Failure of pultruded GRP single-bolt tension joints under hot–wet conditions

An experimental study of the effects of hot–wet conditions on the load carrying capacity of pultruded GRP (glass reinforced plastic) single-bolt tension joints has been carried out. The bolted joints were failed in tension after being immersed in water at three temperatures for two periods of time. Two joint geometries, defined in terms of end distance and width to bolt diameter, were tested with the pultrusion and tension axes coincident. The reductions in the load capacities of the joints due to the hot–wet conditions were quantified and shown to be very large. For example, it was found that more than 60% of the load carrying capacity of a single-bolt tension joint was lost after being immersed in water for 6.5 weeks at 60 °C. This temperature is lower than the manufacturer’s recommended maximum service temperature for this type of pultruded GRP material.     

Thermal preconditioning study for bolted tension joints in pultruded GRP plate

Thermocouples have been used to monitor the time versus temperature rise response at the centre of four sizes of pultruded GRP plate used in bolted tension joints in order to demonstrate that all of the GRP plate is at the required temperature prior to testing the joints to failure. A simple one-dimensional finite element (FE) model has also been developed and used to predict time versus temperature rise response. The experimental and numerical results confirm that a thermal preconditioning period of 30 min duration is more than adequate for all of the pultruded GRP plates to achieve the actual test temperatures of 40, 60 and 80 °C.     

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.     

复合材料拉挤方管型材螺栓节点承载力试验

复合材料型材是采用工业化拉挤工艺生产的截面形状一致、性能稳定的连续构件（如：方形、工字形、槽形等）,其节点连接技术是难点。重点开展了复合材料型材节点螺栓机械连接的试验研究与理论分析,研究了螺栓节点孔径、端距、壁厚等参数对复合材料型材节点极限承载力的影响规律,提出了拉挤复合材料型材螺栓孔的金属垫圈孔壁增强技术,进而拟合了拉挤型材螺栓节点连接的设计公式。研究结果表明：复合材料方管拉挤型材在螺栓连接局部挤压的破坏模式下,其极限承载力与孔径和板厚的乘积（d·t）呈线性关系,接头处的破坏形式和连接接头端距与孔径的比值相关。在挤压破坏模式下,当接头板件壁厚一定时,极限承载力的增量随着孔径的增大而减小。螺栓孔采用金属垫圈增强技术,可以大幅度提高节点承载能力（提高63%）。     

Effect of geometric parameters on the mechanical behavior of PFRP single bolted connection

In recent years, efforts have been made to use pultruded FRP (PFRP) materials in the construction industry. Although PFRP materials have numerous advantages, such as light weight, high specific strength, and better resistance to corrosion, PFRP materials are not frequently used for new civil construction due to some problems, such as the absence of a reliable reference standard and the vulnerability of the connections or joints of the PFRP member. In this paper, the results of an experimental investigation of the mechanical behavior of single-bolt PFRP connections under tension are discussed. Tests for mechanical properties of PFRP and single-bolt connection tests are conducted and reported. In the connection tests, average ultimate loads, average ultimate strengths, and failure modes for 134 connection test specimens are analyzed based on the variables such as geometric parameters.     

Analysis of the bearing response test for polymer matrix composite laminates: bearing stiffness measurement and simulation

This paper is the first part of a project that aims to investigate the mechanical and fracture behaviour of bolted joints in general purpose glass fibre-reinforced polyesters (GRP). In the present study a procedure is set up to measure the bearing stiffness of a GRP laminate in a single-bolt double lap joint. With a three-dimensional finite element model it is shown that the bolt and fixture deformations affect the stiffness results. Hence the experimental displacement data were corrected before calculating the coupon bearing stiffness. The coupon bearing stiffness was also simulated by a two-dimensional finite element model. Provided that bolt–hole clearance, material non-linearity and bolt–hole friction are taken into account, good agreement is observed with experimental data. Bearing strain and bearing stiffness are based on the bearing deformation of the coupon, not on the hole elongation. This makes the stiffness data useful for design and allows an easy installation of the displacement measurement devices.     

