Bolted joints of pultruded sandwich composite laminates

This paper describes the tensile behavior of bolted joints of pultruded sandwich composite laminates. The pultruded sandwich laminates have a skin-core-skin structure. Joint strength of longitudinal specimens was independent of specimen width (w), whereas it increased with w in the case of transverse specimens. The joining efficiency of pultruded sandwich laminates was greater in the longitudinal direction than in the transverse direction. The core layer of longitudinal specimens failed by a combination of bearing and shear-out modes, independent of w. The failure mode of skin layers changed from net-tension to bearing mode with increasing w. In transverse specimens, the failure mode of core and skin layers changed from net-tension to bearing with increasing w. Finite element numerical analysis was carried out to predict the failure mode and joint strength. The numerical results were in good correlation with the experimental results.     

Bolted Joints in Three Axially Braided Carbon Fibre/Epoxy Textile Composites with Moulded-in and Drilled Fastener Holes

Experimental behaviour of bolted joints in triaxial braided (0°/±45°) carbon fibre/epoxy composite laminates with drilled and moulded-in fastener holes has been investigated in this paper. Braided laminates were manufactured by vacuum infusion process using 12 K T700S carbon fibres (for bias and axial tows) and Araldite LY-564 epoxy resin. Moulded-in fastener holes were formed using guide pins which were inserted in the braided structure prior to the vacuum infusion process. The damage mechanism of the specimens was investigated using ultrasonic C-Scan technique. The specimens were dimensioned to obtain a bearing mode of failure. The bearing strength of the specimens with moulded-in hole was reduced in comparison to the specimens with drilled hole, due to the increased fibre misalignment angle following the pin insertion procedure. An improvement on the bearing strength of moulded-in hole specimens might be developed if the specimen dimensions would be prepared for a net-tension mode of failure where the fibre misalignment would not have an effect as significant as in the case of bearing failure mode, but this mode should be avoided since it leads to sudden catastrophic failures.     

螺栓夹持填充孔复合材料层压板层间应力分析

为了研究含螺栓复合材料层压板连接接头的层间应力分布规律,提出了采用填充孔形式和虚拟界面层方法求解层间应力,并建立了三维有限元模型对受面内压缩载荷的螺栓夹持填充孔层压板进行分析。结果表明:层压板层间应力集中不仅发生在孔边,还会在螺栓头边缘附近出现,且夹持力越大螺栓头边缘附近的层间应力集中越严重;合理的螺栓夹持力能改善孔边应力状态,提高孔边抵抗分层的能力,但无法改善螺栓头边缘附近的层间剪切应力集中状态。因此,在进行含螺栓夹持的层压板机械连接结构孔边层间强度设计时,也要考虑螺栓头边缘的层间剪切应力集中问题,以提高复合材料结构的安全性。     

Tensile behaviors of friction bolt connection with bolt head corrosion damage: Experimental research B

In this study, the tensile characteristics of a friction connection using a high-strength bolt, based on the clamping force loss developed by sectional corrosion damage to the bolt head, was examined. Eighteen specimens with bolt connections were tested to evaluate the tensile strength. In these test specimens, artificial corrosion damage was introduced to the bolt head of the bolt connection, creating a clamping force loss. In the tensile strength tests, the tensile failures of connected steel plates were observed; the tensile strength was unaffected by the artificial sectional corrosion damage to the bolt head for this failure mode. However, the friction resistance of the specimens was affected by the clamping force of the bolt, and thus, their friction loads were slightly reduced depending on the sectional damage level of the bolt head. From the friction load–clamping force relationship, friction-resistance equations were used to approximate the friction resistance of the friction bolt connection.     

