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.     

Three-dimensionally printed designable joint for carbon fibre reinforced plastics

Three-dimensional (3D) printing of carbon fibre reinforced plastic (CFRP) components shows promise for building light-weight strong components for industry. Many structures require multiple components to be joined to form the completed parts. However, tightening of metal bolts used to join CFRP can damage the composite and the bonding strength of adhesive bonding is variable. Here, we propose a new joint system, based on all-3D printed parts. Snap-in joints and Ajax-pins induce a mechanical joining force at the interface between parts, together with a bearing force. This joint system offers comparable strength to that of adhesive and metal bolt joints, while adding negligible weight to the parts. Furthermore, 3D printing of the surface structures, holes and pins enables unprecedented control over the joint design, enabling strength optimization through the thickness and in-place directions as required. The Snap-in joint system shows great potential for the fabrication of strong light-weight 3D printed structures.     

Tensile strength of open-hole,pin-loaded and multi-bolted single-lap joints in woven composite plates

The prediction of the tensile strength of multi-bolted joints uses characteristics that are obtained from the open-hole tension (OHT), bolt-filled hole tension (FHT), pin-loaded tension (PLT) and single-bolt single-lap joint (BJ) tests. However, the relative relevance of each of these tests to multi-bolted joints is not clear. This investigation aims to fill the gap by performing these tests on carbon/epoxy and glass/epoxy laminates with quasi-isotropic and cross-ply configurations and on an Al-6065 aluminum alloy. It is found that the highest strength achieved by a multi-bolted joint corresponds to the OHT strength. The number of bolts required to achieve this upper bound depends on the material characteristics. The Al-6065 alloy achieves the OHT strength with two bolts, whereas the composites require up to four bolts. Narrower specimens require fewer bolts to achieve the OHT strength. The stiffness and strength of the BJ and PLT are comparable for Al-6065. However, for the composites, BJ has a lower stiffness but a higher strength than PLT. The pin contact force triggers delamination initiation and propagation in the PLT, whereas the tightened bolt in the BJ suppresses the delamination. In addition, the rotation of the bolt explains the lower stiffness of the BJ.     

Capacity analysis,investigations and retrofitting of a long span steel grid hangar

In this study, a reticulated long space steel hangar that does not meet the allowable maximum deflection limits was investigated. The purpose was to assess, test and reinforce the structure so as to improve its capacity and prevent excessive deformation. Upon site investigations, some members had corroded, bolts are partially tightened onto node ball, mechanical air duct are suspended on the members rather than on joints. The support joint previously designed as a flexible joint translating in the y-dir is partially fixed, and some members experienced bending near support joint. The configuration of crane load path, assumptions of column stiffness effects and errors in designing and construction of joint support configuration contributed to an increment in deflection, internal member forces and bending/rotation on members near to support joints. The structure was incrementally loaded at specified points on the grid to test and evaluate deformation and its loading capacity, and an extensive analysis was carried out before and after retrofitting. Apart from servicing the hangar, other recommended strategies are retrofitting mechanism of welding new steel tubes to existing ones and reinforcing the bolt joints was agreed upon since the hangar was observed to be elastic. The hangar member loading capacity was observed to have improved significantly after the retrofitting. This study urges the stakeholders to be keen on discussed items and on roof erection process. A schedule for monitoring the behavior of the hangar is underway.     

An analytical model for the prediction of through-thickness stiffness in tension-loaded composite bolted joints

This paper presents the development of an analytical model for replicating the through-thickness stiffness of single-bolt, single-lap composite joints subjected to secondary bending. The model is an extension of a spring-based method, where bolts and laminates are represented by a series of springs and masses. The model accounts for extension of the bolt, the stiffness of the clamped region of the joint due to bolt torque, as well as the flexural stiffness and anticlastic curvature within the laminates. In order capture bolt extension and the stiffness of the clamped region, a closed form approach is used. An approximation approach is used to model flexural stiffness and anticlastic curvature within the laminates. The method is validated against detailed three-dimensional finite element models of bolted composite plates and good agreement was obtained. The method is subsequently employed to calibrate the through-thickness stiffness of single-bolt, single-lap joints in highly-efficient numerical models.     

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.     

Relaxation behaviour of gasketed joints during assembly using finite element analysis

Gasketed bolted flange pipe joints are always prone to leakage during operating conditions. Therefore, performance of a gasketed flange joint is very much dependent on the proper joint assembly with proper gasket, proper gasket seating stress and proper pre-loading in the bolts of a joint. For a gasketed flange joint, the two main concerns are the joint strength and the sealing capability. To investigate these, a detailed three-dimensional nonlinear finite element analysis of a gasketed joint is carried out using gasket as a solid plate. Bolt scatter, bolt bending and bolt relaxation are concluded as the main factors affecting the joint’s performance. In addition, the importance of proper bolt tightening sequence, number of passes influence of elastic and elasto-plastic material modelling on joint performance are also presented. A dynamic mode in a gasketed joint is concluded, which is the main reason for its failure.     

