An experimental investigation of the bolt clamping force and friction effect on the fatigue behavior of aluminum alloy 2024-T3 double shear lap joint

In this article, the effect of bolt clamping force on the fatigue life of bolted double shear lap joints was investigated. To do so, fatigue tests were carried out on the bolt clamped double shear lap joint specimens made of aluminum alloy 2024-T3. These fatigue tests were conducted with applied torques of 0.25, 2 and 4 N m at different cyclic longitudinal load levels in un-lubricated and lubricated states. From these tests the stress–life (S–N) data for different clamping forces for un-lubricated and lubricated states were obtained. The results show that clamping force increases fatigue life compared to clearance fit specimens. In general, at higher tightening torque higher fatigue lives were achieved, however, below a certain load level the life improvement was discontinued because of fretting phenomenon. Also lubricating the parts of the specimens reduces the advantage of clamping force or torque tightening.     

Experimental and numerical investigations about the combined effect of interference fit and bolt clamping on the fatigue behavior of Al 2024-T3 double shear lap joints

In this paper the fatigue behavior of double shear lap joints treated by different combinations of interference fit and bolt clamping have been investigated both experimentally and numerically. To do so, specimens made from aerospace structural material of aluminum alloy 2024-T3 plates were interference fitted at the sizes of 1.5% and 4.7% and torque tightened with 2 and 4 N m to be prepared for fatigue tests. Consequently, the joints were subjected to cyclic load at different levels to obtain fatigue life. Finite element (FE) analysis was also performed to find the stress and strain distributions and the results were used to help explain the trends observed in the experimentally obtained S–N data. The experimental tests showed that during the interference fit process a protruded region is created at around the hole in the exit plane due to directional material plastic flow as a consequence of the oversized bolt force fitting. This protruded region has a bigger height for the bigger interference fit size. The finite element results showed that the protruded region generally localizes the compressive effect of bolt clamping and reduces its capability in fatigue life enhancement, by relaxing the clamping force. The fatigue test results showed that a better fatigue life improvement was achieved by employing the combination of a smaller interference fit size and bigger clamping force.     

Prediction of fatigue life in aircraft double lap bolted joints using several multiaxial fatigue criteria

In this research, the effects of torque tightening on the fatigue strength of 2024-T3 aluminium alloy double lap bolted joints have been studied via experimental and multiaxial fatigue analysis. To do so, three sets of the specimens were prepared and each subjected to different levels of torque i.e. 1, 2.5 and 5 N m and then fatigue tests were carried out at various cyclic longitudinal load levels. A non-linear finite element ANSYS code was used to obtain stress and strain distribution in the joint plates due to torque tightening of bolt and longitudinal applied loads. Fatigue lives of the specimens were estimated with six different multiaxial fatigue criteria by means of local stress and strain distribution obtained from finite element analysis. Multiaxial fatigue analysis and experimental results revealed that the fatigue life of double lap bolted joints were improved by increasing the clamping force due to compressive stresses which appeared around the hole.     

Investigation the effect of tightening torque on the fatigue strength of double lap simple bolted and hybrid (bolted–bonded) joints using volumetric method

In this research, the effect of the tightening torque on the fatigue strength of 2024-T3 double lap simple bolted and hybrid (bolted–bonded) joints have been investigated experimentally by conducting fatigue tests and numerically by implementing finite element analysis. To do so, three sets of specimens were prepared and each of them subjected to tightening torque of 1, 2.5 and 5 Nm and then fatigue tests were carried out under different cyclic longitudinal load levels. In the numerical method, the effect of the tightening torque on the fatigue strength of the considered joints has been studied by means of volumetric method. To obtain stress distribution around the notch (bolt hole) which is required for the volumetric method, nonlinear finite element simulations were carried out. In order to estimate the fatigue life, the available smooth S–N curve of Al2024-T3 and the fatigue notch factors obtained from the volumetric method were used. The estimated fatigue life was compared with the available experimental test results. The investigation shows that there is a good agreement between the life predicted by the volumetric method and the experimental results for different specimens with a various amount of tightening torques. The results obtained from the experimental analysis showed that the hybrid joints have a better fatigue strength compared to the simple bolted joints. In addition, the volumetric method and experimental results revealed that the fatigue life of both kinds of the joints were improved by increasing the clamping force resulting from the torque tightening due to compressive stresses which appeared around the bolt hole.     