Joint performance of the glass fiber reinforced polypropylene leaf spring

Commercial utilization of the composite leaf springs in the suspension application is significantly decided by its eye end joint performance. Present work attempts to design and evaluate the performance of double bolted end joint for thermoplastic composite leaf spring. Injection molded 20% glass fiber reinforced polypropylene leaf springs were considered for the joint strength evaluation. Servo hydraulic test facility is utilized to evaluate the static and fatigue performance of the bolted joint. Various bolt sizes were utilized for the joint and its performances were evaluated under static loading condition to understand the effect of fit between bolt and its hole of the joints. Ultimate bearing strength of the joint is found to decrease with the increase in the clearance between bolt and part hole. Joints were subjected to various amplitudes of completely reversed fatigue loads to evaluate the endurance strength. Load–deflection hysteresis plot of the joint under fatigue conditions is continuously measured and used as the bearing damage index of the joint. Inspection of the bearing surface tested under static and fatigue loading condition revealed severe matrix deformation and fibrillation. In spite of unidirectional load being acted at the joint, curved nature of the bearing surface induces bi-axial stresses, which results in severe matrix fibrillation at the bearing surface. Failure morphology under static conditions shows net-tension beside the bearing damage. Failure morphology under fatigue condition revealed net-tension, and shear-out failures besides the bearing damages.     

Bearing strength of autoclave and oven cured Kevlar/epoxy laminates under static and dynamic loading

The pin bearing behaviour of woven Kevlar fibre-reinforced epoxy laminates, prepared by autoclave and oven curing methods, was examined under static and dynamic loading. Static bearing strength was determined as a function of bolt constraint and specimen geometry with particular attention to failure modes. The performance was very sensitive to lateral bolt tightening: a bolt with only a ‘finger-tight’ nut produced ∼100% improvement in bearing strength, compared with the performance of a pin-loaded hole. Some specimens were hygrothermally conditioned and indicated a 10% deterioration in the bearing strength of the bolt-loaded hole. However, this was well below the magnitude of variation in strength which resulted under different methods of production. The fatigue endurance limit (N ∼ 10⁶) at 1 Hz was only achievable at stress levels equivalent to 25% of the maximum static bearing strength.     

国产炭纤维复合材料层合板高温单钉连接性能试验

对国产CCF300/GW300复合材料层合板与铝合金板单钉连接结构在常温以及200℃和300℃高温环境下的拉伸性能进行试验研究,分析了连接试件的高温拉伸破坏行为,以及温度、铺层形式、连接螺栓直径对于条件挤压强度的影响。研究发现,高温越高其条件挤压强度保持率越低,300℃时条件挤压强度仅为常温的30%;90°铺层比例越高...     

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%。     

Effects of fibre/matrix adhesion on carbon-fibre-reinforced metal laminates—I.: Residual strength

The role of interfacial adhesion between fibre and matrix on the residual strength behaviour of carbon-fibre-reinforced metal laminates (FRMLs) has been investigated. Differences in fibre/matrix adhesion were achieved by using treated and untreated carbon fibres in an epoxy resin system. Mechanical characterisation tests were conducted on bulk composite specimens to determine various properties such as interlaminar shear strength (ILSS) and transverse tension strength which clearly illustrate the difference in fibre/matrix interfacial adhesion. Scanning electron microscopy confirmed the difference in fracture surfaces, the untreated fibre composites showing interfacial failure while the treated fibre composites showed matrix failure. No clear differences were found for the mechanical properties such as tensile strength and Young's modulus of the FRMLs despite the differences in the bulk composite properties. A reduction of 7·5% in the apparent value of the ILSS was identified for the untreated fibre laminates by both three-point and five-point bend tests. Residual strength and blunt notch tests showed remarkable increases in strength for the untreated fibre specimens over the treated ones. Increases of up to 20% and 14% were found for specimens with a circular hole and saw cut, respectively. The increase in strength is attributed to the promotion of fibre/matrix splitting and large delamination zones in the untreated fibre specimens owing to the weak fibre/matrix interface.     

