load transfer in hybrid (bonded/bolted) composite single-lap joints

Composite Structures 69 (2005) 35–43

www.elsevier.com/locate/compstruct

Load transfer in hybrid (bonded/bolted) composite single-lap joints

Gordon Kelly *

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

Available online 2 June 2004

Abstract

The load transfer in hybrid (bolted/bonded) joints is complicated due to the difference in stiffness of the alternative load paths.

The load distribution in hybrid composite single-lap joints has been predicted through use of a three-dimensional finite element

model including the effects of bolt–hole contact and non-linear material behaviour. The effect of relevant joint design parameters on

the load transferred by the bolt have been investigated through a finite element parameter study. Joint configurations where hybrid

joining can provide improved structural performance in comparison to adhesive bonding have been identified.

Experiments were performed to measure the distribution of load in a hybrid joint. A joint equipped with an instrumented bolt

was used to measure the load transfer in the joint. The measured bolt load values were compared to predictions from the finite

element model and the results were found to be in good agreement.

� 2004 Published by Elsevier Ltd.

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

1. Introduction

Joining of polymer matrix composite materials hastraditionally been achieved by mechanical fastening or

adhesive bonding. Combining these techniques has been

considered unnecessary in terms of structural perfor-

mance as the adhesive provides a stiffer load path and

thus transfers the majority of the load. However, these

assumptions are often related to high performance

aerospace joints where long overlap lengths (defined

here as overlap length/adherend thickness P 40) andhigh modulus epoxy adhesives are used.

In non-aerospace applications, joining polymer ma-

trix composites using alternative methods such as hybrid

joints combining mechanical fastening and adhesive

bonding could be motivated.

Hybrid joining techniques have previously been

considered in relation to repair and improvement of

damage tolerance [1–3]. Hart-Smith [1,2] conducted atheoretical investigation of combined bonded/bolted

stepped lap-joints between titanium and carbon fibre

reinforced plastic (CFRP). While no significant strength

benefits were found in comparison to perfectly bonded

joints, the combined bonded/bolted joint was found to

be beneficial for repairing damaged bonded joints and

* Tel.: +46-8-790-6445; fax: +46-8-20-7865.

E-mail address: [email protected] (G. Kelly).

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

doi:10.1016/j.compstruct.2004.04.016

limiting damage propagation. Under room temperature

and ambient humidity conditions, 98% of the applied

load was predicted to be transferred by the adhesive.Chan and Vedhagiri [3] investigated the use of bolted,

bonded and combined bolted/bonded joints used in re-

pair. A single strap joint with CFRP adherends was

considered with two bolts used in the overlap region.

The study primarily considered stress distributions in

the laminates and limited consideration was given to

load transfer through the joint. The entire load was as-

sumed to be transferred by the adhesive in the hybridjoint.

Fu and Mallick [4] investigated the static and fatigue

strength of hybrid (adhesive/bolted) joints in structural

reaction injection moulded (SRIM) composite materials.

The authors performed an experimental investigation on

a single-lap joint geometry considering the effect of

different washer designs. It was concluded that the per-

formance of the hybrid joints was dependent upon thewasher design which affected the distribution of the bolt

clamping force. The hybrid joints were shown to have

higher static strength and longer fatigue life than adhe-

sive bonded joints for the studied material system.

Previous studies on hybrid (bolted/bonded) joints

have considered fixed joint geometries and material

systems. Limited consideration has been given to the

measurement or prediction of the load distribution inthe joints. The aim of this paper is to investigate the

mail to: [email protected]

36 G. Kelly / Composite Structures 69 (2005) 35–43

distribution of load in hybrid (bolted/bonded) compos-

ite single-lap joints. The aim is to identify joint config-

urations and conditions where the method offers

improved structural performance.

