quasi-static strength and fatigue life of hybrid (bonded/bolted) composite single-lap joints

Composite Structures 72 (2006) 119–129

www.elsevier.com/locate/compstruct

Quasi-static strength and fatigue life of hybrid(bonded/bolted) composite single-lap joints

Gordon Kelly

Division of Lightweight Structures, Department of Aeronautical and Vehicle Engineering, Kungl Tekniska Hogskolan, S-100 44 Stockholm, Sweden

Available online 2 March 2005

Abstract

The strength and fatigue life of hybrid (bonded/bolted) joints with carbon-fibre reinforced plastic adherends have been investi-

gated. The effect of adhesive material properties and laminate stacking sequence on the joint structural behaviour and failure modes

were determined experimentally. Hybrid joints were shown to have greater strength, stiffness and fatigue life in comparison to adhe-

sive bonded joints. However, the benefits were only observed in joints with lower modulus adhesives which allowed for load sharing

between the adhesive and the bolt. Hybrid joints with high modulus adhesives showed no significant improvement in strength

although increased fatigue life was observed due to the presence of the bolt.

Three distinct stages in the fatigue life of hybrid joints were observed where the adhesive, the bolt and their combination were all

contributing to the load transfer. Fatigue crack initiation was found to occur later in the hybrid joints where the bolt transferred a

significant portion of the load.

The failure mode of the joints was shown to be dependent upon the relation between the hybrid joint strength and the bearing

strength of the laminate. Hybrid joints in which the strength greatly exceeded the laminate bearing strength failed catastrophically in

net-section mode. In contrast, laminates with higher bearing strength failed in a non-catastrophic bearing mode.

Careful comparison of the material bearing strength and adhesive bond strength is necessary to ensure increased joint strength

and non-catastrophic failure.

� 2004 Published by Elsevier Ltd.

Keywords: Composite; Single-lap joint; Hybrid (bonded/bolted); Finite element analysis; Strength; Fatigue life

1. Introduction

Joining of composite laminates is commonly achieved

by adhesive bonding, mechanical fastening or a combi-

nation of the two methods. The combination of adhesivebonding and mechanical fastening is often employed as

a safeguard against defects within the adhesive layer

which may lead to premature or catastrophic failure.

Prediction of the joint fatigue life and the susceptibility

of the adhesive/interface to environmental attack are

additional uncertainties which have resulted in hybrid

joint designs being used in practice.

0263-8223/$ - see front matter � 2004 Published by Elsevier Ltd.

doi:10.1016/j.compstruct.2004.11.002

E-mail address: [email protected]

Hybrid joining is attractive in automotive applica-

tions as the technique can offer benefits during part

manufacture. The bolts can be used as a means to align

and attach different structural parts to one another and

provide fixation during adhesive cure. In addition, hy-brid joints can offer improved performance in compari-

son to adhesive bonded joints during crash loading

where part separation is often undesirable.

Previous studies which have focused on aerospace

applications [1–3] have not regarded hybrid joints to of-

fer any significant increase in strength over adhesive

bonded joints, mainly due to the low fraction of load

transferred by the bolt. The results were based on theo-retical analysis with aerospace joint configurations and

material systems being adopted. However, it was

mailto:[email protected]

tt

φ=6.35mm

L=25mm

a

φ =10mm

CFRP AdherendAdhesive

Huck C6L Lockbolt

120 G. Kelly / Composite Structures 72 (2006) 119–129

acknowledged that bolts can be valuable in heavily

loaded structures through arresting any initial damage.

Montabone et al. [4] investigated a combination of bolt-

ing and cold bonding to join composite sandwich panels

in a butt-strap configuration. Simplified finite element

analysis was used to estimate failure loads with hybridjoints being predicted to be 25% stronger in comparison

to adhesive bonded joints. However, the joint failure

load was limited by peeling of the CFRP and the adhe-

sive material did not exceed the yield stress prior to

failure.

Limited published work exists where the structural

behaviour of hybrid joints is characterised experimen-

tally. Fu and Mallick [5] investigated the static and fati-gue strength of single-lap joints made from structural

injection moulded (SRIM) composite material. Hybrid

joints were designed with large washers which covered

the main part of the overlap region. Higher static

strength and longer fatigue life was observed in compa-

rison to adhesive bonded joints. However, the perfor-

mance of hybrid joints was sensitive to the washer

configuration with larger washer areas yielding greaterstrength and fatigue life. Imanaka et al. [6] investigated

the combination of blind rivets and adhesive bonding in

lap-joints made from high strength steel. The effect of

joint width and the adhesive properties on the fatigue

life were investigated. The fatigue life of bonded and riv-

eted/bonded joints with an acrylic adhesive were found

to be approximately equal but increased fatigue life

was observed in the hybrid joints with an epoxy adhe-sive. The fatigue strength of the joints increased with

reduction in joint width to hole diameter ratio (w/d).