碳纤维增强树脂基复合材料层合板胶螺混合连接失效机制

为研究碳纤维增强树脂基复合材料(CFRP)层合板单搭接双螺栓胶螺混合连接失效机制,采用基于断裂能断裂准则的连续渐进退化方式,仿真CFRP层合板刚度退化,采用基于能量的B-K准则仿真胶层的损伤演化,建立胶螺混合连接结构渐进损伤三维有限元模型,有限元模型预测的最大失效载荷与实验结果吻合较好。搭接长度L_a为影响胶螺混合接头刚度和强度的重要几何参数,螺栓的位置不会明显影响接头的刚度,粘结面积越大,强度越大。胶螺混合接头在拉伸载荷作用下,由于二次弯曲效应的影响,螺栓向左倾斜,搭接区域的胶层损伤起始于搭接区域胶层外侧,并由外侧向内部扩展到钉孔附近,当胶层损伤扩展到钉孔附近时,螺栓承载增加,胶层和螺栓共同承载,此时CFRP层合板开始出现损伤;最终,左侧钉孔处的上层合板和右侧钉孔处的下层合板产生分层损伤并发生断裂。      

含孔CFRP正交层合板的双轴拉伸力学行为

采用加载臂开槽的中心开孔等厚度十字形试样,实验研究了正交对称铺层碳纤维增强聚合物基复合材料（CFRP）层合板在双轴拉伸载荷作用下的力学行为,分析了3种双轴加载比对其拉伸强度和破坏行为的影响。研究表明:纤维被切断的铺层部分在拉伸作用下容易与其相邻铺层脱粘,导致层合板承载力下降;等双轴加载时,在孔边的被切断纤维与连续纤维间基体在横向拉伸和纵向剪切组合作用下首先开裂;非等双轴加载时,在垂直于快速拉伸方向的铺层中沿孔边应力集中处先出现基体裂纹;随着加载比的增大,快速拉伸方向的细观结构损伤随载荷的增大发展更快,刚度下降更快,破坏时主裂纹的扩展方向更趋于垂直于快速拉伸方向;强度包络线的分析表明快速拉伸方向的拉伸强度随加载比的增大呈缓慢增大的趋势。      

Tensile fracture behaviour of notched boron/aluminum composite laminates

This paper presents modifications in the stress fracture criteria (known as the point stress criterion and the average stress criterion), developed by Whitney and Nuismer to predict the tensile strength of composite laminates containing holes and cracks. A simple relation is used for the characteristic lengths to take care of the notch size dependence, which can improve the accuracy while evaluating the notched strength of composite laminates. The applicability of the simple relation is examined by considering a good amount of fracture data on boron/aluminium laminates. The present procedure correlates well with the test data on circular hole specimens as well as various cracked configurations and confirms that the approach can be applied for tensile strength estimations of notched composite laminates.     

单面贴补修理后层合板的拉伸性能

对含孔损伤复合材料层合板单面贴补后进行拉伸试验研究。测量了层合板的应变分布、修理后层合板中心点的离面位移及拉伸强度等, 考察了补片的厚度、大小等因素对修理效果的影响。结果显示, 增加补片的厚度和直径能够提高母板的承拉能力, 但是增加补片的厚度会导致层合板离面位移增大。对无侧边支持的单面贴补层合板进行计算分析时, 必须考虑偏心载荷引起的弯矩的影响。在此基础上, 采用分层损伤判据建立了三维有限元模型, 对单面贴补层合板的破坏机理和拉伸强度进行了计算和分析。结果表明, 修理后层合板的拉伸破坏是由补片或母板内与胶接面相邻的层间分层引起的; 计算结果与试验结果一致。      

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.     