2. Methodology

Two methods were adopted to characterise the

load transfer in the hybrid joints. The first method was

based on numerical simulation using the finite element

method. A parameterised finite element model wasdeveloped and used to predict the load transferred by

the adhesive and the bolt for different joint configura-

tions. The ratio of the load transferred by the bolt load

to total applied load was predicted for different joint

configurations and used as a means of characterising the

load transfer. The material models used the simulations

were based on experimentally determined mechanical

properties. The finite element model was also used toevaluate the influence of the bolt on the stress state in

the adhesive.

The second method involved measurement of the

load transfer in a specially designed hybrid joint. The

load transferred by the bolt was determined through use

of an instrumented bolt which provided bolt shear load

measurements. The experimental load transfer results

were used to validate the predicted values from the finiteelement model.

3. Finite element parameter study

A finite element parameter study was undertaken to

investigate the effect of selected geometric and material

parameters on the load distribution through the joint.The effects of adherend thickness, adhesive thickness,

overlap length, bolt pitch distance and adhesive modu-

lus were investigated. The joint configuration used in the

study was a hybrid single-lap joint as shown in Fig. 1.

The bolt geometry was maintained constant throughout

the analyses.

tt

d =6.35

w =25

L

L =160tot

a

d =10

45o

Fig. 1. Dimensions of the hybrid single-lap joint (dimensions in mm).

In addition to the prediction of load transfer in the

joint, stress analysis was conducted to investigate the

effect of the bolt on the peel and shear stress distribu-

tions in the adhesive.

3.1. Finite element modelling

A finite element model of a hybrid single-lap joint

was developed using the ABAQUS [5] finite element

code. In order to accurately determine the load transfer

between the bolt and the laminate a three dimensional

model was required where the contact between the bolt

and hole was modelled in detail as shown by Ireman [6].

The finite element joint model was developed usingthree dimensional brick elements. The adherends were

modelled using linear brick elements enhanced with

incompatible modes which improve the performance of

the elements in bending. The adhesive was modelled

using fully integrated eight node brick elements. The

mesh was refined adjacent to the hole and at the overlap

ends and a coarser mesh was used in regions further

afield from the overlap. The bolt was modelled usingquadratic brick elements which produced a more accu-

rate representation of the curved surface. A neat fit was

assumed between the bolt and the laminate in all sim-

ulations. Symmetry was adopted along the length of the

joint and thus the model was reduced to a half model

as shown in Fig. 2.

Contact was defined between the bolt and the lami-

nates on the hole surface and on the laminate outersurfaces. The contact pair approach based on a master-

slave algorithm was used with finite sliding allowed be-

tween surfaces in contact. A friction coefficient of 0.2

was assumed between laminates and the bolt. The bolt

shear load was determined through summation of the

nodal forces on the mid-surface of the bolt.

The analyses were performed in two steps. Firstly, a

bolt clamping load was applied through application ofa pre-tension load to the mid-surface of the bolt. A

clamping load of 5 kN (z-direction) was used through-

out the study. Secondly, tensile load was applied to one

end of the joint which was free to move in the longi-

tudinal (x) direction only (uy ; uz ¼ 0). The opposite

end of the joint had clamped boundary conditions

(ux; uy ; uz ¼ 0). The effects of both non-linear material

properties and non-linear geometry were included in theanalyses.

3.1.1. Materials

The material properties of the adherends used in the

analyses were based on carbon fibre/epoxy (T700/Epi-

cote 828LV) laminates with a [0/45/90/-45]sn laminate

stacking sequence. The mechanical properties of the

laminates were homogenised in the model. The elastic

properties of the lamina are listed in Table 1. The in-

Fig. 2. Finite element model of the hybrid single-lap joint.

Table 1

Elastic properties for the carbon fibre/epoxy lamina from [7]

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

G. Kelly / Composite Structures 69 (2005) 35–43 37

plane properties were based upon experimentally

determined values [7] with the out-of-plane properties

being estimated values.

The adhesive used in the study was a two component

polyurethane material (Pliogrip 7400/7410, Ashland

Speciality Chemicals GmbH). The tensile stress–strain

behaviour of the adhesive was determined experimen-

tally according to ASTM D638-02a [8]. The engineeringstress–strain behaviour of the adhesive is illustrated in

Fig. 3. The elastic–plastic properties of the adhesive

material were modelled using the Drucker–Prager yield

criterion. The bolt was assigned steel material properties

(E ¼ 205 GPa, m ¼ 0:3).

