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 clamping force on the delamination onset and growth in bolted composite laminates

Clamping force is a key element that alters the mechanism and sequence of failure in bolted joints of composite laminates. The mode of failure in bolted joints can be controlled by geometrical parameters and the preferred fail safe mode of failure is ‘bearing’ which generally consists of matrix cracks, delamination and fibre microbuckling. Three-dimensional (3-D) pinned (without clamping force) and bolted (1 kN clamping force) joint models were developed in [0/90]_s carbon fibre reinforced plastic (CFRP) laminates to show the clamping force effect on the onset and growth of delamination. It is shown that delamination was resulted from the shear stress components (Mode II & III) at the interface and the contribution of the out-of-plane component (Mode I - opening), so the clamping force, was negligible without modelling the in-plane failure modes and their coupling with delamination, which will be considered in future work.     

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.     

复合材料连续损伤力学模型在螺栓接头渐进失效预测中的应用

提出考虑层合板面内(纤维和基体失效)和层间失效的复合材料连续损伤力学模型,对螺栓接头的渐进失效行为进行预测。基于Tsai-Wu强度准则,发展可以判定复合材料面内和层间失效的强度准则。采用幂指数衰减材料退化模型模拟复合材料的损伤扩展过程。建立连续损伤力学模型用以研究0°铺层比例和螺栓直径对复合材料螺栓接头挤压性能的影响,预测结果与实验结果吻合。结果表明:0°铺层比例过高,接头发生剪切破坏,降低连接结构承载能力;增大螺栓直径,层合板损伤受到抑制,可提高复合材料螺栓接头的挤压强度。      

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.     

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%）。     

Experimental investigation of damage progression and strength of countersunk composite joints

An experimental investigation is conducted into the damage progression and strength of bolted joints with fibre-reinforced composite laminates and countersunk fasteners. The main goal of the experimental investigation is to characterise the effect of the countersink geometry on the load-carrying capacity of single lap joints in comparison to the straight-shank case. The effects of bolt torque, clearance and countersink height ratio on the damage progression and joint strength are also studied. Experimental tests and detailed microscopy studies are conducted on a bearing test specimen with a straight-edged hole, and several single-lap joint configurations with countersunk fasteners. It is found that introduction of the countersunk hole roughly halves the bearing stress, and causes delamination for some configurations. This delamination is primarily located at the start of the countersink region, though is found to be triggered by other damage mechanisms and has only minor influence on the results. Bolt torque increases the density of through-thickness damage though limits its extension from the hole edge, whilst bolt clearance causes localisation of the damage region. Increasing the ratio of the countersink depth to the laminate thickness reduces the extent of bearing and promotes bending, with a change to net section failure at large ratios.     

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.     

Failure of carbon composite-to-aluminum joints with combined mechanical fastening and adhesive bonding

Composite-to-aluminum double lap joints were tested to obtain the failure loads and modes for three types of joints: adhesive bonding, bolt fastening and adhesive-bolt hybrid joining. A film type adhesive FM73 and a paste type adhesive EA9394S were used for aluminum and composite bonding. A digital microscope camcorder was used to monitor the failure of the joints. It was found that hybrid joining improves joint strength when the mechanical fastening is stronger than the bonding, as when the paste type adhesive is used. On the other hand, when the strength of the bolted joint is lower than that of the bonded joint, as when the film type adhesive is used, bolt joining contributes little to the strength of the hybrid joint.     

Numerical experimental analysis of hybrid double lap aluminum-CFRP joints

Due to their reliability and ease of assembly, both the adhesively bonded and the mechanical joints are commonly used in different fields of modern industrial design and manufacturing, to joint composite materials or composites with metals.As it is well known, adhesively bonded joints are characterized by high stiffness and good fatigue life, although delamination phenomena localized near the free edges may limit their use, especially for applications where corrosive environments and/or moisture can lead to premature failure of the bonding. In these cases, a possible alternative is given by the well-known mechanical joints. On the contrary, these last joints (bolted, riveted) require a preliminary drilling of the elements to be joined, that may cause localized material damage and stress concentration, especially for anisotropic laminates characterized by high stress concentration factors and easy drilling damaging, with significant decrease of the load-carrying capacity of the joined elements. In order to exploit the advantages of the bonded joints and those of the mechanical joints, both industrial manufacturing and research activity have been focused recently on the so called hybrid joints, obtained by the superposition of a mechanical joint to a simple adhesively bonded joint.In order to give a contribution to the knowledge of the mechanical behavior of hybrid bonded/riveted joints, in the present work a numerical–experimental study of bonded/riveted double-lap joints between aluminum and carbon fiber reinforced polymer (CFRP) laminates, has been carried out. It has permitted to highlight both the static and the fatigue performance of such joints obtained by using aluminum and steel rivets, as well as to known the particular damage mechanisms related also to the premature localized delamination of the CFRP laminate due to the riveting process.     

