Composite Structures 134 (2015) 190–198

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Composite Structures

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An experimental study on the effect of joining interface condition onbearing response of single-lap, countersunk composite-aluminum boltedjoints

http://dx.doi.org/10.1016/j.compstruct.2015.08.0780263-8223/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +86 10 82315095.E-mail address: zhaiyunong.1988@163.com (Y. Zhai).

Yunong Zhai a,⇑, Dongsheng Li a, Xiaoqiang Li a, Liang Wang b

aResearch Laboratory of Precision Forming and Assembly for Lightweight Structure, School of Mechanical Engineering & Automation, Beihang University, Beijing, Chinab The Third Research Institute of China Aerospace Science & Industry Corporation, Beijing, China

a r t i c l e i n f o

Article history:Available online 28 August 2015

Keywords:Countersunk composite-aluminum jointsSingle-lapShimmingInterface gapBearing tests3D Digital Image Correlation

a b s t r a c t

An experimental study on the effect of joining interface condition (including shimming and interface gap)on bearing response of single-lap, countersunk composite-aluminum bolted joints are presented. Thespecimens consisted of a T700/3068 carbon/epoxy laminate with quasi-isotropic lay-up and anAluminum alloy 7075T651 substrate. Bearing stress/bearing strain behavior were obtained accordingto ASTM standard. Both solid shim and liquid shim were considered and a comparison was made forthem. 3D Digital Image Correlation was utilized to evaluate the effect of shimming on the surface straindistribution and out-of-plane deformation of the joints. One focus of the study was to investigate theeffect of interface gap on the bearing performance of composite bolted joints. The interface gap wasdesigned and characterized by variable parameters, i.e., thickness and span. It is found that comparedto liquid-shim series, specimens with solid shim gain a little better bearing performance because ofhigher joint bending stiffness that benefits from the higher tensile modulus of solid peelable fiberglassshim. The presence of interface gap significantly weakens the bearing performance of single-lap, counter-sunk composite-aluminum joints by making the countersunk hole losing support from aluminum plate atthe shear plane, and meanwhile intensifying the loading eccentricity of single-lap joints.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Due to high specific strength and stiffness, as well as high resis-tance to fatigue and corrosion, composite materials can bringweight and performance benefits to the new generation airlinersand thus have been widely used in the airframe structures, e.g.,53% for the Airbus A350XWB and 50% for the Boeing 787 [1]. Theassembly of composite airframe commonly relies on mechanicalfastening, which is low-cost, reliable and facilitative of disassemblyfor repair. Because of the existence of hole, as well as material ani-sotropy and brittleness, composite bolted joints represent poten-tially weak points in the structures, and significantly influencethe load carrying capacity and integrity of the structures [2]. Thusit is crucial to fully understand the effect of multiple design & man-ufacturing factors on the loading behavior of composite boltedjoints.

So far, numerous researches have mainly focused on jointgeometry [3,4], laminate lay-up [5–7], bolt-hole fit condition

[8–13] and lateral bolt clamping [10,14–16]. Joint geometry,mainly referring to edge distance ratio (e/d) and width diameterratio (w/d), dominates the failure mode of composite bolted joints,i.e., changing the failure mode from shear failure (e/d < 3) tobearing failure (e/d > 3), or from tension failure (w/d < 4) to bearingfailure (w/d > 4). The joints with quasi-isotropic lay-up gain abetter loading performance than that with orthotropic lay-up,and the delamination bearing strength is higher for the lay-up with90� layers on the laminate surface. Bolt-hole clearance fit is foundto result in reduced joint stiffness and an earlier initiation ofdamage due to reduced bolt-hole contact area. Low bolt-hole inter-ference fit can lower the stress concentration and improve theultimate bearing strength as well as fatigue life of composite boltedjoints. Proper clamping pressure and clamping area (washer size)could enhance the bearing performance of composite bolted jointsby restraining the occurrence and development of bearing damagein the vicinity of hole.

