Engineering Failure Analysis 45 (2014) 406–420

Contents lists available at ScienceDirect

Engineering Failure Analysis

journal homepage: www.elsevier .com/locate /engfai lanal

Investigation of bolt clamping force on the fatigue life of doublelap simple bolted and hybrid (bolted/bonded) joints viaexperimental and numerical analysis

http://dx.doi.org/10.1016/j.engfailanal.2014.07.0141350-6307/� 2014 Published by Elsevier Ltd.

⇑ Corresponding author. Tel.: +98 411 3392492.E-mail address: f.esmaeili@tabrizu.ac.ir (F. Esmaeili).

F. Esmaeili ⇑, T.N. Chakherlou, M. ZehsazFaculty of Mechanical Engineering, University of Tabriz, P.O. Box 51666-14766, Tabriz, Iran

a r t i c l e i n f o

Article history:Received 22 January 2014Received in revised form 13 July 2014Accepted 15 July 2014Available online 23 July 2014

Keywords:Torque tighteningClamping forceHybrid jointFatigue lifeBonded joint

a b s t r a c t

In this research, the effect of bolt clamping force on the fatigue life of double lap simplebolted and hybrid (bolted/bonded) joints have been studied both experimentally andnumerically. To do so, two kinds of joints, i.e. double lap simple and hybrid (bolted/bonded)joints were studied. For each kind of the joints, three sets of specimens were prepared andsubjected to tightening torque of 1, 2.5 and 5 N m and then fatigue tests were carried out atdifferent cyclic longitudinal load levels. Experimental tests revealed that the hybrid jointshave higher fatigue life in comparison with the simple bolted joints. In the numericalmethod, finite element code was used to obtain stress distribution in the joint platesdue to clamping force and longitudinal applied loads. Numerical simulation and experi-mental results showed that the fatigue life of specimens was improved by increasing theclamping force due to compressive stresses created around the hole. In addition, the inves-tigation revealed the positive role of clamping force resulting from torque tightening on thefatigue life of both simple and hybrid joints.

� 2014 Published by Elsevier Ltd.

1. Introduction

Detachable joints such as bolted, riveted or pined ones are frequently used in aerospace industry for connecting variousparts. Among the mentioned detachable joints, bolted joints are the most important elements in aerospace structures. How-ever, the existence of geometrical discontinuity in these joints due to essential hole drilling process causes stress concentra-tion and thus increases the tendency of early fatigue crack initiation and grow under cyclic loading [1–4]. On the other hand,easy dismantling that simplifies repairs and permits replacing of damaged components makes these types of joints favoritefor extensive use in aerospace structures. Therefore, it is of great importance to reduce the effect of stress concentration andattain enhanced fatigue life [5]. According to results of previous researches, bolted joints have higher tensile and fatiguestrengths than welded, riveted and pinned joints [6,7].

An alternative method to mechanical fastening is adhesive bonded joints. In order to perceive the advantages of adhesivebonding in fatigue, two fundamental differences are important between the two types of lap joints. First, in a mechanicaljoint, the overlapping areas are attached to one another at discrete points only, i.e. by the fasteners. Obviously, severe stressconcentrations should occur. However, if the connection is made continuously in the full overlapping area by adhesive bond-ing, these stress concentrations do not occur. Because, the adhesive bonded joints do not need to the fasteners and fastening

F. Esmaeili et al. / Engineering Failure Analysis 45 (2014) 406–420 407

holes. Therefore, the stress distributions in the joint are relatively uniform compared with those in the mechanical joint. Sec-ondly, metallic direct contact between the two sheets does not happen in the adhesively bonded joints [7].

In order to employ the advantages and avoid the disadvantages of adhesive bonding, and mechanical joints, and therefore,to obtain high performance joints, a combination of mechanical joints (riveted, bolted, etc.) and an adhesive, namely hybridjoints, are used [8–11]. Hybrid joints are used in many engineering application such as aerospace, automotive, and navalindustries because of their better performance compared to simple joints such as adhesively bonded, welded and mechan-ically fastened joints [12,13]. Hybrid joints have also been used for the repair and improvement of damage tolerance [14,15].

Several researchers [15–20] have investigated the hybrid joints. An analytical investigation was conducted by Hart-Smith[11] on a hybrid joint with stepped lap joints. The strength of hybrid joints was found to be the same as well-designedbonded joints. Chan and Vedhgiri [16] conducted experiments with composite joints as well as a parametric study usingfinite element analysis to study the stacking sequence effect on joint strength. Barut and Madenci [18] developed a semi-analytical solution method for stress analysis of a hybrid joint and they revealed that the adhesive, despite the fact it haslow modulus compared to the bolt, transfers most of the load. Kweon et al. [21] detected a similar phenomenon in theirexperiments, where a double lap hybrid joint was employed using composite and aluminum adherends.

Pirondi and Moroni [28] compared the hybrid weld-bonded, rivet-bonded, clinch-bonded, and simple joints under variousconditions, via experimental analysis. The effect of the material, geometrical factors, and environment on static strength andenergy absorption were evaluated through the analysis of variance.

In other investigation, Gomeza et al. [13] proposed a mechanical model to reproduce the mechanical behavior of a hybrid(riveted-bonded) joint.

