Fatigue and fretting of mixed metal self-piercing riveted joint

The fatigue behavior of self-piercing rivet (SPR) joints joining differing thicknesses of AA6111-T4 aluminum and HSLA340 steel sheets in lap shear geometry was investigated in this paper. Crack initiation in the aluminum sheet was the dominant failure mode, while unexpected rivet shank failure tended to occur at high loading levels. Fretting wear was also observed at interface between aluminum and steel sheets as well as between the rivet and sheets under sinusoidal cyclic tension–tension loading. An Energy Dispersive X-ray (EDX) analysis of fretting debris revealed the presence of oxides of aluminum and zinc. Fretting was shown to be critical to crack initiation. For initiations in the aluminum sheet, micro cracks were found to nucleate early in the fatigue life, and crack initiation life was found to be much shorter than crack growth life.     

TA1异质自冲铆接头力学性能及失效机理

通过自冲铆接对比试验获得接头最优铆接参数,并以此制备TA1钛合金板分别与Al5052铝合金板和H62铜合金板的异质自冲铆接头。通过静力学实验和疲劳实验研究异质接头的力学性能,并运用疲劳三参数经验公式拟合S-N曲线,最后利用扫描电镜和能谱仪进行断口分析和能谱分析进而研究接头的疲劳失效机理。结果表明,TA1-H62(STH)接头静失效载荷优于TA1-Al5052(STA)接头;且前者在低载荷下疲劳寿命优于后者,STA接头则在高载荷下优势明显。STA接头疲劳失效模式为下板断裂,STH接头则出现了两种失效模式;两板间及铆钉与上下板之间接触区域发生的剧烈微震磨损是导致疲劳裂纹萌生的主要原因。     

Fatigue resistance and residual strength of riveted joints in FML

Static and fatigue tests have been carried out on 190-mm-wide fibre metal laminate (FML) riveted lap joints. The specimens were made with a Glare 3-3/2-0.3 material, i.e. a laminate composed of three layers of 2024-T3 aluminium alloy, thickness 0.3 mm and two double layers (0/90) of pre-preg FM-94-27%-S2 glass fibre. Seven configurations were tested which differed in the number of rivet rows (two or three), in the rolling direction of metal layers (perpendicular or parallel to the load direction), in the rivet type (solid or special, such as Hi-Lok or Lock-Bolt), in the rivet material for solid rivets (7050-T73 or 2017-T3), in the rivet diameter (4.8 or 5.5 mm) and in the presence of interlaminar doublers in the overlap area (titanium, aluminium, glass fibre). An additional difference was in the pre-formed rivet head: solid rivets had countersunk head, while Hi-Loks and Lock-Bolts had protruding head. The fatigue tests demonstrated the efficiency of a selective local reinforcement in the overlap area; in some cases, the fatigue resistance was so high that fatigue cracks nucleated in the laminates, rather than in the overlap area, as commonly expected.     

铆钉分布形式对自冲铆接头力学性能的影响

该文采用Landmark试验机对5052铝合金自冲铆接头进行力学性能测试来研究铆钉分布形式对自冲铆接头力学性能的影响。获得了单铆钉自冲铆(简称SR)接头、横向分布双铆钉自冲铆(DRT)接头和纵向分布双铆钉自冲铆(DRL)接头的静力学和疲劳性能,采用正态分布和威布尔分布对试验数据进行分析检验其有效性。结果表明:DRT接头静强度约为SR接头的2倍;DRL接头静强度约为SR接头的1.9倍。DRL接头比DRT接头具有更好的能量吸收能力。双铆钉自冲铆接头的疲劳性能优于SR接头。铆钉分布形式对接头疲劳变形量影响较小,但是对寿命影响较大。当疲劳载荷较高时,DRL接头可提高疲劳寿命。随着疲劳载荷降低,DRT接头可显著增加疲劳寿命。     

A Comparative Study of Friction Self-Piercing Riveting and Self-Piercing Riveting of Aluminum Alloy AA5182-O

In this paper, self-piercing riveting (SPR) and friction self-piercing riveting (F-SPR) processes were employed to join aluminum alloy AA5182-O sheets. Parallel studies were carried out to compare the two processes in terms of joint macrogeometry, tooling force, microhardness, quasi-static mechanical performance, and fatigue behavior. The results indicate that the F-SPR process formed both rivet–sheet interlocking and sheet–sheet solid-state bonding, whereas the SPR process only contained rivet–sheet interlocking. For the same rivet flaring, the F-SPR process required 63% less tooling force than the SPR process because of the softening effect of frictional heat and the lower rivet hardness of F-SPR. The decrease in the switch depth of the F-SPR resulted in more hardening of the aluminum alloy surrounding the rivet. The higher hardness of aluminum and formation of solid-state bonding enhanced the F-SPR joint stiffness under lap-shear loading, which contributed to the higher quasi-static lap-shear strength and longer fatigue life compared to those of the SPR joints.     

