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 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.     

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.     

Health monitoring of bolted joints via electrical conductivity measurements

The use of electrical resistance as a diagnostic parameter in structural health monitoring of bolted joints is discussed in the paper. The proposed method is based on the phenomenon that the electrical constriction resistance of the rough contact interface between two conductive members clamped by a bolt is a sensitive indicator of bolted joint integrity. A simple formula for relationship between relative electrical conductance changes and relative tightening torque changes is analytically obtained. Based on this formula, a methodology is described for detecting a loosening failure of conductive bolted joints.     

The static and fatigue strength of bolted joints in composites with hygrothermal cycling

The effects of hygrothermal cycling upon the performance of a bolted composite joint was examined. Bolt torque relaxed as the number of environmental cycles increased. Comparison with analytical results suggested that the bolt torque “zigzag” behavior probably results from the natural sensitivity of bolted composite joints to the existing ambient temperature and moisture. The washer effect and specimen surface finishes were investigated to study the friction effect on joint bearing performance. Fatigue tests of specimens exposed to hygrothermal cycling exhibited greater hole elongation than specimens not exposed. High preload does improve static failure strength and the fatigue life of specimens under room conditions. Tests run on IM6/3501-6 material specimens with hygrothermal cycling show reasonable declines in fatigue life. The moisture weight gains of composites under hygrothermal cycling exposure were measured and compared to numerical results; good correlation was obtained. As a result, bolt failure occurred in some fatigue tests and is most frequently associated with large thickness-to-diameter ratio. The peak-to-peak stress was defined to study the effect of the R-ratio on bolts for the bolt failure problem.     

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.     

三维六向编织复合材料单剪搭接连接结构在拉伸载荷下的力学性能

通过试验测试与数值模拟相结合的方法对三维六向编织复合材料的螺栓连接性能进行了研究。首先,通过拉伸试验对不同侧向约束螺接方式连接件的连接强度进行了测试。测试结果表明:单搭连接结构的二次弯曲现象明显,连接强度与侧向约束有一定的关系,使用垫片可有效提高连接强度,螺栓拧紧力矩增加对连接强度影响不大;连接结构的破坏模式包括挤压破坏和拉伸破坏,在孔径较小时其主导破坏模式是挤压破坏。随后,基于测试中发现的破坏模式,建立了基于点应力准则的分析模型,并使用升温法实现螺栓拧紧力矩的施加。通过数值结果与试验结果的比较验证了分析模型的可靠性。最后,利用得到验证的分析模型,分析了单搭连接的二次弯曲现象,获得了侧向约束面积、螺栓拧紧力矩及连接平板厚度对单搭单螺栓连接结构力学性能的影响规律。分析结果表明:当侧向约束应力增加时,连接强度表现为先增加后降低的规律。     

Effect of bolt clamping force on the fracture strength of mixed mode fracture in an edge crack with different sizes: Experimental and numerical investigations

To investigate the effect of bolt clamping force, resulting from torque tightening, on the mixed mode fracture (I and II) strength and effective geometry/loading factor of an edge crack with different lengths, experimental and numerical studies have been carried out. In the experimental part fracture tests were conducted on three batches of simple edge crack and bolt torque tightened with 3.5 and 7 N m edge crack at three different crack sizes of Poly methyl-methacrylate (PMMA) rectangular plate. The specimens’ fracture strength was obtained using a tensile test machine at different loading angles by employing a modified Arcan fixture. In numerical part, finite element simulations were employed to model the three test specimen batches at the three crack lengths to obtain their stress intensity geometry/loading factors. The results show that the bolt tightening torque significantly reduces the effective geometry/loading factor, and thus increases the joint fracture strength compared to bolt-less simple edge crack specimens. However, the bolt clamping effect on increasing the fracture strength was almost the same for different crack lengths.     

Determination of forces in bolted joints

We propose a new approach to the determination of forces applied to a bolt in a tightened joint taking into account the variations of its stiffness and present the results of experimental investigations of the stress-strain diagrams of bolts for different values of tightening forces applied to the joints. The proposed procedure for the determination of forces acting on the bolt in the joint subjected to the action of variable loads takes into account the behavior of the stiffness of the joint regarded as a function of the load and tightening forces. We analyze different methods aimed at the determination of forces applied to the bolt according to the results of fatigue tests of bolts in tightened simple joints and present preliminary initial data required for the analysis of variable forces acting on the bolts in tightened joints. By using a new approach, we propose basic principles of design of economically efficient joints.     