0 10 20 30 40 50 60 70 800

5

10

15

20

25

30

Strain (%)

Str

ess

(MP

a)

Fig. 3. Tensile stress–strain behaviour of Pliogrip 7400/7410.

3.2. Stress distribution within the joint

A comparison was made between the stress distribu-

tions in a bonded only and a hybrid bonded/bolted joint

for the case of a single-lap joint with adherend thickness

t ¼ 3:2 mm, adhesive thickness ta ¼ 0:5 mm and an

overlap length L ¼ 20 mm. The effect of the bolt on the

peel and shear stress distributions in the adhesive isillustrated in Fig. 4(a) and (b). The stress distributions

are plotted for an applied joint load of 3 kN. At this

load level, the bolt was predicted to transfer 32% of the

applied load.

The stress distribution along the overlap at three

different locations in the joint width direction are plot-

ted. The position y ¼ 0 corresponds to the centreline of

the joint with y ¼ 12:5 mm corresponding to the outeredge of the joint (see Fig. 2).

The peel stress distribution along the length of the

overlap is shown in Fig. 4(a). It is evident that the

presence of the bolt creates a non-uniform peel stress

distribution in the width direction of the joint. The

clamping force exerted by the bolt imparts a compres-

sive load on the adhesive directly under the bolt head.

However, the effect of the bolt diminishes towards theouter edge of the joint. The bolt is shown to have limited

effect on the magnitude of the maximum peel stress

at the ends of the overlap.

The shear stress distribution along the length of the

overlap is shown in Fig. 4(b). The shear stress in the

adhesive is shown to be significantly reduced when

comparing the stress in the hybrid and the bonded

joints. The maximum shear stress in the adhesive in thehybrid joint was 50% lower in comparison to the

adhesive bonded joint at the given load level. The shear

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

5

10

15

20

She

ar S

tres

s (M

Pa)

Normalised overlap length

BondedHybrid y=12.5mmHybrid y=6mmHybrid y=0mm

Bolt Centre-line

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-20

-15

-10

-5

0

5

10

15

20

25

30

Pee

l Str

ess

(MP

a)

Normalised overlap length

BondedHybrid y=12.5mmHybrid y=6mmHybrid y=0mm

Bolt Centre- line

(a) (b)

Fig. 4. Comparison of stress distributions in bonded and hybrid joints (P ¼ 3 kN). (a) Peel stress distribution (b) Shear stress distribution.


stress distribution was not found to vary significantly

over the width of the joint.

3.3. Bolt load transfer predictions

The load transfer in the joint was characterised by the

ratio of the bolt load (Pb) to the total applied load (Pt).This provided a measure of the effectiveness of the fas-

tener as a means to alter the load transfer in the joint.

Geometric and material parameters in the joint were

varied and predictions made of the load transfer distri-

bution. The ratio Pb=Pt was calculated as the appliedload was increased. The results from the parameter

study are discussed in the following sections.

3.3.1. Effect of the adherend thickness

The effect of the adherend thickness on the load dis-

tribution within the joint is illustrated in Fig. 5(a). Joints

with adherend thickness 1.6, 3.2 and 6.4 mm were

compared using a constant overlap length L ¼ 20 mm

and adhesive thickness ta ¼ 0:5 mm.

Considering the joint with the t ¼ 1:6 mm adherend,

the relation between the bolt load and the total appliedload is initially linear but becomes non-linear as the

applied load is increased. The rate at which the bolt

transfers load with respect to the total applied load is

shown to decrease at higher load levels. The ratio of the

bolt load to the total applied load reaches a maximum

of 32% at Pt ¼ 10 kN.

