Prediction of Cross-Tension Strength of Self-Piercing Riveted Joints Using Finite Element Simulation and XGBoost Algorithm

Self-piercing riveting(SPR)has been widely used in automobile industry,and the strength prediction of SPR joints always attracts the attention of researchers.In this work,a prediction method of the cross-tension strength of SPR joints was proposed on the basis of finite element(FE)simulation and extreme gradient boosting decision tree(XGBoost)algorithm.An FE model of SPR process was established to simulate the plastic deformations of rivet and substrate materials and verified in terms of cross-sectional dimensions of SPR joints.The residual mechanical field from SPR process simulation was imported into a 2D FE model for the cross-tension testing simulation of SPR joints,and cross-tension strengths from FE simulation show a good consistence with the experiment result.Based on the verified FE model,the mechanical properties and thickness of substrate materials were varied and then used for FE simulation to obtain cross-tension strengths of a number of SPR joints,which were used to train the regression model based on the XGBoost algorithm in order to achieve prediction for cross-tension strength of SPR joints.Results show that the cross-tension strengths of SPR steel/aluminum joints could be successfully predicted by the XGBoost regression model with a respective error less than 7.6%compared to experimental values.

Fatigue strength evaluation of self-piercing riveted joints of AZ31 Mg alloy and cold-rolled steel sheets

The application of magnesium alloys to automobiles is increasing due to their superior specific strength and specific stiffness.In this study,an upper sheet of AZ31 magnesium alloy and a lower sheet of cold-rolled steel were joined by self-piercing riveting(SPR),a method commonly used to join automotive panels.A cross-shaped specimen was fabricated with a punching force of 35 kN,which exhibited the best joint strength for the SPR joint specimen geometry.Monotonic and fatigue strengths were evaluated using cross-shaped specimens at loading angles of 0°,45°,and 90°.The load amplitude corresponding to the fatigue endurance limit was assumed to be at 106 cycles,and the fatigue ratios(=fatigue endurance limit/static strength)at the loading angles of 0°,45°,and 90°are 22%,13%,and 9%,respectively.For all three loading angle specimens,fatigue cracks initiated at the triple point where the rivet shank,the upper sheet and the lower sheet are in contact with each other,with the cracks propagating through the thickness of the upper sheet and ultimately leading to fracture.The fatigue lifetimes were evaluated through the von-Mises stress,maximum principal stress,and equivalent stress intensity factor.It was found that the fatigue lifetimes could be evaluated most appropriately through the maximum principal stress.

Mechanical joint performances of friction self-piercing riveted carbon fiber reinforced polymer and AZ31B Mg alloy

Carbon fiber reinforced polymer(CFRP) and AZ31B Mg alloy were joined by the friction self-piercing riveting(F-SPR) with different steel rivet shank sizes. With the increase of rivet shank size, lap shear fracture load and mechanical interlock distance increased. Ultrafine grains were formed at the joint in AZ31B as a result of dynamic recrystallization, which contributed to the higher hardness. Fatigue life of the CFRP-AZ31B joint was studied at various peak loads of 0.5, 1, 2, and 3 kN and compared with the resistance spot welded AZ31B-AZ31B from the open literature. The fatigue performance was better at higher peak load(>2 kN) and comparable to that of resistance spot welding of AZ31B to AZ31B at lower peak loads(<1 kN). From fractography, the crack initiation for lower peak load(<1 kN) case was observed at the fretting positions on the top and bottom surfaces of AZ31B sheet. When peak load was increased, fretting between the rivet and the top of AZ31B became more dominant to initiate a crack during fatigue testing.