接头局部增强的复合材料层合板螺栓连接试验与数值模拟

采用真空辅助成型工艺（VARI）制备了四种局部增强的复合材料层合板螺栓连接试件，通过试验及数值模拟对其力学性能进行了研究。数值研究中将复合材料层合板连接件的拉伸作为一个准静态问题，运用ABAQUS的显示分析算法及所编写用户材料子程序VUMAT对连接件进行了三维渐进失效模拟，同时在有限元模型中采用内聚力单元模拟了层合板与所设增强层的界面分层失效。数值计算结果与试验结果取得了较好的一致，验证了本文中数值方法的有效性。研究结果表明，不同的局部增强方案对复合材料螺栓连接性能的影响较大，设置[0/90/0/90]_S铺层的内置纤维增强层能显著提高层合板的螺栓连接性能。     

碳纤维增强环氧树脂复合材料螺栓连接结构在拉伸载荷下损伤过程的声发射分析

采用声发射技术对不同几何尺寸的碳纤维增强环氧树脂复合材料（CFRP）螺栓连接结构在静力载荷下破坏行为进行了试验研究,比较了不同几何构型下的连接结构的破坏行为与声发射信号之间的映射关系。采用声发射技术对结构损伤过程中的声发射信号进行全程采集与转换,结合CFRP螺栓结构的载荷-位移曲线和宏/细观破坏形貌,分析了幅值、熵曲线和Andrews曲线与破坏行为之间的关系。结果表明:挤压与剪切破坏试件的载荷-位移曲线均呈现出较明显的塑性特征。结构发生挤压和剪切破坏时,声发射信号以中幅值信号为主,并伴随少量高幅值信号;结构发生拉伸破坏时对应的幅值为中幅值信号。根据熵曲线特征将CFRP连接结构破坏过程分为四个阶段,在损伤演化阶段发生纤维断裂、分层等失效模式,在结构失效阶段以分层失效为主。基于Andrews曲线分析得到挤压和拉伸失效模式在损伤演化阶段会出现多种损伤类型,剪切失效模式在结构失效阶段会出现多种损伤类型。      

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.     

结合改进单胞模型的单钉双剪层合板螺栓连接结构挤压性能的多尺度表征分析

为提高螺栓连接层合板结构的可靠性和承载能力,基于ABAQUS软件及用户子程序(USDFLD),结合改进的单胞模型,建立了考虑组分材料失效的多尺度数值模型。利用该模型表征分析了单钉双剪层合板螺栓连接结构的力学性能,研究了铺层形式及几何尺寸对连接结构性能的影响。该模型的预报结果与试验结果吻合较好。结果表明:准各向同性层合板螺栓连接结构的挤压强度高于正交各向异性层合板连接结构的挤压强度,前者的失效模式为挤压失效,后者为剪出失效,该模式导致结构承载能力降低,设计中应避免。层合板边径比大于3时,不同宽径比连接结构的挤压强度趋近稳定值;但相同边径比的连接结构,其挤压强度随宽径比的增大而增大,连接结构设计时应给予考虑。      

Damage mode characterization of mechanically fastened composite joints using carbon nanotube networks

The practice of upgrading metal parts with composites in large structures has led to an increased use of composite joints, particularly mechanical fastenings, due to ease of assembly and replacement. A drawback of mechanical joints is that damage is difficult to detect visually. In this research, an embedded carbon nanotube network has been used to modify the conductivity of bolted composite joints. In situ electrical resistance measurements in conductive composites have potential to provide quantitative evidence of damage as well as insight into the type of damage which occurs during tensile loading. We demonstrate that the electrical resistance of a bolted composite joint is more sensitive to certain modes of damage (e.g., matrix cracking and delamination) than others (bearing and shear-out), due mostly to the varying amount of void space created, thus proving the potential of an embedded carbon nanotube network in the health monitoring of mechanically fastened cross-ply composites.     

Experimental and numerical investigation on bolted joint in glass–fiber reinforced composites

In this paper, vacuum assisted resin injection technique was employed to prepare the composite laminates with different plies ways. The macro-mechanical performances of the bolted joint of the composite laminates were investigated by experimental and finite element simulation. The influence mechanism of different parameters on the joint performance and failure models of the composite laminates were analyzed. A VUMAT subroutine was developed, according to the failure criteria and corresponding stiffness degradation criteria, to describe the progressive damage process of bolted joint composite laminates through finite element analysis method. The results indicated that the failure strength of composite laminates would be improved with the increasing of tightening torque, and the strength of three-ply-way hybrid composite laminates is higher than that of two-way hybrid laminates. The comparisons between numerical simulation results and experimental results showed that the developed subroutine can effectively predict the macroscopic response of bolted joint glass–fiber reinforced composite laminates.     

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.     

基于连续损伤力学的复合材料单剪螺栓连接强度估算策略

针对中国缺少T800级复合材料螺栓连接设计参数的问题,发展一种综合连续损伤力学（CDM）和工程算法的单剪连接强度估算策略,以替代试验,降低研究周期和成本。在该强度估算策略中,首先建立试件的CDM有限元模型,通过数值模拟得到单剪螺栓连接的设计参数,包括单剪挤压强度修正系数、无缺口层合板拉伸强度和应力集中减缓因子等。随后根据上述参数,建立工程算法,估算复合材料单剪螺栓连接的最终挤压强度。结果表明:通过该策略得到的T800级复合材料螺栓连接设计参数和强度估算结果与试验结果有较好的一致性,说明该强度估算策略的可行性。      

CFRP复合材料螺栓连接失效载荷不确定性的评估方法

螺栓连接是先进复合材料结构的薄弱环节.因此,螺栓连接力学性能显著的不确定性不仅阻碍了先进复合材料的高效应用,且给整体结构的安全性和可靠性带来威胁.为定量评估碳纤维增强树脂(CFRP)复合材料螺栓连接失效载荷的不确定性,将数值的渐进损伤模型和区间分析方法结合,提出了一种高效、准确的分析方法.采用该方法预测了典型T800碳...     

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.