湿热环境对CFRP复合材料-铝合金螺栓连接结构静力失效的影响

湿热环境对碳纤维增强树脂(CFRP)复合材料-铝合金螺栓连接结构失效的显著影响给整体结构带来了安全隐患。为准确评估湿热环境对混合螺栓连接静力失效的影响，基于复合材料渐进损伤模型及金属韧性断裂准则，建立了考虑湿热效应的复合材料-金属螺栓连接静力失效预测模型。采用该模型预测了CFRP复合材料-铝合金单钉双剪连接结构在23℃干态、70℃平衡吸湿状态下的静强度和失效模式，与试验结果吻合良好，验证了模型的有效性。在此基础上，进一步揭示了不同湿热工况对CFRP复合材料-铝合金单钉双剪、多钉双剪连接结构静力拉伸失效的影响规律。结果表明:相比于23℃干态条件，23℃平衡吸湿条件、70℃干态条件和70℃平衡吸湿条件下CFRP复合材料-铝合金单钉双剪连接结构的失效载荷分别下降了4.5%、7.2%和13.9%；高温是导致湿热环境中CFRP复合材料层板损伤区域增大的主要因素；随着螺栓数目的增加，70℃平衡吸湿状态时连接结构静强度相比于23℃干态的下降幅度逐渐降低。      

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.     

Mechanical property and failure mechanism of 3D Carbon–Carbon braided composites bolted joints under unidirectional tensile loading

In this paper, the mechanical property and failure mechanism of Carbon–Carbon braided composites (C–Cs) bolted joints structure subjected to unidirectional tensile load were studied by the experimental method and numerical analysis. The braided C–Cs bolted joints with the single-bolt single-lap (SBS) and double-bolt single-lap (DBS) were tested. The dominant failure modes for both C–Cs SBS and DBS joint configurations were bearing failure and net-tension. Additionally, the finite element method (FEM) was utilized to study the mechanical property and failure mechanism of the joints. The FEM results have a good agreement with the test values. Parametrics studies were implemented by finite element (FE) analysis to classify the effects of geometric parameters including the joint width (W), edge distance (e) and the bolt pitch (p) on the SBS and DBS joint configurations. It can be found that present numerical model can be used to predict the experimental mechanical behaviors and failure modes of bolted C–Cs joints with different geometric parameters and joint configurations.     

复合材料多钉连接钉载矢量传感器

在总结复合材料多钉连接钉载测试方法的基础上,针对单向载荷传感器应用中必须已知钉载方向且需要按照钉载方向装设传感器的缺点,提出了钉载矢量传感器方案。钉载矢量传感器无预设安装角度要求,能够测量载荷方向;并对钉载矢量传感器的选材、应变片方案及开槽尺寸进行了设计,制造了钉载矢量传感器。采用复合材料单钉单剪螺栓连接,分别对钉载矢量传感器测量结果的线性符合性、重复性、安装角度影响以及载荷方向测量能力进行了测试研究。测试结果表明:钉载矢量传感器满足工程上对载荷分配测试的技术要求,验证了本文所提出的钉载矢量传感器的适用性。     

Analysis of Bolted Flanged Panel Joint for GRP Sectional Tanks

The performance of flanged panel bolted joints used in Glass Reinforced Plastic (GRP) sectional tanks is investigated using a combination of experimental and computational methods. A four-panel bolted assembly is subjected to varying pressure in a rupture test rig to study damage development at the intersection of the four panels. It is found that cracking initiates at a panel corner at the four panel intersection at a pressure of 35 kPa and propagates to other panel corners with increasing pressure. This is attributed to the excessive deformation at the four panel intersection. The effect of bolt spacing, varying end distances and bolt pre-tension in decreasing the localized deformation and maximum induced stresses are investigated using finite element analysis. It is found that varying the amount of bolt spacing and end distances had a considerable influence on the joint performance whereas varying bolt pretension had very negligible effect. Consequently, this study establishes the maximum pressure which the GRP panel joint can withstand without failure and the corresponding optimum joint parameters.     

