Microstructure investigation and mechanical property analysis in electromagnetic riveting

Electromagnetic riveting (EMR) is a new joining technology based on high-speed magnetic impulse forming and conventional riveting. EMR experiments were conducted on dual-layered 2A12 aluminum alloy sheets with one 2A10 aluminum alloy rivet. Observation of the microstructure and mechanical properties of rivet heading verified the forming quality of the heading. And, mechanical property tests were performed to evaluate the durability of riveted specimens through shear and pull-out loading. Microstructure observation indicated that adiabatic shear bands, where the deformation was highly concentrated, were an important characteristic of deformed structures in rivet headings under electromagnetic riveting. The distribution law of hardness in rivet headings was obtained. The compressive yield strength of rivet headings increased 70 % over that of the original rivets. Riveted specimens were capable of withstanding shear loads of 23.2 kN and pull-out loads of 35 kN. Ruptures characterized by a scanning electron microscope were found to be due to shear failure.     

航空结构宽板铆接搭接件承载性能数值研究

为研究飞机蒙皮铆接搭接结构的力学特性,基于ANSYS软件建立宽板三排铆钉搭接件的三维有限元分析模型。考虑物面间的非线性接触、铆钉与板孔间的干涉配合、铆钉预紧力的影响,应用非线性有限元法对宽板铆接搭接件在恒定远端拉伸载荷作用下的承载性能进行计算。通过读取沿预设路径的位移分布数据,获得宽板搭接件产生的次弯曲和第三弯曲的变形趋势。从搭接板等效应力云图可以看出,靠近宽搭接板侧缘的沉头铆钉孔周围应力集中比较严重。计算每个铆钉的钉传载荷,获得宽板搭接件铆钉承载的分布规律。在调整铆钉预紧力和钉孔干涉配合量的情况下,给出三排铆钉载荷传递比的变化情况。针对宽板铆接搭接件的力学特性计算分析,可以为后续的搭接件结构多处损伤裂纹扩展和疲劳寿命研究奠定基础。     

Experimental and numerical studies of stress/strain characteristics in riveted aircraft lap joints

The fatigue property of riveted lap joint is greatly related to the riveting-induced residual stress, especially the stress distribution on the faying surface. However, an accurate study of the residual stress characteristics in the riveted sheet could be very difficult. In this paper, both numerical and experimental investigations were carried out on the stress/strain characteristics in riveted aircraft lap joints. A special specimen was designed for the test of strain variations on the faying surface of the sheet by microstrain gages. For the numerical simulation, the rivet squeezing process was analyzed using the explicit dynamic finite element (FE) method, whilst a general static FE analysis was employed for the elastic springback after the squeeze force was removed. A comparison of the strain variations between the experimental results and FE simulations shows a general good agreement, although there may be some difference for points measured near the hole surface. The FE analysis reveals that both compressive and tensile residual stresses could be introduced in the riveted sheet. Massive compressive residual stress can be created in the near-surface layer of the hole. However, the stress level is not always increased with increasing the squeeze force, and so is the improvement of fatigue life observed. Further study is still necessary to account for the fatigue life decreasing effect caused by a high squeeze force.

     

Effect of riveting process parameters on the local stress field of a T-joint

Rotorcraft airframes are complex structures designed to satisfy different goals; fatigue life represents one of the most critical issues, which must be guaranteed throughout the design and the construction phase. In this scenario riveted joints play an important structural role especially because of the variable loads they are subjected to. Their fatigue behaviour is strictly influenced by the local stress fields left very near to the holes during riveting manufacturing operations. Therefore, with the aim to improve design awareness, the effects of the different parameters involved during the riveting process are herein investigated. The effects of the squeeze force, the clearance, the rivet length and the clamping angle in the stress field of the joints are considered by means of numerical models. Detailed finite elements models (including rivet forming formation) are validated through experimental tests. These models are aimed at obtaining an accurate stress–strain field in the most stressed zone including the residual stress in the holes. Finally, using the literature fatigue data of Al8090-T81 and the Crossland multiaxial fatigue criterion, the influence of the riveting parameters on the fatigue strength is evaluated for optimisation purposes.     

