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...     

单向碳/环氧复合材料拉—拉疲劳性能研究

本文采用GB3354-84规定的大标距直条试样对单向碳/环氧复合材料的拉一拉(R=0.1)疲劳性能和剩余静拉伸强度进行了研究。用成组试验法和升降法测定了S-N曲线和疲劳极限。试样疲劳破坏是由于沿纤维方向(纵向)疲劳裂纹的扩展引起的。试样剩余静拉伸强度大于0.7σ_b。     

配合精度对复合材料单钉双剪螺栓连接静强度的影响

为研究不同配合精度(间隙配合及干涉配合)对复合材料单钉双剪螺栓连接静强度的影响,基于ABAQUS软件建立了三维渐进损伤有限元分析模型,并将分析结果与实验结果进行比较验证模型的有效性.结果表明,间隙的增加会导致接头挤压强度及刚度的降低,但在0％ ～2％ 间隙范围内极限挤压强度变化不大.间隙配合下螺栓-孔接触面积的减小引发应力集中,致使孔边出现损伤区域集中.适当的干涉配合能增强接头刚度及挤压强度;适当的干涉量能降低孔边应力集中,但过大的干涉量会导致孔周出现严重的初始损伤.     

碳/碳复合材料疲劳损伤失效试验研究

对单向碳/碳复合材料纵向拉-拉疲劳特性及面内剪切拉-拉疲劳特性进行了试验研究; 对三维四向编织碳/碳复合材料的纵向拉-拉疲劳特性及纤维束-基体界面剩余强度进行了试验研究。使用最小二乘法拟合得到了单向碳/碳复合材料纵向及面内剪切拉-拉疲劳加载下的剩余刚度退化模型及剩余强度退化模型, 建立了纤维束-基体界面剩余强度模型。结果显示: 单向碳/碳复合材料在87.5%应力水平的疲劳载荷下刚度退化最大只有8.8%左右, 在70.0%应力水平的疲劳载荷下, 面内剪切刚度退化最大可达30%左右; 三维四向编织碳/碳复合材料疲劳加载后强度及刚度均得到了提高; 随着疲劳循环加载数的增加, 三维四向编织碳/碳复合材料中纤维束-基体界面强度逐渐减弱。      

碳/环氧复合材料缺口层合板拉-拉疲劳特性研究

一、前 言 复合材料结构和工艺的复杂性,使它对疲劳载荷的响应在本质上不同于金属材料。复合材料可能发生脱胶、分层、基体开裂、空隙增长、纤维断裂以及整体断裂等多种疲劳损伤形式,而且复合材料的损伤是由几种形式的损伤共同相互作用的综合结果,其疲劳损伤的发展缺乏规律性。 为了了解复合材料的疲劳特性,我们做了碳/环氧复合材料层合板的拉-拉疲劳试验研究,对多向复合材料层合板在循环拉-拉载荷作     

碳纳米管对碳纤维/环氧树脂复合材料力学性能的影响

为了探讨碳纳米管(CNTs)对碳纤维/环氧树脂复合材料(CF/ER复合材料)力学性能与疲劳寿命的影响,利用静态拉伸实验和拉-拉疲劳实验沿纤维方向对CF/ER复合材料和CNTs增强CF/ER复合材料(CNTs/CF/ER复合材料)进行了对比研究,同时利用X射线仪与扫描电镜对试样进行了观察.研究结果表明,CNTs的加入,虽然对CF/ER复合材料的拉伸力学性能影响不明显,但可以提高高周疲劳寿命约4倍,使各种实验应力水平下的裂纹密度降低9.5％以上,并可观察到试样中CNTs的拔出、破裂及桥联作用.由此可见,CNTs的加入可明显改善CF/ER复合材料的疲劳寿命.     