Experimental and numerical study of fatigue crack growth of aluminum alloy 2024-T3 single lap simple bolted and hybrid (adhesive/bolted) joints

In this paper, the effects of tightening torque (clamping force) on the fatigue crack growth rate and stress intensity factors in cracked single lap simple bolted and hybrid (adhesive/bolted) joints have been studied experimentally and numerically. To do so, series of fatigue crack growth tests for two different amounts of tightening torque in Aluminum alloy 2024-T3 pre-cracked joints have been carried out to record the fatigue crack growth and also the fatigue life of specimens. In the numerical part, finite element method was employed to obtain the stress intensity factors and also the effective stress intensity factor ranges for different crack lengths to explain the behavior of fatigue crack propagation. It was found that the hybrid joint has longer fatigue crack growth life compared to the simple bolted joint at a given bolt tightening torque. The results also showed that a higher bolt tightening torque provides improved fatigue crack growth life for both types of the joints.     

Experimental and numerical investigations into the effect of an interference fit on the fatigue life of double shear lap joints

In this research the effect of bolt interference fit on the fatigue life of lap joints in double shear was investigated by conducting experimental fatigue tests and also analytically by FE simulation. In the experimental part, fatigue tests were carried out on specimens made from aluminium alloy 2024-T3 plates joined together as double lap joints and secured using bolts having fits ranging from zero clearance to different levels of interference. The results demonstrate how the failure is affected using different levels of interference fit. In the numerical study, 3-D FE models were used to simulate the different pin in hole fits considered and the results have been used to help explain the trends which were observed in the experimentally obtained S–N curve behaviour.     

Effect of cold expansion on the fatigue life of Al 2024-T3 in double shear lap joints: Experimental and numerical investigations

In this paper the effect of cold expansion on fatigue life improvement of aluminum alloy 2024-T3 plates used in double shear lap joints is investigated experimentally by conducting fatigue tests and numerically by implementing finite element simulations. In the experimental part, fatigue tests were carried out on the plates with cold expansion levels of 0%, 1.5% and 4.7% which were used in double shear lap joints. In the numerical study, three-dimensional finite element models were employed to predict stress distributions in the cold expanded plates used in the double shear lap joint. The results obtained from finite element simulation, have been employed to explain the trends which were observed in the experimentally attained S–N data and the fatigue crack initiation location. The experimental and numerical results showed that cold expansion improves fatigue life at low load levels and the life enhancement is more for the bigger cold expansion size. However, the fatigue life improvement is smaller in double shear lap joints compared to a single cold expanded plate.     

Experimental Investigations on the Effects of Torque Tightening on the Fatigue Strength of Double‐lap Simple Bolted and Hybrid (Bolted/Bonded) Joints

In this paper, the effects of bolt torque tightening on the fatigue strength of double‐lap simple bolted and hybrid (bolted/bonded) joints have been studied experimentally. To do so, two types of joints, that is, double‐lap simple and hybrid (bolted/bonded) joints, were studied. For each type of joints, three sets of specimens were prepared and subjected to the tightening torque of 1, 2.5 and 5 Nm, and then, fatigue tests were carried out at different cyclic longitudinal load levels. Experimental tests results showed that the hybrid joints have better fatigue performance in comparison with the simple bolted joints. In addition, the investigation revealed the positive role of tightening torque on the fatigue life of both simple and hybrid joints.     