铆钉铆接装配应力的分析和计算

分析了铆钉铆接时的成形过程。对铆接成形力和铆钉铆接成形后对板孔圆柱面上的径向分布压力及铆钉冒对板圆环面上的分布压力进行了计算。该分析和计算对工程实际具有一定的参考价值。同时，在对一些具有铆钉连接的板或梁进行有限元分析和计算时，解决了其力边界条件的问题。     

铜铝异种金属板压印连接与接头拉剪载荷研究

采用一种新的塑性成形连接方法--压印连接方法来实现铜、铝合金板的连接,通过拉剪实验研究了连接接头的力学性能,并分析了铜、铝合金板的排列顺序对接头拉剪载荷的影响。实验结果表明,对于铜、铝复合接头,铜板在上时接头具有较高的拉剪载荷(3kN),是铝板在上时拉剪载荷的4.6倍。根据接头失效形式对复合接头进行优化后发现：下层铝板的厚度增大时,接头的拉剪强度略有增大,拉剪过程中的能量吸收明显增大;提高下板材料强度时,接头的拉剪载荷明显增大;铜板作为上板进行压印连接时,失效形式为颈部断裂失效,且接头的拉剪强度较大。     

某型电子设备连接铆钉断裂分析

针对某型电子设备在使用中出现的铆钉断裂现象,建立了结构的全尺寸有限元模型,并根据断面特征定量反推了输入载荷。随后通过仿真分析,准确预测了铆钉的断裂位置,还原了铆钉断裂过程并给出了铆钉的疲劳寿命。计算结果显示过大装配应力和来自载机的振动载荷共同导致了铆钉疲劳破坏。     

Numerical analysis of static performance comparison of friction stir welded versus riveted 2024-T3 aluminum alloy stiffened panels

Most researches on the static performance of stiffened panel joined by friction stir welding（FSW） mainly focus on the compression stability rather than shear stability. To evaluate the potential of FSW as a replacement for traditional rivet fastening for stiffened panel assembly in aviation application, finite element method（FEM） is applied to compare compression and shear stability performances of FSW stiffened panels with stability performances of riveted stiffened panels. FEMs of 2024-T3 aluminum alloy FSW and riveted stiffened panels are developed and nonlinear static analysis method is applied to obtain buckling pattern, buckling load and load carrying capability of each panel model. The accuracy of each FEM of FSW stiffened panel is evaluated by stability experiment of FSW stiffened panel specimens with identical geometry and boundary condition and the accuracy of each FEM of riveted stiffened panel is evaluated by semi-empirical calculation formulas. It is found that FEMs without considering weld-induced initial imperfections notably overestimate the static strengths of FSW stiffened panels. FEM results show that, buckling patterns of both FSW and riveted compression stiffened panels represent local buckling of plate between stiffeners. The initial buckling waves of FSW stiffened panel emerge uniformly in each plate between stiffeners while those of riveted panel mainly emerge in the mid-plate. Buckling patterns of both FSW and riveted shear stiffened panels represent local buckling of plate close to the loading corner. FEM results indicate that, shear buckling of FSW stiffened panel is less sensitive to the initial imperfections than compression buckling. Load carrying capability of FSW stiffened panel is less sensitive to the initial imperfections than initial buckling. It can be concluded that buckling loads of FSW panels are a bit lower than those of riveted panels whereas carrying capabilities of FSW panels are almost equivalent to those of riveted panels with identical geometries. Finite elem     

Acoustic-Emission Testing of Failure in Samples of CFRP Exposed to Static and Heat Loads

The results of testing samples made of T700 carbon fiber reinforced plastic with a stress concentrator in the form of a hole 14 mm in diameter are considered. Some of the samples were heated to a temperature of +100°C, followed by exposure to a static tensile load, with the remaining samples statically loaded at a temperature of +20°C. In the process of loading, information was recorded using the method of acoustic emission. At first, the samples were loaded statically up to 50 kN, which was 50% of the average breaking load. Then the load was increased in increments ΔP = 10 kN until sample failure. It was observed that the simultaneous exposure of samples to static and heat loads decreased the load capacity of the samples.