A similar investigation conducted by Schvechkov [29] on the effects of adhesive mechanical properties on the fatiguestrength of hybrid (riveted-bonded) joints by means of experimental analysis.

In separate investigations, Kelly [17,30] studied the static and fatigue strength of the hybrid (bonded–bolted) hybridsingle lap using different modulus adhesives. The results of studies, revealed that, the hybrid joints with lower modulusadhesives allowed for load sharing between the adhesive and the bolts.

Imanaka [31] showed that the fatigue strength of the adhesive joint can be improved through combination with a rivetwhose fatigue strength is at least the one of the corresponding bonded joint.

A similar investigation conducted by Fu and Mallick [9]. They experimentally showed that the hybrid (bolted–bonded)single lap joints have a higher static and fatigue strength in comparison with only adhesively bonded joints.

In the hybrid joint, mechanical fastening is added to the bonded joint. The hybrid joint uses mechanical fastening in addi-tion to bonding in an effort to overcome the potential weaknesses of adhesive bonding.

The hybrid joints may include weld-bonded, clinch-bonded and rivet-bonded connections [22–27]. It is important to notethat, even though some limited research have been conducted on the analysis of hybrid joints, still static and fatigue strengthdata for the hybrid joint are lacking.

When a nut and bolt are used to join mechanical members together, the nut or bolt can be pre-tensioned by applyingtightening torque using a torque wrench and then the bolt and nut are pulled toward each other. Pre-tension or clampingforce is the technical term for the tension caused by tightening the nut that holds the assembled part together [32–38].

Previous works revealed that the clamping force can reduce the stress concentration at the bolted hole region, and there-fore improve the strength of the joint considerably [32,39]. Collings [39] investigated the effects of bolt clamping pressure onthe strength of bolted joints in CFRP laminates. Stockdale and Matthews [40] investigated the effect of clamping pressure onbolt bearing load in glass fiber-reinforced plastics experimentally. Deng and Hutchinson [38] investigated the residualclamping stress exerted by the rivets on the joint. The relation between the clamping and applied force was analyzed usingfinite element methods in the small strain framework.

In this research, the effects of clamping force on the fatigue strength of double lap simple bolted and hybrid (bolted/bonded) joints have been investigated both experimentally and numerically. To do so, two kinds of joints, i.e. double lap sim-ple and hybrid (bolted/bonded) joints were studied. For each kind of the joint, three sets of specimens were prepared andsubjected to tightening torque of 1, 2.5 and 5 N m and then fatigue tests were carried out on them at different cyclic longi-tudinal load levels. In the numerical method, finite element ANSYS code was used to obtain stress distribution in the jointplates due to clamping force and longitudinal applied loads.

2. Experimental procedures

The specimens employed in this investigation were made from 2024-T3 aluminum alloy with thickness of 2 mm. Table 1lists the mechanical properties of the aluminum alloy obtained from tension (static) tests, while Table 2 presents the chem-ical compositions of the used aluminum alloy.

Table 1Mechanical properties of 2024-T3 aluminum alloy.

Young’s modulus (GPa) Yield stress (MPa) Tensile strength (MPa) Poisson’s ratio Elongation (%)

72 315 550 0.33 18

Table 2Chemical composition of 2024-T3 aluminum alloy (the values are in percentages).

Cu Mg Mn Fe Si Cr Zn Ti Al

4.82 1.67 0.58 0.18 0.07 0.02 0.06 0.15 Bal

408 F. Esmaeili et al. / Engineering Failure Analysis 45 (2014) 406–420

Two different types of joints i.e. double lap simple and hybrid (bolted/bonded) joints were prepared. Test specimens’ con-figurations and dimensions for both types of the joints are illustrated schematically in Fig. 1.

The hybrid joints were fabricated using the structural two component epoxy adhesive, namely Loctite 3421[41], preparedby mechanical mixing of the resin and hardener in equal amount by weight. This adhesive was selected due to its highstrength and long working life.

In order to obtain the tensile stress–strain curve of the adhesive, several dog-bone specimens were prepared according toASTM D638-02 [42]. The adhesives were injected into a mold, as shown in Fig. 2, and left to cure at room temperature for24 h. Finally, the prepared specimens were tested on a 100 kN Zwick/Z100 static testing machine with a crosshead speed of5 mm/min (see Fig. 3). The engineering stress–strain curve of the adhesive is shown in Fig. 4.

(b) H

(a

yb

) Si

rid

imp

(bo

le b

nd

olte

ed-

d j

bol

oint

lted

t

) joint

Fig. 1. Configurations and dimensions of the joints. (a) Simple bolted joint, (b) hybrid (bonded–bolted) joint.

Fig. 2. Mold for dog-bone specimens.

Fig. 3. The adhesive dog-bone specimen under tensile testing.

0

10

20

30

40

0 0.01 0.02 0.03 0.04 0.05 0.06

Stre

ss (

MP

a)

Strain (mm/mm)

Fig. 4. Engineering stress–strain curve of Loctite 3421 adhesive.