Impact of Stack Orientation on Self-Piercing Riveted and Friction Self-Piercing Riveted Aluminum Alloy and Magnesium Alloy Joints

Self-piercing riveting(SPR)is a mature method to join dissimilar materials in vehicle body assembling.Friction self-piercing riveting(F-SPR)is a newly developed technology for joining low-ductility materials by combining SPR and friction stir spot welding processes.In this paper,the SPR and F-SPR were employed to join AA6061-T6 aluminum alloy and AZ31B magnesium alloy.The two processes were studied in parallel to investigate the effects of stack orientation on riveting force,macro-geomet-rical features,hardness distributions,and mechanical performance of the joints.The results indicate that both processes exhibit a better overall joint quality by riveting from AZ31B to AA6061-T6.Major cracking in the Mg sheet is produced when riveting from AA6061-T6 to AZ31B in the case of SPR,and the cracking is inhibited with the thermal softening effect by friction heat in the case of F-SPR.The F-SPR process requires approximately one-third of the riveting forces of the SPR process but exhibits a maximum of 45.4%and 59.1%higher tensile-shear strength for the stack orientation with AZ31B on top of AA6061-T6 and the opposite direction,respectively,than those of the SPR joints.The stack orientation of riveting from AZ31B to AA6061-T6 renders better cross-section quality and higher tensile-shear strength and is recommended for both processes.

Mechanical and fatigue properties of self-piercing riveted joints in high-strength steel and aluminium alloy

Static tensile and fatigue tests were performed on shear and tensile self-piercing riveted aluminium-steel structures to evaluate their mechanical and fatigue properties. The influences of the thickness and the strength of the high strength steel on mechanical and fatigue performances were investigated based on the tensile and F-N curves of the joints. The results show that mechanical and fatigue properties of the shear self-piercing riveted joints are much better than those of the tensile self-piercing riveted joints. Mechanical and fatigue performances of the two joints were significantly influenced by the thickness and strength of the steel sheet, and were markedly improved when the thickness of steel sheet increased. The steel strength showed significantly different effects on shear and tensile riveted structures, i. e. , when the steel strength increased, the strength of the shear structure greatly increased while the tensile structure just had a slight increase in the strength. Fatigue failure generally occurred in the sheet materials and the fatigue crack location changed with increasing the sheet thickness and the sheet strength.

Combined strengthening mechanism of solid-state bonding and mechanical interlocking in friction self-piercing riveted AA7075-T6 aluminum alloy joints

A recently developed friction self-piercing riveting(F-SPR)technique based on the combination of fric-tion stir processing and riveting has been reported to possess both solid-state bonding and mechanical fastening characteristics.However,there is still a lack of quantitative understanding of the hybrid en-hancement mechanism,hindering its engineering application.To fill in this gap,the current research investigated the microstructure evolution,microhardness distribution,and miniature-tensile performance of the aluminum alloy AA7075-T6 F-SPR joints by experiments.An accurate numerical simulation model was established to quantitatively evaluate the individual contributions of microstructure,local bonding strength,and macro interlocking to the performance of the joint,which could well explain the experi-mental results.It was found that due to the friction stirring of the rivet,solid-state bonding driven by dynamic recrystallization is realized between the trapped aluminum in the rivet cavity and the bottom aluminum sheet.The solid-state bonding zone has 75%yield strength,81%ultimate tensile strength,and 106%elongation compared to the base material.This solid-state bonding enables the internal interlock-ing between the trapped aluminum and the rivet to withstand the additional load,which forms a novel dual-interlock fastening mechanism and increases the peak cross-tension force by 14.3%compared to the single-interlock joint.

Microstructural evolution in friction self-piercing riveted aluminum alloy AA7075-T6 joints

Friction self-piercing riveting(F-SPR)is an emerging technique for low ductility materials joining,which creates a mechanical and solid-state hybrid joint with a semi-hollow rivet.The severe plastic deformation of work materials and localized elevated temperatures during the F-SPR process yield complex and heterogeneous microstructures.The cut-off action of the work materials by the rivet further complicates the material flow during joint formation.This study employed the F-SPR process to join AA7075-T6 aluminum alloy sheets and systematically investigated the microstructural evolutions using electron backscatter diffraction(EBSD)techniques.The results suggested that as the base material approached the rivet,grains were deformed and recrystallized,forming two distinct fine grain zones(FGZs)surrounding the rivet and in the rivet cavity,re s pectively.Solid-state bonding of aluminum sheets occurred in the FGZs.The formation of FGZ outside the rivet is due to dynamic recrystallization(DRX)triggered by the sliding-to-sticking transition at the rivet/sheet interface.The FGZ in the rivet cavity was caused by the rotation of the trapped aluminum,which created a sticking affected zone at the trapped aluminum/lower sheet interface and led to DRX.Strain rate gradient in the trapped aluminum drove the further expansion of the sticking affected zone and resulted in grain refinement in a larger span.