Quasi-static behaviour of composite joints with protruding-head bolts

An experimental study of the quasi-static behaviour of composite bolted joints under tensile and compressive loading has been performed. The joint plates made of quasi-isotropic and 0°-dominated lay-ups were fastened by two, four, or six titanium bolts. Two specimen configurations were tested: single lap and double lap. Strain gauges were applied in order to measure strain between the bolt rows and to calculate the load transfer of the bolt rows. Bolt-movement measurements were performed using extensometers to examine bolt behaviour under static loading. The obtained experimental results showed that bolted joints with four and six bolts perform better resistance to quasi-static loading than joints with two bolts. Strain distribution between two bolts is significantly affected by the bolt presence and their behaviour under loading. The load-transfer calculations showed that different bolt rows transfer slightly different amounts of load.     

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.     

Stress analysis of multi-bolted joints for FRP pultruded composite structures

In the last decades, FRP composites have been widely used for constructing entire civil structures. One of the challenging issues for building with pultruded FRP composites is understanding the behaviour of bolted joints. In this paper, the results of a numerical analysis performed on different types of bolted composite joints with different geometry and subjected to tensile loads are reported. The aim of this study is to examine the distribution of shear stresses among the different bolts by varying the number of rows of bolts as well as the number of bolts per row. The study also considers the presence of variable diameter washers and their influence on the bearing stresses of composites with different fibre orientations. For verification of the validity of the analytical models, numerical results are compared to experimental results reported elsewhere. The results of this study showed that in multi-bolt joints, the load is not distributed equally due to varying bolt position, bolt-hole clearance, bolt-torque or tightening of the bolt, friction between member plates and at washer-plate interface. The results also indicated that in the presence of washers, the stress distributions in the fibre direction, varying fibre inclinations, are decreasing for each value of washer pressure.     

声时差法测定螺栓的预紧应力

本文利用超声波沿轴向传播时的波速同轴向应力以及三阶弹性常数的关系，推导了声时差与应力的关系，采的时差法多探头同时测量同一法兰上的各螺栓的预紧应力，整个过程中各探头位置不变，可以克服多次耦合误差并使测量过程简便，精度高，本文还应研究了温度变化对声时差的影响，提出并且从实验数据算出几个只与材料有关，与螺栓的夹紧距离，直径以及总长我关的常数，使测量和应力计算变得简单，实验结果表明应力低于２４０ＭＰａ，夹     

Evaluation of the Clamping Force of Bolted Joints Using Local Mode Characteristics of a Bolt Head

It is important to determine and control the clamping force of a bolted joint. Due to its simple setup, the torque control method is typically used to control the clamping force when tightening bolts. After tightening, hammer tests, ultrasonic techniques and methods employing sheet materials as sensors are often used. Many methods have been proposed, but using them to determine and control the clamping force during or after tightening bolts is labor intensive or expensive. Here we conduct impact tests with an impulse hammer combined with experimental modal analysis to determine the clamping force by interpreting the change in the local mode frequency of a bolt head in the high frequency region as a function of the clamping force. To demonstrate the applicability of our method, we also investigate its limits with regard to bolt sizes.     

复合层压板单螺栓连接的刚度建模

针对复合层压板单螺栓单搭接结构,提出一种更合理的四阶段刚度模型。该模型是在传统三阶段刚度分析模型的基础上,采用部分滑移理论对模型粘滞阶段的刚度进行了改进;同时考虑到层压板之间、垫片与层压板之间的异步滑动,增加了部分滑移阶段,给出新的刚度模型的理论分析过程和使用条件。为了验证改进模型的准确性,在ABAQUS中建立了对应的三维有限元模型,同时对比了层压板单独建模和均质建模的优缺点,表明单独建模更符合实际,更精确;最后通过分析螺栓孔间隙、扭矩、铺层厚度、宽径比、液态垫片及铺层角度对复合材料螺栓连接各阶段刚度的影响。结果表明:螺栓孔间隙会导致螺杆发生二次弯曲,从而降低其接头刚度;相对于宽径比的影响,铺层厚度对螺栓接头刚度的影响更显著;不同铺层角度对螺栓接头刚度的影响机制也不同。      

The effect of bolt length in the fatigue strength of M24×3 bolt studs

In this study we evaluated the effect of bolt length on the fatigue behavior of joints with bolt studs used in the flange/housing assembly of a gas compressor for polyethylene polymerization. The bolt stud used is the M24×3 class 8.8. Two different bolt lengths were evaluated. A slim spread sleeve was used in the case of the longer bolts. The torque control was used to achieve the tightening preload. However, the preload and cyclic strains were monitored. The fatigue strength increases with increasing bolt stud length, even with the use of the slim spread sleeve. The study showed a difference between the theoretical and the experimental load factor for the case of short joints.     