The propagation rate of fatigue cracks was reduced in

riveted/bonded joints in comparison with adhesive

bonded joints.

It was recently shown by Kelly [7] that hybrid com-

posite single-lap joints can be designed where the bolt

transfers a significant part of the load. The joint geom-etry and adhesive mechanical properties were found to

be significant parameters governing the load transfer

distribution in the joint. As a continuation of the previ-

ous study, the present paper addresses the static

strength, failure mechanisms and fatigue resistance of

the joints. The performance of the joints is compared

to that of adhesive bonded joints. An important aim

of the present study was to determine if the presenceof the bolt affected the initiation of fatigue damage in

the joints. The number of cycles to damage initiation

was measured for different joint configurations using

the backface strain technique [8].
w= 25mm
L =160mmtot

Fig. 1. Test specimen geometry for bonded and hybrid composite

joints.

2. Experimental

An experimental investigation was undertaken to

measure the quasi-static strength and fatigue life of

bonded and hybrid (bonded/bolted) single-lap joints.

Measurements of joint compliance and relative displace-

ment between the adherends were performed to aid in

understanding the load transfer in the joint. The joint

failure modes were characterised together with the initi-

ation of fatigue damage, providing valuable knowledgerequired for hybrid joint design.

2.1. Materials and specimen manufacture

Specimens were machined from carbon fibre epoxy

laminates manufactured by resin transfer moulding

(RTM). A continuous carbon fibre (Toray T700) non-

crimp fabric from heavy tow yarn was used togetherwith epoxy resin (Shell Epicote LV 828). Two different

laminate thicknesses were used in the study with the

stacking sequences [0/45/90/�45]s (t = 1.6 mm) and [0/

45/90/�45]s2 (t = 3.2 mm), respectively. Laminate plates

of size 300 mm · 140 mm were bonded together in a sin-

gle-lap joint configuration. The adherend surfaces were

abraded using 400 grit sand paper and rinsed with ace-

tone prior to bonding. Spacer plates of thickness 0.5mm were used to control the thickness of the adhesive

and fillets were manufactured at the ends of the overlap.

Test specimens were machined from the plates to the

geometry illustrated in Fig. 1 using a diamond tip saw.

Two different adhesives were considered in the pres-

ent study. The first adhesive was a two component struc-

tural polyurethane adhesive (Pliogrip 7400/7410,

Ashland Specialty Chemicals). The adhesive had a lowelastic modulus and a large strain to failure. The adhe-

sive was cured for 30 min at a temperature of 100 �C.The second adhesive was a two component epoxy sys-

tem (Epibond 1590 A/B, Vantico). The adhesive had a

higher elastic modulus and lower failure strain than

the polyurethane adhesive. The curing condition for

the Epibond adhesive was 25 min at room tempera-

ture followed by 1 h at 80 �C. The tensile stress–strain

Strain Gauge

CFRP

Brackets to attach Extensiometer

1mm

CFRP

Adhesive

Huck bolt

Fig. 3. Experimental setup.

G. Kelly / Composite Structures 72 (2006) 119–129 121

behaviour of the adhesives was measured according to

ASTM D638-02a [9] and the results are shown in Fig. 2.

The manufacture of the hybrid joints was performed

in two steps. First, a bonded joint was manufactured

and cured as described. Thereafter, holes of diameter

6.35 mm were drilled in the centre of the overlap regionin the bonded joints using a dagger drill. A plate was

used at the rear side of the joint to limit any delamina-

tion caused by the drilling process. Stainless steel Huck

C6L lock-bolts were then inserted into the holes and

installation was performed using a pneumatic fastening

system which was used for automated installation of

the bolts. The bolts were located at the centre of the

overlap which gave edge distance to bolt diameter (e/d)and width to bolt diameter (w/d) ratios of 2 and 4,

respectively. The values were chosen to obtain bearing

failure mode, based on the results from bearing strength

tests on the same material [10].

A series of bonded, bolted and hybrid joint configu-

rations were manufactured to allow for comparison

between the different joining techniques. The configura-

tions considered in the present study are listed in Table1. The joint width and overlap length were both 25 mm.

0 10 20 30 40 50 600

5

10

15

20

25

30

35

40

Strain (%)

Str

ess

(MP

a)

Epibond 1590 A/BPliogrip 7400/7410

Fig. 2. Tensile stress–strain behaviour of the Pliogrip and Epibond

adhesive systems.