The load transferred by the bolt is shown to increase

with increasing adherend thickness. Increasing theadherend thickness increases the level of shear stress in

the adhesive. The out-of-plane displacement due to

secondary bending is also reduced and this is coupled

with an increase in the relative displacement between the

adherends in the x-direction. The increase in the relative

displacement allows for the bolt to transfer a greater

part of the applied load. The t ¼ 3:2 mm joint transfers

10% more load than the t ¼ 1:6 mm joint at an applied

load of Pt ¼ 10 kN.However, comparison of the t ¼ 6:4 mm joint and

t ¼ 3:2 mm joint reveals similar load transfer charac-

teristics. For a given joint configuration and material

system, there is an indication of a limited bolt load

transfer capacity which is dependent on the adherend

properties.

3.3.2. Effect of the adhesive thickness

The effect of the adhesive thickness on the load

transferred by the bolt is illustrated in Fig. 5(b). Theload transferred by the bolt is shown to increase with

increasing adhesive thickness. Increasing the thickness

of the adhesive in the joint results in a reduction of the

shear and peel stresses in the adhesive layer. The shear

stress distribution becomes more uniform with no sharp

peaks at the ends of the overlap. The peel stress con-

centration is also reduced as a result of the increased

flexibility at the ends of the overlap. However the peelstress level in the vicinity of the bolt is unaffected by the

change in adhesive thickness.

The most significant difference observed as a result of

the increase in adhesive thickness was an increase in the

relative displacement between the adherends. Increasing

the adhesive thickness results in a more flexible joint.

The increase in relative displacement between the adh-

erends allows the bolt to come in to contact with theadherends and transfer more load. At Pt ¼ 10 kN, the

load transferred by the bolt in the ta ¼ 1:5 mm joint

is 50% higher than in the ta ¼ 0:5 mm joint.

3.3.3. Effect of the overlap length

The effect of the joint overlap length on the load

transfer is illustrated in Fig. 5(c). Increasing the overlap

length reduces the load transferred by the bolt. As the

overlap length of the joint is increased, the average shear

stress in the centre of the joint is reduced with the load

0 2 4 6 8 10 120

10

20

30

40

50

Load (kN)

Bol

t loa

d/T

otal

Loa

d (%

)t=1.6mmt=3.2mmt=6.4mm

0 2 4 6 8 10 120

10

20

30

40

50

60

70

80

Load (kN)

Bol

t loa

d/T

otal

Loa

d (%

)

ta=0.5mm

ta=1.0mm

ta=1.5mm

0 2 4 6 8 10 120

10

20

30

40

50

60

60

Load (kN)

Bol

t loa

d/T

otal

Loa

d (%

)

L=20mmL=30mmL=40mm

0 2 4 6 8 10 120

10

20

30

40

50

60

Load (kN)

Bol

t loa

d/T

otal

Loa

d (%

)

w/d=4w/d=6w/d=8

(a) (b)

(c) (d)

Fig. 5. Effect of joint geometry on the bolt load transfer. (a) Effect of the adherend thickness (ta ¼ 0:5 mm, L ¼ 20 mm). (b) Effect of the adhesive

thickness (t ¼ 3:2 mm, L ¼ 20 mm). (c) Effect of the overlap length (t ¼ 3:2 mm, ta ¼ 0:5 mm)] (d) Effect of the pitch distance (t ¼ 3:2 mm, ta ¼ 0:5

mm, L ¼ 20 mm).


transfer mainly occurring in the adhesive at the ends of

the overlap. Increasing the overlap length also reduces

the relative displacement between the adherends for a

given load. This reduction in relative displacement re-

sults in less load being transferred to the bolt. The rate

at which the bolt load increases with total applied load is

also shown to decrease significantly with increasingoverlap length.

The hybrid joining method is most beneficial for joints

with shorter overlap length. Increasing the overlap from

20 to 40 mm results in a significant reduction in the load

transferred by the bolt. Including bolts in joints with long

overlaps will only serve to improve the damage tolerance

and prevent debonding of the adherends. The bolt will

not make any significant contribution to the structuralperformance under service loading.

3.3.4. Effect of the pitch distance

An important parameter in the design of both bolted

and hybrid joints is the bolt pitch distance which is

defined as the distance between bolts in a row.