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.     

Progressive damage and nonlinear analysis of pultruded composite structures

This study combines a simple damage modeling approach with micromechanical models for the progressive damage analysis of pultruded composite materials and structures. Two micromodels are used to generate the nonlinear effective response of a pultruded composite system made up from two alternating layers reinforced with roving and continuous filaments mat (CFM). The layers have E-glass fiber and vinylester matrix constituents. The proposed constitutive and damage framework is integrated within a finite element (FE) code for a general nonlinear analysis of pultruded composite structures using layered shell or plate elements. The micromechanical models are implemented at the through-thickness Gaussian integration points of the pultruded cross-section. A layer-wise damage analysis approach is proposed. The Tsai–Wu failure criterion is calibrated separately for the CFM and roving layers using ultimate stress values from off-axis pultruded coupons under uniaxial loading. Once a failure is detected in one of the layers, the micromodel of that layer is no longer used. Instead, an elastic degrading material model is activated for the failed layer to simulate the post-ultimate response. Damage variables for in-plane modes of failure are considered in the effective anisotropic strain energy density of the layer. The degraded secant stiffness is used in the FE analysis. Examples of progressive damage analysis are carried out for notched plates under compression and tension, and a single-bolted connection under tension. Good agreement is shown when comparing the experimental results and the FE models that incorporate the combined micromechanical and damage models.     

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.     

Static and high-rate loading of single and multi-bolt carbon–epoxy aircraft fuselage joints

Single-lap shear behaviour of carbon–epoxy composite bolted aircraft fuselage joints at quasi-static and dynamic (5 m/s and 10 m/s) loading speeds is studied experimentally. Single and multi-bolt joints with countersunk fasteners were tested. The initial joint failure mode was bearing, while final failure was either due to fastener pull-through or fastener fracture at a thread. Much less hole bearing damage, and hence energy absorption, occurred when the fastener(s) fractured at a thread, which occurred most frequently in thick joints and in quasi-static tests. Fastener failure thus requires special consideration in designing crashworthy fastened composite structures; if it can be delayed, energy absorption is greater. A correlation between energy absorption in multi-bolt and single-bolt joint tests indicates potential to downsize future test programmes. Tapering a thin fuselage panel layup to a thicker layup at the countersunk hole proved highly effective in achieving satisfactory joint strength and energy absorption.     

Progressive failure analysis of bolted single-lap composite joint based on extended finite element method

In this paper, extend finite element method (XFEM) is used to predict the failure of single-lap bolted joints. To simplify calculation of XFEM model, composite laminates of joint are modeled using linear elastic properties. Three-dimensional equivalent material properties are calculated by the MATLAB code written by author. Progressive failure of bolted single-lap composite joint is investigated, and the failure load of joint simulated by XFEM is compared with experiments in literature. Then the influences of geometric parameters on failure load of one bolt single-lap composite joint are studied. Two geometric parameters include plate width-to-hole diameter ratio (W/D) and the edge-to-hole diameter ratio (E/D). At last the failure of single-lap joints with one bolt and two bolts are compared.     

Numerical Investigation of Dynamically Loaded Bolted Joints in Carbon Fibre Composite Structures

This paper reports on recent experimental work to investigate the response of bolted carbon fibre composite joints and structures when subjected to constant dynamic loading rates between 0.1 m/s and 10 m/s. Single fastener joints were tested in both the bearing (shear) and pull-through (normal) loading directions. It was found that the joints exhibited only minor loading rate dependence when loaded in the pull-through direction but there was a significant change in failure mode when the joints were loaded in bearing at or above 1 m/s. Below 1 m/s loading rate the failure mode consisted of initial bolt bearing followed by bolt failure. At a loading rate of 1 m/s and above the bolt failed in a ‘tearing’ mode that absorbed significantly more energy than the low rate tests. A simple composite structure was created to investigate the effect of loading rate on a more complex joint arrangement. The structure was loaded in two different modes and at constant dynamic loading rates between 0.1 m/s and 10 m/s. For the structure investigated and the loading modes considered, only minor loading rate effects were observed, even when the dominant contribution to joint loads came from bearing. It was observed that the load realignment present in the structural tests allowed the joints to fail in a mode that was not bearing dominant, and hence the loading rate sensitivity was not expressed.     

