1198862 (119879 minus 1198863)] + 1 + 11988612 (6)

where 119864119891(119879) is the tensile modulus of CFRP composite atspecific temperature119879119864119891 is the room-temperaturemodulus1198861 1198862 and 1198863 are the residual values of tensile modulusconstants that describe central temperature and severity ofproperty degradation with temperature which are 08690106 and 52207 respectively according to the regressionanalysis

Figure 7 depicts how the normalized interfacial fractureenergy (119866119891(119879)119866119891) varies with the normalized temperature(119879119879119892) A slight decrease of interfacial fracture energy 119866119891(119879)was observed before119879119892 approached whereas a rapid decreaseoccurred when the temperature exceeds 119879119892 of the adhesive

ExpTheo

Gf(T

)G

f

10 15 20 2505TTg

00

02

04

06

08

10

Figure 7 Interfacial fracture energy of specimens at elevatedtemperatures

This decrease is mainly attributed to themechanical degrada-tion of the adhesive layer According to the regression anal-ysis the temperature-dependent interfacial fracture energycan be written as

119866119891 (119879)119866119891 = 1 minus 11988712 sdot tanh[minus1198872 ( 119879119879119892 minus 1198873)] + 1 + 11988712 (7)

where 119866119891 refers to the interfacial fracture energy at ambienttemperature and the regression values of parameters 1198871 1198872and 1198873 are 0 2762 and 1257 respectively

43 Shear Stress-Slip Mode The interfacial shear behaviorof adhesively bonded joint is usually characterized by therelationship of the local shear stress and the relative slipbetween the two adherends In the double strap joint testthe bond stress between the 119894-1th strain gauge and the 119894thstrain gauge 119894 120591119894 can be obtained based on the internal forceequilibrium

120591119894 = 119864119891119905119891 (120576119894 minus 120576119894minus1)Δ119909 119894 isin (2 8) (8)

where 120576119894 and 120576119894minus1 are measured strains in the CFRP sheet atadjacent strain gauge locations 119894 and 119894minus1which are separatedby a distance Δ119909

Supposing that there is no relative slip at the free end ofCFRP sheet thus the relative slip 119904119894 at gauge location 119894 isexpressed as

119904119894 = Δ1199092 (1205761 + 2119894minus1sum119895=2

120576119895 + 120576119894)119894 isin (2 8) 119895 isin (2 7)

(9)

where 1205761 is the strain at the free end of CFRP sheet 120576119895 is thestrain of the 119895th gauge attached on the CFRP sheet 120576119894 is the


strain at the loaded end of CFRP sheet Using (8) and (9) thelocal bond-slip relationships along the CFRP-steel interfacecan be obtained for specimens at ambient temperature

For specimens at elevated temperatures without straingauges the bond-slip curve can be obtained from the rela-tionship between the CFRP strain and slip at the loaded endAccording to the literature [9 10 12 21 26ndash28] the bond-slip curves for CFRP-steel bonded joints are similar to thosefor CFRP-concrete bonded joints both have a bilinear shapeAnd their developments of shear stress are the same At alow load the interface is in an elastic stage during which theshear stress is the largest at or very close to the loaded endand gradually decreases towards the free end As the loadincreases part of the interface enters into softening stageduringwhich the shear stress at the loaded end decreases afterit has approached its maximum value When the shear stressat the loaded end reduces to zero the ultimate load of thespecimen is reached and keeps almost constant debondingcommences and propagates along the interface towards thefree end with the peak shear stress moving gradually towardsthe free end On the other hand the strain-slip curves atthe loaded end in this study derived from the load-sliprelationship for CFRP-steel bonded joints are similar tothose for CFRP-concrete bonded joints Therefore a similarexpression linked to an exponential term is proposed for therelationship of the CFRP strain and interfacial slip in thissection based on existing analytical model [21]

120576 = 119891 (119904) = 120572 (1 minus 119890minus120573119904) (10)

where 120572 and 120573 are regression parametersFigure 8 shows the comparison between regressing curves

and experimentally observed relationships It is found that allthe 120576-119904 curves predicted for specimens at elevated tempera-tures match well the experimental relationships plotted