In addition, the manufacturing processes of composite compo-nents inevitably introduce variations to nominal geometries, e.g.,process-induced deformation [17,18] and thickness variations (upto 7–10%) [19], leading to assembly gaps in the joining interface.

3.78

4

80

25

16

30

ø4.76

D

H

Tab

Aluminum CompositeLaminate

40

Point 1 Point 2

XY

A

B

Fig. 1. Specimen geometry (all dimensions in mm): (A) shimming specimens; (B) interface gap specimens.

Table 1Test configuration for shimming.

Test parameter R L0.3 L0.6 L1.1 S0.6 S1.1

Type n/a Liquid Liquid Liquid Solid SolidThickness (mm) 0 0.3 0.6 1.1 0.6 1.1

Table 2Test configuration for interface gap.

Test parameter R C1 C2 C3 C4 C5

Thickness H (mm) 0 0.2 0.5 0.8 0.5 0.8Span D (mm) 0 12 12 12 22 22

Y. Zhai et al. / Composite Structures 134 (2015) 190–198 191

Shimming is generally adopted to compensate these gaps in thejoining interface [20], which shall influence the behavior of com-posite bolted joints at some level.

To understand the effect of shimming on the structural behaviorof composite bolted joints, several authors have conducted exper-imental and numerical studies. Hühne et al. [2] investigated theinfluence of liquid shim layer thickness on the strength and struc-tural behavior of bolted CFRP-joints by performing a test programand constructing a progressive damage FE model. It was concludedthat increasing shim layer thickness led to lower stiffness, but didnot yield a clear decreasing trend in the ultimate load or designload. Comer et al. [21] conducted thermo-mechanical fatigue testson double-bolt composite-aluminum hybrid joints to evaluate the

Component A

Interface g

Span

Thickness

Fig. 2. Joining interface gaps in the a

performance of commercially available liquid shim under repre-sentative service conditions of a commercial aircraft. Resultsshowed that there was no degradation in terms of mechanical stiff-ness and no significant damage on the bearing plane for the liquidshim. Dhôte et al. [22] performed an experiment to study the effectof a liquid-shim layer on the in-plane strain and out-of-planedeformation of single-lap composite bolted joints. It was shownthat the introduction of a liquid-shim layer magnified the out-of-plane deformation and modified the in-plane strain distribution,potentially leading to higher tensile strains in the laminates.

While the previous studies on shimming in composite boltedjoints mainly relate to liquid shim, solid shim is generally specifiedto fill bigger gaps in the joining interface of composite airframes.But few studies have dealt with the effect of the introduction ofsolid shim, as well as a comparison with liquid shim. On the otherhand, shimming disposition is not required for the interface gapswithin design allowables [20], leaving these gaps in the compositebolted joints. However, the effect of joining interface gap on thebearing behavior of composite bolted joints have not been pub-lished in open literature. Consequently, a broader experimentalprogram on the joining interface condition including shimmingand interface gap would be required.

In this paper, the effect of joining interface condition on bearingresponse of single-lap, countersunk composite-aluminum joints (arepresentative type of joints in composite airframes) were investi-gated systematically through a series of quasi-static tests. Bothsolid shim and liquid shim were considered and a comparisonwas made for them. A commercial 3D Digital Image Correlation(DIC) system was utilized to evaluate the effect of shimming on

Component Bap

ssembly of composite airframe.

A

B

C

D

(a) Testing machine and VIC-3D system: (A) specimen; (b) Specimen applied speckle(B) CCD cameras; (C) light source; (D) synchronous data transmission device.

Fig. 3. Test set-up. (a) Testing machine and VIC-3D system: (A) specimen, (B) CCD cameras, (C) light source, (D) synchronous data transmission device; (b) specimen appliedspeckle.