Table 1

Test specimen configurations

Joint Adhesive Adherend thickness

(mm)

Adhesive thickness

(mm)

Bonded Pliogrip 1.6 0.5

Bonded Pliogrip 3.2 0.5

Bonded Epibond 3.2 0.5

Bolted – 1.6 –

Bolted – 3.2 –

Hybrid Pliogrip 1.6 0.5

Hybrid Pliogrip 3.2 0.5

Hybrid Epibond 3.2 0.5

2.2. Testing procedure

The quasi-static tests were conducted using a univer-

sal testing machine (Instron 4505) with a 100 kN load

cell. The system was fully computer controlled and al-

lowed for acquisition of load, displacement and strain

data. The tests were run in displacement control at a rate

of 1 mm/min. The experimental setup is illustrated in

Fig. 3.

Fatigue testing was performed using a Schenk PSA-10 servo hydraulic universal testing machine with a 10

kN load cell. The load and grip displacement were re-

corded throughout the tests using a PC data acquisition

system. The tests were conducted in load control mode

with a sinusoidal waveform at a frequency of 5 Hz.

The joints were subjected to tension–tension fatigue with

a stress ratio (R = rmin/rmax) equal to 0.1. Several spe-

cimens were instrumented with strain gauges (ShowaN11-FA-2-120-11, gauge length = 2 mm) in order to de-

tect damage initiation using the backface strain tech-

nique [8]. This technique relies on the fact that

damage, such as fracture of the adhesive layer, results

in a change in the strain distribution on the surface of

the adherends. The strain gauges were positioned at

both ends of the overlap as shown in Fig. 3 with the

gauge locations being based on the recommendationsby Crocombe et al. [11].

3. Finite element analysis

Finite element analysis was used to study the behav-

iour of the joints and predict the load transfer and stress

distribution in the joints at different load levels. A three-dimensional model of a hybrid single-lap joint was

developed using the ABAQUS finite element code

[12] with the model including the effects of large-

deformations, bolt-hole contact and non-linear adhesive

material properties. The model was verified experimen-

tally and shown to be able to predict load transfer in

hybrid single-lap joints [7].

The laminate adherends were modelled using linearbrick elements with quadratic elements being used for

Fig. 4. Finite element model of the hybrid single-lap joint.

Table 2

Elastic properties for the carbon fibre/epoxy lamina from [15]

E11 (GPa) E22 (GPa) E33 (GPa) G12 (GPa) G13 (GPa) G23 (GPa) m12 m13 m23

98 7.8 7.8 4.7 4.7 3.2 0.34 0.34 0.44

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

-10

-5

0

5

10

15

20

25

30

35

Normalised Overlap length

Max

imum

Prin

cipa

l Str

ess

(MP

a)

Epibond t=3.2mmPliogrip t=3.2mmPliogrip t=1.6mm

Fig. 5. Stress distribution in the adhesive for the hybrid single-lap

joints (P = 4 kN).


the bolt. A neat fit was assumed between the bolt and

the laminate and this condition was found to provide

a higher bound value for the load transferred by the bolt

[7]. Contact between the bolt and the adherends was

modelled based on the contact pair approach in ABA-

QUS with finite sliding allowed between surfaces in con-

tact. A friction coefficient of 0.2 was assumed between

laminates and the bolt. The bolt shear load was pre-dicted through summation of the nodal forces on the

mid-plane of the bolt. Symmetry was adopted along

the length of the joint and thus the model was reduced

to a half model as shown in Fig. 4.

The CFRP adherends were modelled using linear

elastic material properties detailed in Table 2. The true

stress–strain properties of the adhesive were modelled

as shown in Fig. 2. The bolt was modelled as a linearelastic isotropic material and assigned steel material

properties (E = 205 GPa, m = 0.3).

The analyses were performed in two steps. An initial

bolt clamping load of 5 kN, which was set by the pneu-

matic installation system, was applied as a pre-tension

load to the mid-surface of the bolt. Thereafter, a tensile

load was applied to one end of the joint which was free

to move in the longitudinal (x) direction only with theopposite end of the joint having clamped boundary con-

ditions (ux,uy,uz = 0).

3.1. Stress analysis and load transfer predictions

The maximum principal stress along the centreline of

the adhesive bond is plotted in Fig. 5. The stresses are

plotted from toe-to-toe and at the mid-width of thejoint. The stress distributions in the adhesive layer of

the different joints are similar in the central region where

a compressive stress state exists. At the ends of the over-

lap, stress concentrations exist and the stress state in the

adhesive is tensile. The stress concentrations are most

evident in the joint with the higher modulus epoxy adhe-

sive. Peaks are evident in the region directly adjacent to

the embedded adherend corner and at the toe of the fil-

let. The peak stress in the epoxy joints is approximately

75% greater than the peak stress in the polyurethane

joints with the same adherend thickness. A single stress

peak is evident in the polyurethane joints with the peakstress occurring in the region adjacent to the embedded

adhesive corner. The stress is lower in the fillet region.