On the other hand the equation of forces equilibriumacting on the CFRP sheet can be written as

119889120590119891119889119909 minus 120591119905119891 = 0 (11)

where 120590119891 and 120591 are the axial stress in the CFRP sheet andshear stress in the adhesive layer at any location respectivelyCombining (10) and (11) and neglecting the strain of the steelplate yield the following equation for the 120591-119904 curve120591 = 119905119891 119889120590119891119889119909 = 119864119891 (119879) 119905119891 119889120576119889119909 = 119864119891 (119879) 119905119891119891 (119904) 119889119891 (119904)119889119904= 1205722120573119864119891 (119879) 119905119891119890minus120573119904 (1 minus 119890minus120573119904)

(12)

because

119866119891 (119879) = intinfin0120591119889119904 = 121205722119864119891 (119879) 119905119891 (13)

The 120591-119904 relationship can be rewritten as

120591 = 2119866119891 (119879) 120573119890minus120573119904 (1 minus 119890minus120573119904) (14)

CFRP

stra

in (120583

120576)

0

1000

2000

3000

4000

5000

6000

7000

01 02 03 04 05 0600Slip (mm)

S-T40S-T50S-T60

S-T80S-T100S-T120

Figure 8 Relationships of CFRP strain and slip at loaded end

Therefore the shear stress as a function of relative slip canbe determined with 119866119891(119879) calculated by (7) and regressionparameter 120573

Figure 9 shows how the normalized maximum shearstress (120591max(119879)120591max) and the corresponding normalized slip(119904max(119879)119904max) vary with the normalized temperature (119879119879119892)The effect of elevated temperature is easier to characterizeshowing typically a decrement of maximum shear stress withlarger corresponding relative interfacial slip The maximumbond stress decreased in agreement with the lower interfacialfracture energy while the increase in slip meant degradationin the interfacial stiffness because of the softening of theadhesive

5 Conclusions

The experimental results of 21 double strap joints subjectedto elevated temperatures are presented It is found thatthe CFRP-steel interface is significantly weakened and thedegradation is affected by thermal exposure The followingconclusions are reached within the scope of this research

(1) The failure mode of specimens was affected bythe elevated-temperature exposure It was likely tochange from debonding along steel-adhesive fail-ure to debonding along CFRP-adhesive failure Thechange of failure mode is dominated by the degrada-tion and volume decrease of the adhesive

(2) The thermal exposure tended to decrease the initialstiffness and ultimate load for specimens especiallyat temperatures above 119879119892 of the adhesive

(3) A model was developed based on the Hart Smithmodel and the Chowdhury model for predicting theultimate bearing capacity

(4) The interfacial fracture energy decreased gently attemperature below 119879119892 but much sharply when the


120591m

ax(T

)120591

max

10 15 20 2505TTg

00

02

04

06

08

10

(a)

10 15 20 2505TTg

0

2

4

6

8

10

s max(T

)s m

ax

(b)

Figure 9 Effect of elevated temperatures on (a) 120591max and (b) 119904max

temperature exceeded 119879119892 A degradation model ofinterfacial fracture energy was presented

(5) Bond-slip relationship that approximates the effect ofthermal exposure through regression parameters cal-ibrated by experimental data was suggested CFRP-steel interfaces at elevated temperatures showed adecrement of the maximum bond stress attained at alarger slip

Competing Interests

The authors declare no competing interests

Acknowledgments

The tests reported herein were made possible by the financialsupport from the Natural Science Foundation of China(51578428) The authors wish to thank the civil engineeringtechnicians of Wuhan University for their help throughoutthe test process

References

[1] L C Hollaway ldquoA review of the present and future utilisationof FRP composites in the civil infrastructure with referenceto their important in-service propertiesrdquo Construction andBuilding Materials vol 24 no 12 pp 2419ndash2445 2010