High cycle fatigue mechanisms of aluminum self‐piercing riveted joints

A study examining the fatigue failure mechanism of self‐piercing riveted (SPR) joints between aluminum alloy 6111‐T4 and 5754‐O is presented in this paper. In particular, the high‐cycle fatigue behavior of the SPR joints in the lap‐shear configuration is characterized. Experimental fatigue testing revealed that failure of SPR joints occurred because of cracks propagating through the sheet thickness at locations away from the rivet. In‐depth postmortem analysis showed that significant fretting wear occurred at the location of the fatigue crack initiation. Energy dispersive X‐ray of the fretting debris revealed the presence of aluminum oxide that is consistent with fretting initiated fatigue damage. High‐fidelity finite element analysis of the SPR process revealed high surface contact pressure at the location of fretting‐initiated fatigue determined by postmortem analysis of failed coupons. Furthermore, fatigue modeling predictions of the number of cycles to failure based on linear elastic fracture mechanics supports the conclusion that fretting‐initiated fatigue occurred at regions of high surface contact pressure and not at locations of nominal high‐stress concentration at the rivet.     

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.     

Fatigue properties of self‐piercing riveted multi‐rivet joints in steel and aluminum sheets

Self‐piercing riveting (SPR) is an important joining technology for connecting steel and aluminum sheets. In this paper, AA6111 aluminum alloy and DP780 high‐strength steel were adopted to study the influence of fatigue on remaining static strength and energy absorption properties on self‐piercing riveting multi‐rivet joints. The results showed that energy absorption capacity of the specimens decreased significantly after high cycle fatigue. Fatigue reduced the remaining static lap shear strength of riveted specimens. Scanning electron microscopy (SEM) was used to analyze the cross section of fatigue specimens fractured by static tension. The results showed that fretting wear was found at the contact area between rivet and aluminum sheets. Fatigue bands and fatigue cracks appeared in fatigue specimens after high cycle fatigue, while those with low cycle fatigue specimens did not appear. Small cracks weaken the strength of the aluminum sheet, resulting in the static tensile strength of the riveted specimen with high cycle fatigue is lower than that of other fatigue specimens.     

Fatigue behaviour of clinched joints in a steel sheet

Clinch joining has been used in sheet metal work owing to its simplicity and because it facilitates the joining dissimilar metal sheets. In this study, monotonic and fatigue tests were conducted using coach‐peel and cross‐tension type specimens to evaluate the fatigue strength of clinch joints in a cold‐rolled mild steel sheet. The monotonic experimental results reveal that the coach‐peel specimen exhibits the lowest monotonic strength among the three specimen configurations. The coach‐peel and cross‐tension specimen geometries exhibit very low fatigue ratios, compared with the tensile‐shear specimen. The maximum von Mises and principal stresses at the fatigue endurance limit are much higher than the engineering tensile strengths of the steel sheet used to determine the three specimen geometries. Compared with the effective stress and maximum principal stress, the Smith–Watson–Topper fatigue parameter can be used for an appropriate prediction of the current experimental fatigue life. With regard to the coach‐peel specimen geometry, all samples exhibit pull‐out failure mode in the fatigue testing range. However, for the cross‐tension specimen geometry, mixed (pull‐out and interface) and interface failure modes occurred, depending on the number of cycles to failure.     

TA1纯钛与1420铝锂合金异质薄板自冲铆接微动疲劳特性

基于自冲铆接技术研究TA1工业纯钛(TA1)与1420铝锂合金(AL1420)异质薄板组合的可铆性,并采用硬度≥44HRC和≥46HRC的铆钉分别制备TA1/AL1420 (TAF),AL1420/TA1(ATF)和TA1/AL1420(TAS) 3组接头。在拉伸-剪切实验的基础上进行了高周疲劳实验,拟合出各组接头的F-N曲线,进一步利用电子扫描显微镜和能谱仪分析了各组接头的微动磨损机理。结果表明:ATF接头的疲劳强度相对较优,TAS接头则在中高载荷水平下优于TAF接头;各组接头的疲劳失效均始于微动磨损区域,微动磨损导致疲劳裂纹的萌生;微动磨屑现象的剧烈程度是影响疲劳强度的重要因素。在不同疲劳载荷水平下,发生剧烈微动磨损的区域不同,使得疲劳裂纹的萌生区域存在差异,最终导致同种接头出现不同的失效模式。     

Analytical and experimental investigation of fatigue and fracture behaviors for anti-double dog-bone riveted joints

Load-transferred experiments and fatigue tests of the anti-double dog-bone riveted joints were carried out, and the fracture surfaces were observed by scanning electron microscopy (SEM) in this paper. Based on the finite element analysis (FEA), the multi-axial fatigue life of the anti-double dog-bone riveted joints has been predicted by Smith–Watson–Topper (S–W–T) method and Wang–Brown (W–B) method respectively. And the results of FEA were compared with the experiment results. The results show that because of the deformation of the rivets and the holes, the load transferred by the rivets decreases with external load increase. The fracture of joints all occur at rivet holes, and the crack initiates at the dog-bone specimen closed to the rivet head. Compared with the result of W–B method, the result of S–W–T method is more conservative and reliable.     