F. Esmaeili et al. / Engineering Failure Analysis 45 (2014) 406–420 409

To eliminate any possible surface scratches, the surface of the plates (specimens) were polished using different grinding(sand) papers with grits of 400, 600 and 1000 at first.

To prepare the specimens, fastener holes with diameter of 5 mm, were drilled and reamed in the joint plates. A hex headM5 (class 10.9) steel bolt was used for the mechanical fastening and suitable types of steel washers and nuts were used toprepare the joint as illustrated in Fig. 1. Finally, the nut is tightened by applying torque using a torque-wrench up to requiredamounts of torques, i.e. 1 N m, 2.5 N m and 5 N m.

As mentioned earlier, aluminum alloy 2024-T3 sheets were used as an adherend to prepare hybrid joints in thisinvestigation.

410 F. Esmaeili et al. / Engineering Failure Analysis 45 (2014) 406–420

The preparation of the hybrid joints has been implemented in two main steps. Firstly, a double lap bonded joint was con-structed. In order to obtain high strength joint, the joint plates were cleaned with acetone and then they were let dry, prior tousing the adhesive layer. In order to achieve the constant thickness of adhesive layer, 0.5 mm thick sheets were usedbetween adherends.

The prepared bonded joints were left in ambient temperature for 72 h, in accordance with the adhesive manufacturersuggestion.

In the second step of preparing the hybrid joints, the double lap bonded specimens with the adhesive were bolt tightenedusing the same amounts of torques as the simple bolted joints, i.e. 1 N m, 2.5 N m and 5 N m.

2.1. Clamping force measurement

In order to measure the clamping force or bolt pretension resulting from the torque tightening, at different applied tor-ques, for both types of the joints, i.e. simple bolted and hybrid joints, a special experimental method was designed using asteel bush that was placed between the nut and the plate. At the bush outer surface, two strain gauges were stuck to measurethe compressive axial strain and so the stress in the bush using Hooke’s stress–strain law. Having the bush cross-sectionalarea at hand and the axial stress, the axial force in the bush and therefore the clamp force has been determined. The usedmethod and the bush dimensions were shown in Fig. 5.

To calibrate the applied torque and clamping force, torques were applied in 1 N m increments from 1 to 7 N m to the nutusing a torque wrench, and then the axial strains were recorded for each value of the torques. This test was repeated threetimes for each case to obtain the mean value of compressive strains (em), and determine the corresponding clamping forcesusing Eq. (1).The elastic modulus for the bush material (Ebush) was also experimentally determined in order to obtain theaccurate values for the mean axial clamping force.

Fcl ¼ EbushAbushem ¼ 204;188� p4ð92 � 52Þem ¼ 89:8� 105emðNÞ ð1Þ

where Abush is the area of the bush cross section. The relation between the calculated clamping forces and the applied torquesfor the specimen is shown in Fig. 6. As the figure shows, there is a linear relation between the clamping force and the appliedtorque. This confirms that the bush material is still in its elastic region, even under the maximum applied torque.

2.2. Fatigue tests

A total number of 36 specimens were tested in this investigation to study the effect of tightening torque on the fatiguelives of both types of the joining method.

Fatigue tests have been carried out using constant amplitude loads in a servo-hydraulic 250 kN Zwick/Roell fatigue test-ing machine with a frequency of 10 Hz and stress ratio (load ratio) of 0.1 (Fig. 7). Fatigue tests have been performed on thedouble lap simple bolted specimens with three different amounts of tightening torques, i.e. 1 N m, 2.5 N m and 5 N m whichcreated clamping forces equal to Fcl = 976, 2440 and 4880 N respectively, according to the linear equation obtained fromFig. 6. In each case, six fatigue tests were performed with different maximum remote longitudinal loads to final failure.The fatigue test results for double lap simple bolted specimens have been shown in Fig. 8 in terms of the number of cyclesto failure (full separation of specimen).

In addition, in the case of double lap hybrid specimens, fatigue tests have been performed with the same amounts oftightening torques, as simple bolted specimens, i.e. 1 N m, 2.5 N m and 5 N m which created clamping forces equal to

Fig. 5. Measuring clamping force with the load cell.

Fcl = 987.8 T

Fcl = 841.43 T

0

1000

2000

3000

4000

5000

6000

7000

8000

0 1 2 3 4 5 6 7 8

Bol

t C

lam

ping

For

ce (

N)

Applied Tightening Torque (Nm)

Simple Bolted Joint

Hybrid Joint

Fig. 6. The tightening torque–clamping force relation.

Fig. 7. Servo-hydraulic 250 kN Zwick/Roell fatigue testing machine and fatigue test specimen.

60

80

100

120

140

160

180

200

1.0E+04 1.0E+05 1.0E+06 1.0E+07

App

lied

Stre

ss R

ange

(MPa

)

Fatigue Lives (Cycles)

Simple Bolted Joint: Clamped by T=1 N.m

Simple Bolted Joint: Clamped by T=2.5 N.m

Simple Boltedd Joint: Clamped by T=5 N.m

Fig. 8. Fatigue test results for double lap simple bolted specimens.