Effect of torque tightening on the fatigue strength of bolted joints

The effect of tightening torques on the life of plates bolted using single and double lap joints was investigated. The effect of plate thickness using an aircraft grade aluminium alloy with double lap joints was also studied. Constant amplitude fatigue tests, under load control, were carried out, with a near zero stress ratio, on plain specimens (for bench mark purposes) and on both single and double lap joint specimens, for which several torque levels were applied on the bolted joints. The objective of the fatigue tests was to demonstrate failure trends for each joint type, material thickness and torque loading, rather than the generation of comprehensive S–N curves. Possible factors that affected the fatigue life of the bolted joints are discussed and conclusions are drawn with respect to the beneficial effects of tightening torques on the bolted single and double lap joints.     

预腐蚀铝合金典型螺栓单搭接件疲劳寿命研究

模拟飞机服役环境,对航空LY12 铝合金板材典型螺栓单搭接试验件做了预腐蚀试验,然后通过疲劳试验得到了未腐蚀试验件和预腐蚀后试验件的疲劳寿命。通过对试验件分解检查,分析了腐蚀对航空LY12 铝合金典型螺栓单搭接件的影响。建立了考虑腐蚀影响的基于断裂力学的沉孔螺栓搭接件疲劳寿命计算模型,对预腐蚀搭接件的疲劳寿命进行了计算和分析。结果表明:螺栓孔附近和搭接面腐蚀较表面其他部位严重,腐蚀产物的膨胀作用下产生了“枕垫效应”;计算模型得到的疲劳寿命与试验寿命吻合较好,其中模型1 最大相对误差为36.7%,模型2 的最大相对误差为25.7%。搭接件不同的腐蚀程度要选择不同的计算模型才能得出精确的结果。在分析较严重的腐蚀对搭接件疲劳寿命影响时,应考虑搭接面的多腐蚀损伤。     

Fatigue life estimation of bolt clamped and interference fitted-bolt clamped double shear lap joints using multiaxial fatigue criteria

In this paper seven different multiaxial fatigue criteria based on stress, strain and energy were employed to estimate the fatigue lives of double shear lap joint specimens of aluminum 2024-T3 with bolt clamped and interference fitted-bolt clamped fastener holes. Detailed finite element (FE) simulations were conducted to obtain the stress and strain distributions in the joint to be used to as basic data in estimating the fatigue lives using multiaxial fatigue criteria. The estimated lives were compared with available experimental fatigue test results to investigate the capability of the criteria in predicting the fatigue lives. The results showed that the accuracy of life estimation for any criterion varies for different batches of specimens and applied load ranges. However, including the fatigue crack growth life can improve the fatigue estimation life and reduce the absolute error of multiaxial fatigue criteria.     

Effect of hole lubrication on the fretting fatigue life of double shear lap joints: An experimental and numerical study

In this research the affect that lubrication at a hole and pin connection has on the fatigue life of a double shear lap joint is studied both experimentally and numerically. The study focuses on the joint middle plate item, which is connected via a central hole to the outer plates by means of a clearance fitting pin, thereby placing the hole in double shear. In the experimental work three identical batches of fatigue specimens, which are made from aluminum alloy 2024-T3, were fatigue tested. In the first batch the surface of the fastener hole was not lubricated whilst the hole in the other two batches was lubricated – each batch using a different lubricant. The three batches of double shear lap joint specimens were fatigue tested and their S–N curves established. The results show that the specimens in which the holes were lubricated have better fatigue lives than the non-lubricated hole specimens. In the numerical study, FE simulations were performed to include hole lubrication effect on the stress distribution by using different friction coefficient at the interface of the hole and its fastener (pin). The FE results have helped to gain an understanding of the reasons for fatigue life improvement and also have helped to quantify the level of improvement.     