Finite element investigations of bolt clamping force and friction coefficient effect on the fatigue behavior of aluminum alloy 2024-T3 in double shear lap joint

In this paper, the effect of bolt clamping force on the fatigue life of bolted double shear lap joints was investigated numerically. To do so, finite element simulation results were used to illustrate the trends occurred in experimental fatigue tests showing the effect of bolt clamping on improving the fatigue life of double shear lap joints. The results show that clamping force decreases the resultant longitudinal stress at the hole edge thus the fatigue life increases compared to clearance fit specimens. In general, at higher tightening torque longer fatigue lives were achieved, however, below a certain load level the life improvement was discontinued because of fretting occurrence. Also lubricating the specimens reduces the advantages of the clamping force.     

Strength prediction in CFRP woven laminate bolted double-lap joints under quasi-static loading using XFEM

The current paper is concerned with modelling damage and fracture in woven fabric composite double-lap bolted joints that fail by net-tension. A 3-D finite element model is used, which incorporates bolt clamp-up, to model a range of CFRP bolted joints, which were also tested experimentally. The effects of laminate lay-up, joint geometry, hole size and bolt clamp-up torque were considered. An Extended Finite Element (XFEM) approach is used to simulate damage growth, with traction–separation parameters that are based on previously reported, independent experimental measurements for the strength and toughness of the woven fabric materials under investigation. Good agreement between the predicted and measured bearing stress at failure was obtained.     

Determination of forces in bolted joints

We propose a new approach to the determination of forces applied to a bolt in a tightened joint taking into account the variations of its stiffness and present the results of experimental investigations of the stress-strain diagrams of bolts for different values of tightening forces applied to the joints. The proposed procedure for the determination of forces acting on the bolt in the joint subjected to the action of variable loads takes into account the behavior of the stiffness of the joint regarded as a function of the load and tightening forces. We analyze different methods aimed at the determination of forces applied to the bolt according to the results of fatigue tests of bolts in tightened simple joints and present preliminary initial data required for the analysis of variable forces acting on the bolts in tightened joints. By using a new approach, we propose basic principles of design of economically efficient joints. Translated from Problemy Prochnosti, No. 5, pp. 126–134, September–October, 1997.     

Spectrum fatigue of composite bolted joints

The new Swedish fighter JAS39 Gripen has a large number of primary structures made of composites. On those structures a large number of bolted joints are used which during the aircraft's service life will be subjected to spectrum fatigue loading. Consequently it is important to study the spectrum fatigue life of bolted joints. Specimens with a double-lap configuration and six bolts have been fatigue loaded at the load ratios R=−0.2 and R=−5. Specimens were also fatigue loaded with a vertical fin spectrum which had different amounts of elimination of load cycles. A linear damage rule, Miner's rule, was used to predict the spectrum fatigue life. The experimental results show that the shortest fatigue life occurs for specimens loaded at R=−1 followed by specimens loaded at R=−0.2. The longest fatigue life occurred for specimens loaded at R=−5. It was found that 50% elimination of load cycles in the spectrum can be used without affecting the fatigue life. The Miner's rule predictions appeared to overestimate the spectrum fatigue life. From bolt failure it was found that the first bolt row transfers the largest amount of load in the specimens.     

Fatigue crack growth analyses of offshore structural tubular joints

This study investigated various aspects of a fatigue crack growth analysis, ranging from the stress intensity factor solutions to the simulation of a fatigue crack coalescence process of a tubular joint weld toe surface flaw. Fracture mechanics fatigue crack growth analyses for offshore structural tubular joints are not simple, because of the difficulty to calculate the stress intensity factors due to their geometric complexity. The fully mixed-mode stress intensity factors of nine weld toe surface cracks of an X-shaped tubular joint under tension loading were calculated by detailed three-dimensional finite element analyses. Using these stress intensity factor solutions, a fatigue crack growth study was performed for the X-joint until (the crack surface length grew to two times the tube thickness. Through this study, the crack shape change during the fatigue crack propagation was investigated in detail. Fatigue life calculations were also performed for a range of crack geometries using the stress intensity factor solutions of the nine flaws. These calculations indicate that the natural fatigue crack growing path for a crack is its quickest growing path. The study demonstrated that detailed fracture mechanics fatigue analyses of tubular joints can be practical using the finite element method.     