The investigation of the tensile behaviors of single L‐joint type bonded with adhesive and rivet

In this study, tensile behaviour of single “L” type joints was investigated experimentally and numerically. Aluminium pieces with 2 mm, 3 mm and 4 mm thicknesses were used as joint materials. Overlap lengths were chosen as 10 mm, 15 mm and 20 mm. Joints were executed with two different rivets as steel and aluminium and 3 M 2216 epoxy based adhesive. Experiments were carried out by means of Shimadzu universal testing machine with 5 kN load cell. While the numerical study was carried out, using ANSYS Workbench, finite element analysis program, models were formed with the same properties as experimental works. Results showed that the samples with 20 mm overlap length had the highest strength of all models. Models with steel rivet had higher strength than models with aluminium rivets for all overlap lengths.     

Fatigue strength of self‐piercing riveted joints in lap‐shear specimens of aluminium and steel sheets

The self‐piercing riveting (SPR) process is gaining popularity because of its many advantages. This study investigated the fatigue strength of SPR joints in tensile‐shear specimens with dissimilar Al‐5052 and steel sheets. A structural analysis of the specimen was conducted. For this specimen, the upper steel sheet withstood applied load in a monotonic test and played a major role in the low‐cycle region. In the high‐cycle region, however, the harder surface of the upper steel sheet reduced the fatigue strength by enhancing fretting crack initiation on the opposite softer aluminium surface. Therefore, the fatigue endurance of the specimen was reduced. The fatigue endurance of a SPR joint with the combination of steel and aluminium sheets was found to be governed by the strength of the lower sheet, which is more vulnerable to the applied loading. Thus, it is desirable to use a stronger metal sheet as the lower sheet with regard to the fatigue performance. Scanning acoustic microscopy was effectively used to reveal and prove the formation and growth of subsurface cracks in SPR joints. The structural stress can predict the fatigue lifetimes of the SPR joint specimens within a factor of three.     

The driven rivet head dimensions as an indication of the fatigue performance of aircraft lap joints

Solid aluminium alloy rivets are used in joints of aircraft structures. Riveting implies a squeezing process of the rivet stem with large plastic deformations to form the driven rivet head. The dimensions of the driven rivet head (diameter D and height H) depend on the applied squeeze load F_sq. High squeeze loads are beneficial for the fatigue properties of riveted joints. The present investigation is focused on the relation between F_sq and the rivet head dimensions during controlled squeezing. Measurements of D or H will then allow estimation of the applied squeeze load for quality control of the riveting process with respect to the fatigue properties of the joint. Extensive test series were carried out with five rivet materials, three rivet diameters and two sheet materials (2024-T3 and Glare). An equation was derived for F_sq(D) and F_sq(H) based on assuming a zero volume change of the rivet during plastic deformation. The two material constants in the equations were empirically determined for the rivet materials. A satisfactory correlation was obtained.     

Rotation friction pressing riveting of AZ31 magnesium alloy sheet

A kind of joining method for magnesium alloys, rotation friction pressing riveting (RFPR), is proposed in this paper. In RFPR operation, a rivet with a plug rotating at high speed is brought to contact with the riveted sheets, generating frictional heat between the rivet and riveted sheets, which softens the sheet materials and enables the rivet to be drilled into the sheets under reduced force. When fully inserted, the rivet is stopped rotating, and the plug is immediately pressed into the shank of the rivet by a punch. The expansive deformation of the rivet shank occurs under the action of the plug, thereby forming a mechanical interlock between the rivet and the sheets to fasten the sheets together. The studies show that RFPR of AZ31 magnesium alloy sheet can be carried out at ambient temperature, and provides the joints with superior shear strength and fatigue property when compared with self-piercing riveting (SPR). The effects of the operating parameters of RFPR process on the quality of the joints were investigated in the study. The results shows that while the rivet rotation speed little affects the shear strength of RFPR joints, the punch pressure has a significant influence on the mechanical properties of the RFPR joints. A numerical analysis was also performed to understand the effect of the punch pressure on the interlock between the rivet and the sheets, and the stress and strain distribution inside the sheet materials around the rivet. The results show that the interlock increased with the punch pressure and there is residual compressive stress inside the sheet materials, which seems to explain the good fatigue property of RFPR joints observed.     