The load transfer distribution in the hybrid single-lap

joints is illustrated in Fig. 6. The load transfer is charac-

terised by the ratio of the load transferred by the bolt to

the total applied load. The load transfer distribution is

shown to differ in the joints with the adhesive elastic

modulus having a significant effect. The load transferredby the bolt in the hybrid joint with the epoxy adhesive is

0 2 4 6 8 10 120

5

10

15

20

25

30

35

40

45

50

Applied load (kN)

Bol

t loa

d /to

tal l

oad

(%)

Pliogrip, t=1.6mmPliogrip, t=3.2mmEpibond, t=3.2mm

Fig. 6. Predicted load transfer in the hybrid single-lap joints.


low with the adhesive transferring approximately 98% of

the applied load. The bolt load transfer is low as a result

of the small relative displacement which occurs betweenthe adherends in the joint. In contrast, the load trans-

ferred by the bolt in the joints with is significantly

higher. Initially, the bolt load increases linearly with

the applied load. At higher load levels, the rate at which

the bolt load increases with the applied load decreases.

0 1 2 3 4 5 60

2

4

6

8

10

12

14

16

Load

(kN

)

Grip Displacement (mm)

BondedBoltedHybrid

0 1 2 30

2

4

6

8

10

12

14

16

Load

(kN

)

Grip Displacem

Fracture of first fillet

Fracture of sec

(a) (b

(c)

Fig. 7. Load displacement behaviour of the bonded, bolted and hybrid sing

adhesive, t = 3.2 mm. (c) Epoxy adhesive, t = 3.2 mm.

The load transferred by the bolt is slightly higher in

the joint with the thicker adherends with the load trans-

fer being related to the relative displacement between the

adherends.

4. Results

4.1. Quasi-static tests

The load displacement behaviour of each of the joint

configurations is illustrated in Fig. 7(a)–(c). The behav-

iour of the bolted, bonded and hybrid joints are com-

pared for the different laminate stacking sequences(and thickness) and adhesive systems.

The load displacement curves of the joints based on

the [0/45/90/�45]s laminates and polyurethane adhesive

are illustrated in Fig. 7(a). It is evident that the bolted

joint configuration has a significantly lower load capac-

ity than the bonded and hybrid joints. The low load

capacity occurs as result of the small area over which

the bolt bearing load is transferred. The thin and rela-tively flexible composite laminates also allow for the bolt

to tilt which increases the stress concentration at the

edge of the hole further reducing the bearing strength.

Adhesive bonding was both a stiffer and stronger joining

0 1 2 3 4 5 60

2

4

6

8

10

12

14

16

Load

(kN

)

Grip Displacement (mm)

BondedBoltedHybrid

4 5 6ent (mm)

BondedBoltedHybrid

ond fillet

)

le-lap joints. (a) Polyurethane adhesive, t = 1.6 mm. (b) Polyurethane


method for this laminate configuration. Linear elastic

behaviour was observed in the adhesive joint up to

40% of the failure load and the joint strength was

approximately 60% greater than that of the bolted joint.

The hybrid joint was shown to possess the highest stiff-

ness and strength in comparison to the bolted andbonded joints. The joint stiffness was equal to that of

the bonded joint at lower load levels which suggested

that the adhesive was the main load bearing component.

However at load levels greater than 50% of the ultimate

failure load, the hybrid joint was shown to be signifi-

cantly stiffer in comparison to the bonded joint. The in-

crease in stiffness was attributed to the load sharing

between the bolt and the adhesive with the bolt restrict-ing the relative displacement between the adherends.

The ultimate strength of the hybrid joint was 11% higher

than that of the bonded joint.

Somewhat different behaviour was observed in the

joints with the [0/45/90/�45]s2 adherend stacking se-

quence and the polyurethane adhesive (see Fig. 7(b)).