     

X-cor夹层结构剪切强度的有限元分析

利用有限元软件分析了X-cor夹层结构的剪切强度及失效机理,提出了失效判据和材料刚度退化规则,明确了X-cor夹层结构的失效过程和模式;根据失效判据,采用有限元模型中发生刚度退化的单元及其分布模拟失效类型及扩展路径,说明了X-cor夹层结构的剪切失效机理。结果表明:在剪切载荷下,树脂区首先失效,然后是Z-pin拔出面板、Z-pin被剪断及Z-pin屈曲多重失效模式并存;有限元分析结果与试验数值吻合较好,说明了失效判据和刚度退化规则选择的有效性。     

基于ANSYS/FE-SAFE的单搭自冲铆接头的疲劳分析

采用有限元分析软件ANSYS建立单搭自冲铆接头模型,对接头进行静力分析,在此基础上结合ANSYS/FE-SAFE,基于正弦激励载荷,对单搭自冲铆接头的疲劳寿命和安全系数进行了分析,得出自冲铆接头的最大应力位置与疲劳破坏位置一致,均为上板与铆钉头接触的部位。从安全系数云图中可得到工作安全系数大于许用安全系数,满足条件要求。用ANSYS/FE-SAFE软件来进行疲劳分析可以为下一步研究提供一定的参考。     

轻型车身自冲铆接头拉伸剪切实验的研究

自冲铆接是一种很有潜力的连接工艺,尤其对于轻型材料(轻合金、复合材料等)的连接。文中对2 mm厚铝合金板5052的自冲铆连接试件进行了研究,分析了整个铆接过程;采用有限元分析软件ANSYS分析了自冲铆接结构;利用AG-IS力学实验机对铆接试件进行了拉伸剪切实验,通过8组实验数据分析对比,分析了铆接试件的断裂形式;客观地评价了自冲铆连接技术对汽车轻量化制造的重大意义。     

粘接剂对酸性环境中自冲铆接头静力学性能的影响

以AA5052-AA5052自冲铆接头（MJ）和AA5052-AA5052粘-铆复合接头（ZM）为研究对象,0.02 mol/L的NaHSO3溶液作为腐蚀液,通过干湿周浸实验、静力学测试和能谱分析,对失效载荷、失效形式以及形貌变化进行分析,对比两组接头的静力学性能。结果表明：粘接剂的使用,对接头静强度有一定的促进作用;由于腐蚀产物的影响,接头静强度均呈现先减小后增大的趋势;粘接剂减小接头的能量吸收值,腐蚀时间对接头缓冲吸震的能力未产生较大影响;MJ失效形式均为T型失效,ZM分为T型和C型失效,接头失效形式也受粘接剂的影响;板材表面成分和铆钉腐蚀后也发生明显变化。     

铝合金自冲铆成形过程最大冲压载荷的反演模型

基于材料塑性变形虚功原理并结合上限定理,展开自冲铆成形过程最大冲压载荷的反演分析.以成形接头几何特征参数(钉脚张开度、铆钉几何尺寸参数、被连接材料的厚度)、铆钉和被连接材料力学性能参数以及冲头与铆钉的接触面接触条件为反演计算模型的控制量,建立最大冲压载荷的反演计算模型;同时,针对多种铝合金自冲铆成形开展最大冲压载荷的验...     