[2] J-G Dai Y-L Bai and J G Teng ldquoBehavior and modeling ofconcrete confined with FRP composites of large deformabilityrdquoJournal of Composites for Construction vol 15 no 6 pp 963ndash973 2011

[3] Y Y Lu N Li and S Li ldquoBehavior of FRP-confined concrete-filled steel tube columnsrdquo Polymers vol 6 no 5 pp 1333ndash13492014

[4] D Schnerch and S Rizkalla ldquoFlexural strengthening of steelbridges with high modulus CFRP stripsrdquo Journal of BridgeEngineering vol 13 no 2 pp 192ndash201 2008

[5] A H Al-Saidy F W Klaiber and T J Wipf ldquoStrengthening ofsteel-concrete composite girders using carbon fiber reinforcedpolymer platesrdquoConstruction and BuildingMaterials vol 21 no2 pp 295ndash302 2007

[6] J G Teng T Yu and D Fernando ldquoStrengthening of steelstructures with fiber-reinforced polymer compositesrdquo Journalof Constructional Steel Research vol 78 no 6 pp 131ndash143 2012

[7] R Haghani ldquoAnalysis of adhesive joints used to bond FRPlaminates to steel membersmdasha numerical and experimentalstudyrdquo Construction and Building Materials vol 24 no 11 pp2243ndash2251 2010

[8] S Fawzia R Al-Mahaidi and X-L Zhao ldquoExperimental andfinite element analysis of a double strap joint between steelplates and normal modulus CFRPrdquo Composite Structures vol75 no 1ndash4 pp 156ndash162 2006

[9] S H Xia and J G Teng ldquoBehaviour of FRP-to-steel bondedjointsrdquo in Proceedings of the International Symposium on BondBehaviour of FRP in Structures (BBFS rsquo05) pp 411ndash418 HongKong December 2005

[10] T Yu D Fernando J G Teng and X L Zhao ldquoExperimentalstudy on CFRP-to-steel bonded interfacesrdquo Composites Part BEngineering vol 43 no 5 pp 2279ndash2289 2012

[11] C Wu X L Zhao W H Duan and R Al-Mahaidi ldquoBondcharacteristics between ultra high modulus CFRP laminatesand steelrdquoThin-Walled Structures vol 51 no 2 pp 147ndash157 2012

[12] S Fawzia X-L Zhao and R Al-Mahaidi ldquoBond-slip modelsfor double strap joints strengthened by CFRPrdquo CompositeStructures vol 92 no 9 pp 2137ndash2145 2010

[13] D Cree T Gamaniouk M L Loong and M F Green ldquoTensileand lap-splice shear strength properties of CFRP composites athigh temperaturesrdquo Journal of Composites for Construction vol19 no 2 Article ID 04014043 2015

[14] E U Chowdhury R Eedson L A Bisby M F Green andN Benichou ldquoMechanical characterization of fibre reinforcedpolymers materials at high temperaturerdquo Fire Technology vol47 no 4 pp 1063ndash1080 2011

[15] A P Mouritz and A G Gibson Fire Properties of PolymerComposite Materials Springer Dordrecht The Netherlands2006


[16] S Cao Z Wu and X Wang ldquoTensile properties of CFRP andhybrid FRP composites at elevated temperaturesrdquo Journal ofComposite Materials vol 43 no 4 pp 315ndash330 2009

[17] T-C Nguyen Y Bai X-L Zhao and R Al-Mahaidi ldquoMechani-cal characterization of steelCFRPdouble strap joints at elevatedtemperaturesrdquo Composite Structures vol 93 no 6 pp 1604ndash1612 2011

[18] A Al-Shawaf R Al-Mahaidi andX-L Zhao ldquoEffect of elevatedtemperature on bond behaviour of high modulus CFRPsteeldouble-strap jointsrdquo Australian Journal of Structural Engineer-ing vol 10 no 1 pp 63ndash74 2009

[19] L J Hart Smith ldquoAdhesive-bonded double-lap jointsrdquo TechRepNASACR-112235DouglasAircraftCompany LongBeachCalif USA 1973