Bearing Strain, %

Bea

ring

Stre

ss (M

Pa)

0 10 20 30 40 500

100

200

300

400

500

600

R L0.3

L0.6

L1.1

Bearing Strain, %

Bea

ring

Stre

ss (M

Pa)

0 10 20 30 40 500

100

200

300

400

500

600

R S0.6

S1.1

(a) Liquid-shim series (b) Solid-shim series

Bearing Strain, %

Bea

ring

Stre

ss (M

Pa)

0 10 20 30 40 500

100

200

300

400

500

600

S0.6

L0.6

Bearing Strain, %

Bea

ring

Stre

ss (M

Pa)

0 10 20 30 40 500

100

200

300

400

500

600

S1.1

L1.1

(c) 0.6 mm series (d) 1.1 mm series

Fig. 4. Bearing stress/bearing strain curves under various shimming conditions. (a) Liquid-shim series; (b) solid-shim series; (c) 0.6 mm series; (d) 1.1 mm series.

192 Y. Zhai et al. / Composite Structures 134 (2015) 190–198

Y. Zhai et al. / Composite Structures 134 (2015) 190–198 193

the surface strain distribution and out-of-plane deformation of thejoints. One focus of the study was to investigate the effect of inter-face gap on the bearing performance of composite bolted joints.

2. Experimental program

2.1. Specimen preparation

The specimens were single-lap, hybrid joints consisted of a car-bon/epoxy laminate and an Aluminum alloy 7075T651 substrate.The carbon/epoxy laminates were manufactured from T700/3068carbon fiber/epoxy unitapes (nominal ply thickness 0.125 mm)with quasi-isotropic lay-up [G2/+45/0/�45/0/(+45/0/�45/90)/+45/902/�45/0/90]s. SW100/3068 woven glass fabric prepregs (nominalply thickness 0.1 mm) were used to avoid galvanic corrosion. Thespecimen geometry is shown in Fig. 1 and was designed to inducebearing failure according to the recommendations in ASTM D5961standard [24]. The carbon/epoxy laminates were drilled with acemented carbide drill-counterbore tool on a CNC machine toensure the holes consistent and accurate. Glass/epoxy laminateswere utilized to avoid damage on the exit side of the specimens.The holes were examined for damage using ultrasonic test instru-ment. The fastener systems used were aerospace grade Titaniumalloy countersunk fasteners (HST11AG6-9) with Aluminum alloyHi-lock collars (HST79CY6). Bolt-hole clearance was within 1% ofthe nominal hole diameter. The bolt clamping pressure was appliedby torquing off the collars with 3 N m bolt torque. This joint

Shim thickness (mm)

Join

tStif

fines

s(M

Pa)

0 0.3 1.10.670

75

80

85

90

95

100

R(mean value) R Liquid(mean value) Liquid Solid(mean value) Solid

(a) Joint stiffness versus shimming conditions (b

Shim

Ulti

mat

eB

earin

gSt

reng

th(M

Pa)

0 0.3500

510

520

530

540

550

560

R(mean value) R Liquid(mean va Liquid Solid(mean valu Solid

(c) Ultimate bearing streng

Fig. 5. Joint stiffness and bearing strength under various shimming conditions. (a) Joint sconditions; (c) ultimate bearing strength versus shimming conditions.

configuration is a representative type of joints in composite air-frames, and the experimental results will be commonly instructive.

The test configurations are presented in Tables 1 and 2. The con-figuration R was regarded as the reference. In terms of shimming,shim type and shim thickness were investigated. Solid peelablefiberglass shim and Hysol EA 9394 liquid shim [25], currently usedin the composite airframe assembly, were employed in this study.The shim thickness chosen were 0.3, 0.6 and 1.1 mm, which are therepresentative conditions in aircraft industry. Solid shim wasdirectly bonding to the Aluminum plates. Liquid shim was firstlymixed of epoxy resin and curing agents at the weight ratio of100:17, and then applied to the Aluminum plates. After curingfor 5 days at 25 �C, the shimmed Aluminum plates were drilled.