Experimental analysis of the friction coefficient effect of the zinc‐lamella coated fasteners on bolt preload and tightening moment

Basic parameters of the bolt‐nut joints, which are ones of the most important elements of assembly processes, are the torque, bolt preload and friction coefficients between bolt and nut interfaces. In bolted joints tightened with torque and angle‐controlled method, friction coefficients of the fasteners are highly significant because they affect final torque and bolt preload values directly, creating a large uncertainty in regard to meet the minimum requirements on preloads considering the safety of joints and further systems, in case of this study, the vehicles being assembled. Also, the range of the lower and upper limits of friction coefficients of the coated fasteners affect process quality considerably in bolted joints tightened with torque and angle‐controlled technique. In this study, the effect of the friction coefficients on the bolt preload and final torque values in the vehicle chassis joints, which are created using torque and angle‐controlled tightening, were investigated experimentally. Therefore, bolt specimens which have both low and high friction coefficients, were tightened by the torque and angle‐controlled tightening method especially using high angle torque parameters on the vehicle chassis test bench. The torque and preload values obtained have been compared to each other and correlated in terms of the friction coefficients occurred.     

A study of fatigue crack growth from artificial corrosion pits at welded joints under complex stress fields

The paper studies the effects of artificial corrosion pits and complex stress fields on the fatigue crack growth of full penetration load‐carrying fillet cruciform welded joints with 45° inclined angle. Parameters of fatigue crack growth rate of welded joints are obtained from S‐N curves under different levels of corrosion. A numerical method is used to simulate fatigue crack growth using different mixed mode fatigue crack growth criteria. Using polynomial regression, the crack shape correction factor of welded joints is fitted as a function of crack depth ratios. Because the maximum circumferential stress criterion is simple and easy to use in practice, fatigue crack growth rate is modified using this criterion. The relationship of effective stress intensity factor, crack growth angle and crack depth is studied under different corrosion levels. The simulated crack growth path obtained from the numerical method is compared with the actual crack growth path observed by fatigue tests. The results show that fatigue cracks do not initiate at the edge or bottom of pits but at the weld toes where the maximum stress occurs. The artificial corrosion pits have little effect on the effective stress intensity factor ranges and crack growth angle. The fatigue crack growth rates of welded joints with pits 1 and 2 are 1.15 times and 1.40 times larger than that of the welded joint with no pit, respectively. The simulated crack growth path agrees well with the actual one. The fatigue life prediction accuracy using the modified formulation is improved by about 18%. The crack shape correction factor obtained using the maximum circumferential stress criterion is recommended being used to calculate fatigue life.     

Fatigue of bolted joints in (0/90) CFRP laminates

The results of an experimental study of the fatigue behaviour of double-lap bolted joints using 1 mm thick (0/90)_2s and 2 mm thick (0/90)_4s laminates from the XAS/914 CFRP system are described. The damage development under fatigue loading was monitored qualitatively by microscopy, and quantitatively by recording the bearing stiffness and permanent joint deformation with cycling. The possibility of a stiffness-based failure criterion is investigated.The importance of the bolt clamp-up torque on the fatigue performance of the laminates was also studied It is shown that damage development under fatigue loading is considerably reduced at higher clamp-ups.     

Experimental characterization of hybrid composite-to-metal bolted joints under flexural loading

Presented is an investigation of the structural performance of hybrid composite-to-metal bolted joints loaded in flexure. The main goal was to develop a watertight, hybrid connection to resist bending loads. The effect of bolt type, doubler plate geometry and foam inserts was studied. Fourteen different joint configurations were tested including standard bolted joints and bolted joints with doubler plates. The performance of connections loaded cyclically in flexure was assessed by comparing: (1) the initial damage and failure loads; (2) the types of failure modes; and (3) the joint initial rotational stiffness. Instability in the hysteresis loops, where the load drops in subsequent cycles, is used as an indication of damage. A joint using a short doubler plate, a foam insert, and a single row of bolts, was found to be at least 33% stronger and 29% stiffer when compared to a standard bolted joint with two rows of the same diameter bolts. Use of doubler plates and foam inserts in a bolted joint resulted in higher strength and stiffness and can effectively mitigate joint opening, which improves the ability to seal the joint and maintain watertight integrity.     

Failure analysis of timber bolted joints by fracture mechanics

This study concerns the failure of dowel-type joints in glued laminated timber under static loading. Failure of joints with a single bolt or dowel due to cracking parllel to the grain direction is considered. Results presented concern only the first mode of cracking,i.e. splitting under tension perpendicular to the grain. Fracture is analyzed by the use of linear elastic fracture mechanics concepts; the crack propagation condition is assumed to be based upon the critical energy release rate G_Ic. A simplified average stress criterion allows predicting the onset of splitting. An experimental program was carried out on joints for different structural parameters and bolt diameters. The experimental stable crack growth allowed obtaining the load-crack length curves. Another experimental program was carried out in order to compare the fracture energy of a few CIB-type specimens with the critical energy release rate G_Ic being used in the crack propagation simulation. The comparison between experimental and numerical results for the simulation of fracture in joints shows that Linear Elastic Fracture Mechanics provides a good approximation of the load-bearing capacity of bolted joints and may help improve design codes.