The bolted joint demonstrated almost equal load capa-

city to the bonded joint. However, the stiffness of thebolted joint was lower than that of both the bonded

and hybrid joints. The bonded and hybrid joints demon-

strated similar trends to those observed in the [0/45/90/

�45]s laminates. The stiffness of the joints were equal

in the early stages of loading with the hybrid joint being

stiffer at higher load levels. The difference in stiffness was

related to the relative displacement between the adher-

ends. A typical plot of applied load versus relative dis-placement for the bonded and hybrid joints with the

polyurethane adhesive is shown in Fig. 8. The relative

displacement between the adherends, Ur, was equal for

both joints below an applied load of 5 kN. Thereafter

the effect of the bolt in the hybrid joint was evident, lim-

iting the relative displacement between the adherends

and consequently increasing the joint stiffness. The bolt

0 0.4 0.8 1.20

2

4

6

8

10

12

14

16

Load

(kN

)

Relative displacement between adherends, Ur (mm)

Hybrid JointBonded Join

Fig. 8. Relative displacement measured between the adherends in bonded

7400/7410 adhesive.

load predicted from the finite element analysis was 23%

of the total load at the point where the relative displace-

ment curves for the bonded and hybrid joint diverged.

The ultimate strength of the hybrid joint was 22%

higher than that of the bonded joint. Thus, the benefits

of the hybrid joint in terms of stiffness and strength weregreater for the [0/45/90/�45]s2 laminate in comparison to

the [0/45/90/�45]s laminate.

A comparison of the joints with the [0/45/90/�45]s2stacking sequence and epoxy adhesive is illustrated in

Fig. 7(c). The ultimate strengths of the bolted and

bonded joints were found to be similar. The hybrid joint

illustrated similar behaviour to the adhesive bonded

joint even after fracture of the fillet at one end of theoverlap. The main deviation in the behaviour occurred

at fracture of the second fillet when a sudden shift in

load transfer to the bolt occurred. After fracture of the

adhesive layer, the entire load was transferred by the

bolt and the load bearing capacity of the joint increased

to the laminate bearing strength. No significant increase

in strength was observed in the hybrid joints with both

the bolt and the adhesive working independently. Therelative displacement between the adherends in the

bonded and hybrid joints was small with negligible dif-

ference being observed between both joint configura-

tions. This supported the low bolt load transfer

predictions from the finite element analysis as shown

in Fig. 6.

A summary of the ultimate strengths for each of the

joint configurations is listed in Table 3. A minimum ofthree specimens were tested for each joint configuration.

The failure modes were consistent for each of the tested

joint configurations.

4.1.1. Failure modes in bonded joints

The joint failure modes are important with regard to

design, repair and eventual damage tolerance. Failure of

1.6

t

Ur

and hybrid joints with [0/45/90/�45]s2 stacking sequence and Pliogrip

Table 3

Quasi-static joint strengths

Joint type Adhesive Adherend thickness

(mm)

Ultimate strength

(kN)

Bonded Pliogrip 1.6 11.62 ± 0.79

Bonded Pliogrip 3.2 12.48 ± 0.54

Bonded Epibond 3.2 11.83 ± 0.83

Bolted – 1.6 7.56 ± 0.41

Bolted – 3.2 11.66 ± 0.78

Hybrid Pliogrip 1.6 13.19 ± 0.61

Hybrid Pliogrip 3.2 15.62 ± 0.46

Hybrid Epibond 3.2 12.66 ± 0.52


the bonded joints was catastrophic with separation of

the adherends. The joints with the polyurethane adhe-

sive failed with damage initiating at the embedded adhe-

sive corner and a crack propagating through the

adhesive layer toward the opposite adherend (see Fig.

9(a)). Examination of the failure surface revealed that

the crack had propagated into the composite adherendand through the fibre bundle of the surface ply. The ini-

tial crack at the adherend corner was visible at appro-

ximately 60–70% of the ultimate failure load. Thus,

there was a defined period of damage propagation even

during the quasi-static test.

Failure of the joints bonded with the epoxy adhesive

occurred with the fracture initiating toward the toe of

the adhesive fillet as illustrated in Fig. 9(b). This wassupported by the results from the finite element analysis

which predicted high stresses in this region.

A period of unstable crack growth in the top ply of

the composite was observed prior to catastrophic frac-

ture of the joint. Examination of the failure surfaces re-

vealed fracture through the fibre bundles in the surface

ply, as observed in the joints bonded with the polyure-

thane adhesive. The intralaminar failure mode suggestedthat good adhesion was achieved between both adhe-

sives and the composite adherends.

4.1.2. Failure modes in hybrid joints

Failure initiation in the hybrid joints occurred in a

similar manner to that in the bonded joints. The hybrid

joints failed in two stages with the first stage involving

failure of the adhesive and the second stage involvingfailure of the bolted joint. Failure of the hybrid joints

with the polyurethane adhesive initiated at the embed-

ded adhesive corner as illustrated in Fig. 9(a). Cracks

propagated from both ends of the overlap toward the

centre region of the joint.