One important decision for this study was the design of inter-face gap to examine. As shown in Fig. 2, the shape of interfacegap in reality is irregular and random, but approximates to tapersand can be characterized by thickness and span. In this study,interface gap was represented by machining a highly small slopeannulus on the surface of aluminum plate. The annulus was con-centric with the hole as illustrated in Fig. 1. The variable parame-ters were thickness H and span D. The values for thickness Hwere 0.2, 0.5 and 0.8 mm. 0.2 mm is within aeronautic designallowable (generally 0.3 mm) [20]. The latter two were used tofully understand the response of the joints. The values for span Dwere 12 and 22 mm. After tightened, the specimens with interfacegap were examined for damage and no initial damage wasdetected.

Shim thickness (mm)

2%O

ffse

tBea

ring

Stre

ngth

(MPa

)

0 0.3 1.10.6340

360

380

400

420

440

R(mean value) R Liquid(mean value) Liquid Solid(mean value) Solid

) 2% offset bearing strength versus shimming conditions

thickness (mm)1.10.6

lue)

e)

th versus shimming conditions

tiffness versus shimming conditions; (b) 2% offset bearing strength versus shimming

-0.21 0.240.180.13-0.014 0.071-0.042-0.098-0.15

Out-of-plane displacement(mm)

Y

0.350.29

R L0.6

40 30 20 10 0-0.2

-0.1

0.0

0.1

0.2

0.3

0.4R S0.6

L0.6

S1.1

L1.1

Out

of p

lane

dis

plac

emen

t (m

m)

Y(mm)

S0.6 S1.1 L1.1

(a) Out-of-plane displacement

-0.24 0.160.110.06-0.04 0.01-0.09-0.14-0.19εY(%)

Y

0.260.21

R L0.6S0.6 S1.1 L1.1

(b) The axial strain distribution

-0.14 0.090.060.03-0.02 0-0.05-0.08-0.11ε X(%)

X

0.150.12

R L0.6S0.6 S1.1 L1.1

(c) The transverse strain distribution

Fig. 6. Surface stain distribution and out-of-plane deformation under various shimming conditions. (a) Out-of-plane displacement; (b) the axial strain distribution; (c) thetransverse strain distribution.

194 Y. Zhai et al. / Composite Structures 134 (2015) 190–198

Y. Zhai et al. / Composite Structures 134 (2015) 190–198 195

2.2. Quasi-static testing

All tests were carried out on an MTS E45 hydraulic testingmachine (Fig. 3a) with a load capacity of 100 kN. The tests wererun in displacement control at a rate of 1 mm/min. Loadingstopped after the load dropped 20% from the peak load or excessivedisplacement has occurred. Each test configuration was repeatedwith three specimens. Bearing stress/bearing strain curves wereacquired in accordance with ASTM D5961 Standard [24].

The VIC-3D DIC system from Correlated Solutions Inc. (CSI) wasused to perform full-field, 3D measurements of surface strain andout-of-plane deformation of the joints as shown in Fig. 3a. TwoCCD (charge coupled device) cameras (with 1624 � 1224 pixelsand 25 mm lens) and a light source were required for measure-ment. To ensure data synchronization, DIC system was connectedwith the testing machine by a synchronous data transmissiondevice. A white background was painted on the upper surface ofthe joints using spray paint, and later small black spray paint dro-plets were randomly applied with an appropriate density to makea high-contrast pattern (Fig. 3b). The position of the cameras rela-tive to the test specimens was established through a calibrationprocess to enable accurate measurements.

During the tests, images of speckle pattern were recorded at afrequency of 10 Hz. The VIC 3D software calculated the

Bearing Strain, %

Bea

ring

Stre

ss (M

Pa)

0 10 20 30 40 500

100

200

300

400

500

600

R C1

C2

C3

(a) 12 mm Span series

Bearing Strain, %

Bea

ring

Stre

ss (M

Pa)

0 10 20 30 40 500

100

200

300

400

500

600

C2

C4

(c) 0.5 mm Thickness series

Fig. 7. Bearing stress/bearing strain curves under various interface gap conditions. (a) 1Thickness series.

accumulated movement by comparing the subsequent imageswith the first image, which was taken as the reference image.The strain fields were obtained by local derivative calculations.

