过盈配合和预紧力的混合作用对复合材料机械连接结构的影响及其机制

为研究钉孔过盈配合情况和紧固件预紧力及二者混合作用对复合材料连接强度的影响及其机制,提出了一种基于试验验证的有限元方法,在有限元模拟结果与两组不同配合复合材料与钛合金单剪双钉连接拉伸试验吻合良好的基础上,进一步模拟6种不同螺栓预紧力和7种不同钉孔配合模式的组合,共42种不同情况的结构承载能力。通过对模拟结果的分析发现：一定的干涉配合值和预紧力虽然可能导致钉孔周围在受载较小时提前出现较小破坏,但在载荷较大时反而可以改善孔周的接触条件,从而减小孔周的纤维破坏范围,并最终提高连接结构的破坏载荷。而且钉孔配合情况和螺栓预紧力对连接结构承载能力会相互影响,即在不同钉孔配合情况下所得到的紧固件预紧力优化值会有所不同,反之亦然。因此在进行复合材料机械连接结构承载能力优化时,必须同时考虑不同参数的影响,才能获得最优的结构设计。     

双剪切连接件钉传载荷分析及其均化处理

采用试验和有限元分析相结合的方法开展双剪切连接件钉传载荷研究。通过有限元软件ABAQUS开展双剪切连接件数值模拟研究,数值模拟中综合考虑连接件的摩擦、接触和预紧力。数值模拟得到的钉传载荷分配比率与动态应变采集仪测得的试验结果相吻合,验证了有限元模型的正确性。在此模型基础上分析了过盈和间隙配合对钉传载荷分配比率均化效果的影响。分析结果表明:双剪切连接件搭接板最大应力发生在中板的靠近单板钉孔处,合理安排各孔的过盈及间隙配合能够均化各钉传载荷比率,提高孔的承载能力。     

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.     

胶层厚度对碳纤维/双马来酰亚胺树脂复合材料平-折-平混合连接接头力学性能的影响

以碳纤维/双马来酰亚胺(BMI)树脂复合材料平-折-平(FJF)连接接头为对象,通过试验对比分析了特定胶层厚度下碳纤维/BMI树脂复合材料FJF连接接头的静强度和疲劳性能,并探究了胶层厚度对碳纤维/BMI树脂复合材料FJF混合接头力学性能的影响。利用背面应变技术对碳纤维/BMI树脂复合材料FJF混合接头搭接区端部胶层开裂进行监测。利用有限元软件ABAQUS对不同胶层厚度下碳纤维/BMI树脂复合材料FJF混合接头搭接区胶层应力分布进行了分析。结果表明,碳纤维/BMI树脂复合材料FJF混合接头的平均拉伸极限载荷、搭接区端部胶层开裂平均循环次数和平均疲劳寿命均随着胶层厚度在0.1~0.3 mm范围内增加而增大。不同胶层厚度的碳纤维/BMI树脂复合材料FJF混合接头均经历相同的失效阶段,即搭接区胶层端部开裂,胶层沿搭接区断裂扩展,最终靠近加载端孔边拉伸断裂,呈±45°断口。随着胶层厚度在0.1~0.3 mm范围的增加,搭接区端部胶层剥离应力、剪切应力及孔边胶层压缩应力均减小。在胶层厚度为0.1~0.3 mm范围内,剪应力是胶层破坏的控制因素。      

微动损伤对复合材料螺栓连接预紧力松弛的影响

开展了横向循环载荷下复合材料螺栓连接件预紧力松弛试验,探究了材料疲劳损伤与接触面微动磨损联合作用下螺栓连接预紧力松弛过程。基于有限元软件ABAQUS,采用Archard磨损模型和ALE（Arbitrary Lagrangian-Eulerian）自适应网格技术,编制了适用于连接结构微动磨损的UMESHMOTION子程序,建立了分析连接支承面微动磨损的计算模型;利用子程序UMAT编制了Shokrieh和Lessard提出的疲劳累积损伤定量分析程序。在此基础上分析了复合材料疲劳损伤、螺孔伸长及接触磨损的耦合作用下预紧力随循环周次变化的机制,与实验结果进行对比,验证了所提出的预测方法的合理性、有效性。      

配合方式对复合材料单钉单剪螺栓连接接头刚度的影响及其机制

针对单钉单剪复合材料螺栓连接,研究了间隙与干涉2种配合方式对接头刚度的影响及其机制。通过与试验结果的对比,验证了所采用的有限元方法的有效性。结果表明:对于复合材料单钉单剪螺栓连接,间隙配合导致接头刚度变小, 3%钉直径的间隙量导致接头刚度下降16%~17%,并且随着间隙量的增大,接头刚度基本呈线性下降;而对于装配过程中没有产生孔边分层损伤的干涉配合连接件, 0.5%钉直径的干涉量使得接头刚度增加了约15%。配合方式不同,连接件孔周接触应力峰值方向及厚度方向接触应力分布不均匀的程度不同,进而影响了接头刚度。      