(a) Polyurethane adhesive

Fig. 9. Failed modes in the bonded and hybrid jo

4.1.3. [0/45/90/�45]s laminate and polyurethane adhesive

The hybrid joints in the [0/45/90/�45]s laminate failed

by net-section failure. The load level in the hybrid joint

after adhesive failure greatly exceeded the bolted joint

bearing strength and thus the joint failed catastrophi-

cally. Together with the high bolt load and possible dy-namic effects, the undamaged adhesive in the net-section

may contribute to a reduction in the notched tensile

stress. This failure mode has important implications

with regard to the allowable service load and redun-

dancy. The increased static joint strength of a hybrid

joint for this laminate configuration should not be used

to motivate an increase in the service load, but rather be

viewed as an increased factor of safety. The catastrophicfailure was not observed in the bolted joint which failed

in bearing failure mode. While increasing the bolt pitch

distance may reduce the risk for net-section failure, it re-

duces the benefits of using the hybrid joint concept

through reduction of the load transferred by the bolt

as shown in [7].

4.1.4. [0/45/90/�45]s2 laminate and polyurethane

adhesive

The hybrid joints with the [0/45/90/�45]s2 laminate

stacking sequence and polyurethane adhesive failed in

a non-catastrophic manner. Crack initiation in the adhe-

sive was visible at 60% of the ultimate load and a stable

crack growth allowed for the load to be transferred

gradually to the bolt. Final fracture of the adhesive layer

resulted in a sudden load drop of approximately 5 kN.Thereafter, the joint continued to carry load and be-

haved as a bolted joint with the ultimate failure occur-

ring due to bearing failure at the bolt hole. Typical

micro-mechanical damage mechanisms in the hybrid

joints are illustrated in the micrographs shown in Fig.

10. The initial crack propagation was evident in the

adhesive with the cracks propagating through the resin

rich surface layer of the composite laminate. The prop-agation of damage into the composite laminate was con-

sistent with the observations of the fracture surfaces of

the bonded joints. The ultimate failure of the joint was

bearing failure at the bolt hole. The failure was attrib-

uted to compression failure in the 0� oriented ply which

was aligned in the loading direction and thus contrib-

uted significantly to the load bearing capacity of the

laminate. Micro-buckling followed by compressive fail-ure of plies oriented in the load direction was previously

shown to be a failure mechanism responsible for loss of

(b) Epoxy adhesive

ints. Dashed lines illustrate the crack path.

Fig. 10. Micrograph illustrating damage in a hybrid joint at ultimate failure load (Pliogrip adhesive).


load bearing capacity in the same carbon fibre non-

crimp fabric based laminates [10].

4.1.5. [0/45/90/ �45]s2 laminate and epoxy adhesive

Failure of the hybrid joints with the epoxy adhesive

occurred in stages. Initial failure occurred in the fillet

at one end of the overlap. The joint continued to carryload with no significant loss of stiffness until fracture oc-

curred in the second adhesive fillet. The second fracture

resulted in a significant load drop and with the entire

load being transferred by the bolt. Thereafter, the joint

behaviour resembled that of the bolted joint with ulti-

mate failure occurring due to bearing failure at the bolt

hole.

4.2. Fatigue tests

The results from the fatigue tests are presented in the

form of load-lifetime diagrams in Fig. 11(a)–(c). The

graphs illustrate failure of the adhesive and ultimate fail-

ure of the hybrid joints, and ultimate failure of the adhe-

sive bonded joints. In general, the hybrid joints

demonstrated higher resistance to fatigue than thebonded joints. Extended fatigue life was observed at

all of the tested load levels with the improvement being

more pronounced in the joints with thicker adherends.

At 50% of the static failure load, the improvement in

the fatigue life is in excess of a factor 10 in comparison

to the bonded joints. At lower loads (630�40% Pstatic),

several specimens did not fail and were considered run-

out specimens. The run-out specimens are indicated by

the arrows in the graphs. Static tensile tests were con-

ducted on the run-out specimens to determine the resid-

ual joint strengths. The residual strength of the run-outspecimens are presented in Table 4. The residual

strength of the joints in the [0/45/90/�45]s laminate

was similar to the bolted joint strength. The behaviour

was reasonable, given that the ultimate strength of

bolted joints in composite laminates has been shown

to be relatively insensitive to initial bolt-hole clearance

[10,13,14]. In contrast, the residual strength of the joints

in the [0/45/90/ �45]s2 laminate exceeded the strength ofboth the bonded and bolted joints. This result was not

anticipated and there was no apparent reason for the in-

creased strength. A possible explanation could be the

relaxation of residual stresses within the laminate,

although there was no evidence to support this

hypothesis.