DIC measurement technique has been widely used in researchand industry, and proven over and over to be accurate and reliableby comparing to experimental data and valid FEA models [21–23].Consequently, extensometry or strain gauges were not utilized inthis study. The elongation of the joints was measured via recordingthe Y-displacements of two points (Fig. 1) on the upper surface ofthe joints.

3. Results and discussion

3.1. Effect of shimming

3.1.1. Bearing stress/bearing strain behaviorThe bearing stress/bearing strain curves of single-lap, counter-

sunk composite-aluminum joints under various shimmingconditions are systematically shown in Fig. 4.

For both liquid and solid shim in Fig. 4a and b, the shim thick-ness gives a clearly influence on the stress/strain behavior. Withincreased shim thickness, the joint stiffness loss occurs earlierand the peak load decreases. This is because increasing the shimthickness leads to a higher eccentricity of load path in the

Bearing Strain, %

Bea

ring

Stre

ss (M

Pa)

0 10 20 30 40 500

100

200

300

400

500

600

R C4

C5

(b) 22 mm Span series

Bearing Strain, %

Bea

ring

Stre

ss (M

Pa)

0 10 20 30 40 500

100

200

300

400

500

600

C3

C5

(d) 0.8 mm Thickness series

2 mm Span series; (b) 22 mm Span series; (c) 0.5 mm Thickness series; (d) 0.8 mm

196 Y. Zhai et al. / Composite Structures 134 (2015) 190–198

single-lap joints. Higher loading eccentricity can aggravate the tiltand bending of the countersunk bolt, making a smaller bolt-holecontact area and more concentrated load, and leading to earlieroccurrence of bearing damage as well as premature bearing failureof the specimens. As shown in Fig. 4c and d, it seems that thespecimens with solid shim gain a little better bearing performance.

The effect of shimming condition on the joint stiffness, 2% offsetbearing strength and ultimate bearing strength of single-lap, coun-tersunk composite-aluminum joints are given in Fig. 5. As illus-trated in Fig. 5a, joint stiffness decreases with increased shimthickness. Compared to the configuration R, the joint stiffnessreduction for liquid-shim series L0.3 � L1.1 are 7.3%, 14.2% and24.2% respectively, and for solid-shim series S0.6 � S1.1 are 10.7%and 14.2%. The results agree with the tests by Hühne et al. [2]and Dhôte et al. [22]. This is because joint stiffness largely dependson bolt-hole contaction. Increasing the shim thickness aggravatesthe tilt of the countersunk bolt, leading to reduced bolt-hole con-tact area. Obviously, both 2% offset bearing strength and ultimatebearing strength show a statistical trend with shim thickness, i.e.,decreasing with increased shim thickness as illustrated in Fig. 5band c. The reduction of 2% offset bearing strength and ultimatebearing strength for L1.1 are 18.5% and 8.4% respectively. The sametrend can be seen in the solid-shim series. This is in line with theresults that mentioned above in the bearing stress/bearing straincurves, i.e., earlier occurrence of joint stiffness loss and lower peakload with thicker shim. It is worth noting that with equal shimthickness, the joint stiffness and bearing strength of solid-shim

Gap thickness(mm)

Join

t Stif

fines

s (M

Pa)

0 0.2 0.80.570

80

90

100

R(mean value) R 12mm (mean value) 12mm 22mm (mean value) 22mm

(a) Joint stiffness versus varying interface gap (b

Gap thickn

Ulti

mat

e B

earin

g St

reng

th (M

Pa)

0 0.2

500

520

540

560

R(mean value) R 12mm (mean value) 12mm 22mm (mean value) 22mm

(c) Ultimate bearing strength ve

Fig. 8. Joint stiffness and bearing strength under various interface gap conditions. (a) Joininterface gap; (c) ultimate bearing strength versus varying interface gap.

specimens are obviously higher than that of liquid-shim speci-mens. This is probably due to the difference in mechanical propertybetween solid peelable fiberglass shim and epoxy resin.