F. Esmaeili et al. / Engineering Failure Analysis 45 (2014) 406–420 411

412 F. Esmaeili et al. / Engineering Failure Analysis 45 (2014) 406–420

Fcl = 840, 2100 and 4200 N respectively, according to the linear equation obtained from Fig. 6. The fatigue test results fordouble lap hybrid specimens have been shown in Fig. 9 in terms of the number of cycles to failure.

3. Finite element analysis

In order to obtain the stress distribution in the joint plates for both kinds of the joints, three-dimensional finite elementmodels were simulated by ANSYS 9.0 general finite element code [43]. All of the adherends and adhesive layer are meshedwith eight-node hexahedral structural solid elements Solid45 [44]. The meshed model of the hybrid specimens consisted of19,316 elements and 21,614 nodes. In addition, the minimum mesh size in edge of the bolt hole was 0.041 mm. The finiteelement mesh of the double lap hybrid joint specimens is presented in Fig. 10, together with its corresponding loading andboundary conditions. The nodes located at the left edge of the FE model were considered to have all their degrees of freedomconstrained. Only one quarter of the specimen has been modeled, due to double symmetry (with respect to X–Z and X–YCartesian planes) and symmetric displacement boundary condition has been applied to the corresponding planes as shownin the figure.

In order to transfer the pressure between the contacting surfaces of the bolt head (or nut) and the plate, flexible-to-flexible contact state was used. Each contact pair consisted of target element and contact element. TARGET 170was used as a target element and CONTACT 174 was used as a contact element [45,46]. The friction effect betweenthe surfaces of the washer (bolt head) and Al-alloy plates was included in the FE model using Elastic Coulomb modelwith friction coefficient of l = 0.29 which was obtained from experimental tests based on the sliding of the washerunder its own weight on the sloped surface from Al-alloy plate. Also based on the similar experiments, the friction coef-ficient was found to be l = 0.4 for the contact between the plates in simple bolted specimens. Furthermore, the overlapsurfaces of the aluminum plates and the adhesive were modeled using surface-to-surface contact with bonded option forfinite element models of hybrid specimens. Bonded contact permanently holds the target and contact surfaces togetherand prevents the sliding of connected surfaces. This surface-to-surface contact model uses Gauss integration points as adefault, which generally provides more accurate results than the nodal detection system, which uses the nodes them-selves as the integration points. This type of contact model transmits contact pressure between Gauss points, and notforce between nodes.

To represent the aluminum alloy 2024-T3 stress–strain behavior, an elastic–plastic multi-linear kinematic hardeningmaterial model with Von Mises criterion was used. This behavior of the material was obtained from simple tensile testsand shown in Fig. 11. The elastic modulus and Poisson’s ratio were measured to be E = 72 GPa and m = 0.33 respectively. Also,for adhesive layer the elastic–plastic multi-linear isotropic material model was used and the Poisson’s ratio was consideredequal to 0.35. Furthermore, for the steel bolt a linear elastic material relation was assumed with Young’s modulus of 207 GPaand Poisson’s ratio of 0.30 as it was observed that the bolt material remained in elastic region when it was subjected to max-imum applied torque (8 N m). As the bolt and its washer have approximately the same material properties, the geometricalmodel of the washer was added to the bolt head in order to minimize the contact element use (with ignoring contact ele-ments between the bolt head and the washer). This simplification considerably reduces computation processing time withvery good approximation.

Numerical analyses were carried out in two main steps including the application of the clamping force, which was fol-lowed by a longitudinal load to the aluminum alloy. In the first step of loading, axial displacement was applied to the bottomface of the bolt shank to simulate the clamping force. This process was completed for three initial clamping forces resultingfrom the different amounts of tightening torques for both kinds of the joints using a trial and error method. Then the solutionwas restarted for each state and in the second step, the value of maximum force in each cyclic loading (as in experimentaltest) was applied to the end of the plate in the model as a longitudinal static load.

60

80

100

120

140

160

180

200

1.0E+04 1.0E+05 1.0E+06 1.0E+07

App

lied

Stre

ss R

ange

(MPa

)

Fatigue Lives (Cycles)

Hybrid Joint: Clamped by T=1N.m

Hybrid Joint: Clamped by T=2.5 N.m

Hybrid Joint: Clamped by T=5N.m

Fig. 9. Fatigue test results for double lap hybrid specimens.

Fig. 10. Three dimensional finite element model with loading and boundary conditions.

0

100

200

300

400

500

600

0 0.05 0.1 0.15 0.2

Stre

ss (

MPa

)

Strain (mm/mm)

Fig. 11. True stress–strain curve of 2024-T3 aluminum alloy.

F. Esmaeili et al. / Engineering Failure Analysis 45 (2014) 406–420 413

4. Results and discussion

4.1. Experimental analysis results

Series of experimental tests were carried out to investigate the effects of torque tightening on the fatigue life of simpledouble lap bolted and hybrid (bolted/bonded) joints subjected to longitudinal loads. Three values of tightening torques werechosen to apply on the bolt shank for tightening the joints. It is noted that, the fatigue tests were run until the failure (fullseparation) occurred in the main plate. This investigation shows that in the case of simple bolted joints, the fatigue crack isusually initiated from the edge of the bolt hole in the net section due to stress concentration at the edge (middle plane of themain plate) of the bolt hole and propagates through the plate thickness and plate width semi-elliptically. When the crackpropagation (in the plate thickness direction) reaches the plate surfaces then the crack propagates as through thickness crackin a path perpendicular to the applied longitudinal load. In the case of the hybrid joints, failure occurred in two stages. In thefirst stage, the adhesive bond is fractured from the end of the overlap toward the center of the overlapped region. In the sec-ond stage, a failure occurs in the main plate (as explained previously in the simple bolted specimens). For better illustrationand comparison purpose the effect of tightening torque on the fatigue strength of specimens, the resulting fatigue lives weredisplayed in a semi-log S–N diagram in Fig. 12 against tightening torques for both types of the joints.