复合材料层合板连接件力学性能影响因素分析

在位移载荷作用下,基于二维有限元模型,通过编写材料失效准则和刚度退化规则的ABAQUS用户子程序USFLD,并考虑剪切非线性效应,分析了一般铺层下配合间隙、挤压面切向摩擦系数和开孔形状对复合材料层合板单钉螺栓连接力学性能的影响。结果表明:小的配合间隙、大的挤压面切向摩擦系数和开孔形状采用圆孔时均有利于提高连接强度。     

孔口缝合补强对含孔层板应变集中影响的实验研究

对复合材料开口缝合补强结构进行了实验研究。测试了不同缝合参数(针距、行距、边距、单重和双重缝合) 补强的含孔复合材料层板的拉伸强度, 研究了孔边应力集中、应变集中对强度、刚度等力学性能参数的影响, 分析讨论了孔边及邻近区域应变集中及应变分布的规律, 得到不同缝合参数、孔边不同位置以及不同载荷条件下的应变集中系数。结合实验结果和分析讨论, 给出了含孔复合材料层板缝合补强的缝合设计参数。      

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。      

干涉对复合材料层合板连接系统的极限挤压强度影响

本文对复合材料层合板单面螺纹抽钉紧固件干涉配合连接结构和高锁螺栓间隙配合连接结构的静挤压强度进行了试验研究.分别考虑了不同干涉量配合和间隙配合对紧固件连接结构极限挤压强度的影响.在静拉伸试验中,采用了卡口引伸计来实时监测孔的变形量.根据ASTM D 5961挤压试验方法标准,得到极限挤压强度及偏移量为2%的2%偏移挤压...     

Pin-bearing strength of glass mat reinforced plastics

An experimental investigation was carried out, in order to find simple analytical methods for the prediction of pin-bearing strength of glass mat reinforced plastics fabricated by hand lay-up.The specimens tested exhibited different failure modes, consisting of net-tension, cleavage, and bearing, depending on the geometry adopted.The parameters affecting the bearing strength were the ratio of the specimen width to the hole diameter D, and the ratio of the edge distance to D. The maximum bearing strength was attained when a pure bearing failure happened. The latter was the only safe failure mode: after its occurrence, the joint was still able to support about 70% of its virgin strength. The experimental data obtained were used to assess a simple procedure for the calculation of the joint load carrying capacity, previously proposed.Since a large scatter was found in the bearing strength of the material, the data were analysed statistically, assuming a probability of failure varying according to a two-parameter Weibull distribution. An excellent agreement was found between the experimental results and the theoretical model. Similar conclusions were drawn applying the previous approach to the residual strength after bearing failure.A limited amount of tests was performed to verify the influence of possible damage induced during hole drilling on the bearing behaviour. It was found that a 20% decrease in bearing strength occurs when delamination is induced by inaccurate drilling. However, the residual bearing strength was substantially unaffected by the drilling damage.     

接头尺寸对玻璃纤维/热塑性树脂复合材料机械连接性能的影响接头尺寸对玻璃纤维/热塑性树脂复合材料机械连接性能的影响

对玻璃纤维/聚酰胺（GF/PA）、玻璃纤维/聚甲醛（GF/POM）、玻璃纤维/聚丙烯（GF/PP）这三种玻璃纤维增强热塑性树脂基复合材料进行机械连接试样的常规拉伸试验,以及低周疲劳拉伸试验,并对疲劳前后的试样断裂面进行SEM观察,研究了接头尺寸（宽径比w/d （试样宽度/开孔直径）和端径比e/d （试样端距/开孔直径））对机械连接件破坏载荷和破坏模式的影响。实验结果表明:玻璃纤维增强纤热塑性树脂复合材料机械连接件的承载能力在一定的宽径比时会随着e/d的增加而增加,当w/d≥3、e/d≥2时趋于稳定;破坏模式以拉伸破坏为主;低周疲劳拉伸对GF/POM和GF/PA机械连接试样拉伸强度产生一定的影响,而对GF/PP的拉伸强度无明显影响,低周疲劳拉伸对玻璃纤维增强热塑性树脂复合材料机械连接试样的破坏模式没有影响。SEM观察显示,随着疲劳载荷水平的增加,GF/POM和GF/PA的断裂面上被抽拔纤维数量增加,而GF/PP断裂面纤维与基体的存在状态无明显变化。