An Overview of Self-piercing Riveting Process with Focus on Joint Failures, Corrosion Issues and Optimisation Techniques

Self-piercing riveting(SPR)is a cold forming technique used to fasten together two or more sheets of materials with a rivet without the need to predrill a hole.The application of SPR in the automotive sector has become increasingly popular mainly due to the growing use of lightweight materials in transportation applications.However,SPR joining of these advanced light materials remains a challenge as these materials often lack a good combination of high strength and ductility to resist the large plastic deformation induced by the SPR process.In this paper,SPR joints of advanced materials and their corresponding failure mechanisms are discussed,aiming to provide the foundation for future improvement of SPR joint quality.This paper is divided into three major sections:1)joint failures focusing on joint defects originated from the SPR process and joint failure modes under different mechanical loading conditions,2)joint corrosion issues,and 3)joint optimisation via process parameters and advanced techniques.

Friction self-piercing riveting(F-SPR)of aluminum alloy to magnesium alloy using a flat die

Friction self-piercing riveting(F-SPR)process based on a pip die has been invented to solve the cracking problems in riveting high-strength and low-ductility light metals,such as magnesium alloys,cast aluminum,and 7 series aluminum alloys.In this paper,in order to solve quality issues caused by the misalignment between rivet and pip-die in F-SPR,a flat-die based F-SPR process was proposed and employed to join 1.27 mm-thick AA6061-T6 to 3 mm-thick AZ31B.The results indicate that a 1.0 mm die distance is effective to avoid rivet upset and insufficient flaring.As the feed rate increases,the heat input in the whole process decreases,resulting in a larger riveting force,which in turn increases both the bottom thickness and interlock amount.Besides,solid-state bonding,including Al-Mg intermetallic compounds(IMCs),Al-Mg mechanical mixture,and Al-Fe atom interdiffusion was observed at the joint interfaces.The upper Al layer was softened,but the lower Mg layer was hardened,and both sheets exhibited a narrowed affected region with the increase of feed rate,while the rivet hardness shows no obvious change.Three fracture modes appeared accompanying the variations in lap-shear strength and energy absorption as the feed rate increased from 2 mm/s to 8 mm/s.Finally,the F-SPR process using a flat die was compared to those using a pip die and a flat bottom die to show the advantage of flat die on coping with the misalignment problem.

Progressive Failure Analysis of Composite/Aluminum Riveted Joints Subjected to Pull-Through Loading

Out-of-plane mechanical properties of the riveted joints restrict the performance of the wing box assembly of airplane.It is necessary to investigate the pull-through performance of the composite/metal riveted joints in order to guide the riveting design and ensure the safety of the wing box assembly.The progressive failure mechanism of composite/aluminum riveted joint subjected to pull-through loading was investigated by experiments and finite element method.A progressive damage model based on the Hashin-type criteria and zero-thickness cohesive zone method was developed by VUMAT subroutine,which was validated by both open-hole tensile test and three-point bending test.Predicted load-displacement response,failure modes and damage propagation were analysed and compared with the results of the pull-through tests.There are 4 obvious characteristic stages on the load-displacement curve of the pull-through test and that of the finite element model:first load take-up stage,damage stage,second load take-up stage and failure stage.Relative error of stiffness,first load peak and second load peak between finite element method and experiments were 8.1%,-3.3%and 10.6%,respectively.It was found that the specimen was mainly broken by rivet-penetration fracture and delamination of plies of the composite laminate.And the material within the scope of the rivet head is more dangerous with more serious tensile damages than other regions,especially for 90°plies.This study proposes a numerical method for damage prediction and reveals the progressive failure mechanism of the hybrid material riveted joints subjected to the pull-through loading.

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.