Effects of joint geometry and bolt torque on the structural performance of single bolt tension joints in pultruded GRP sheet material

This paper contains the details of an experimental investigation into double lap single bolt tension joints made from 6.35 mm thick pultruded fibre reinforced plastic flat sheet. The joint geometry [edge distance to bolt diameter (E/D) and width to diameter (W/D) ratio] was varied and the effect of bolt clamping torque was investigated. Failure loads, critical end distances and critical widths were found to increase as the bolt clamping torque increased. After an initial bolt movement, the load vs bolt displacement plots are linear until the joints fail or the stiffness reduces significantly. The load at which the joint stiffness reduces has been called the damage load. This damage load is thought to be a useful quantity on which to base design. A simple statistical analysis has been carried out on the damage loads and damage load capacities for single bolt joints have been determined for prescribed confidence levels.     

复合材料单螺栓双剪搭接干涉配合疲劳强度评价

该文采用有限元软件ANSYS,建立了复合材料单螺栓双剪搭接干涉配合的三维有限元模型,研究了当连接模型中施加了预紧力时,预紧力对干涉配合的影响.首先分析了无载荷作用下复合材料板在干涉配合孔边切向应力分布特点;之后比较了双剪搭接在10kN 和15kN 两种远程纵向载荷作用下,孔边切向应力的分布情况.结果表明：远程纵向循环载荷作用下,预紧力能有效均匀各板所受载荷,并可消除在无预紧力时中板上的局部应力集中点,而且适当增大预紧力能提高干涉配合连接的效率;此外研究显示双剪搭接连接在不同的载荷下,疲劳危险点也不同;最后研究了复合材料干涉配合连接时,各板的交叉区域孔边切向应力分布,因此得出各板在高低不同的载荷下疲劳危险点的分布.     

On the Variation of Clamping Force in Bolted Double Lap Joints Subjected to Longitudinal Loading: A Numerical and Experimental Investigation

Abstract: This study investigates the variation of clamping force and its concomitant effects on the performance of bolted double lap joints subjected to longitudinal loading. Two different amounts of clamping force were applied to bolted double lap joints made of Aluminium 2024‐T3, and variations of clamping force were measured under the application of longitudinal loading. Finite element modelling was also performed to compare with experiments. The results unanimously revealed a gradual initial reduction of clamping force followed by a significant increase as the longitudinal load was increased. Also affected, was the load transfer mechanism in the joint resulting in variation of friction force between the plates, but in a different trend compared to clamping force. Finally, the key parameters have been discussed and highlighted pertaining to the performance of the joint.     

The influence of fatigue on the stiffness and remaining static strength of self-piercing riveted aluminium joints

Self piercing riveting (SPR) is one of the major joining technologies for aluminium structures due to its advantages over some of the more traditional joining technologies. In this paper, the mechanisms of crack initiation and growth during fatigue and the influence of fatigue on the stiffness and remaining static strengths of SPR joints in both lap shear and T peel configurations were studied. The results showed that cracks could initiate and develop from different locations on the substrate materials depending on load levels and test types. Fatigue increased the remaining static lap shear strength and stiffness of specimens due to the increased friction force at the top/bottom sheet interfaces around the tip of punched hole through fretting; however, fatigue reduced the remaining static T peel strength of specimens due to crack initiation and development; T peel fatigue at high load levels also increased the stiffness of specimens due to geometry change through large plastic deformation.     

Fatigue behaviour of double lap riveted joints assembled with and without interfay sealant