3.1.2. Surface strain distribution and out-of-plane deformationThe effect of shimming condition on the surface stain distribu-

tion and out-of-plane deformation of single-lap, countersunkcomposite-aluminum joints are presented in this section. Surfacestain fields include the strain field ey in axial direction and thestrain field ex in transverse direction. Out-of-plane deformation isreferred to as secondary bending of single-lap joints due to eccen-tric load path in the joints. Surface stain distribution and out-of-plane deformation are both given at a level of 3.5 kN, which is inthe linear region of the stress/strain curves for all the specimens.

The out-of-plane displacement is shown in Fig. 6a. A profilealong the loading direction is plotted for each case at a distanceof 1.5d from the centerline of the hole. The deflection indicatesthe bending of the laminates, i.e., secondary bending due to eccen-tric load path. It shows that thicker shim results in higher deflec-tion for both solid-shim and liquid-shim series due to highereccentric loading. In addition, the bending degree of solid-shimspecimens is found to be lower. It can be seen that compared toliquid-shim specimens, the solid-shim specimens gain a higherbending stiffness due to the higher tensile modulus of solid pee-lable fiberglass shim. This is the reason for that specimens withsolid shim gain a little better bearing performance, as mentionedabove in the bearing stress/bearing strain behavior.

Gap thickness(mm)

2% O

ffset

Bea

ring

Stre

ngth

(MPa

)

0 0.2 0.80.5

300

320

340

360

380

400

420

440 R(mean value) R 12mm (mean value) 12mm 22mm (mean value) 22mm

) 2% offset bearing strength versus varying interface gap

ess(mm)0.80.5

rsus varying interface gap

t stiffness versus varying interface gap; (b) 2% offset bearing strength versus varying

R C2C1 C3

Fig. 9. The initial bearing damage for R and C1 � C3.

Y. Zhai et al. / Composite Structures 134 (2015) 190–198 197

The axial strain field ey are shown in Fig. 6b. The area around thehole displays highly stain concentration, corresponding to highlystress concentration. The side above the hole (gripped edge side)shows tensile strain concentration which is due to tensile loadingand the existence of hole. The side below the hole (free edge side)shows compressive strain concentration caused by bolt-hole con-taction. The middle area of the plate shows slightly compressivestate due to bending of the plate caused by eccentric load path. Itshows that the axial stain concentration for both solid-shim andliquid-shim series are more and more severe as the shim thicknessincreases. This is because higher loading eccentricity leads toreduced bolt-hole contact area and intensifies the bending of theplate, resulting in more severe stress concentration around thehole and the middle area. It also indicates that compared toliquid-shim series, the solid-shim series show a little slighter axialstrain concentration. This is due to the higher bending degree ofsolid-shim series as mentioned above.

The transverse strain field ex are shown in Fig. 6c. Stain concen-tration is clearly seen around the hole. The area above the hole dis-plays highly compressive state while the area below displayshighly tensile state. In other word, the area above appears to bedepression while the area below looks like bulge in the transversedirection. This is caused by bolt tilt, i.e., compressing the plateabove the hole while jacking up the plate below. The compressivestrain concentration beside the hole is probably due to the exis-tence of 45� plies. Increasing the shim thickness could aggravatethe bolt tilt, resulting in more concentrated transverse strain distri-bution. The transverse strain concentration degrees are nearlyequal for solid and liquid shim specimens with same shim thick-ness due to identical loading eccentricity.