Fatigue strength reduction factors at rivet holes for aircraft fuselage lap joints

Fatigue test results to be utilized in a fatigue life prediction model for aircraft fuselage riveted lap joints are presented. A series of fatigue tests on aluminium alloy sheet coupons with an open hole and filled hole subjected to remote tension, pin loading and out-of-plane bending have been carried out. Such loading conditions were conceived to replicate local effects of the bypass load, transfer load and secondary bending respectively at the critical rivet row. It was shown that the fatigue response of the open hole specimens could be predicted with a satisfactory accuracy employing the Neuber fatigue notch factor. For the filled hole specimens considerably longer fatigue lives were observed than for the open hole specimens under the same type of loading; however, a reduction of the notch effect due to the hole filling was found to depend on the type of loading, interference magnitude and applied load level. A concept of utilizing the obtained results to quantitatively account in the fatigue life prediction model for filling the hole by the rivet shank, in addition to geometry parameters, is presented. Finally, consideration is given to load transmission by frictional forces and its implication for the fatigue properties of riveted lap joints.     

泡沫金属夹层板自冲铆接头的疲劳性能及失效机理

对铝合金自冲铆接头及泡沫金属夹层结构自冲铆接头进行疲劳实验,通过三参数经验公式采用S-N曲线拟合法绘制接头的F-N曲线,分析接头的疲劳寿命及泡沫金属夹层对自冲铆接头疲劳性能的影响;采用扫描电子显微镜对接头的疲劳失效断口进行观测,分析其微观失效机理。结果表明:泡沫金属夹层缩短了自冲铆接头的疲劳寿命,不同泡沫金属夹层对自冲铆接头疲劳性能的影响具有差异性,在高疲劳载荷下泡沫铜夹层接头疲劳性能更优。三组接头疲劳失效形式都为下板断裂,在高疲劳载荷下裂纹易在铆扣区域萌生,在中低疲劳载荷下裂纹萌生于下板一侧,沿铆扣区域下侧向板材另一侧扩展。     

TA1钛合金单搭自冲铆接头微动磨损机理

对TA1钛合金单搭自冲铆接头进行疲劳实验研究接头失效形式;用扫描电子显微镜和X射线能谱线扫描研究铆钉各部位微动磨损程度的差异和接头微动磨损机理;采用威布尔分布验证数据有效性.结果表明:接头疲劳失效形式主要为上板断裂,高周疲劳均为上板断裂,低周疲劳为上下板混合断裂;微动磨屑包含氧、钛、锌和锡元素,铆钉头部微动磨损程度高于铆钉腿部.微动磨损区出现严重脱层、微动磨屑堆积和微裂纹萌生等现象,随着微动磨损及剪切力共同作用导致接头断口部位出现大量微裂纹并逐步沿深度和宽度方向扩展为宏观裂纹,最终导致接头疲劳失效.     

铝锂合金T型自冲铆接头疲劳特性及失效机理

为研究T型自冲铆接头的疲劳特性,以铝锂合金(AL1420)同种及其与铜合金(H62)和不锈钢(410)异种组合制备的AA、HA和SA三组T型自冲铆接头为研究对象,基于其静力学试验采用正弦波进行拉-拉加载疲劳试验,用二参数威布尔分布对所得疲劳数据的有效性进行验证,采用最小二乘拟合直线得到拟合的F-N曲线,并采用SEM扫描电镜对各组典型疲劳失效断口进行微观分析。结果表明:SA组疲劳寿命对载荷变化最敏感,并且疲劳寿命随相对滑移量的增加而减小;由三组T型接头失效形式可看出接头薄弱环节有由铆钉向板材转移的趋势;从微观分析疲劳断口可知裂纹萌生区多为形貌特征较为平坦的准解理,裂纹瞬断区形貌更加突出,多为沿晶断裂或韧窝形貌。     

铝/钢搅拌摩擦辅助铆接接头界面特性研究

目的探究搅拌摩擦辅助铆接铝/钢接头界面的结合特征,解决传统铆接力学性能较低的问题。方法采用搅拌摩擦焊技术实现了对3mm厚不锈钢板和4mm厚的铝合金板的搭接点焊,采用OM及SEM对铝/钢接头界面结合情况进行分析,并利用EDS对界面处元素分布进行分析。结果搅拌头转速1180 r/min、焊接时间120 s、下压量0.2 mm时,铝/钢接头界面结合较好,平均拉剪力达到6519 N,且在铝/钢界面处产生FeAl金属间化合物。受摩擦热作用的影响,位于下板的铝母材晶粒发生长大变粗,铝铆钉与铝板结合紧密,铝铆钉与铝板的结合情况受搅拌头压力的影响更为显著。结论搅拌摩擦辅助铆接铝/钢异种合金,实现了铆钉与铝板和钢板的有效冶金结合,在铝钢结合界面处存在原子的互扩散现象,且有相应的金属间化合物生成。