The failure modes and fatigue behaviour were found

to differ depending on the joint configuration. In gen-eral, the bonded joints failed in a similar manner to

the quasi-static tests. The main differences in the fatigue

behaviour of the bonded joints was attributed to the

adhesive system. The bonded joints with the polyure-

100

101

102

103

104

105

106

107

0

2

4

6

8

10

12

14

16

18

Max

imum

Loa

d (k

N)

Number of Cycles, N

Bonded Joint FailureHybrid Bond FailureHybrid Joint Failure

(a)

100

101

102

103

104

105

106

107

0

2

4

6

8

10

12

14

16

18

Max

imum

Loa

d (k

N)

Number of Cycles, N


(b)

100

101

102

103

104

105

106

107

0

2

4

6

8

10

12

14

16

18

Max

imum

Loa

d (k

N)

Number of Cycles, N


(c)

Fig. 11. Load lifetime diagrams for bonded and hybrid single-lap joints. (Arrows indicate specimen run-out.) (a) Polyurethane adhesive, t = 1.6 mm.

(b) Polyurethane adhesive, t = 3.2 mm. (c) Epoxy adhesive, t = 3.2 mm.


thane adhesive had a period of visible, stable crackgrowth which was observed as an increase in the speci-

men compliance. In contrast, the bonded joints with

the epoxy adhesive illustrated no change in compliance

prior to catastrophic failure. Only specimens loaded at

low load levels (30% Pstatic) had a period of sustained

crack growth prior to failure.

Three distinct stages were observed in the fatigue life

of the hybrid joints as shown in Fig. 12. The initial stageconsisted of fatigue of the adhesive. After damage initi-

ation in the adhesive, a second stage was observed where

Table 4

Residual strength of hybrid joints after fatigue run-out

Adhesive Adherend

thickness (mm)

Maximum fatigue

load (kN)

Residual

strength (kN)

Pliogrip 1.6 2.5 7.8



Pliogrip 3.2 4.5 13.9

Pliogrip 3.2 5.0 14.3

Pliogrip 3.2 6.0 13.4

Epibond 3.2 5.0 13.2

Epibond 3.2 6.0 15.2

Epibond 3.2 7.0 14.0

Epibond 3.2 7.0 13.6

damage propagated in the adhesive layer and the jointcompliance increased and thereby transferred load onto

the bolt. Finally, the third stage was characterised by fa-

tigue loading of a bolted joint with the adhesive layer

having failed. The relative length of the stages was

dependent upon the applied load and adhesive charac-

teristics. As the load level increased, the first and second

stages of fatigue of the hybrid joints shortened with the

main part of the fatigue life being governed by thebolted joint. However, at lower loads the first stage

involving fatigue of the adhesive was a significant part

of the joint fatigue life. Due to the brittle behaviour of

the epoxy adhesive, the second stage (or transition stage)

was extremely short with the cracks propagating

through the adhesive at a high rate.

Comparison of the bonded and hybrid joints revealed

a significant difference in the behaviour of the joints withthe epoxy and polyurethane adhesives. Plots of ampli-

tude versus number of cycles revealed that failure of

the adhesive in the hybrid joints with the epoxy adhesive

occurred after a similar number of cycles as in the

bonded joints. Thus, the behaviour of the two load

transfer methods (adhesive and bolt) work essentially

independently of one another and in series. In con-

trast, failure of the adhesive in the polyurethane joints

100

101

102

103

104

105

106

107

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Number of Cycles, N

Am

pliti

de,

∆u (

mm

)

Stage 1 Stage 3Stage 2

Adhesive bond failure

Bolted joint failure

P=4.5kN

P=6.5kN

Fig. 12. Fatigue behaviour of hybrid single-lap joints.

0 1 2 3 4 5 6 7

x 104

0.25

0.2

0.15

0.1

0.05

Number of Cycles, N

Bac

kfac

e S

trai

n (%

)

Bonded Side 1Bonded Side 2Hybrid Side 1Hybrid Side 2

(a)

0 0.5 1 1.5 2 2.5 3 3.5

x 104

2.5

2

1.5

1

0.5

0

0.5

1

Bac

kfac

e S

trai

n (%

)

Number of Cycles, N

Bonded Side 1Bonded Side 2Hybrid Side 1Hybrid Side 2

2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2

x 104

0.5

0.45

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.05

Bac

kfac

e S

trai

n (%

)

Number of cycles, N

A

B

(b)

Fig. 13. Backface strain measurements for bonded and hybrid joints

(t = 3.2 mm). (a) Polyurethane adhesive, (Pmax = 6 kN). (b) Epoxy

adhesive, (Pmax = 6 kN).


occurred after a greater number of cycles in the hybrid

joint. While the number of cycles until the first crack ap-peared to be greater, no apparent change in the crack

propagation rate was observed through measurement

of the specimen compliance.