Fatigue tests on ‘double shear’ riveted specimens were carried out. Material was aluminium alloy, 7075‐T73, thickness 3 mm. Specimen surfaces were protected by chromic acid anodization and epoxy primer. The mating surfaces of some specimens were sealed by a two‐component manganese‐dioxide cured, polysulfide compound. Specimens were pre‐assembled by inserting a temporary spring fastener in a pilot hole. The holes were reamed to the final diameter after sealant curing; then the Hi‐Loks were installed. Comparative fatigue tests were carried out on dry assembled specimens. Quite surprisingly, the fatigue resistance of sealed specimens was very low when compared with the results of dry assembled specimens. Hysteresis cycles measured in sealed and un‐sealed specimens clearly indicated a lubricant effect of the sealing layer, which justified the results obtained. Additional tests were performed on specimens assembled by applying different clamping force during the sealant curing time. The results obtained indicated an independence of the fatigue resistance on this parameter. Different failure modes were observed in sealed and un‐sealed specimens: fatigue cracks nucleated outside the hole at a location in the shadow of the rivet in the un‐sealed joints, while fatigue cracks nucleated at both sides of the holes in sealed joints. Finite element calculations carried out under different values of the friction coefficient between the mating surfaces confirmed the different location of the fatigue critical areas in sealed and un‐sealed specimens.     

Experimental and numerical investigations on fatigue behavior of aluminum alloy 7050-T7451 single lap four-bolted joints

The fatigue behavior of aluminum alloy 7050-T7451 single lap four-bolted joints was studied by high-frequency fatigue test and finite element (FE) methods. The fatigue test results showed that a better enhancement of fatigue life was achieved for the joints with high-locked bolts by employing the combinations of cold expansion, interference fit, and clamping force. The fractography revealed that fatigue cracks propagated tortuously; more fatigue micro-cliffs, tearing ridges, lamellar structure were observed, and fatigue striation spacing was simultaneously reduced. The evaluation of residual stress conducted by FE methods confirmed the experimental results and locations of fatigue crack initiation. The extension of fatigue lives can be attributed to the evolution of fatigue damage and effect of beneficial compressive residual stresses around the hole, resulting in the delay of crack initiation, crack deflection, and plasticity-induced crack closure.     

Evaluation of the Effects of Corrosion on Fatigue Life of Clad Aluminum Alloy 2024-T3-Riveted Lap Joints with Acoustic Emission Monitoring

Corrosion affects the fatigue life of clad aluminum alloy-riveted lap joints, such as those found on an aircraft fuselage structure. Single-, double-, and triple-column-riveted lap joint specimens were fabricated and corroded in a Q-Fog accelerated corrosion chamber for five months using an ASTM G85-A5 prohesion test. Specimens were taken out of the chamber every 4?weeks, and the corrosion products which had been deposited on them were removed by immersion in concentrated nitric acid. For each corroded specimen, the mass loss with corresponding corrosion rate was determined. The specimens were fatigue loaded to failure on an MTS Universal Testing Machine with acoustic emission monitoring. Results indicate that exposure of lap joint specimens to this corrosive environment increased corrosion (mass loss), corrosion rate, and significantly reduced fatigue life. For a prolonged exposure in the corrosive environment, the fatigue life was reduced to zero, which has significant implication for aging aircraft. Acoustic emission monitoring successfully detected fatigue failure. Two failure modes, multisite crack damage and shear of the rivets, were observed.     

含多处损伤宽板螺接搭接件疲劳寿命研究

对含多处损伤（Multiple Side Damage,MSD）宽板搭接件做了等幅疲劳试验和断口分析,得到搭接件的疲劳寿命和孔边MSD裂纹的形成特点、裂纹前沿形状及扩展历程。结果表明,搭接件的疲劳破坏具有一定的隐蔽性,其疲劳寿命的绝大部分消耗在螺栓头下裂纹扩展阶段,当孔间裂纹出现首次连通时,搭接件剩余寿命约为总寿命的0.7%~9.4%。基于有限元软件FRANC2D/L和裂纹扩展分析软件AFGROW,建立了考虑钉载、第二弯矩和孔间裂纹干涉等影响因素的含MSD宽板搭接件疲劳寿命计算模型,并对孔边多裂纹的扩展寿命进行了计算分析。计算结果与试验结果的对比表明,该文所建寿命计算模型具有一定的精度,能满足工程需要,计算结果和结论可作为该类结构损伤容限设计的参考依据。