[20] L Bisby Fire behavior of fiber-reinforced polymers (FRP) rein-forced or confined concrete [PhD thesis] Queens UniversityOntario Canada 2003

[21] J-G Dai W Y Gao and J G Teng ldquoBond-slip model for FRPlaminates externally bonded to concrete at elevated tempera-turerdquo Journal of Composites for Construction vol 17 no 2 pp217ndash228 2013

[22] ISO ldquoMetallic materials-tensile testing-part 1 method of test atroom temperaturerdquo ISO 6892-12009 BSI London UK 2009

[23] Standard Test Method for Tensile Properties of Polymer MatrixComposite Materials ASTM D3039-07 ASTM West Con-shohocken Pa USA 2008

[24] R Haghani and M Al-Emrani ldquoA new design model foradhesive joints used to bond FRP laminates to steel beamspart B experimental verificationrdquo Construction and BuildingMaterials vol 30 no 5 pp 686ndash694 2012

[25] Y Y Lu W J Li S Li X Li and T Zhu ldquoStudy of the tensileproperties of CFRP strengthened steel platesrdquo Polymers vol 7no 12 pp 2595ndash2610 2015

[26] H Yuan J G Teng R Seracino Z S Wu and J Yao ldquoFull-range behavior of FRP-to-concrete bonded jointsrdquo EngineeringStructures vol 26 no 5 pp 553ndash565 2004

[27] J G Dai T Ueda and Y Sato ldquoDevelopment of the nonlin-ear bond stress-slip model of fiber reinforced plastics sheet-concrete interfaceswith a simplemethodrdquo Journal of Compositesfor Construction vol 9 no 1 pp 52ndash62 2005

[28] J G Dai T Ueda and Y Sato ldquoUnified analytical approachesfor determining shear bond characteristics of FRP-concreteinterfaces through pullout testsrdquo Journal of Advanced ConcreteTechnology vol 4 no 1 pp 133ndash145 2006

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materials


Journal ofNanomaterials


high modulus CFRP-steel bonded joints using three differentepoxy resins at temperatures between 20∘C and 60∘C Attemperatures near and greater than119879119892 as a result of increasedadhesiversquos softening and degradation in properties the pre-vailing failure mode was debonding failure a significantdecrease in the ultimate joint load was observed and thestrain level along the CFRP surface almost totally declinedBut beyond these studies on the bond behavior betweenCFRP and steel at elevated temperatures remain lacking andfurther investigation is needed to better understand the per-formance of CFRP-steel interface at elevated temperatures

This paper presents results concluded from a series ofCFRP-steel double strap joints tested in tension at temper-atures ranging from 27∘C up to 120∘C Comparisons arepresented on failure modes joint ultimate bearing capacitiesbetween specimens at different temperatures Combiningthe Chowdhury model [14] and Hart Smith model [19] ananalytical model was developed for predicting the ultimatebearing capacity of CFRP-steel joints Moreover a model wasproposed based on the literature [9 20 21] to characterizethe temperature dependence of CFRP-steel joint behaviorsubjected to elevated temperatures

2 Experimental Program

21 Test Specimens A total of 21 double strap joints dividedinto 7 groups were tested Three identical samples wereprepared for each specific temperature For each compositejoint the temperature is the only variable that is changedthroughout the experiment (ie 119879 = 27 40 50 60 80 100and 120∘C) All double strap bond specimens were named inthe form of S-T119909 For example specimen S-T50 strengthenedwith CFRP composite was tested at 50∘C (T50)

22 Preparation of Specimens Figure 1 shows a schematicview of the CFRP-steel double strap joints investigated inthis study including steel substrate CFRP composite andadhesive layer For each specimen two CFRP sheets werebonded symmetrically to the 3mm thickness steel surfaceover a length of 150mm In the preparation of the specimenthe surface of the steel plate was uniformly roughened byemery wheel and cleaned with acetone to remove grease oiland rust to ensure proper bonding between the compositesystem and steel Two CFRP sheets were first cut at 30mmwidth and fully infiltrated After the steel plate was placed inposition and the adhesive was applied on the steel surfacethe epoxy-bonded CFRP sheets were attached to the desiredregion and rolled on the adhesive until a uniform bondinglayer was formed All specimens were then cured in thelaboratory at ambient temperature for a week