液体垫片对复合材料单搭接螺栓接头力学性能的影响

复合材料构件由于存在制造误差,装配时常常产生间隙,消除间隙的一种基本手段是向其中填充液体垫片。以复合材料单搭接螺栓连接接头为研究对象,设计了拉伸实验,选取一种改进的失效准则与对应的材料退化准则建立了渐进损伤有限元分析（FEA）模型,在此基础上研究了液体垫片对复合材料单搭接接头强度、刚度等力学性能的影响及复合材料孔内损伤演化的过程,此外还研究了液体垫片孔边的应力-应变状态。由实验与有限元结果可以得出：随着液体垫片厚度的增加,接头的拉伸刚度与峰值载荷均有所降低;相同载荷下复合材料孔内损伤加剧,孔内单元产生初始损伤时对应的载荷降低;但液体垫片厚度的增加可以降低垫片孔边的应力与塑性应变峰值,并使其分布更加均匀化,改善液体垫片孔边受力状态。     

T300/QY8911 π/4层压板机械连接干涉疲劳

本文对T300/QY8911π/4层压板机械连接干涉配合进行了静拉伸挤压和拉-拉疲劳试验。与滑配合比较,干涉使静挤压强度略有提高,但能使疲劳寿命增加数倍。文中用有限元方法计算了两种配合的孔边应力分布。并对疲劳损伤用X射线作了检测,初步分析了干涉配合的增寿原因。      

复合材料舱段连接构件三维应力分析及设计

基于有限元软件ANSYS,建立了舱段间连接结构的有限元分析模型进行非线性静力分析.给出了其中典型槽形复合材料构件的应力分布规律,并预测构件的破坏部位为舱段端框孔边内拐角部位,在设计时需要重点考虑.对槽形复合材料构件长短支厚度、连接螺栓预紧力以及垫片对构件应力分布的影响研究表明,增加长短支厚度、选择适当预紧力、选择刚性垫片、增加垫片的厚度以及选择合适的垫片形状均可以对拐角部位的强度有所改善.本文的工作为复合材料舱段连接结构的设计提供理论参考.     

碳纤维增强树脂基复合材料预紧力单齿接头的静态及疲劳性能试验

通过试验研究了预紧力、齿长和载荷水平对碳纤维增强树脂基复合材料（CFRP）预紧力单齿接头（PTSTC）静态和疲劳性能的影响。试验结果表明:CFRP PTSTC的疲劳极限承载力可以达到静态极限承载力的80%~85%,与螺栓等传统连接方式相比,其疲劳性能具有一定优势;预紧力可以显著改善CFRP单齿接头的静态和疲劳性能;CFRP PTSTC的静态极限承载力随齿长增加而升高,但是在相同载荷水平下增加齿长不一定可以延长接头的疲劳寿命,尤其是在低载荷水平下齿长增加反而会缩短疲劳寿命;在加载初期,PTSTC的疲劳裂纹快速萌生,之后扩展缓慢,在接近破坏前的几次循环中又骤然增大,预紧力可以减缓疲劳损伤的累积速率;疲劳过程中PTSTC的刚度衰退不明显,在前95%疲劳寿命阶段仅下降1%~4%,预紧力也可以减缓刚度的衰退速率。所得研究成果可为复合材料接头抗疲劳设计提供参考依据。      

Fretting fatigue behavior and life prediction of automotive steel bolted joint

Fatigue tests of bolted joints of SAPH400 automotive steel plate were carried out. Effect of groove on fretting fatigue strength was investigated by introducing various geometries of grooves at contact edge. The fretting fatigue strength was improved by introducing groove: the fatigue strength increased with increasing groove depth. As the next step, the applicability of the tangential stress range–compressive stress range diagram to the bolted joints was investigated using the tangential stress range–compressive stress range diagram obtained from conventional laboratory-type SAPH400 steel specimens. The result showed that the fretting fatigue strength of actual component, i.e. the bolted joint could be successfully predicted based on the tangential stress range–compressive stress range diagram.     