Study of galvanic corrosion and mechanical joint properties of AZ31B and carbon-fiber–reinforced polymer joined by friction self-piercing riveting

A new testing methodology was developed to quantitively study galvanic corrosion of AZ31B and thermoset carbon-fiber–reinforced polymer spot-joined by a friction self-piercing riveting process.Pre-defined areas of AZ31B in the joint were exposed in 0.1 M NaCl solution over time.Massive galvanic corrosion of AZ31B was observed as exposure time increased.The measured volume loss was converted into corrosion current that was at least 48 times greater than the corrosion current of AZ31B without galvanic coupling.Ninety percent of the mechanical joint integrity was retained for corroded F-SPR joints to 200 h and then decreased because of the massive volume loss of AZ31B。

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 element method for simulating static performances of FSW and riveted stiffened panels is proposed and evaluated and some beneficial conclusions are obtained, which offer useful references for analysis and application of FSW to replace rivet fastening in aviation stiffened panel assembly.

Analysis of Fretting Fatigue Crack Initiation in a Riveted Two Aluminum Specimen

Based on the existing experiment results, the fretting fatigue contact geometry of a riveted two aluminum specimen was studied using the finite element method. The contact stress fields of the inner and outer contact edges on the two specimen’s up and down surface under different contact friction coefficient and the fatigue loads were analyzed, the influences of the contact friction coefficient and remote stress on crack initiation were discussed. The results were in well agreement with the observations of the existing experiments, that is, the crack initiated places of the first aluminum specimen change from the area of 900 to 450, and the crack initiated places of the second aluminum specimen change from the area of 900 to 1350 with the increase of the friction coefficient and the remote stress.

Numerical investigation on the position of holes for reducing stress concentration in composite plates with bolted and riveted joints

This paper studies the effects of fiber orientaion and holes position on stress concentration and the determination of weakened areas in the composite of glass fiber reinforced epoxy resin around the hole for joints by using the finite element method.In this study,for the observation of areas affected by stress concentration Tsai-Wu failure criterion is used to determine the failed elements and ANSYS Software is implemented for modeling.In order to compare the effect of geometric parameters on stress concentration around the holes,two types of hole position arrangement along with fibers orientation have been studied.Results show that the stress concentration coefficient is lower in the second type of holes arrangement in comparison with the first type for the same component dimensions.Increasing the distance from hole center to upper or lower edge of the sample and also decreasing the distance between holes,would result in an increase in the stress concentration.

铆接铝合金板铆钉失效缺陷检测方法研究

针对车身用铝合金板内部铆钉缺陷特征提取难度大、缺陷类型与程度识别准确率低的问题,提出一种基于高斯卷积深度信念网络与双向长短期记忆网络相结合的铆钉失效缺陷诊断模型与检测方法。首先,面向5种铆钉断裂缺陷设计试件并搭建自动检测系统,通过规划和调整探头姿态有效地降低提离效应对检测信号的影响。其次,设计双网络融合诊断模型提取和学习多维度缺陷特征信息,解决检测曲线中由时序变化特性和空间分布状态表征的缺陷信息提取难题。实验结果表明,与传统卷积网络及单一深度信念网络相比,优化后算法诊断模型的平均准确率为99.85%,相比提升了14.54%,且具有良好的通用性和鲁棒性,可实现铆钉内部缺陷的在线诊断。

轴向荷载作用下小规格拉铆钉和普通螺栓疲劳性能对比研究

拉铆钉因其良好的防松动、抗疲劳和防腐性能以及较强的环境适应性等特点而被广泛应用于航空航天、轨道交通、桥梁建筑等领域。为解决光伏支架中螺栓连接易发生疲劳的问题,现研究一种国产新型拉铆钉的轴向疲劳性能,用以代替普通螺栓连接进而提高光伏支架的疲劳寿命。对LMY8和LMY10小规格拉铆钉及同规格的普通螺栓进行了轴向荷载作用下的静力试验和疲劳试验,并利用ABAQUS软件进行了拉铆钉铆接过程的模拟,利用FE-Safe软件对拉铆钉疲劳寿命进行了模拟。研究结果表明:在同样条件下,铆钉的疲劳性能优于普通螺栓的疲劳性能;拉铆钉在轴向荷载作用下的疲劳断口位于套环和锁紧环槽啮合的第一圈螺纹处,此处应力集中是拉铆钉发生疲劳失效的主要原因;拉铆钉的疲劳寿命会随着最小荷载的增大而提高,随最大荷载的增大而降低;由疲劳云图可知,拉铆钉在承受轴向循环荷载作用时,铆钉头部下方R角过渡处也会产生较大的应力,此区域也是易发生疲劳失效的部位。