3.2. Effect of interface gap

The bearing stress/bearing strain curves of single-lap, counter-sunk composite-aluminum joints under various interface gap con-ditions are shown in Fig. 7. It shows that the presence of interfacegap significantly weakens the bearing performance of the speci-mens. As the gap thickness increases, joint stiffness loss occurs ear-lier and peak load decreases. For C1 � C3 in Fig. 8b and c, the 2%offset bearing strength decrease by 8.4%, 22% and 26%, and the ulti-mate bearing strength decrease by 1.3%, 4.6% and 6.4% compared tothe configuration R. The same trend is seen for C4 and C5. This isbecause the presence of interface gap made the countersunk holelosing support from the aluminum plate at the shear plane, andmeanwhile intensified the loading eccentricity of single-lap joints.Without supporting at the shear plane, the countersunk hole gotslightly sunken and deformed when tightening the countersunkbolt, and extensive interlaminar stress was induced in the vicinityof hole. Because of secondary bending in single-lap joints, boltbearing load is almost entirely taken by the cylindrical portion ofthe countersunk hole [9]. Thus bolt bearing load combining with

the interlaminar pre-stress led to earlier occurrence of bearingdamage in the cylindrical portion. Further more, the bearing dam-age developed more rapidly and extensively in the laminate due tolack of suppression from the mating plate at the shear plane, ulti-mately resulting in early bearing failure. As the gap thicknessincreases, higher loading eccentricity makes the above situationseverer as the mentioned in the previous section. As shown inFig. 9, the initial bearing damage (only loaded to the onset ofnon-linearity) at the shear plane is more extensive and severe forspecimens with thicker interface gap.

In addition, interface gap slightly influences the joint stiffness.Fig. 8a shows that as the gap thickness increases, the reductionof joint stiffness for C1 � C3 is 4.1%, 6.7% and 8.6% respectively, aswell as 7.0% and 6.3% for C4 and C5. The main reason is similar tothat in shimming, i.e., increased loading eccentricity. Besides, thereduced bolt-hole contact area in the aluminum plates resultingfrom additional machining also made some contribution, but itdoes not happen in reality.

Compared to gap thickness, the gap span seems to have littleeffect on the bearing behavior of single-lap, countersunkcomposite-aluminum joints as illustrated in Fig. 7c and d. But itcan be observed in Fig. 8 that with equal gap thickness, increasedgap span brings a beneficial trend to slightly relieve the adverseeffect of interface gap. This is probably because increasing gap spandoes not affect the loading eccentricity, but could alleviate theinterlaminar pre-stress concentration caused by bolt clampingpressure.

4. Conclusions

In this study, the effect of joining interface condition on thebearing response of single-lap, countersunk composite-aluminumjoints were investigated experimentally. Both solid-shim speci-mens and liquid-shim specimens were considered and compared.A commercial 3D DIC system was utilized to evaluate the effectof shimming condition on the in-plane strain distribution andout-of-plane deformation of the joints. One focus of the studywas to investigate the effect of interface gap on the bearing perfor-mance of composite bolted joints. The shape of interface gap wasdesigned and characterized by variable parameters, i.e., gap thick-ness and gap span. Based on the experimental study, the followingconclusions are made:

The bearing response of single-lap, countersunk, composite-aluminum bolted joints is significantly influenced by shimmingcondition. Increasing the shim thickness intensifies the loadingeccentricity of single-lap joints and results in reduced bearing stiff-ness and bearing strength, as well as more severe in-plane strainconcentration and out-of-plane deformation. Compared to liquid-shim series, specimens with solid shim gain a little better bearingperformance because of higher joint bending stiffness that benefitfrom the higher tensile modulus of solid peelable fiberglass shim.

198 Y. Zhai et al. / Composite Structures 134 (2015) 190–198

The presence of interface gap significantly weakens the bearingperformance of single-lap, countersunk composite-aluminumjoints by making the countersunk hole losing support from the alu-minum plate at the shear plane, and meanwhile intensifying theloading eccentricity of single-lap joints. As the parameter gapthickness increases, higher loading eccentricity makes the situa-tion severer. Increasing the gap span seems to have little effectbut brings a beneficial trend to slightly relieve the adverse effectof interface gap.

From above all, it can be inferred that joining interface condi-tion significantly influence the stress status and damage develop-ment of single-lap, countersunk composite-aluminum joints.Future work will conduct 3D stress analysis and damage character-ization by employing a FE model.

Acknowledgement

The authors wish to acknowledge the National Natural ScienceFoundation of China (51205014) for the provision of financialsupport.

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