铝合金压印接头的强度研究

首次提出了用于汽车生产中分瓣模压印连接接头强度和失效形式的预测方法。根据接头静力学测试中的颈部断裂失效和上下板拉脱失效两种失效形式分别建立了压印接头的两个强度预测公式,2pπ2N N NF A R t t()和2p pπt b s F R,公式以接头颈部厚度Nt和镶嵌量Ut为重要的中间变量。强度预测公式表明:对于颈部断裂的压印接头,颈部厚度值tN越大,接头强度越高;对于拉脱失效的压印接头,接头强度取决于颈部厚度tN和镶嵌量tU,两者之和越大,接头强度越高,并且镶嵌量对接头强度的影响与颈部厚度相比更大。对颈部厚度变化范围为0.35mm~0.56mm、镶嵌量变化范围为0.045mm~0.45mm的15种组合接头,根据强度预测公式计算了接头强度,并进行了拉伸-剪切试验。将计算结果与试验结果进行对比,结果表明二者吻合较好,最大接头强度误差为8.9%。这说明本文建立的接头强度预测公式能够准确地预测压印接头拉伸-剪切过程的强度和破坏形式。     

钛合金异质自冲铆接头力学性能及失效机理分析

对钛合金异质单搭及T型自冲铆接头进行拉伸-剪切实验和剥离实验,对比接头的静力学性能,用扫描电子显微镜对接头典型断口进行研究,分析其微观失效机理。结果表明:TA1-Q215接头的最大失效载荷大于相同搭接类型的TA1-AA5052接头;TA1-AA5052单搭接头因下板断裂失效,为韧性断裂特征;TA1-Q215单搭接头上板铆钉孔撕裂处为韧窝形貌,铆钉与下板分离过程中,下板发生塑性变形并与铆钉产生严重刮擦现象;TA1-AA5052T型接头因铆钉与下板分离失效,下板出现明显的撕裂棱;当TA1-Q215T型接头失效形式为铆钉与上板分离时,上板撕裂处为准解理断裂机制;当铆钉与下板分离时下板撕裂处为韧性断裂,铆钉从下板剥离过程中产生刮痕。     

铝合金 2A12韧性断裂机制的实验研究

用轴对称圆棒静拉伸实验研究了铝合金2A12韧性断裂的特点,并探讨了临界空穴扩张比参数-VGC在实验材料上的适用性。实验测试了实验材料的拉伸特性参数及VGC参数。实验证实实验材料在不同应力状态下会发生断裂形式的转变,光滑试样表现为剪切破坏,而缺口试样为拉伸破坏,VGC参数对材料的断裂形式较为敏感。用SEM观测了试样断口显示缺口试样为空穴聚合型断裂机制。VGC参数只适用于以拉伸型断裂为主的缺口试样,对以剪切断裂为主的光滑拉伸试样不适用。     

Ti-6Al-4V在高应变率下的动态剪切特性及失效机理

采用基于霍普金森压杆的新型加载技术对Ti-6Al-4V材料的动态剪切特性及失效机理进行了测试研究。获得了Ti-6Al-4V材料在超过104 s-1应变率下的剪应力-剪应变曲线及失效参数。研究发现,材料的流动应力存在明显的应变率强化效应;随着应变率的增加,材料的失效应力逐渐增大,而失效应变逐渐减小。采用ABAQUS/Explicit对加载过程进行了数值模拟。结果显示,剪切区材料基本处于平面剪切状态,应力应变场分布较为均匀,计算得到的剪应力-剪应变曲线与实验结果吻合较好。经断口分析可知,随着应变率的升高,Ti-6Al-4V的失效机理存在由韧窝、拉伸韧窝至台阶及河流花样的演化过程,材料的失效模式主要表现为韧性断裂。     