纵肋与横隔板交叉构造细节穿透型疲劳裂纹扩展特性及其加固方法研究

纵肋与横隔板交叉构造细节是正交异性钢桥面板最易发生疲劳开裂的构造细节,通过建立有限元数值模型,采用断裂力学方法,研究栓接角钢加固方式对该处疲劳易损细节穿透型裂纹的加固效果。基于疲劳试验足尺节段模型相对应有限元模型,建立了纵肋与横隔板焊接处穿透型疲劳裂纹模型,针对栓接角钢和纵肋外侧栓接钢板两种加固技术的加固效果进行评估。研究结果表明:钢桥面板纵肋与横隔板交叉构造细节的疲劳裂纹扩展至一定长度后将发展成穿透型裂纹,裂纹面受力复杂,纵肋腹板内外侧疲劳裂纹扩展特性表现的不一样,但是随着裂纹扩展的逐步进行,裂纹尖端的开裂模式均以复合型开裂为主;栓接角钢加固方式主要抑制纵肋与横隔板交叉构造细节易损部位疲劳裂纹的I型开裂,因此能很好地抑制短裂纹的扩展,但对于该细节处以复合形式扩展的穿透型疲劳裂纹的加固效果并不显著;在纵肋外侧栓接半U形钢板的加固方法能有效改善穿透型疲劳裂纹的等效应力强度因子,并且加固之后均保持在裂纹扩展阈值以下,表明该加固方式对穿透型疲劳裂纹有良好加固效果。     

The static and fatigue strength of bolted joints in composites with hygrothermal cycling

The effects of hygrothermal cycling upon the performance of a bolted composite joint was examined. Bolt torque relaxed as the number of environmental cycles increased. Comparison with analytical results suggested that the bolt torque “zigzag” behavior probably results from the natural sensitivity of bolted composite joints to the existing ambient temperature and moisture. The washer effect and specimen surface finishes were investigated to study the friction effect on joint bearing performance. Fatigue tests of specimens exposed to hygrothermal cycling exhibited greater hole elongation than specimens not exposed. High preload does improve static failure strength and the fatigue life of specimens under room conditions. Tests run on IM6/3501-6 material specimens with hygrothermal cycling show reasonable declines in fatigue life. The moisture weight gains of composites under hygrothermal cycling exposure were measured and compared to numerical results; good correlation was obtained. As a result, bolt failure occurred in some fatigue tests and is most frequently associated with large thickness-to-diameter ratio. The peak-to-peak stress was defined to study the effect of the R-ratio on bolts for the bolt failure problem.     

多孔复合材料机械连接件弹性接触内力和应力分析

本文用有限元混合法给出了多孔复合材料机械连接件弹性接触内力和应力分析。文中详细讨论了销钉弹性、层合板铺设方式和摩擦对各孔钉载分配及孔边应力的影响,并给出了多孔连接件向单孔连接件简化的条件。      

基于振动疲劳试验的复合材料螺栓连接预紧力松弛特性

在强迫弯曲振动试验的基础上,建立了基于模态参数(共振频率和阻尼比)表征螺栓连接结构动态性能的分析方法和试验测试手段;通过施加不同初始预紧力和激振频率,探究碳纤维/环氧复合材料螺栓连接预紧力松弛的时变行为及其影响因素。结果表明:在10h振动疲劳过程中,螺栓初始预紧力越小,激振频率越大,连接件预紧力松弛程度越大;振动疲劳损伤会导致连接结构刚度衰退、阻尼增加;复合材料螺栓连接松弛受到材料黏弹性以及界面摩擦的共同影响,其中约50%的松弛是由复合材料黏弹性效应引起的。      

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.     

Effect of torque tightening on the fatigue strength of bolted joints

The effect of tightening torques on the life of plates bolted using single and double lap joints was investigated. The effect of plate thickness using an aircraft grade aluminium alloy with double lap joints was also studied. Constant amplitude fatigue tests, under load control, were carried out, with a near zero stress ratio, on plain specimens (for bench mark purposes) and on both single and double lap joint specimens, for which several torque levels were applied on the bolted joints. The objective of the fatigue tests was to demonstrate failure trends for each joint type, material thickness and torque loading, rather than the generation of comprehensive S–N curves. Possible factors that affected the fatigue life of the bolted joints are discussed and conclusions are drawn with respect to the beneficial effects of tightening torques on the bolted single and double lap joints.     

Experimental and numerical analysis of aluminum-aluminum bolted joints subject to an indentation process

The increasing interest of the industry (especially automotive, aviation and marine) in the fastener joints (riveted, bolted, etc.) between metallic materials, has re-opened the study on the possibility to improve the performance of the drilled structure using plastic deformation processes.Indentation process, performed before the drilling operation, creates circumferential compression stresses around the hole which increase significantly the mechanical performance of the drilled structures.In this paper, static and the fatigue performances of aluminum–aluminum (AW 6082-T6) single-lap bolted joints are studied. In particular, the study compares the mechanical strength of only drilled single-lap bolted joints (OD specimens) and single-lap bolted joints subject to an indentation process (IP specimens). In order to determine the cycles to failure and the corresponding Wöhler diagram, several fatigue tests are performed. The analyses allow to determine the mechanical performance and the failure mode of the analyzed joints.Several numerical analysis, conducted in ANSYS environment on three-dimensional models of the single-lap joint, are focused on the evaluation of the residual stress on the indented plate and, in particular, to compare the stress distribution on both type of analyzed joints.     

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.