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.     

带Z字形悬臂梁段拼接的装配式钢框架节点抗震性能试验研究

提出一种适用于装配式钢结构的带Z字形悬臂梁段拼接的梁柱节点。以滑移耗能的理念设计了基础试件,通过关键参数的变化得到六组试件。对六组试件进行了低周往复荷载作用下的试验研究,对节点的破坏模式和拼接区的滑移情况进行了分析,获得了节点的滞回曲线、骨架曲线、耗能能力、转动能力以及刚度退化等抗震性能。结果表明:带Z字形悬臂梁段拼接的梁柱节点属于半刚性连接节点,节点变形性能良好。节点能有效利用拼接区摩擦面滑移、螺栓和孔壁挤压以及板件屈服实现耗能。适当减少翼缘高强螺栓数目能有效提高节点延性性能和塑性转动能力。翼缘和腹板螺孔开大孔和使用三角形垂直加劲肋对节点的耗能能力和承载能力影响很小。     

Modeling of the dynamic characteristics of two‐bolt‐joints

Abstract

In this paper, a method to identify the damping and stiffness properties of bolted joints from a substructure synthesis scheme is presented. The easily measured frequency response functions of substructures and the assembled structure are the only data needed in this method. A synthesis formula used to predict the frequency response functions of two‐bolt‐joint structures is proposed. The formula makes use of the extracted properties of a single‐bolt‐joint. Some experiments with two free‐free steel beams jointed with one or two bolts are made to check this method. The close correlation between predicted and measured results demonstrates that this method is acceptable.     

一种四角钢组合约束型防屈曲支撑外围连接的设计方法研究:理论推导

在四角钢组合约束型防屈曲支撑中,螺栓是连系外围各个约束角钢的关键元件,在支撑发生侧向弯曲变形时承受拉力、剪力的共同作用。该文对单向弯曲变形状态下的外围螺栓内力计算方法展开了研究。首先介绍了内核构件与外围构件之间侧向挤压力的计算方法。然后,将该侧向挤压力分解为对称受力状态与反对称受力状态,螺栓在这两种受力状态中分别起到防止外围约束角钢发生分离的作用以及保持各角钢变形协调、共同抗弯的作用。基于这两种受力状态,考虑螺栓连接附近角钢板件局部变形所产生的撬力作用,经理论推导分别给出了螺栓内力的简化计算公式。再考虑螺栓内力的生成机制,对这两种受力状态分别计算得到的螺栓内力进行组合,得到了实际受力状态中螺栓内力的计算公式。最后,还介绍螺栓间距对螺栓内力的影响。     

八角垫高压密封结构预紧力分析和试验研究

石油、化工、核电行业法兰螺柱预紧过程中, 螺柱预紧应力计算与保证垫片应力均匀分布的预紧方法是工程实际中值得研究的问题. 该文通过对螺柱垫片系统进行物理结构分析, 受力过程分析, 提出了一种基于垫片线性变形的螺柱应力分布数学模型及预紧方法. 研究表明模型计算结果与试验结果相符合, 满足实际规律. 因此, 该数学模型能用于分析预紧过程中螺柱应力变化情况;计算出螺柱具有相同密封应力的初始载荷及只需一次加载就可以达到理论密封应力的预紧优化方法. 这使得在预紧过程中不必采用边预紧边测试应变的方法来进行指导, 更有效保证密封性能.     

节理面附近锚杆力学模型及其数值流形方法应用

建立了节理面附近锚杆的梁单元模型。该锚杆模型不仅能够准确模拟结构面附近锚杆与结构面的相互作用,而且可以综合反映锚杆的拉压、剪切、弯曲变形以及锚杆进入塑性屈服后的硬化特性。基于该锚杆模型以及数值流形方法的基本原理,发展了锚固节理岩体的流形元模拟方法。由于在数值流形方法中锚杆构件可作为物理网格输入,因此锚杆单元的布设与流形单元网格无关,从而简化了锚杆单元的前处理工作,可以实现大规模锚杆的精细化模拟。通过与加锚节理面直剪试验结果的对比分析,两者较为吻合,验证了该锚杆模型有效性。最后,将该锚杆模型运用于水布垭地下厂房围岩的支护研究,算例表明,该模型较好地反映了锚固岩体的变形行为以及锚杆的加固效果。