内螺纹冷挤压强化对高强度钢疲劳性能的影响

研究了高强度人螺纹挤压强化对疲劳性能的影响。利用高精度云纹干涉法和扫描电镜,分别对螺纹牙根残余应力和疲劳试件口进行了测试和观察。疲劳对比试验结果表明,在应力为５００和６００ＭＰａ的条件下,经挤压强化的３００Ｍ高强度钢内方的寿命是切削螺纹的４～３０倍。     

不同背压方式对静液挤压过程影响的模拟研究

目的研究两种常用的背压方式在静液挤压工艺中对材料变形过程的影响。方法通过选取合理的背压工艺和压力数值,减小静液挤压过程中坯料的拉应力区域,降低材料开裂的风险。基于有限元分析(FEA)软件ANSYS,采用压力加载的方式对顶杆背压和静水背压静液挤压工艺中坯料内部应力应变分布以及材料流动性能的影响开展分析和讨论。结果两种背压方式均存在临界值,适当的压力范围可以抑制裂纹,并且可以获得较好的挤压样品;顶杆背压较静水背压更明显地抑制了轴向拉应力,但会导致其余两项拉应力的滋生;静水背压对三向拉应力都有一定的抑制效果。结论顶杆背压适合于在塑性材料的大挤压比加工中抑制表面纵向裂纹的萌生,也适用于塑性一般的材料的静液挤压变形;静水背压对三向拉应力都有一定的抑制作用,适用于加工变形能力较差的材料。     

多轴复杂应力形式下TB6高强钛合金耳片的微动疲劳断裂研究

本实验研究了TB6高强钛合金在拉-扭多轴应力条件下的微动疲劳断裂机理,并结合宏微观观察、残余应力测试、计算机模拟分析了不同工艺条件的耳片在拉-扭疲劳试验应力下的微动疲劳寿命差异,研究了微动疲劳裂纹的萌生及扩展特性,揭示了耳片裂纹萌生机制。结果表明:二次挤压耳片的断口扩展区面积比例较大,说明其拉-扭疲劳扩展更为充分。经二次挤压后,耳片的拉-扭微动疲劳总寿命得到延长,且裂纹萌生寿命均占总寿命的95%以上;二次挤压后耳片的裂纹萌生驱动力较低,呈现较高的裂纹萌生抗力。各断口均起源于耳片内孔微动磨损印迹较重处,大面积连续片状的微动磨损印迹表明二次挤压后的钛合金微动疲劳敏感性大大降低。经挤压强化后,孔壁形成了较高幅值的残余压应力层和组织强化层,可以有效地抑制裂纹的萌生和扩展。耳片内孔微动磨损层的形成是机械诱导机制和热诱导机制共同作用的结果,其最终磨损形式为疲劳磨损。     

加载循环数对2D针刺C/SiC复合材料疲劳剩余强度的影响

为研究陶瓷基复合材料的低周疲劳失效机理,通过试验和细观分析对其疲劳特性进行了探讨。研究了室温下加载循环数对2D针刺C/SiC复合材料拉-拉疲劳剩余强度的影响,并采用光学显微镜和扫描电子显微镜对该材料的断口形貌和微观结构进行了观察。结果表明:2D针刺C/SiC复合材料具有较好的抗疲劳特性,在85%极限拉伸强度(UTS)载荷下的循环数超过106;随着加载循环数的增加,剩余强度先增大然后下降。断口分析表明:纤维拔出长度随着加载循环数的增加而增加,说明在疲劳加载过程中,纤维/基体的界面结合强度降低,减缓了材料内部受力的不均匀性,提高了材料的承载能力,使2D针刺C/SiC复合材料出现了疲劳强化现象。      

有机玻璃边缘连接结构疲劳寿命可靠性分析

针对有机玻璃边缘连接结构弯曲疲劳实验结果,提出了结构低周疲劳寿命模型,对有机玻璃边缘连接结构可靠性进行分析.考虑材料属性、疲劳幅值载荷的随机性,分别采用Monte-Carlo法和四阶矩法,借助有限元分析软件Marc对结构进行了可靠性分析,得到低周疲劳载荷作用下结构疲劳寿命的可靠度,比较两种计算结果,发现四阶矩法计算结果与Monte-Carlo法计算偏差很小,适用于有机玻璃边缘连接结构疲劳可靠性分析;同时分析了基本变量均值与标准差影响结构可靠性的灵敏度,得到载荷为影响结构疲劳寿命可靠性的主要因素.     