The pitch distance affects the load capacity of the

joint and the mode in which the joint will fail. For small

pitch distances, bolted joints fail catastrophically by net-

section failure where fracture occurs in the laminate

between the bolts, perpendicular to the load direction.

As the pitch distance is increased the failure mode

changes to bearing failure and a maximum value ofbearing strength is attained.

The effect of the pitch distance on the load transfer

through the hybrid joint is illustrated in Fig. 5(d). Joint

width to bolt diameter ratios (w=d) of 4, 6 and 8 have

been simulated with the bolt diameter being kept con-

stant. The pitch distance is shown to affect the load

transferred by the bolt and the rate at which the bolt

load changes with total applied load. Increasing thepitch distance reduces the load transferred by the bolt

and the rate at which the bolt load increases with ap-

plied load. The reduction in load is directly attributed to

the increase in size of the bonded area which results in

the adhesive transferring a greater share of the applied

load.


The bolt pitch distance can be used to govern the

distribution of load between the adhesive and bolts in a

hybrid joint. Adjusting the pitch distance, or the number

of bolts, can provide a factor of safety for the joint,or redundancy in the design.

3.3.5. Effect of the adhesive modulus

The effect of the adhesive modulus on the load dis-tribution in the joint was investigated by comparing the

behaviour of joints with three different adhesives. Two

different epoxy adhesives: Betamate 1496-V and Beta-

mate 1040 (Dow Automotive Inc.) with elastic moduli of

1.6 and 2.9 GPa respectively, were compared with the

results from the joint with the polyurethane adhesive

(E ¼ 0:6 GPa).

The adhesive tensile stress–strain curves used in theanalyses were determined experimentally according to

ASTM D638-02a [8] and are illustrated in Fig. 6(a). The

Betamate 1040 adhesive has high strength (rmax ¼ 48

MPa) although it is relatively brittle (�f ¼ 3%). The

Betamate 1496-V adhesive has a slightly lower tensile

strength (rmax ¼ 27 MPa) in comparison to Betamate

1040 although it is significantly more ductile (�f ¼ 12%).

The polyurethane adhesive is extremely ductile (�f ¼60%) with a maximum tensile strength of 23 MPa. The

results were compared for the case of a joint with

adherend thickness t ¼ 3:2 mm, adhesive thickness

ta ¼ 0:5 mm and an overlap length L ¼ 20 mm.

The effect of the adhesive modulus on the load

transfer is shown in Fig. 6(b). The adhesive modulus had

a significant effect on the load distribution in the hybrid

joint. The stiffer epoxy adhesives transferred almost allthe load in the hybrid joints. The load transferred by the

bolt in the Betamate 1040 joint is limited to approxi-

mately 2%. The load distribution did not change sig-

nificantly as the load is increased and the adhesive

remained the main load carrying component. The load

0 10 20 30 40 50 600

5

10

15

20

25

30

35

40

45

50

Strain (%)Strain (%)

Str

ess

(MP

a)S

tres

s (M

Pa)

Betamate 1496 V Betamate 1496 V

Betamate 1040 Betamate 1040

Pliogrip 7400/7410 Pliogrip 7400/7410

Bo

lt lo

ad/T

ota

l Lo

ad (

%)

Fig. 6. Effect of the adhesive modulus on the bolt load transfer. (a) Tensile

mm, L ¼ 20 mm).

transferred by the bolt in the Betamate 1496-V joint was

also shown to be relatively low. At low load levels, the

behaviour was similar to that observed in the Betamate

1040 joints. However, as the applied load exceeded 7kN, the load transferred by the bolt increased. The in-

crease in load was attributed to plastic deformation in

the adhesive.

In contrast, the hybrid joint is more efficient in dis-

tributing the load between the adhesive and the bolt in

the polyurethane joint. The low modulus polyurethane

adhesive contributes to a more flexible joint with larger

relative displacement between the adherends. This dis-placement allows for a significant transfer of the load

to the bolt.