Effect of variations in the riveting process on the quality of riveted joints

Sheet metal parts are widely used in the assembly of aircraft. The most common method of assembling sheet metal parts is through riveting. Failure of a rivet can have severe consequences in terms of loss of human lives and money. There are many parameters associated with a riveting process that directly affect the quality of rivets including squeeze force, rivet length, rivet diameter, and hole diameter tolerance. Incorrect selection or variations in these parameters could induce excessive residual stresses that result in stress concentration sites and initiate cracks, and also result in improper rivet head deformation leading to loose rivets. This paper presents a study of the effect of the aforementioned riveting parameters on the quality of a formed rivet using finite element simulation. The study was part of a larger project carried out for the local aircraft industry and hence, uses the inch system. The research is conducted on a 1/8″ (0.125″) diameter rivet and a 0.064″ thick aluminum sheet. The results indicate that under normal variations in the riveting process parameters, when using the recommended countersunk depth of 0.042″ for rivet hole, most rivets that are formed will not meet the quality requirements. This is primarily due to the existence of a gap between the formed rivet and the hole. Decreasing the countersunk depth to 0.032″ allows for a higher range of hole and rivet diameter tolerances, as well as an increase in the range of squeeze force that can be applied without violating the quality requirements.     

三维四向编织复合材料接头承载能力分析

针对三维四向编织复合材料单耳承力接头,用细观力学和均匀化方法分析三维编织复合材料的细观结构,用MATLAB程序对复合材料弹性性能进行参数化求解,利用ANSYS估算两种几何尺寸试样的破坏载荷.有限元计算结果与试验值有较好的吻合,显示文中方法可以满足工程设计和分析的要求.     

Static and dynamic behaviour of composite riveted joints in tension

The tensile response and failure of composite riveted joints are studied experimentally in the present paper. Seven joint configurations for aircraft application are tested at quasi-static, 4 and 8 m/s nominal loading rates. Joint specimens are made of CFRP in a number of lay-ups of unidirectional tapes and woven fabrics. A dynamic tensile test method is designed to give reliable test results. It is shown that the variation of tensile strength with loading rate is negligible for the tested composite riveted joints. However, for most of the tested specimens, the average total energy absorption of the composite joint increases with increasing loading rate. Various failure modes are identified for various joint designs and it was found that joint failure modes can change with varying loading rates.     

钎焊金刚石工具钎焊强度评价的研究

利用高频感应钎焊技术,在不同的钎焊时间和真空度条件下制备金刚石工具试样,在自制的装置上测试试样单颗金刚石磨粒的剪切破坏力大小。结合对破坏颗粒的微观观察发现,金刚石磨粒在受剪切力时一般有三种破坏形式:滑移、折断和拔出。由此得出:制备钎焊金刚石工具应选用高级别的金刚石磨粒及高真空度下较长时间的钎焊。     

胶层缺陷对胶接-拉铆搭接剪切接头力学性能影响研究

近年来胶接-拉铆技术在载运工具和航空航天等领域得到了广泛应用,而大型结构连接部件的制备过程会不可避免地在胶层内部产生局部缺陷,进而影响接头服役性能。以胶接-拉铆单/双搭接接头为研究对象,通过在胶层区域设置不同形式的人工缺陷,模拟胶层制备过程中产生的胶层内部缺陷,研究其对胶铆接头搭接剪切力学性能的影响规律。以6061-T6铝合金为基底制备胶接-拉铆接头试件,考虑单、双搭接试件形式,胶层内部预先放置不同厚度、面积、形状、位置的聚四氟乙烯缺陷片,对固化后的接头进行准静态拉伸破坏试验,并采用宏观观测和扫描电镜分析胶层失效表面宏观和微观样貌,从而对胶层内部缺陷对其失效模式影响机理进行评价。试验结果表明,接头主要失效模式为内聚失效,胶层中缺陷的位置、面积、厚度会对接头失效载荷以及失效强度产生不同程度影响,而缺陷的形状对接头力学性能影响不大。无缺陷的双搭接试件胶层失效载荷是单搭接试件1.62倍,单搭接试件胶层失效强度是双搭接试件1.24倍。有圆形缺陷的双搭接试件失效载荷是单搭接试件2倍,而失效强度没有受较大影响。基于现有试验结果,可以为实际结构设计中含胶层缺陷胶铆复合接头失效强度提供有效评估方法。