Effect of hole lubrication on the fretting fatigue life of double shear lap joints: An experimental and numerical study

In this research the affect that lubrication at a hole and pin connection has on the fatigue life of a double shear lap joint is studied both experimentally and numerically. The study focuses on the joint middle plate item, which is connected via a central hole to the outer plates by means of a clearance fitting pin, thereby placing the hole in double shear. In the experimental work three identical batches of fatigue specimens, which are made from aluminum alloy 2024-T3, were fatigue tested. In the first batch the surface of the fastener hole was not lubricated whilst the hole in the other two batches was lubricated – each batch using a different lubricant. The three batches of double shear lap joint specimens were fatigue tested and their S–N curves established. The results show that the specimens in which the holes were lubricated have better fatigue lives than the non-lubricated hole specimens. In the numerical study, FE simulations were performed to include hole lubrication effect on the stress distribution by using different friction coefficient at the interface of the hole and its fastener (pin). The FE results have helped to gain an understanding of the reasons for fatigue life improvement and also have helped to quantify the level of improvement.     

Investigation of the fatigue life of pre- and post-drilling hole in dog-bone specimen subjected to laser shot peening

The influence of processing sequence of laser shot peening (LSP) on the fatigue properties of fastener hole was investigated with finite element method and experiments. The results show that different processing sequences lead to different residual stress distributions and different fatigue lives. The compressive residual stresses (CRS) are squeezed into two-sided surface layers of fastener hole by two sided laser shot peening, and the ellipse CRS fields are found on both sided surfaces of sample. However, when the pre-drilling hole in dog-bone specimen is subjected to LSP, the tensile stresses appear at its mid-thickness region, while the CRS distribute in the entire thickness region of the post-drilling hole after LSP. The fatigue crack initiation of specimens treated by LSP stems from the subsurface layer of hole edge. The fatigue striation spacing of specimen with post-drilling hole after LSP is narrower in comparison with that of case with pre-drilling hole before LSP. The fatigue life of post-drilling hole is longer than that of the pre-drilling hole.     

EXTENDING THE FATIGUE LIFE OF MULTI-LAYER METAL JOINTS

Abstract The influences of both hole cold expansion and interference-fit fasteners for extending the fatigue life of multi-layer aluminium alloy joint specimens under variable-amplitude loading have been examined experimentally. Improvements in fatigue life were markedly dependent on the degree of load transfer in the specimen joint. Secondary bending in the joint also had a major influence, reducing the effectiveness of these life improvement techniques. Depending on the joint configuration, improvements in fatigue life ranged from nothing at all to a factor of about 40. Fretting appeared to be involved in the initiation of all cracks and where this occurred on faying surfaces only small, if any, improvements in fatigue life were obtained.     

Effect of detail design on fatigue performance of fastener hole

In the present study, a series of tests were conducted on aluminum alloy 2024 to investigate the effect of detail design on the fatigue behavior of fastener holes in specimens. Two types of detail designs were concerned. One was the mode of fastener holes (countersunk rivet or countersunk bolt), the other was drilling process (traditional air-drilling process or one step compound cutting process). The fracture surfaces were observed by means of an optical microscope. Finite element method (FEM) was employed to analyze the distribution of stress around the fastener holes. The results showed that crack always initiated at the hole edge where the stress concentration occurred. Crack initiation was induced by stress concentration. Crack initiation life accounted for 80% of total fatigue life of fastener holes. The fatigue life of fastener hole using countersunk rivet was longer than that using countersunk bolt. Contrasted to traditional air-drilling process, the fatigue life of fastener hole could be improved by 44–55% using one step compound cutting process. However, the dispersibility of fatigue life became increasingly severe when fatigue life was prolonged.     

