Failure Analysis of Bolted Joints in Cross-ply Composite Laminates Using Cohesive Zone Elements

A strength prediction method is presented for double-lap single fastener bolted joints of cross-ply carbon fibre reinforced plastic (CFRP) composite laminates using cohesive zone elements (CZEs). Three-dimensional finite element models were developed and CZEs were inserted into subcritical damage planes identified from X-ray radiographs. The method makes a compromise between the experimental correlation factors (dependant on lay-up, stacking sequence and joint geometry) and three material properties (fracture energy, interlaminar strength and nonlinear shear stress-strain response). Strength of the joints was determined from the predicted load-displacement curves considering sub-laminate and plylevel scaling effects. The predictions are in a reasonable agreement with the experimental data.     

Influence of stamping on the compressive behavior and the damage mechanisms of C/PEEK laminates bolted joints under severe conditions

Bolted joint tests have been performed in order to evaluate the influence of stamping on the behavior of thermoplastic-based woven-ply laminates subjected to structural loadings under severe service conditions (120 °C after hygrothermal aging). Compressive tests have been carried out on carbon fabrics reinforced PolyEtherEtherKetone (PEEK) laminates to investigate fibers buckling due to changes induced by stamping on the non-planar interply structure of woven-ply laminates. As compressive strength decreases by 13% in stamped laminates, it facilitates the plastic buckling of 0° and ±45° oriented fibers due to compressive loads in bolted joints. Contrary to double-lap joints, stamping does not affect the strength of single-lap joints, as the geometry of single-lap joints is non-symmetric. Stamping modifies the damage mechanisms of PEEK-based laminates under bolt-bearing loadings, such as the failure of stamped bolted joints is dominated by bearing failure mode.     

Hybrid titanium–CFRP laminates for high-performance bolted joints

This paper presents an experimental and numerical investigation of the mechanical response of bolted joints manufactured using new hybrid composite laminates based on the substitution of CFRP plies with titanium plies. The local hybridization of the material is proposed to increase the efficiency of the bolted joints in CFRP structures. Two modeling strategies, based on non-linear finite element methods, are proposed for the analysis of the bolt-bearing and transition regions of the hybrid laminates. The bolt-bearing region is simulated using a three-dimensional finite element model that predicts ply failure mechanisms, whereas the free-edge of the transition region is simulated using plane stress and cohesive elements. The numerical and experimental results indicate that the use of hybrid composites can drastically increase the strength of CFRP bolted joints and therefore increase the efficiency of this type of connection. In addition, the numerical models proposed are able to predict the failure mechanisms and the strength of hybrid composite laminates with a good accuracy.     

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.     

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.     

Evaluation of impact damage mechanism of multi-axial stitched CFRP laminate

This study focuses on multi-axial stitched fabric, which is a thick, high performance reinforcement for large-scale composite structures. The effects of impact damage on multi-axial stitched CFRP laminates molded by vacuum-assisted resin transfer molding (VARTM) method were evaluated. Impact damage within material was evaluated by ultrasonic scanning device and optical cross-sectional observations. Probed images obtained by both non-destructive and destructive methods were compared, and internal damage distributions of multi-axial stitched CFRP laminates were clarified. In addition, residual compressive strength and fatigue property of impact-damaged CFRP laminates were evaluated by in situ damage growth monitoring using the thermo-elastic stress analyzer (TESA). Three-dimensional damage distribution of impacted CFRP laminate was obtained from ultrasonic C-scan images and cross-sectional photographs. Damage progress behavior was observed on a destructive and non-destructive basis by post-impact fatigue (PIF) test.     

Instantaneous mechanical fastening of quasi-isotropic CFRP laminates by a self-piercing rivet

A modified self-piercing rivet (SPR) has been proposed to mechanically fasten CFRP laminates. The modified SPR consists of a rivet body and two flat washers. The two flat washers were used to suppress delamination in the CFRP laminates at the point of piercing. The advantages of the modified SPR for fastening CFRP laminates are instantaneous process time and low cost. Any pretreatments such as surface treatments or hole drilling are not required. In this study, the viability of the modified SPR for a quasi-isotropic CFRP laminate was investigated by tensile and fatigue tests on the single lap joints. The experimental results showed that the tensile strength of a modified SPR joint was slightly higher than a bolted joint. In tension–tension fatigue tests, a fatigue limit at N_f = 10⁷ cycles was about 50% of the tensile joint strength. Experimental results showed that the modified SPR was one of the promising fasteners for future mass-production CFRP automobiles.     