The effect of the adhesive modulus can be correlated

with the stress distribution in the adhesive. As the

adhesive modulus is increased, the shear stress at the end

of the joint increases and thus the main part of the load

is transferred at the ends of the overlap leaving the

centre of the joint relatively unloaded.

4. Experimental verification

The aim of the experimental program was to measure

the load distribution in the hybrid joint and validate the

results obtained from the finite element model. The finiteelement model used in the parameter study was verified

through comparison with a specially instrumented joint.

The joint was fitted with an instrumented bolt which was

used to measure the shear load. The dimensions of the

joint were restricted by the design of the instrumented

bolt. The mid-length of the bolt shank must lie in the

shear plane of the joint to ensure correct measurement

of the bolt shear load. This placed a restriction on thethickness of the adherends used in the joint. The single-

lap joint configuration used in the experimental verifi-

0 2 4 6 8 10 120

10

20

30

40

50

60

Load (kN)

Pliogrip 7400/7410Betamate 1496VBetamate 1040

stress–strain curves. (b) Load transfer behaviour (t ¼ 3:2 mm, ta ¼ 0:5

Table 2

Joint configuration for experimental verification (dimensions in mm)

Laminate

thickness

Overlap

length

Width Adhesive

thickness

Fastener

diameter

4.16 20 25 0.5 6.0

WashersInstrumented bolt

Adhesive

CFRP

CFRP

Fig. 7. Experimental set-up for testing of the hybrid joints.


cation is illustrated schematically in Fig. 1 with the exact

dimensions listed in Table 2.

The laminate adherends were fabricated from thecarbon fibre/epoxy unidirectional prepreg system HTA/

6376 (Hexcel Composites). The stacking sequence [(±45/

0/90)4]s32 was used for each laminate with the basic

lamina properties listed in Table 3. The laminates were

machined to size using a diamond tipped saw. Bonded

single-lap joints were manufactured using a two com-

ponent structural polyurethane adhesive (Pliogrip 7400/

7410). The bonding surfaces were grit-blasted and de-greased prior to bonding and the joints were cured

according to the manufacturer’s specification. A hole of

diameter of 6 mm was machined in the joint using a

dagger drill with a backing plate used to prevent back

face delamination.

The joint was subject to quasi-static tensile testing

which was conducted using a universal testing machine

(Instron 4505) with a 100 kN load cell. The system wasfully computer controlled and allowed for acquisition of

load, displacement and strain data. The tests were run in

displacement control at a rate of 0.5 mm/min. Loading

was stopped at sub-critical load levels in order to pre-

vent damage to the instrumented bolt. The experimental

setup used for the hybrid joint tests is illustrated in

Fig. 7.

4.1. Bolt load measurement

The load transferred by the bolt in the hybrid joint

was measured using a specially instrumented bolt. The

technique was previously used to measure the bolt load

transfer in multi-row lap shear joints [9]. The bolt was a

titanium hi-torque lockbolt which was adapted to in-

clude load measurement instrumentation. The bolt was

instrumented with two strain gauge rosettes on each sideof the bolt as shown in Fig. 8. The strain gauges were

located equidistant from the shear plane of the joint.

Connection of the strain gauges in a full Wheatstone

bridge eliminated the effect of the axial bolt strains on

the shear strain measurement.

The signal from the measurement bolt was calibrated

by testing a single-lap bolted joint manufactured from

Table 3

Lamina elastic properties for the HTA/6376 material from [6]

E11 (GPa) E22 (GPa) E33 (GPa) G12 (GPa) G13 (GPa

140 10 11 5.2 5.2

the same adherend material. The measurement bolt was

inserted in the hole and finger tightened to limit the ef-fects of lateral clamping. Tensile load was applied to the

bolted joint and the signal from the measurement bolt

and testing machine load cell recorded. The procedure

was repeated three times in order to obtain an average

signal characteristic. The measurement signal from the

bolt was correlated to the load cell reading providing a

calibration curve for the bolt.