As it can be seen from Fig. 12, increasing the tightening torque, leads to a significant improvement in the fatigue life asexpected. The following can be pointed out according to the obtained results. The results obtained from the experimentaltests, revealed that the fatigue life joints were improved by increasing the tightening torque due to compressive stresses,which appeared around the hole. More details about the effects of compressive stress on the fatigue life improvement havebeen discussed in next section.

60

80

100

120

140

160

180

200

1.0E+04 1.0E+05 1.0E+06 1.0E+07

App

lied

Stre

ss R

ange

(MPa

)

Fatigue Lives (Cycles)

Hybrid Joint: Clamped by T=1N.m

Hybrid Joint: Clamped by T=2.5 N.m

Hybrid Joint: Clamped by T=5N.m

Simple Bolted Joint: Clamped by T=2.5 N.m

Simple Bolted Joint: Clamped by T=1 N.m

Simple Boltedd Joint: Clamped by T=5 N.m

Fig. 12. S–N curve obtained from experimental fatigue tests for both types of the joints.

414 F. Esmaeili et al. / Engineering Failure Analysis 45 (2014) 406–420

In addition, in the hybrid joints, the stress concentration around the hole is reduced significantly. As a result, the localstress at the edge of the hole is lower, since some portion of the total load is transmitted by adhesive layer.

Finally, the comparison of the obtained results from the experimental tests, confirms that the hybrid joints have betterfatigue performance than simple joints for all levels of the tightening torque. In addition, it should be noted that, in thehybrid joints, direct metallic contact between the plates does not occur (due to the adhesive layer presence), therefore,

(a) simple bolted

(b) hybrid

Fig. 13. The typical fractured specimens under fatigue tests with tightening torque of T = 1 N m that subjected to maximum remote longitudinal load equalto 7.2 kN (a) simple bolted, (b) hybrid joints.

Hybrid Joint Simple Joint

(a) Clamped by T=1 Nm

Hybrid Joint Simple Joint

(b) Clamped by T=2.5 Nm.

Hybrid Joint Simple Joint

(c) Clamped by T=5 Nm.

Fig. 14. Distribution of resultant compressive stress rz in MPa due to tightening torque of (a) 1 N m, (b) 2.5 N m and (c) 5 N m for simple and hybrid joints.

F. Esmaeili et al. / Engineering Failure Analysis 45 (2014) 406–420 415

the possible fretting fatigue (that causes an early fatigue crack initiation) is eliminated. The typical fractured specimens forsimple and hybrid joints under fatigue tests with tightening torque of T = 1 N m that subjected to maximum remote longi-tudinal load equal to 7.2 kN is shown in Fig. 13.

4.2. Numerical analysis results

As it was mentioned previously, three different tightening torque values were selected to apply. To do so, the correspond-ing clamping force, i.e. Fcl = 976, 2440 and 4880 N for simple bolted joints, and Fcl = 840, 2100 and 4200 N for hybrid joints

Fig. 15. Typical nominating for smallest cross section area: top and middle lines with directions of Cartesian axes.

50

100

150

200

250

300

350

400

0 2 4 6 8 10

Sx (M

Pa)

Distance from hole edge (mm)

mid-5Nm-Hybridmid-2.5Nm-Hybridmid-1Nm-Hybridmid-1Nm-Simplemid-2.5Nm-Simplemid-5Nm-Simple

(a)

50

100

150

200

250

300

350

400

0 2 4 6 8 10

Sx (M

Pa)

up-5Nm-Hybridup-2.5Nm-Hybridup-1Nm-Hybridup-1Nm-Simpleup-2.5Nm-Simpleup-5Nm-Simple

(b)

Distance from hole edge (mm)

Fig. 16. Distribution of longitudinal stress after clamping for smallest cross section area for different planes (a) mid-plane, (b) top plane; subjected toremote longitudinal tensile stress of 168 MPa.

416 F. Esmaeili et al. / Engineering Failure Analysis 45 (2014) 406–420

were to be applied on the plates. Therefore, a displacement boundary condition in �Z direction was applied on the lower faceof the bolt shank to achieve the desired clamping forces equal to experimental test results. The magnitude of the requireddisplacement was found after a few trial and error processes to achieve the desired reaction forces or the clamping forcesresulting from tightening torques.

According to the results of the first load step solution of the finite element analysis, some beneficial compressive stresseswere observed near the hole of the plates. The compressive stress contours around the bolt hole of the main plate, createddue to 1, 2.5 and 5 N m tightening torques are shown in Fig. 14. As it can be seen, the most compressive stresses are observedat the edge of the hole, which increased from �20 to �101 MPa when the clamping force increased from 976 to 4880 N.