23 Material Properties Mechanical property of steel platewas obtained through tensile coupon test as per ISO 6892-1 [22] CFRP sheet was a unidirectional material and itsproperties were obtained in accordance with ASTM D3039[23] A two-part epoxy adhesivewith the glass transition tem-perature (119879119892) being 50∘C was applied for all test specimensThe saturant resin (Part A) and hardener (Part B) were mixed

Adhesive

CFRP sheet Steel plate

30mm

150mm 3mm

70mm

Figure 1 CFRP composite double strap joint specimen

with a ratio of four to one by weight Material properties ofthe two adherends and adhesive are summarized in Table 1

24 Test Setup and Instrumentation For control specimenseight strain gaugeswere attached on the top of theCFRP sheetwith spacing of 20mm in the bonded zone and one straingauge in the unbonded zone was used to record the tensileload For specimens at elevated temperatures strain measurewith adhesively bonded strain gauges is difficult due to thesusceptibility of glue and strain gauge to heat Neverthelessthe displacement values calculated from strain readingsbasically agree well with the LVDT readings reported byothers [10] Therefore two L-shaped copper plates whichhad a low coefficient of thermal expansion (165 times 10minus5∘C)were attached symmetrically for measuring displacementat the loaded end A device using electric coil for heatingwas designed for elevated temperature tests Specimens werepreheated in the device to their target temperatures for aperiod of 20 minutes and kept inside throughout the testprocess The arrangements of strain gauges and L-shapedcopper plates are shown in Figure 2All specimenswere testedindividually in direct tension to failure under displacementcontrol (05mmmin) using a universal hydraulic testingmachine (capacity of 100 kN) The schematic view of testsetup is shown in Figure 3

3 Experimental Results

31 Failure Mode Two predominant failure modes couldbe observed and described as failure Mode I and Mode IIMode I failure refers to debonding between steel substrateand the adhesive Mode II failure was debonding betweenCFRP sheet and the adhesive The failure modes of allspecimens were summarized in Table 2 with selective imagesof failed specimens presented in Figure 4 Specimens tested attemperatures between 27∘C and 50∘C were representative ofMode I failure The debonding initiated from the loaded endand headed to the free end along the steel surfaceThe resultssuggest that at temperatures below 119879119892 the adhesive used isbrittle and glassy and has microcracking propensity flawsand imperfections This will weaken the interfacial adhesivelayer and lead to the debonding failure between steel and theadhesive [18] When heated to tested temperatures from 60to 120∘C the failure was characteristic of Mode II failurewith a little island-like adhesive left on the steel plate Inaddition more residual epoxy was attached on the surfaceof the steel plate as the temperature increased A similarfailure mode was observed in previous researches [9 18 24]The epoxy remnant on the steel plate is indicative of poor





Yieldstrength(MPa)


Failurestrain()

Shearstrength(MPa)


Strain gauge

820mm

(a)

LVDT

(b)



Clamp

Figure 3 Test setup






119875119906 = 120593119887119891radic2120591119886119905119886 (12120574119890 + 120574119901) 4119864119891119905119891 (1 + 2119864119891119905119891119864119904119905119904 ) (1)




Ultimate load(kN)

Predicted load(kN)

Failuremode


(a) (b)



0

1

2

3

4

5

6

Load

(kN

)


S-T80S-T100S-T120


in which

120593 = 1 119871 ge 119871119890119871119871119890 119871 lt 119871119890

119871119890 = 1198751199062120591119886119887119891 + 2120582120582 = radic119866119886119905119886 ( 1119864119891119905119891 + 2119864119904119905119904)

(2)








ExpTheo

40 60 80 100 12020Temperature (∘C)