4.2. Comparison between the FE model and experiments

A comparison between the measured and predictedbolt load is shown in Fig. 9. The experimental bolt load

curve was repeatable indicating no effects of hysteresis

) G23 (GPa) m12 m13 m23

3.9 0.3 0.3 0.5

A A

5.1

4.16

R 0.7510.1

1.5

0.7

G

G2

G3

G4

Side 2Side 1

Bolt Bolt

Shear

Section A-A

plane

Fig. 8. Details of the instrumented bolt (dimensions in mm).

0 1 2 3 4 5 6 7 80

0.5

1

1.5

2

2.5

3

Total Load (kN)

Bol

t loa

d (k

N)

Experiment 1Experiment 2FEA (Neat Fit)FEA (0.065mm Clearance)FEA (0.1mm Clearance)

Fig. 9. Comparison between measured and predicted bolt loads.


or inelastic deformation. The curve of bolt load versus

total applied load is shown to be non-linear with the

bolt load increasing at a greater rate as the joint is

loaded.

The predicted bolt load curves for three different

levels of bolt–hole clearance are illustrated. Prediction

of the load transfer for the neat fit condition revealed anover prediction of the bolt load. A possible reason for

the difference in the results was the tolerance between

the bolt and the hole. Larger tolerance between the bolt

and the hole in the experimental joint resulted in a delay

in the load transfer to the bolt. Measurement of the

instrumented joint revealed the actual hole diameter to

be 6.065 mm. Adjustment of the finite element model to

account for the actual fit between the bolt and the holeresulted in a more accurate prediction of the bolt load.

An additional simulation with a larger bolt–hole

tolerance (0.1 mm) highlighted the sensitivity of the re-

sults to manufacturing tolerances. Clearance between

the bolt and the hole delays the load transfer to the bolt

and subsequently the performance of the joint.

5. Discussion and conclusions

The load distribution in hybrid (bonded/bolted) joints

was investigated numerically through use of finite ele-

ment analysis. A three-dimensional finite element modelwas developed including the effects of bolt–hole contact,

non-linear material behaviour and large deformations.

The simulation model was verified experimentally using

an instrumented joint. The results from the experiments

were found to be in good agreement with those obtained

from finite element analysis.

A parameter study was undertaken to investigate the

effect of selected geometrical and material parameters onthe load distribution in the hybrid single-lap joint. The

results of the simulations conducted in the parameter

study are based on a neat fit between the bolt and the

hole. Thus, the results represent the highest possible bolt

load transfer for the respective joint configurations.

The results from the parameter study can be sum-

marised as follows:

• The load transferred by the bolt increases with

increasing adherend thickness.

• The load transferred by the bolt increases with

increasing adhesive thickness.

• The load transferred by the bolt decreases with

increasing overlap length.

• The load transferred by the bolt decreases with

increasing pitch distance.• The load transferred by the bolt decreases with

increasing adhesive modulus.

The benefit of adding bolts to a bonded joint is

greater if the joint is flexible either as a result of the

adhesive material or joint design. However, the method

could also provide performance improvements for a

wide range of joints in adverse environments with bothelevated temperature and moisture reducing the per-

formance of the adhesive.

The current work has shown that the combination of

mechanical fastening and adhesive bonding can be


advantageous for specific joint geometries and material

combinations. Additional benefits may be obtained with

regard to damage tolerance with the bolt preventing

catastrophic failure of the joint through separation ofthe adherends.

Acknowledgements

Dr. Stefan Hallstr€om is gratefully acknowledged for

valuable discussions regarding this work and for sug-

gesting improvements to the manuscript. This work has

been financially supported by the Commission of the

European Union through Growth Project TECABS

(Technologies for Carbon Fibre Reinforced Modular

Automotive Body Structures) and by the Swedish

Foundation for Strategic Research through the nationalSwedish research program ‘Integral Vehicle Structures’.

Betamate and Pliogrip adhesives were kindly supplied

by Dow Automotive and Ashland Speciality Chemicals

GmbH.

References

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load transfer in hybrid (bonded/bolted) composite single-lap joints

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