In the second load step, in according to the experimental test process, a tensile remote stress was applied to the FE modelsto simulate the tensile loading of the specimens. Therefore, a tensile (remote) stress equal to the applied cyclic load rangewas applied on the right end of the main plate while the displacement of the left end of the connector plates was constrained.The stress distributions along the lines perpendicular to the loading directions in the top and mid planes (as shown in Fig. 15)of the smallest cross-sectional area, which has the maximum tendency for crack initiation and propagation, have been plot-ted. The resultant longitudinal stress distribution in the smallest cross section area for these paths are shown in Figs. 16 and17 for different clamping forces under the application of maximum remote stress equals to Smax = 168 MPa, respectively. As

Hybrid Joint Simple Joint(a) Clamped by T=1 Nm

Hybrid Joint Simple Joint(b) Clamped by T=2.5 Nm

Hybrid Joint Simple Joint(c) Clamped by T=5 Nm

Fig. 17. Distribution of longitudinal normal stress rx in MPa due to tightening torque (a) T = 1 N m, (b) T = 2.5 N m and (c) T = 5 N m and applied remotelongitudinal tensile stress of 168 MPa.

F. Esmaeili et al. / Engineering Failure Analysis 45 (2014) 406–420 417

expected, the maximum stress values occurred at the edge of the hole, which decreased from 402 to 378 MPa for simplebolted specimens and 278–252 MPa for hybrid specimens when the tightening torque increased from 1 to 5 N m.

The improvement in fatigue life can be related to compression around the hole caused by the compression of the plates bythe bolt pretension. Such negative stresses can reduce the total amount of resultant stresses that cause fatigue crack initi-ation and propagation in the plate due to the applied tensile external loads. In other words, the higher clamping compressivestresses because of higher applied tightening torques could further reduce the net effect of the destructive cyclic longitudinaltensile loads, which cause fatigue crack initiation and propagation. It is noted that, once the bolt has been tightened to therequired preload, the bolt will be in tension and stretched and the jointed plate will be in compression. During this process

0

200

400

600

800

1000

1200

1400

-1000 1000 3000 5000 7000 9000 11000 13000

Fric

tion

forc

e (N

)Applied longitudinal Remote Load (N)

Simple(T=1Nm)

Simple(T=2.5Nm)

Simple(T=5Nm)

Fig. 18. The load transmitted by friction between the plates versus the applied longitudinal load for three different applied tightening torques.

0

2000

4000

6000

8000

10000

0 2000 4000 6000 8000 10000 12000

Adh

esiv

e La

yer

forc

e (N

)

Applied longitudinal Remote Load (N)

Hybrid(T=1Nm)

Hybrid(T=2.5Nm)

Hybrid(T=5Nm)

Fig. 19. The load transmitted by adhesive layer versus the applied longitudinal load for three different applied tightening torques.

418 F. Esmaeili et al. / Engineering Failure Analysis 45 (2014) 406–420

the material under the bolt washer will be compressed transversally (in thickness) and due to Poisson’s ratio the materialwill radially stretched. However, the far away material will resist against stretching thus compressive radial and compressivetangential pre-stress are created around the hole. The improvement in fatigue life can also be related to the compressive pre-stress stress surrounding the bolt hole caused by the compression of the member material by the bolt pretension. As thetorque is increased the pretension is also increased, so two thing will happen; the bolt will be stretched more and the mem-ber material will be placed under great compression. This compressive pre-stress will be beneficial to prevent fatigue cracksinitiation and growing (the stress concentration around the hole) from the bolt hole as explained in Ref. [32]. It can, there-fore, be assumed that the fatigue life improvement is benefited by a combination of the load transfer method and the assis-tance of the compressive pre-stresses surrounding the bolt hole. This is in complete agreement with available results offatigue life in simple bolted plate and double shear lap bolted joints clamped with tightening torques [32–38].

The improvement in fatigue life specimens can also be attributed to the method that the joint transmit the applied load.In the case of simple bolted specimens, as the tightening torque is increased, a larger part of the load is transmitted by fric-tion (at the plate faces). The remaining part of the load is transmitted by the reaction of the bolt against the bolt hole surface.The transmitted load between the connection plates (as a result of friction) versus the applied longitudinal load for threedifferent applied tightening torques has been shown in Fig. 18.

As it can be seen from the figure, the transmitted load by means of friction initially increases as the longitudinal load isincreased. At the beginning of the loading, the total of applied longitudinal load is transmitted only by means of frictionbetween the plates. However, the friction force stops to increase when the bolt touches the hole after which the applied loadis divided between the bolt and the plates. This is because of an initial gap between the bolt shank and the bolt hole. Clamp-ing force in the joints creates friction on the contact surfaces. This friction changes the load transfer mechanism in the joint.Therefore, friction force reduces a portion of load transferred through the bolt as shown in Fig. 18. As a result, the local stressat the edge of the hole is lower, since part of the total load is absorbed (transmitted) by friction depending on the clampingforce and the friction coefficient of the contact surface. Additionally, in the case of hybrid joints the transmitted load bymeans of adhesive layer versus the applied longitudinal load has been shown in Fig. 19. As it can be seen from this figure,at the beginning of to the loading, total of applied load is transmitted via adhesive layer. On the other hand, when the boltreaches the bolt hole, some of the applied load is transmitted by bolt. By comparing the Figs. 18 and 19, it can be concludedthat the lower portion of the applied longitudinal load is transmitted by the bolt in the hybrid specimens in comparison withthe simple bolted specimens. Consequently, the stress concentration due to the bearing of the bolt on the edge of the bolthole is considerably reduced, which is helpful with regard to any fatigue crack initiation and propagation.

