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.     

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.     

A Stacked-Shell Finite Element Approach for Modelling a Dynamically Loaded Composite Bolted Joint Under in-Plane Bearing Loads

This paper presents the results of a study into a novel application of the “stacked-shell” laminate modelling approach to dynamically loaded bolted composite joints using the explicit finite element code PAM-CRASH. The stacked-shell approach provides medium-high fidelity resolution of the key joint failure modes, but is computationally much more efficient than full 3D modelling. For this work, a countersunk bolt in a composite laminate under in-plane bearing loading was considered. The models were able to predict the onset of damage, failure modes and the ultimate load of the joint. It was determined that improved debris models are required in order to accurately capture the progressive bearing damage after the onset of joint failure.     

屈曲约束支撑钢筋混凝土框架结构干式柔性梁柱节点的试验研究

建议了一种由受弯框架与采用梁柱柔性节点的屈曲约束支撑框架并联组成的装配式混凝土双重抗侧力体系,设计并完成了4个装配式梁柱节点试件和1个现浇节点试件的往复加载试验。研究了混凝土楼板、螺栓孔形式等因素对该柔性节点受力性能和破坏模态的影响,并与现浇试件进行了对比。试验结果表明:混凝土楼板对柔性节点刚度及承载力有很大的影响,在设计时不可忽略;螺栓孔形式在结构位移较小时对节点的受力性能没有影响,但当结构位移较大时会影响节点的破坏模式;与现浇节点相比,装配式柔性节点具有很大的变形能力。     

Single-bolt tension joint tests on pultruded GRP plate - effects of tension direction relative to pultrusion direction

The literature on single-bolt tension joints in structural grade pultruded GRP plate is reviewed and the differences between the main investigations are highlighted. Details are given of the test setup and the joint configurations used in a series 54 such tests on single-bolt joints in which the angle between the pultrusion and tension axes (the off-axis angle) and the joint geometry are varied. Ultimate strength, intial stiffness, initial bolt slip and bolt displacement at failure data are presented as functions of the joints' principal geometric ratios. The observed joint failure modes show that, for off-axis angles ≥ 30 °, bearing failure (a relatively benign failure mode) does not arise. Instead, tension mode failure predominates and cracks tend to propagate parallel to the rovings diagonally across the width of the joint. It is concluded that the rovings play a significant role in controlling the crack propagation and this has implications for joint layout and design in pultruded GRP plate.     

A study of fastener pull-through failure of composite laminates. Part 1: Experimental

Of the extensive number of investigations examining mechanically fastened composite joints, all but a few are limited to consideration of in-plane modes of failure. An experimental and numerical investigation of fastener pull-through failure of composite joints has therefore been undertaken. The experimental program included an investigation of the influence fastener head geometry, laminate thickness, stacking sequence and material system have upon the pull-through loading response. Circular specimens were transversely loaded to pre-determined displacements of the fastener, sections through the specimen taken and their failure mechanisms investigated with an optical microscope. Pull-through failure was found to be characterised by substantial internal damage similar to that observed for low-velocity impacted composite panels. Failure is not evident from inspection of the laminate surfaces. Damage is manifested in the form of a conically distributed network of matrix cracking and delaminations extending through-the-thickness from the fastener head outer edge, directed away from the fastener hole. The internal/barely visible nature of failure represents a significant departure from that generally considered to distinguish fastener pull-through failure. The means by which to increase resistance to pull-through failure are discussed. This research constitutes work performed as part of the Cooperative Research Centre for Advanced Composite Structures (CRC-ACS) task on highly loaded joints.     

AA5052薄板磁脉冲胶焊复合接头动态力学性能研究

目的 研究AA5052铝合金薄板在高速冲击载荷下的磁脉冲胶焊复合接头的动态力学性能,探究不同载荷速率对该胶焊复合接头力学和失效行为的影响规律.方法 利用磁脉冲焊接系统成功制备了胶焊复合连接试件.采用万能拉伸试验机、高速拉伸试验系统,结合全场应变测量系统,获得胶焊复合接头的力学性能规律,以及渐进失效过程和搭接区应变变化....     

Static and dynamic failure behaviour of bolted joints in carbon fibre composites

An experimental test series of mechanically fastened bolted joints with countersunk head in quasi-isotropic carbon/epoxy composite laminates under quasi-static and dynamic loads with velocities up to 10 m/s has been conducted in order to investigate potential strain rate effects on the failure behaviour. The test campaign covered bolt pull-through tests, single lap shear tests with one and two bolts and coach peel tests. Identical test equipment has been used for the whole range of test velocities to avoid influences of different test machines. No rate sensitivity occurred for most test configurations. Only the single lap shear tests with two bolts showed a change of failure mode at the highest test velocity enabling higher energy absorption.     