4.2.1. Backface strain measurements

Damage initiation in the bonded and hybrid joints

was detected through use of the backface strain tech-

nique. The technique was used to investigate the influ-ence of the bolt on the number of cycles to damage

initiation, Ni, in the hybrid joints with the polyurethane

and epoxy adhesive systems.

A typical plot of the backface strain versus number of

cycles for the bonded and hybrid joints with the polyure-

thane adhesive is illustrated in Fig. 13(a). The onset of

failure is shown to be delayed in the hybrid joint with

the number of cycles to failure initiation, Ni, beingapproximately double that of the bonded joint. It was

also evident that damage initiated at both ends of the

overlap at approximately the same number of cycles.

The steady increase in the backface strain after failure

initiation indicated a progressive transfer of load from

the adhesive to the bolt.

In contrast, the failure of the bonded joints with the

epoxy adhesive was sudden with fracture at one end ofthe overlap propagating rapidly through the joint (see

Fig. 13(b)). Similar behaviour was observed in the hy-

brid joint as shown in the zoomed region of Fig.

13(b). Fracture in the fillet at one end of the overlap

(A) resulted in a 50% increase in the backface strain level

at the opposite end (B). The backface strain at point B

continued to increase at a higher rate until fracture of

the fillet occurred. The sudden fracture resulted in anabrupt shift of load from the adhesive to the bolt.

Comparison of the number of cycles to failure initia-

tion in the bonded and hybrid joints with the epoxy

adhesive did not reveal any significant benefit of the hy-brid joint. While the number of cycles to failure initia-

tion was greater for the hybrid joint in comparison to

the bonded joint, in general the values of Ni were similar

for the bonded and hybrid joints. Thus, the combination

of a high modulus adhesive and a hybrid joining tech-

nique only provides benefits to structural performance/

durability after failure of the adhesive layer.

5. Conclusions

The quasi-static strength and fatigue resistance of hy-

brid (bonded/bolted) joints in composite laminates was

investigated. The load transfer distribution in the joints

was predicted using finite element analysis and the joint

structural behaviour and failure modes characterised.The effects of the adherend thickness and adhesive mate-

rial system were identified as critical parameters in the

design of hybrid joints.

It was shown that hybrid joining in combination with

a stiff epoxy adhesive offered only limited improvements


in structural performance. Results from numerical simu-

lations revealed that only a small fraction of the load

was transferred by the bolt and no significant increase

in quasi-static strength was observed with the adhesive

and bolt functioning as separate joining elements. The

fatigue resistance of the joints was improved due tothe presence of the bolt. However, the bolt had limited

influence on the number of cycles to initiation of

damage.

In contrast, the combination of hybrid joining with a

flexible polyurethane adhesive produced both higher

joint strength and extended fatigue life. Bolt load trans-

fer ratios up to 35% were predicted by numerical simu-

lation and an increase in strength of between 11% and22% was measured for the hybrid joints in comparison

to adhesive bonded joints. Greater strength increase

was noted for the joints with the [0/45/90/�45]s2 lami-

nate stacking sequence in comparison to those with

the [0/45/90/�45]s stacking sequence as a result of the

higher bearing load capacity in the former laminate.

The hybrid joints with the [0/45/90/�45]s adherends

failed ultimately by net-section failure with the load lev-els in excess of those sustainable in a bolted joint. This

failure mode imposes design restrictions if redundancy

or non-catastrophic failure is required. The performance

of the hybrid joint was improved in joints with the [0/45/

90/�45]s2 adherends which had a higher bearing load

capacity.

The fatigue resistance of the hybrid joints with the

polyurethane adhesive was greater in comparison toadhesive bonded joints. Three distinct stages in the fati-

gue life were identified and backface strain measure-

ments revealed that fracture in the adhesive initiated

after a larger number of cycles in the hybrid joints for

a given load.

Hybrid joining was shown to offer potential improve-

ment in strength and fatigue life in comparison to adhe-

sive bonded joints. However, material selection andjoint design must be carefully combined to ensure an

appropriate distribution of load between the adhesive

and the bolt which is necessary to achieve improved

strength and fatigue resistance.

Acknowledgements

This work has been financially supported by the

Commission of the European Union through Growth

Project TECABS (Technologies for Carbon Fibre Rein-

forced Modular Automotive Body Structures). Dr. Ste-

fan Hallstrom is gratefully acknowledged for suggesting

valuable improvements to the paper. Ashland Speciality

Chemicals GmbH and Vantico Ltd. are gratefully

acknowledged for provision of Polyurethane and Epoxyadhesives. The Huck fastening equipment was kindly

supplied by Danielsson and Stickler AB.

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