Effect of carbon nanotube modified epoxy adhesive on CFRP-to-steel interface

The effects of carbon nanotube (CNT) modified epoxy adhesive on CFRP-to-steel interfaces were investigated using double strap joints. The bond behaviours studied were failure modes, bond interface at microlevel, bond strength, effective bond length, CFRP strain distribution and bond-slip relationships.For the first time, a novel type of failure in the CFRP-steel joint was discovered, attributable to weak bonding between woven mesh and CFRP fibres. This failure mode prevented exploitation of the full potential of the carbon fibres and the CNT modified epoxy adhesive. Joints bonded with CNT-epoxy adhesive had an effective bond length of about 60 mm, whereas that of joints bonded with pure epoxy was about 70 mm. The CNT-epoxy adhesive can transfer more load from the host structure to the bonded CFRP laminates, consequently modifying bond behaviour. It is therefore expected that CNT-epoxy nanocomposites will assist in the strengthening and rehabilitation of steel infrastructures using CFRP laminates.     

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。      

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.     

Damage evolution behavior of CFRP laminates under post-impact fatigue with water absorption environment

In this study, the damage evolution behavior was evaluated considering the effect of the textile structure and water absorption. Damage observation was conducted by the integration of non-destructive and direct observation methods. Candidate textile reinforcements were T300-3k plain woven fabric (PW) and T700S-12k multi-axial knitted fabric (MA). The effect of water absorption on the performances of compression after impact (CAI) and PIF were small in PW CFRP laminates. Conversely, PIF properties of water-absorbed MA CFRP laminates drastically decreased than that of dry ones. CAI strength was not affected by water absorption. PIF performance of dry MA CFRP was fairly higher than that of the others. From the precise observation, some evidences of interfacial deterioration caused by water absorption were confirmed in both PW and MA CFRP laminates.     

Edge Delamination and Residual Properties of Drilled Carbon Fiber Composites with and without Short-Aramid-Fiber Interleaf

Edge delamination is frequently observed in carbon fiber reinforced plastic (CFRP) laminates after machining, due to the low fracture toughness of the resin interfaces between carbon fiber plies. In this study, the effects of incorporating tough aramid fibers into the brittle CFRP system are quantified by measuring the residual properties of bolted CFRP. By adding short-aramid-fiber interleaves in CFRP laminates, the residual tensile strength have been substantially increased by 14 % for twill-weave laminates and 45 % for unidirectional laminates respectively. Moreover, tensile failure was observed as the major mode of toughened laminates, in contrast to shear failure of plain laminates. The qualitative FEM results agreed well with the experimental results that edge delamination would cause relatively higher shear stress and therefore alter the failure mode from tensile failure to shear failure.     

T800碳纤维增强复合材料双剪单钉连接的拉伸试验及强度估算

为系统地研究T800碳纤维增强复合材料螺栓连接的力学性能,首先,对T800碳纤维增强复合材料双剪单钉连接进行了面内准静态拉伸试验,探讨了铺层比例、铺层顺序、螺栓直径以及固化工艺对复合材料螺栓连接刚度和2%偏移挤压强度的影响;然后,根据试验结果得到了T800碳纤维增强复合材料螺栓连接的应力集中减缓因子;最后,结合相应铺层比例的无缺口层合板的应力集中减缓因子和拉伸强度,建立了复合材料连接最终挤压强度的工程算法。结果表明:当螺栓断裂时,连接的最终挤压强度由螺栓剪切强度和螺栓直径/板厚比决定;连接存在挤压和剪切2种失效形式,与±45°铺层比例有关;工程算法的计算结果与试验结果吻合良好。所得试验结果和工程算法可为T800碳纤维增强复合材料螺栓连接的初步设计提供理论依据和技术支持。      