Behaviors of a cracked lapped joint under mixed mode loading

Fatigue failure of steel connections is a common failure mechanism, especially for structures which sustain heavy cyclic loads like steel bridges. In this paper, lapped bolted joints were modeled numerically to study the effect of a crack on the ultimate response of the joint. The site of crack initiation was located under different mixed mode loading in single and multiple bolts joints. The effects of axial to transverse loading ratio or load mixity (LM = F_x/F_y), friction coefficient (μ), and bolt diameter were analyzed. For a single bolt pin-joint, by increasing LM, the crack initiation site angle (γ) increased up to a certain value at which it became constant (γ_f) independent of LM. This value γ_f depended only on the coefficient of friction and the bolt hole diameter. Stress intensity factor and crack path of a propagated crack emanating from the predicted crack initiation site were analyzed in the lapped joints under either mode I or mixed mode loading. It was found that, for multiple-bolt joints, loaded with mixed mode loading, the crack path remained approximately horizontal like that for mode I loading. For pin joints, the crack path remained at the direction of the crack initiation. The numerical model developed was validated using existing experimental results for the initial stiffness of the bolted joint and using theoretical prediction of the stress intensity factor. A parametric study for different bolt diameters and numbers was developed to study the behavior of these connections under double and single side cracks. It was found that the crack advancement in a specific bolt hole may cause crack to initiate in other bolt holes, due to the increase of the stress concentration factor (SCF), K_t.     

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。     

Investigating the Potential of Using Off-Axis 3D Woven Composites in Composite Joints’ Applications

The effect of circular notch has been evaluated for three different architectures of three-dimensional (3D) carbon fibre woven composites (orthogonal, ORT; layer-to-layer, LTL; angle interlock, AI) through open-hole quasi-static tension and double-lap bearing strength tests in the off-axis (45°) direction. Damage characterisation is monitored using Digital Image correlation (DIC) for open-hole testing and X-ray Computed Tomography (CT) for double-lap bearing strength test. The off-axis notched 3D woven composites exhibits minor reduction (less than 10 %) of the notched strength compared to the un-notched strength. DIC strain contour clearly show stress/strain localisation regions around the hole periphery and stress/strain redistribution away from the whole due to the z-binder existence, especially for ORT architecture. Up to 50 % bearing strain, no significant difference in the bearing stress/bearing strain response is observed. However when ORT architecture was loaded up to failure, it demonstrates higher strain to failure (~140 %) followed by AI (~105 %) and lastly LTL (~85 %). X-ray CT scans reveal the effect of the z-binder architecture on damage evolution and delamination resistance. The study suggests that off-axis loaded 3D woven composites, especially ORT architecture, has a great potential of overcoming the current challenges facing composite laminates when used in composite joints’ applications.     

Experimental study on clamping effects on the tensile strength of composite plates with a bolt-filled hole

An experimental study was performed to assess the effects of clamp-up on the net-tension failure of laminated composite plates with bolt-filled holes. Graphite/epoxy prepreg of T800/3900-2 was selected for fabricating the laminates for the tests. The tensile strength and failure response of specimens with an open hole and a bolt-filled hole were evaluated. Both 100% bypass load (no bolt bearing load) and no bypass load (100% bolt bearing load) were considered during the experiments. X-radiographs were taken for specimens after pre-loading at different stress levels for the purpose of characterizing the failure modes and damage progression inside the composite.Experimental results showed that the bolt clamping force can significantly reduce the notch tensile strength of composite laminates which are prone to fiber-matrix splitting and delamination. A reduction in failure load of up to 20% was observed. Higher clamping pressure resulted in higher reductions of notch strength. However, for bolted joints which failed in a net-tension mode, clamping improved the joint strength regardless of the ply orientation.     

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.     

平板支座抗震性能的有限元研究

为了研究水平荷载下不同砂浆层厚度的平板支座节点的抗震性能及受力机理,该文利用有限元软件ABAQUS,模拟分析了8个砂浆层厚度为0 mm~50 mm的平板支座节点在单调和循环荷载下的破坏模式及承载力曲线,并将模型破坏模式及极限承载力计算值与试验结果进行对比,结果证明该模型可有效对平板支座的滞回性能进行计算分析。有限元计算结果表明,随着砂浆层厚度增加,支座节点极限承载力降低,支座锚栓由剪切变形变为弯曲变形;支座锚栓近似均匀受力;循环加载下平板支座的承载力相比于单调加载降低约30%。在有限元参数分析的基础上,综合考虑了支座锚栓的破坏机制及支座锚栓的变形对平板支座水平承载力的影响,总结平板支座的水平承载力主要由支座锚栓承担的剪力、支座锚栓轴力的水平分量及摩擦力组成,提出了平板支座节点水平承载力的计算公式。同时,在公式中引入折减系数0.7,考虑循环加载对平板支座节点水平承载力的降低。     