F. Esmaeili et al. / Engineering Failure Analysis 45 (2014) 406–420 419

5. Conclusion

In this investigation, the effects of torque tightening on the fatigue strength of double lap simple bolted and hybrid(bolted/bonded) joints have been studied experimentally and numerically.

A series of experimental tests were carried out to investigate the effects of torque tightening on the fatigue life of doubleshear lap joints subjected to longitudinal load. Three values of tightening torques were chosen to apply on the bolt for tight-ening the joint. In a general trend, increasing the tightening torque or clamping force on the joint, leads to improve the fati-gue life. In addition, finite element analysis was used to obtain stress distribution in joint plates due to clamping force andlongitudinal applied loads and to afford a detailed clarification of the joint’s performance. According to the obtained resultsfrom the test of the two different specimens consisting of simple bolted joints and hybrid joints, the later joints have longerfatigue lives compared to the simple bolted joints. Numerical simulation and experimental results revealed that the fatiguelife of double lap bolted joints were improved by increasing the tightening torque due to compressive stresses whichappeared around the hole. The improvement in fatigue life can be attributed to the method that the joint transmits theapplied load. As the tightening torque is increased, a large part of the load is transmitted by friction. The remaining partof the load is transmitted by bearing, through the interaction of the bolt with its hole. The improvement in fatigue lifecan also be related to compression around the hole caused by the compression of the plates by the bolt pretension. Suchnegative stresses can reduce the total amount of resultant stresses that cause fatigue crack initiation and propagation inthe plate due to the applied tensile external loads. Furthermore, in the hybrid joints, direct metallic contact between theplates does not occur; therefore, the possible fretting fatigue is eliminated.

References

[1] Chakherlou TN, Mirzajanzadeh M, Abazadeh B, Saeedi K. An investigation about interference fit effect on improving fatigue life of a holed single plate injoints. Eur. J. Mech. A – Solid. 2010;29:675–82.

[2] Cloud GL. 2012 William M. Murray lecture: some curious unresolved problems, speculations, and advances in mechanical fastening. Exp Mech53(7);2013:1073–104.

[3] Iancu F, Ding X, Cloud GL, Raju BB, Hahn GT. Three-dimensional investigation of thick single-lap bolted joints. Exp Mech 2005;45(4):351–8.[4] Bray A, Levi R. Factors affecting the tightening characteristics of bolts. Exp Mech 1966;6(8):409–19.[5] Aragon, Alegre JM, Gutierrez-Solana F. Effect of clamping force on the fatigue behaviour of punched plates subjected to axial loading. Eng Fail Anal

2006;13:271–81.[6] Ireman T. Design of composite structures containing bolt holes and open holes. Report No. 99-03. Royal Institute of Technology; 1999.[7] Essam A, Bahkali A. Finite element modeling for thermal stresses developed in riveted and rivet-bonded joints. Int. J. Eng. Technol. IJET-IJENS

2011;11(6):106–12.[8] Camanho PP, Tavares CML, Oliveira Rd, Marques AT, Ferreira AJM. Increasing the efficiency of composite single-shear lap joints using bonded inserts.

Composites: Part B 2005;36:372–83.[9] Fu M, Mallick PK. Fatigue of hybrid (adhesive/bolted) joints in SRIM composites. Int J Adhes Adhes 2001;21:145–59.

[10] Gomez S, Onoro J, Pecharroman J. A simple mechanical model of a structural hybrid adhesive/riveted single lap joint. Int J Adhes Adhes 2007;27:263–7.[11] Hart-Smith LJ. Bonded–bolted composite joints. J Aircraft 22(11);1985:993–1000[12] Allan RC, Bird J, Clarke JD. The use of adhesives in the repair of cracks in ship structures. Structural adhesives in engineering. Bristol, UK: Institution of

Mechanical Engineers; 1986.[13] Gomeza S, Onoro J, Pecharroman J. A simple mechanical model of a structural hybrid adhesive/riveted single lap joint. Int J Adhes Adhes

2007;27:263–7.[14] Camanho PP, Tavares CML, Oliveira Rd, Marques AT, Ferreira AJM. Increasing the efficiency of composite single-shear lap joints using bonded inserts.

