Investigation of bolt clamping force on the fatigue life of double lap simple bolted and hybrid (bolted/bonded) joints via experimental and numerical analysis PDF

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Engineering Failure Analysis 45 (2014) 406–420 Contents lists available at ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal Investigation of bolt clamping force on the fatigue life of double lap simple bolted and hybrid (bolted/bonded) joints via exper...

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Investigation of bolt clamping force on the fatigue life of double lap simple bolted and hybrid (bolted/bonded) ... Firooz Esmaeili

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Engineering Failure Analysis 45 (2014) 406–420

Contents lists available at ScienceDirect

Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal

Investigation of bolt clamping force on the fatigue life of double lap simple bolted and hybrid (bolted/bonded) joints via experimental and numerical analysis F. Esmaeili ⇑, T.N. Chakherlou, M. Zehsaz Faculty 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 2014 Received in revised form 13 July 2014 Accepted 15 July 2014 Available online 23 July 2014 Keywords: Torque tightening Clamping force Hybrid joint Fatigue life Bonded 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 simple bolted and hybrid (bolted/bonded) joints have been studied both experimentally and numerically. 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 and subjected to tightening torque of 1, 2.5 and 5 N m and then fatigue tests were carried out at different cyclic longitudinal load levels. Experimental tests revealed that the hybrid joints have higher fatigue life in comparison with the simple bolted joints. In the numerical method, finite element code was used to obtain stress distribution in the joint plates due to clamping force and longitudinal applied loads. Numerical simulation and experimental results showed that the fatigue life of specimens was improved by increasing the clamping force due to compressive stresses created around the hole. In addition, the investigation revealed the positive role of clamping force resulting from torque tightening on the fatigue life of both simple and hybrid joints. Ó 2014 Published by Elsevier Ltd.

1. Introduction Detachable joints such as bolted, riveted or pined ones are frequently used in aerospace industry for connecting various parts. Among the mentioned detachable joints, bolted joints are the most important elements in aerospace structures. However, the existence of geometrical discontinuity in these joints due to essential hole drilling process causes stress concentration 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 favorite for extensive use in aerospace structures. Therefore, it is of great importance to reduce the effect of stress concentration and attain enhanced fatigue life [5]. According to results of previous researches, bolted joints have higher tensile and fatigue strengths 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 adhesive bonding in fatigue, two fundamental differences are important between the two types of lap joints. First, in a mechanical joint, the overlapping areas are attached to one another at discrete points only, i.e. by the fasteners. Obviously, severe stress concentrations should occur. However, if the connection is made continuously in the full overlapping area by adhesive bonding, these stress concentrations do not occur. Because, the adhesive bonded joints do not need to the fasteners and fastening ⇑ Corresponding author. Tel.: +98 411 3392492. E-mail address: [email protected] (F. Esmaeili). http://dx.doi.org/10.1016/j.engfailanal.2014.07.014 1350-6307/Ó 2014 Published by Elsevier Ltd.

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holes. Therefore, the stress distributions in the joint are relatively uniform compared with those in the mechanical joint. Secondly, 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 hybrid joints, are used [8–11]. Hybrid joints are used in many engineering application such as aerospace, automotive, and naval industries because of their better performance compared to simple joints such as adhesively bonded, welded and mechanically 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-designed bonded joints. Chan and Vedhgiri [16] conducted experiments with composite joints as well as a parametric study using finite element analysis to study the stacking sequence effect on joint strength. Barut and Madenci [18] developed a semianalytical solution method for stress analysis of a hybrid joint and they revealed that the adhesive, despite the fact it has low modulus compared to the bolt, transfers most of the load. Kweon et al. [21] detected a similar phenomenon in their experiments, 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 various conditions, via experimental analysis. The effect of the material, geometrical factors, and environment on static strength and energy 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 fatigue strength 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) hybrid single lap using different modulus adhesives. The results of studies, revealed that, the hybrid joints with lower modulus adhesives 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 rivet whose 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 addition 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 note that, even though some limited research have been conducted on the analysis of hybrid joints, still static and fatigue strength data 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 applying tightening torque using a torque wrench and then the bolt and nut are pulled toward each other. Pre-tension or clamping force 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 therefore improve the strength of the joint considerably [32,39]. Collings [39] investigated the effects of bolt clamping pressure on the strength of bolted joints in CFRP laminates. Stockdale and Matthews [40] investigated the effect of clamping pressure on bolt bearing load in glass fiber-reinforced plastics experimentally. Deng and Hutchinson [38] investigated the residual clamping stress exerted by the rivets on the joint. The relation between the clamping and applied force was analyzed using finite 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 simple and hybrid (bolted/bonded) joints were studied. For each kind of the joint, three sets of specimens were prepared and subjected to tightening torque of 1, 2.5 and 5 N m and then fatigue tests were carried out on them at different cyclic longitudinal load levels. In the numerical method, finite element ANSYS code was used to obtain stress distribution in the joint plates 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 1 lists the mechanical properties of the aluminum alloy obtained from tension (static) tests, while Table 2 presents the chemical compositions of the used aluminum alloy.