搭接长度和铺层方式对CFRP复合材料层合板胶接结构连接性能和损伤行为的影响

针对不同搭接长度和铺层方式的碳纤维增强树脂（CFRP）复合材料层合板单搭胶接结构进行了拉伸试验,观察了试件的受力过程和失效形态,获得了载荷-位移曲线;同时基于连续损伤力学模型和三维Hashin失效准则模拟了CFRP复合材料层合板的层内损伤形成和演化,并利用内聚力模型来模拟层间及胶层的失效损伤,对CFRP复合材料层合板单搭胶接结构在拉伸作用下的失效强度和损伤机制进行了预测,通过对比验证了该数值方法的有效性;通过数值试验比较不同搭接长度和铺层方式的单搭胶接结构及双搭胶接结构的连接强度和损伤行为,并提出了一种优化的CFRP复合材料层合板胶接结构。结果表明:CFRP复合材料层合板胶接结构的极限失效载荷随着搭接长度的增大逐渐增加并趋于稳定值,且结构的失效形式逐渐从胶层自身剪切失效过渡到邻近胶层的层合板层间分层失效;CFRP复合材料层合板胶接结构的连接强度和损伤行为随着铺层方式的不同而改变,通过对3种铺层方式的对比和分析,得到性能最好的铺层方式是[0₃/90₃]_2S;在搭接长度为5~20 mm时,通过对搭接长度进行优化,得到单搭胶接结构的最优搭接长度是17 mm,双搭胶接结构的最优搭接长度是19.3 mm,与搭接长度为20 mm相比,单搭胶接结构和双搭胶接结构的连接强度分别提高了13.26%和0.43%。      

Damage resistance and damage tolerance of hybrid carbon-glass laminates

The influence of impact energy and stacking sequence on the damage resistance and Compression After Impact (CAI) strength of Carbon and Glass Fibre Reinforced Plastic (CFRP and GFRP respectively) hybrid laminates is investigated. CAI tests demonstrate that, in comparison to fully CFRP laminates, hybrid laminates show increases in structural efficiency of up to 51% for laminates subject to a 12J impact and 41% for those subject to an 18J impact. Laminates displaying the highest stresses at failure are those that exploit stacking sequences and GFRP content to prevent delaminations from forming close to the outer surface of the laminate during impact. This favourable damage morphology inhibits both sublaminate-buckling-driven delamination propagation and anti-symmetric laminate buckling failures.     

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.     

自定义粘接元接触技术在复合材料连接强度分析中的应用

为解决复合材料接头强度分析中由于接触造成的收敛困难问题, 自定义了具有特殊本构关系的粘接元用来模拟接触, 该单元只能传递压力而不能传递拉力。自定义的粘接元被置于两接触对之间用来传递接触压力, 从而保证了接触体之间在接触面上的变形协调关系。同时, 建立了复合材料单钉接头的三维累积损伤有限元模型。基于文献中材料性能退化模型, 对纤维及基体压缩破坏的刚度退化模式做了改进, 并利用自定义粘接元接触技术及改进后的刚度退化方法对模型进行了有限元模拟。最后对四组相应试验件进行了试验验证, 试验结果和模拟结果吻合较好, 证明了自定义粘接元接触技术以及文中模型和算法的有效性。      

Quantification of flexural fatigue life and 3D damage in carbon fibre reinforced polymer laminates

Carbon fibre reinforced polymer (CFRP) laminated composites have become attractive in the application of wind turbine blade structures. The cyclic load in the blades necessitates the investigation on the flexural fatigue behaviour of CFRP laminates. In this study, the flexural fatigue life of the [+45/−45/0]_2s CFRP laminates was determined and then analysed statistically. X-ray microtomography was conducted to quantitatively characterise the 3D fatigue damage. It was found that the fatigue life data can be well represented by the two-parameter Weibull distribution; the life can be reliably predicted as a function of applied deflections by the combined Weibull and Sigmodal models. The delamination at the interfaces in the 1st ply group is the major failure mode for the flexural fatigue damage in the CFRP laminate. The calculated delamination area is larger at the interfaces adjacent to the 0 ply. The delamination propagation mechanism is primarily matrix/fibre debonding and secondarily matrix cracking.     

Probabilistic assessment of pin joint strength in CFRP laminates

Composite materials have more scattering material properties than conventional metals. Probabilistic design and analysis (PDA) method has been applied to develop more efficient designs of composites. In this study, an approach combining finite element damage analysis and a stochastic technique was applied to pin joints in CFRP laminates to analyze the probabilistic strength of the joints. A compressive strength parameter inherent to the material itself and a controllable dimension parameter were evaluated and compared for their effects on the strength.     

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。