Composites: Part B 2005;36:372–83.[15] Hart-Smith LJ. Design methodology for bonded–bolted composite joints. Technical Report AFWAL-TR-81-3154. Douglas Aircraft Company; 1982.[16] Chan WS, Vedhagiri S. Analysis of composite bonded/bolted joints used in repairing. J Compos Mater 2001;35:1045–61.[17] Kelly G. Load transfer in hybrid (bonded/bolted) composite single-lap joints. Compos Struct 2005;69(1):35–43.[18] Barut A, Madenci E. Analysis of bolted–bonded composite single-lap joints under combined in-plane and transverse loading. Compos Struct

2009;88:579–94.[19] Paroissien E, Sartor M, Huet J, Lachaud F. Analytical two-dimensional model of a hybrid (bolted/bonded) single-lap joint. J Aircraft 2007;44(2):573–82.[20] Stewart ML. An experimental investigation of composite bonded and/or bolted repairs using single lap joint designs. AIAA-97-1339;1997:2752–60.[21] Kweon J, Jung J, Kim T, Chai J, Kim D. Failure of carbon composite-to aluminum joints with combined mechanical fastening and adhesive bonding.

Compos Struct 2006;75:192–8.[22] Darwish SM, Niazi A, Ghania A, Kassem ME. Formulation effects on some properties of structural epoxy resin adhesives. 3rd Applied Mechanical

Engineering Conf., Military Technical College 1988:147–155 [Egypt].[23] Kilik S, Davies R, Darwish SM. Thermal conductivity of epoxy resin adhesives. Int J Adhes Adhes 1989;9(4):219–33.[24] Darwish SM, Niazi A, Gahanya A, Kassem ME. Improving electrical conductivity of structural epoxy resin adhesives. Int J Adhes Adhes

1990;11(1):37–41.[25] Darwish SM, Al-Abbas RK, Sadek MM. Design rationale of industrial containment joints. Int J Adhes Adhes 1991;11(2):65–70.[26] Darwish SM, Niazi A, Ghanya A. Phase stability of duralumin machined with bonded and brazed metal cutting tools. Int J Mach Tool Manufact

1992;32(4):593–600.[27] Al-Samhan Ali M. Analysis of adhesively bonded riveted joints. J King Saud Univ Eng Sci 2005;18(1):57–66 [Riyadh (1426H./2005)].[28] Pirondi A, Moroni F. Clinch-bonded and rivet-bonded hybrid joints: application of damage models for simulation of forming and failure. J Adhes Sci

Technol 2009;23:1547–74.[29] Schvechkov EI. Failure patterns and cycle longevity of rivet-bonded joints. Masinovedenie 1984;2:71–6.[30] Kelly G. Quasi-static strength and fatigue life of hybrid (bonded/bolted) composite single-lap joints. Compos Struct 2006;72:119–29.[31] Imanaka M, Haraga K, Nishikawa T. Fatigue strength of adhesive/rivet combined lap joints. J Adhes 1995;49:197–209.[32] Chakherlou TN, Abazadeh B, Vogwell J. The effect of bolt clamping force on the fracture strength and the stress intensity factor of a plate containing a

fastener hole with edge cracks. Eng Fail Anal 2009;16:242–53.[33] Chakherlou TN, Alvandi-Tabrizi Y, Kiani A. On the fatigue behavior of cold expanded fastener holes subjected to bolt tightening. Int J Fatigue

2011;33:800–10.[34] Oskouei RH, Chakherlou TN. Reduction in clamping force due to applied longitudinal load to aerospace structural bolted plates. Aerosp Sci Technol

2009;13:325–30.

420 F. Esmaeili et al. / Engineering Failure Analysis 45 (2014) 406–420

[35] BudynasR G, Nisbett JK. Shigley’s mechanical engineering design. 9th ed. McGraw-Hill; 2011.[36] Yan Y, Wen WD, Chang FK, Shyprykevich P. Experimental study on clamping effects on the tensile strength of composite plates with a bolt-filled hole.

Composites Part A 1999;30:215–1229.[37] Shankar K, Dhamari R. Fatigue behaviour of aluminium alloy 7075 bolted joints treated with oily film corrosion compounds. Mater Des

2002;23:209–16.[38] Deng X, Hutchinson JW. The clamping stress in a cold driven rivet. Int J Mech Sci 1998;40:683–94.[39] Collings TA. The strength of bolted joints in multi-directional CFRP laminates. Composites 1977;8(1):43–54.[40] Stockdale JH, Matthews FL. The effect of clamping pressure on bolt bearing loads in glass fiber-reinforced plastics. Composites 1976;7(1):34–9.[41] NN. Technical Data Sheet. Product 3421, Loctite Corp. Dublin; 2000.[42] ASTM D638-02a Standard Test Method for Tensile Properties of Plastics. American Society for Testing and Materials; 2002.[43] Swanson Analysis Systems Inc., ANSYS, Release 9; 2004.[44] Swanson Analysis Systems Inc., ANSYS, User’s Guide for Revision 9, ANSYS 9 Documentation. ANSYS Element Reference, Part I: Element Library,

SOLID45; 2004.[45] Swanson Analysis Systems Inc., ANSYS, User’s Guide for Revision 9, ANSYS 9 Documentation. Theory Reference, Element Library, TARGET170 – 3D

Target Segment; 2004.[46] Swanson Analysis Systems Inc., ANSYS, User’s Guide for Revision 9, ANSYS 9 Documentation, Theory Reference, Element Library. CONTA174 – 3D

Surface-to-Surface Contact; 2004.