Table 1 Mechanical 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

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Table 2 Chemical 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

Two different types of joints i.e. double lap simple and hybrid (bolted/bonded) joints were prepared. Test specimens’ configurations 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], prepared by mechanical mixing of the resin and hardener in equal amount by weight. This adhesive was selected due to its high strength and long working life. In order to obtain the tensile stress–strain curve of the adhesive, several dog-bone specimens were prepared according to ASTM D638-02 [42]. The adhesives were injected into a mold, as shown in Fig. 2, and left to cure at room temperature for 24 h. Finally, the prepared specimens were tested on a 100 kN Zwick/Z100 static testing machine with a crosshead speed of 5 mm/min (see Fig. 3). The engineering stress–strain curve of the adhesive is shown in Fig. 4.

(a) Siimple bolted jointt

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

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

409

Fig. 2. Mold for dog-bone specimens.

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

Stress (MPa)

40 30 20 10 0 0

0.01

0.02

0.03

0.04

0.05

0.06

Strain (mm/mm) Fig. 4. Engineering stress–strain curve of Loctite 3421 adhesive.

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 head M5 (class 10.9) steel bolt was used for the mechanical fastening and suitable types of steel washers and nuts were used to prepare the joint as illustrated in Fig. 1. Finally, the nut is tightened by applying torque using a torque-wrench up to required amounts 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 this investigation.

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The preparation of the hybrid joints has been implemented in two main steps. Firstly, a double lap bonded joint was constructed. In order to obtain high strength joint, the joint plates were cleaned with acetone and then they were let dry, prior to using the adhesive layer. In order to achieve the constant thickness of adhesive layer, 0.5 mm thick sheets were used between adherends. The prepared bonded joints were left in ambient temperature for 72 h, in accordance with the adhesive manufacturer suggestion. In the second step of preparing the hybrid joints, the double lap bonded specimens with the adhesive were bolt tightened using 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 torques, for both types of the joints, i.e. simple bolted and hybrid joints, a special experimental method was designed using a steel bush that was placed between the nut and the plate. At the bush outer surface, two strain gauges were stuck to measure the compressive axial strain and so the stress in the bush using Hooke’s stress–strain law. Having the bush cross-sectional area at hand and the axial stress, the axial force in the bush and therefore the clamp force has been determined. The used method 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 nut using a torque wrench, and then the axial strains were recorded for each value of the torques. This test was repeated three times for each case to obtain the mean value of compressive strains (em), and determine the corresponding clamping forces using Eq. (1).The elastic modulus for the bush material (Ebush) was also experimentally determined in order to obtain the accurate values for the mean axial clamping force.

F cl ¼ Ebush Abush em ¼ 204; 188 

p 4

ð92  52 Þem ¼ 89:8  105 em ðNÞ

ð1Þ

where Abush is the area of the bush cross section. The relation between the calculated clamping forces and the applied torques for the specimen is shown in Fig. 6. As the figure shows, there is a linear relation between the clamping force and the applied torque. 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 fatigue lives 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 testing machine with a frequency of 10 Hz and stress ratio (load ratio) of 0.1 (Fig. 7). Fatigue tests have been performed on the double lap simple bolted specimens with three different amounts of tightening torques, i.e. 1 N m, 2.5 N m and 5 N m which created clamping forces equal to Fcl = 976, 2440 and 4880 N respectively, according to the linear equation obtained from Fig. 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 cycles to failure (full separation of specimen). In addition, in the case of double lap hybrid specimens, fatigue tests have been performed with the same amounts of tightening 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.

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Bolt Clamping Force (N)

8000 Simple Bolted Joint

7000

Hybrid Joint

6000 Fcl = 987.8 T

5000 4000

Fcl = 841.43 T

3000 2000 1000 0 0

1

2

3

4

5

6

7

8

Applied Tightening Torque (Nm) Fig. 6. The tightening torque–clamping force relation.

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

200

Applied Stress Range (MPa)

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

180

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

160

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

140 120 100 80 60 1.0E+04

1.0E+05

1.0E+06

1.0E+07

Fatigue Lives (Cycles) Fig. 8. Fatigue test results for double lap simple bolted specimens.

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

3. Finite element analysis

Applied Stress Range (MPa)

In order to obtain the stress distribution in the joint plates for both kinds of the joints, three-dimensional finite element models were simulated by ANSYS 9.0 general finite element code [43]. All of the adherends and adhesive layer are meshed with eight-node hexahedral structural solid elements Solid45 [44]. The meshed model of the hybrid specimens consisted of 19,316 elements and 21,614 nodes. In addition, the minimum mesh size in edge of the bolt hole was 0.041 mm. The finite element mesh of the double lap hybrid joint specimens is presented in Fig. 10, together with its corresponding loading and boundary conditions. The nodes located at the left edge of the FE model were considered to have all their degrees of freedom constrained. Only one quarter of the specimen has been modeled, due to double symmetry (with respect to X–Z and X–Y Cartesian planes) and symmetric displacement boundary condition has been applied to the corresponding planes as shown in the figure. In order to transfer the pressure between the contacting surfaces of the bolt head (or nut) and the plate, flexibleto-flexible contact state was used. Each contact pair consisted of target element and contact element. TARGET 170 was used as a target element and CONTACT 174 was used as a contact element [45,46]. The friction effect between the surfaces of the washer (bolt head) and Al-alloy plates was included in the FE model using Elastic Coulomb model with friction coefficient of l = 0.29 which was obtained from experimental tests based on the sliding of the washer under its own weight on the sloped surface from Al-alloy plate. Also based on the similar experiments, the friction coefficient was fou...