Experimental and computed natural frequencies of square pultruded GRP plates: effects of anisotropy, hole size ratio and edge support conditions

Experiments have been carried out to determine the free vibration frequencies and mode shapes of 3.2 mm thick, pultruded GRP, square plates with six combinations of clamped (C), simply supported (S) and free (F) edge supports. Comparison of experimental and theoretical/numerical frequencies confirms that thin homogeneous orthotropic/anisotropic plate theory provides a reasonable model for predicting the free vibration response of pultruded GRP plates. Additional vibration experiments were carried out on plates with central circular cutouts. The hole size ratios were varied from about 0.1 to 0.4 for three combinations of clamped (C) and simply supported (S) edge conditions. Finite-element (FE) frequency and mode shape predictions based on orthotropic plate theory were again shown to be in reasonable agreement with the experimental frequencies and modes.     

Experimental Investigation of Dynamically Loaded Bolted Joints in Carbon Fibre Composite Structures

This paper reports on recent experimental work to investigate the response of bolted carbon fibre composite joints and structures when subjected to constant dynamic loading rates between 0.1 m/s and 10 m/s. Single fastener joints were tested in both the bearing (shear) and pull-through (normal) loading directions. It was found that the joints exhibited only minor loading rate dependence when loaded in the pull-through direction but there was a significant change in failure mode when the joints were loaded in bearing at or above 1 m/s. Below 1 m/s loading rate the failure mode consisted of initial bolt bearing followed by bolt failure. At a loading rate of 1 m/s and above the bolt failed in a ‘tearing’ mode that absorbed significantly more energy than the low rate tests. A simple composite structure was created to investigate the effect of loading rate on a more complex joint arrangement. The structure was loaded in two different modes and at constant dynamic loading rates between 0.1 m/s and 10 m/s. For the structure investigated and the loading modes considered, only minor loading rate effects were observed, even when the dominant contribution to joint loads came from bearing. It was observed that the load realignment present in the structural tests allowed the joints to fail in a mode that was not bearing dominant, and hence the loading rate sensitivity was not expressed.     

一种新型装配式梁柱节点抗震性能试验研究

提出一种新型装配式全栓接的方钢管柱H型梁梁柱节点。为研究节点的抗震性能,对6个1:2试件进行了拟静力试验。分析轴压比、抗弯、抗剪螺栓预拉力、槽形钢厚度、狗骨式连接等参数对节点的破坏模式、滞回性能、延性的影响。研究结果表明:在0.2~0.4范围内提高轴压比,节点的极限承载力略有降低,但耗能能力和延性均有提高;降低抗剪螺栓的预拉力,节点的极限承载力,耗能能力与延性均有降低;降低槽形钢的板厚,节点承载力微有提高,节点梁柱相对转角提高,但耗能能力降低;降低抗弯螺栓预拉力,节点的极限承载力提升,但耗能能力与延性均有降低;采用狗骨式连接,虽然梁翼缘削弱部位在加载后期出现撕裂,但节点表现出良好的延性和耗能能力以及稳定的刚度退化性能。节点层间位移延性系数μ=2~2.66,弹性层间位移角φ_y=0.0208~0.0327,弹塑性层间位移角φ_u=0.0486~0.079,梁柱相对极限塑性转角θ_u=0.05~0.087。极限荷载时等效粘滞阻尼系数h_e=0.287~0.45,试验结果表明节点具有良好的抗震性能。