Fatigue life prediction model for riveted lap joints

Applicability of currently available approaches to predict the fatigue life of riveted joints in aircraft structures is limited to cases when the actual joint, for which the predictions are made, and the reference joint, for which the prediction model has been tuned, differ only in the geometry. It is required that the riveting process should be similar for the actual and reference case, but the similarity criterion cannot be formulated in a precise way. In the prediction model developed by the authors and presented in this paper the influence of riveting on the fatigue life of a joint is unambiguously characterized by measurable quantities, namely rivet hole expansion and the load transfer distribution. Hence, the similarity of the riveting processes for the reference and actual joint is no longer required, which considerably extends the transferability of the reference results. A validation of the model is performed by comparing fatigue lives computed and observed in over 80 fatigue tests on aluminium alloy, lap joint specimens with three rivet rows, typical for aircraft fuselage skin connections in the longitudinal direction. Various combinations of production variables, such as sheet material and thickness, the squeeze stress and rivet type, were involved. A significant improvement in the present model prediction accuracy compared to a model which disregards the effect of riveting has been noted. Specifically, underestimates or overestimates of the fatigue life observed for the latter model in the case when hole expansion of the actual joint is larger or smaller respectively than for the reference joint are avoided with the present approach.     

Improving fretting fatigue behaviour of Al 7075-T6 bolted plates using electroless Ni–P coatings

In this paper, the effect of electroless nickel–phosphorous coatings on the fatigue and fretting fatigue behaviour of Al 7075-T6 bolted plates has been investigated. A double-lap bolted joint specimen was designed and manufactured from the aluminium plates and subsequently coated with Ni–P coatings of 40 μm in thickness with a high phosphorous content of 10–13 wt.%. Then, different tightening torques were applied to clamp the plates together with the aim of studying the effect of clamping force on the fretting fatigue life of the joints. Ni–P coatings were found to protect Al 7075-T6 clamped plates against fretting fatigue damages even at low fatigue loads where the uncoated joints showed a large reduction in their fatigue life due to the fretting effects. Fretting fatigue life of moderately and firmly clamped plates was successfully improved by approximately 30–40% and 50–60% respectively after the application of Ni–P coatings. Furthermore, fatigue cracks were found at the outer surface of the aluminium substrate at the edge of the hole associated with nodular defects whose deleterious effect was more pronounced at high cyclic loads. The Ni–P deposit presented a very good adhesion to the substrate at low and moderate loads; however, considerable delaminations and fracture of the coating film at high cyclic loads was observed.     

Analysis of the crack growth propagation process under mixed-mode loading

In the present paper, a computational model for crack growth analysis under Mode I/II conditions is formulated. The focus is on two issues – crack path simulation and fatigue life estimation. The finite element method is used together with the maximum principal stress criterion and the crack growth rate equation based on the equivalent stress intensity factor. To determine the mixed-mode stress intensity factors, quarter-point (Q-P) singular finite elements are employed. For verification purposes, a plate with crack emanating from the edge of a hole is examined. The crack path of the plate made of 2024 T3 Al Alloy is investigated experimentally and simulated by using the finite element method with the maximum tangential stress criterion. Then, the validation of the procedure is illustrated by applying the numerical evaluation of the curvilinear crack propagation in the polymethyl methacrylate (PMMA) beam and the Arcan specimen made of Al Alloy for which experimental results are available in the literature. In order to estimate fatigue life up to failure of the plate with crack emanating from the edge of a hole, the polynomial expression is evaluated for the equivalent stress intensity factor using values of stress intensity factors obtained from the finite element analysis. Additionally, the fatigue life up to failure of the Arcan specimen is analyzed for different loading angles and compared with experimental data. Excellent correlations between the computed and experimental results are obtained.     

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。      

Finite element investigations of bolt clamping force and friction coefficient effect on the fatigue behavior of aluminum alloy 2024-T3 in double shear lap joint

In this paper, the effect of bolt clamping force on the fatigue life of bolted double shear lap joints was investigated numerically. To do so, finite element simulation results were used to illustrate the trends occurred in experimental fatigue tests showing the effect of bolt clamping on improving the fatigue life of double shear lap joints. The results show that clamping force decreases the resultant longitudinal stress at the hole edge thus the fatigue life increases compared to clearance fit specimens. In general, at higher tightening torque longer fatigue lives were achieved, however, below a certain load level the life improvement was discontinued because of fretting occurrence. Also lubricating the specimens reduces the advantages of the clamping force.