A novel predictive model for mechanical behavior of single-lap GFRP composite bolted joint under static and dynamic loading

Mechanical connection of composite is critical due to its complicated meso-structure and failure mode, which has become a bottleneck on reliability of composite material and structure. Although many researches on composite bolted joints have been carried out, the theory and experiment on mechanical behavior of such a joint structure under dynamic loading were rarely reported. Here, we propose a novel predictive model for quasi-static and dynamic stiffness of composite bolted joint by introducing the strain rate dependent elastic modulus into the mass spring model. Combined with the composite laminate theory and Tsai-Hill theory, the present model was capable of predicting the strain rate dependent stiffness and strength of the composite bolted joint. Quasi-static and impact loading experiments were carried out by Zwick universal hydraulic testing machine and split Hopkinson tension bar, respectively. The stiffness and strength predicted by our model showed good accordance with the experiment data with errors below 12% under quasi-static loading and below 30% under impact loading. The results indicated that under impact loading, stiffness and strength of the composite bolted joint were significantly higher than their quasi-static counterparts, while the failure mode of the joint structure trended towards localization which was mainly bearing failure. Among various lay-up ratios studied, the optimal lay-up ratio for quasi-static and dynamic stiffness was 0:±45:90 = 3:1:1.     

碳纤维/环氧树脂复合材料L型接头拉伸失效机制

设计了单L型(LS)及双L型(LD)两种重量相近的L型接头。采用试验与数值模拟相结合的方式对两种接头的拉伸失效机制进行了研究。通过自行设计的试验夹具在伺服液压试验机上将两种L型接头准静态加载至破坏,分析其破坏机制及应变分布。研究发现,两种L型接头存在不同的失效机制,在破坏阶段单L型接头表现出更好的延展性。单L型接头加载至峰值载荷时,在靠近加载侧的内侧螺栓孔附近首先出现破坏,随后损伤向外侧螺栓孔附近扩展,直至完全失效。双L型接头加载至峰值载荷的50%左右时,L型框体和L型片之间的胶膜首先发生破坏,随后载荷继续增加至峰值载荷时,L型框螺栓孔附近发生破坏,损伤向框体边缘扩展,载荷大幅下降。此外,两种接头的应变随载荷的增加存在不同的变化趋势。采用一种新型复合材料初始失效准则及刚度折减方法,编写用户自定义子程序(UMAT),结合内聚区模型建立复合材料L型接头的渐进损伤模型。基于ABAQUS软件进行计算,得到接头的预测失效载荷及破坏形式。结果表明：有限元分析所得复合材料L型接头的损伤位置及失效模式与试验吻合,预测载荷与试验值相差较小,证明了有限元模型的适用性。     

Experimental investigation of bond characteristics between CFRP fabrics and steel plate joints under impact tensile loads

In recent years, the use of adhesively-bonded fibre-reinforced composite materials has attracted widespread attention as a viable alternative for the retrofitting of civil infrastructure such as buildings and bridges. This has been particularly the case for concrete structures. The retrofitting of metallic bridges and buildings with FRP materials, however, is still in its early stages. In real life, these structures are subjected to dynamic loads. Therefore, it is necessary to understand the bond behaviour between steel and the strengthening materials for both static and dynamic loads. To examine the bond between steel plates and carbon fibre-reinforced polymers (CFRP) fabrics, this paper describes the experimental procedures and results of double strap steel joints loaded at different loading rates (2 mm/min, 3.35, 4.43 and 5 m/s). In this test program, ultimate load-carrying capacity, effective bond length, failure mechanism and strain distribution were examined for all loading rates. Different numbers of CFRP layers with different bond lengths were investigated. Experimental findings reveal that the maximum improvement in joint capacity occurs at a rate of 3.35 m/s. It was observed that the effective bond length is insensitive to loading rate for both joints. The failure modes and strain distributions, however, exhibit little difference between static and dynamic conditions.     

复合材料中厚板沉头连接结构强度与损伤失效

针对复合材料沉头螺栓连接结构的强度与损伤问题,开展了两种厚度复合材料层合板凸头与沉头螺栓连接结构挤压强度对比试验研究。试验结果表明,增加层合板厚度会引起连接结构挤压强度下降,但沉头连接结构下降比例小于凸头连接结构。通过数值模拟方法对复合材料中厚板沉头连接结构的强度及损伤失效进行分析。提出一种非线性面内连续损伤与三维混合失效模型,模型考虑了复合材料基体剪切非线性特征并改进了纤维损伤失效判据,有效解决了数值模拟中沉头复合材料连接结构难于收敛的问题。对比分析表明:沉头连接结构的数值计算结果与试验结果吻合良好,极限强度最大计算误差8.62%。     

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%）。     

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.