0

2

4

6

8

10

12

Ulti

mat

e loa

d (k

N)



12119875119906 = 119887119891radic2119866119891119864119891119905119891 (4)


119866119891 (119879) = [(12) 119875119906 (119879)]221198872119891119864119891 (119879) 119905119891 (5)





ExpTheo

Gf(T

)G

f

10 15 20 2505TTg

00

02

04

06

08

10









119904119894 = Δ1199092 (1205761 + 2119894minus1sum119895=2

120576119895 + 120576119894)119894 isin (2 8) 119895 isin (2 7)

(9)





120576 = 119891 (119904) = 120572 (1 minus 119890minus120573119904) (10)




119889120590119891119889119909 minus 120591119905119891 = 0 (11)


(12)

because

119866119891 (119879) = intinfin0120591119889119904 = 121205722119864119891 (119879) 119905119891 (13)



CFRP

stra

in (120583

120576)

0

1000

2000

3000

4000

5000

6000

7000

01 02 03 04 05 0600Slip (mm)

S-T40S-T50S-T60

S-T80S-T100S-T120




5 Conclusions







120591m

ax(T

)120591

max

10 15 20 2505TTg

00

02

04

06

08

10

(a)

10 15 20 2505TTg

0

2

4

6

8

10

s max(T

)s m

ax

(b)




Competing Interests


Acknowledgments


References





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials







Yieldstrength(MPa)


Failurestrain()

Shearstrength(MPa)


Strain gauge

820mm

(a)

LVDT

(b)



Clamp

Figure 3 Test setup






119875119906 = 120593119887119891radic2120591119886119905119886 (12120574119890 + 120574119901) 4119864119891119905119891 (1 + 2119864119891119905119891119864119904119905119904 ) (1)




Ultimate load(kN)

Predicted load(kN)

Failuremode


(a) (b)



0

1

2

3

4

5

6

Load

(kN

)


S-T80S-T100S-T120


in which

120593 = 1 119871 ge 119871119890119871119871119890 119871 lt 119871119890

119871119890 = 1198751199062120591119886119887119891 + 2120582120582 = radic119866119886119905119886 ( 1119864119891119905119891 + 2119864119904119905119904)

(2)








ExpTheo

40 60 80 100 12020Temperature (∘C)

0

2

4

6

8

10

12

Ulti

mat

e loa

d (k

N)



12119875119906 = 119887119891radic2119866119891119864119891119905119891 (4)


119866119891 (119879) = [(12) 119875119906 (119879)]221198872119891119864119891 (119879) 119905119891 (5)





ExpTheo

Gf(T

)G

f

10 15 20 2505TTg

00

02

04

06

08

10









119904119894 = Δ1199092 (1205761 + 2119894minus1sum119895=2

120576119895 + 120576119894)119894 isin (2 8) 119895 isin (2 7)

(9)





120576 = 119891 (119904) = 120572 (1 minus 119890minus120573119904) (10)




119889120590119891119889119909 minus 120591119905119891 = 0 (11)


(12)

because

119866119891 (119879) = intinfin0120591119889119904 = 121205722119864119891 (119879) 119905119891 (13)



CFRP

stra

in (120583

120576)

0

1000

2000

3000

4000

5000

6000

7000

01 02 03 04 05 0600Slip (mm)

S-T40S-T50S-T60

S-T80S-T100S-T120




5 Conclusions







120591m

ax(T

)120591

max

10 15 20 2505TTg

00

02

04

06

08

10

(a)

10 15 20 2505TTg

0

2

4

6

8

10

s max(T

)s m

ax

(b)




Competing Interests


Acknowledgments


References





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






Ultimate load(kN)

Predicted load(kN)

Failuremode


(a) (b)



0

1

2

3

4

5

6

Load

(kN

)


S-T80S-T100S-T120


in which

120593 = 1 119871 ge 119871119890119871119871119890 119871 lt 119871119890

119871119890 = 1198751199062120591119886119887119891 + 2120582120582 = radic119866119886119905119886 ( 1119864119891119905119891 + 2119864119904119905119904)