改良铆钉缝合法在后腹腔镜下肾部分切除术中的应用

目的探讨改良铆钉缝合法在后腹腔镜下肾部分切除术中的临床效果。方法选取2019年3月至2022年3月因肾细胞癌于郑州人民医院泌尿外科接受后腹腔镜下肾部分切除术的患者,共81例,回顾其临床资料,根据缝合方法分为研究组(改良铆钉缝合,41例)和对照组(倒刺线连续缝合,40例),总结临床资料,比较两组患者的热缺血时间、术中出血量、术后引流量、术后尿漏发生率、术后肾周血肿发生率、术前术后血红蛋白下降情况。结果研究组热缺血时间短于对照组,术中出血量、术后引流量、术后尿漏及肾周血肿发生率均低于对照组(P<0.05)。术后研究组血红蛋白水平高于对照组,差异有统计学意义(P<0.05)。对照组中出现1例术后尿瘘和2例术后肾周血肿,经过治疗后好转,研究组未出现并发症。结论改良铆钉缝合法在后腹腔镜下肾部分切除术中效果确切,可缩短肾脏热缺血时间,减少术中出血量及术后并发症,值得在临床推广。

基于改进DETR的机器人铆接缺陷检测方法研究

铆接作为铁道车辆结构件的主要连接方式,合格的铆接质量是车辆安全稳定运行的重要保证。针对现有铆接缺陷检测方法存在检测精度低、检测点位少、检测智能化水平不高等问题,提出一种基于改进DETR的机器人铆接缺陷检测方法。首先,搭建铆接缺陷检测系统,依次采集工件尺寸大、铆钉尺寸小工况下的铆接缺陷图像。其次,为了增强DETR模型在小目标中的图像特征提取能力和检测性能,以EfficientNet作为DETR中的主干特征提取网络,并将3-D权重注意力机制SimAM引入EfficientNet网络,从而有效保留图像特征层的镦头形态信息和铆点区域的空间信息。然后,在颈部网络中引入加权双向特征金字塔模块,以EfficientNet网络的输出作为特征融合模块的输入对各尺度特征信息进行聚合,增大不同铆接缺陷的类间差异。最后,利用Smooth L1和DIoU的线性组合改进原模型预测网络的回归损失函数,提高模型的检测精度和收敛速度。结果表明,改进模型表现出较高的检测性能,对于铆接缺陷的平均检测精度mAP为97.12%,检测速度FPS为25.4帧/s,与Faster RCNN、YOLOX等其他主流检测模型相比,在检测精度和检测速度方面均具有较大优势。研究结果能够满足实际工况中大型铆接件的小尺寸铆钉铆接缺陷实时在线检测的需求,为视觉检测技术在铆接工艺中的应用提供一定的参考价值。

螺栓连接和拉铆连接在剪切循环荷载作用下的松动行为研究

拉铆连接凭借其优秀的防松性能,被应用于越来越多的工程领域。为研究国产新型拉铆钉在剪切循环荷载作用下的松动性能,以应用最为广泛的螺栓连接为对照,分别对规格为8 mm的4.8级普通螺栓、4.8级防松螺栓以及小规格拉铆钉进行剪切循环荷载下的松动试验研究,分析了三类连接在剪切循环荷载作用下的松动过程,研究了荷载幅值和频率对螺栓连接和拉铆连接松动行为的影响。结合微观测试手段分析其损伤形貌,进一步研究剪切循环荷载下螺栓和拉铆钉的防松原理及微动损伤机制。研究表明:在同种工况下拉铆连接的防松性能优于螺栓连接的防松性能;当荷载幅值增大或荷载频率减小时,螺栓连接和拉铆连接的松动程度增大;相较于螺栓,拉铆钉独特的圆弧结构牙型和铆接成型后牙型配合面的过盈配合机制,使其防松性能有明显提升;螺栓与拉铆钉的磨损机制主要是磨粒磨损和疲劳磨损。