(2)








ExpTheo

40 60 80 100 12020Temperature (∘C)

0

2

4

6

8

10

12

Ulti

mat

e loa

d (k

N)



12119875119906 = 119887119891radic2119866119891119864119891119905119891 (4)


119866119891 (119879) = [(12) 119875119906 (119879)]221198872119891119864119891 (119879) 119905119891 (5)





ExpTheo

Gf(T

)G

f

10 15 20 2505TTg

00

02

04

06

08

10









119904119894 = Δ1199092 (1205761 + 2119894minus1sum119895=2

120576119895 + 120576119894)119894 isin (2 8) 119895 isin (2 7)

(9)





120576 = 119891 (119904) = 120572 (1 minus 119890minus120573119904) (10)




119889120590119891119889119909 minus 120591119905119891 = 0 (11)


(12)

because

119866119891 (119879) = intinfin0120591119889119904 = 121205722119864119891 (119879) 119905119891 (13)



CFRP

stra

in (120583

120576)

0

1000

2000

3000

4000

5000

6000

7000

01 02 03 04 05 0600Slip (mm)

S-T40S-T50S-T60

S-T80S-T100S-T120




5 Conclusions







120591m

ax(T

)120591

max

10 15 20 2505TTg

00

02

04

06

08

10

(a)

10 15 20 2505TTg

0

2

4

6

8

10

s max(T

)s m

ax

(b)




Competing Interests


Acknowledgments


References





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




ExpTheo

40 60 80 100 12020Temperature (∘C)

0

2

4

6

8

10

12

Ulti

mat

e loa

d (k

N)



12119875119906 = 119887119891radic2119866119891119864119891119905119891 (4)


119866119891 (119879) = [(12) 119875119906 (119879)]221198872119891119864119891 (119879) 119905119891 (5)





ExpTheo

Gf(T

)G

f

10 15 20 2505TTg

00

02

04

06

08

10









119904119894 = Δ1199092 (1205761 + 2119894minus1sum119895=2

120576119895 + 120576119894)119894 isin (2 8) 119895 isin (2 7)

(9)





120576 = 119891 (119904) = 120572 (1 minus 119890minus120573119904) (10)




119889120590119891119889119909 minus 120591119905119891 = 0 (11)


(12)

because

119866119891 (119879) = intinfin0120591119889119904 = 121205722119864119891 (119879) 119905119891 (13)



CFRP

stra

in (120583

120576)

0

1000

2000

3000

4000

5000

6000

7000

01 02 03 04 05 0600Slip (mm)

S-T40S-T50S-T60

S-T80S-T100S-T120




5 Conclusions







120591m

ax(T

)120591

max

10 15 20 2505TTg

00

02

04

06

08

10

(a)

10 15 20 2505TTg

0

2

4

6

8

10

s max(T

)s m

ax

(b)




Competing Interests


Acknowledgments


References





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






120576 = 119891 (119904) = 120572 (1 minus 119890minus120573119904) (10)




119889120590119891119889119909 minus 120591119905119891 = 0 (11)


(12)

because

119866119891 (119879) = intinfin0120591119889119904 = 121205722119864119891 (119879) 119905119891 (13)



CFRP

stra

in (120583

120576)

0

1000

2000

3000

4000

5000

6000

7000

01 02 03 04 05 0600Slip (mm)

S-T40S-T50S-T60

S-T80S-T100S-T120




5 Conclusions







120591m

ax(T

)120591

max

10 15 20 2505TTg

00

02

04

06

08

10

(a)

10 15 20 2505TTg

0

2

4

6

8

10

s max(T

)s m

ax

(b)




Competing Interests


Acknowledgments


References





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




120591m

ax(T

)120591

max

10 15 20 2505TTg

00

02

04

06

08

10

(a)

10 15 20 2505TTg

0

2

4

6

8

10

s max(T

)s m

ax

(b)




Competing Interests


Acknowledgments


References





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials
























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



research article effect of temperature variation on bond characteristics between...

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