高强铝合金的动态拉伸断裂行为实验研究

在分离式霍普金森拉杆、三点弯曲和平板撞击加载下对棒材铝合金（2024-T4、7075-T6）进行动态拉伸断裂实验研究。实验结果表明：1）一维应力动态加载下 7075-T6铝合金的初始屈服应力与断裂应变明显高于2024-T4铝合金,但三点弯曲和平板撞击层裂实验中发现2024-T4铝合金相比于7075-T6铝合金具有更好的抗裂纹扩展与层裂失效能力,这表明应力状态对两种铝合金拉伸断裂行为有明显的影响; 2）断口的光学与扫描电镜分析发现：2024-T4铝合金主要表现出脆性断裂行为,起因于孔洞或裂纹主要成核于晶内强化相形成穿晶断裂;而7075-T6铝合金则展现出韧性和脆性混合断裂特征,原因是部分孔洞或裂纹在晶界成核增长发生沿晶断裂,部分在晶内强化相周围形成孔洞从而造成穿晶断裂。, 在分离式霍普金森拉杆、三点弯曲和平板撞击加载下对棒材铝合金（2024-T4、7075-T6）进行动态拉伸断裂实验研究。实验结果表明：1）一维应力动态加载下 7075-T6铝合金的初始屈服应力与断裂应变明显高于2024-T4铝合金,但三点弯曲和平板撞击层裂实验中发现2024-T4铝合金相比于7075-T6铝合金具有更好的抗裂纹扩展与层裂失效能力,这表明应力状态对两种铝合金拉伸断裂行为有明显的影响; 2）断口的光学与扫描电镜分析发现：2024-T4铝合金主要表现出脆性断裂行为,起因于孔洞或裂纹主要成核于晶内强化相形成穿晶断裂;而7075-T6铝合金则展现出韧性和脆性混合断裂特征,原因是部分孔洞或裂纹在晶界成核增长发生沿晶断裂,部分在晶内强化相周围形成孔洞从而造成穿晶断裂。     

铝合金泡沫金属夹层板自冲铆接头力学性能研究

对AA5052铝合金泡沫金属夹层板进行自冲铆接试验,对接头的可连接性进行探究。通过静力学试验对比各组接头的静力学性能,分析不同种类泡沫金属对自冲铆接头力学性能的影响;用扫描电子显微镜对接头典型断口进行研究,分析其微观失效机理。结果表明:自冲铆技术可以实现泡沫金属夹层板的有效连接,接头具有良好的成形质量;泡沫金属夹层提高了自冲铆接头的静力学性能,其中泡沫镍夹层使AA5052自冲铆接头静失效载荷提高了4.1%;夹层板自冲铆接头失效形式均为铆钉与下板分离,其中泡沫铁镍夹层自冲铆接头下板铆扣断裂区域粘附在铆钉脚底部,为韧性断裂机制;下板内锁区域失效时铆钉与下板发生剧烈刮擦,呈现具有分层现象的片状组织。     

航空铆钉动态加载失效实验

为研究航空铆钉连接元件在动态加载下的失效模式,依托高速液压伺服材料试验机,通过特殊设计的实验夹具,进行了6类航空铆钉在不同加载速度和不同加载模式下的动态失效实验,获得了铆钉元件在纯拉伸加载、纯剪切加载、30°拉-剪耦合加载和60°拉-剪耦合加载状态下的失效实验数据。实验结果表明,铆钉元件的失效模式和失效载荷与加载速度、载荷的作用形式密切相关,并基于实验结果拟合得到铆钉元件的失效本构参数,获得了可表征铆钉在一般情形下失效的本构方程。     

复合材料层合板单搭胶接力学性能检测技术与分析

胶接工艺缺陷对单搭胶接接头的拉伸剪切性能有着重要的影响。为了研究不同单搭接胶接层厚度对不同材质复合材料层合板胶接性能的影响规律,通过喷水穿透法超声C扫描对试样的剪切区域进行无损检测,并分别采用1mm、2mm、4mm的胶层厚度,以碳纤维/玻璃纤维复合材料层合板为被粘物,进行单搭胶接拉伸剪切性能试验。检测及试验结果表明:当胶层厚度h1mm时,对于相同材料的被粘物,胶层厚度越大,试件胶接接头剪切强度越小;相同的粘接剂厚度,以碳纤维增强复合材料板为被粘物的试件胶接接头剪切强度大于以玻纤增强复合材料板为被粘物的试件胶接接头强度;胶粘剂与碳纤维被粘物表面的润湿效果要优于胶粘剂与玻纤被粘物表面的润湿效果。     

含材料非线性的复合材料单钉接头累积损伤分析

发展了静拉伸复合材料接头层合板三维逐渐损伤模型,考虑了单层复合材料在材料1-2面及3-1面上具有明显非线性剪切应力-应变关系的叠层非线性效应,结合有限元技术即应力分析、失效判定准则及损伤过程中材料性能退化等,对接头层合板损伤扩展进行了模拟,结果表明考虑材料非线性的影响与实验结果吻合更好.     

固体薄膜拉伸分叉行为的研究进展

薄膜材料被广泛应用到各个领域.柔性电子产品中大量使用薄膜组件和薄膜连接导线,在使用过程中会受到反复的拉伸、卷曲和折叠.薄膜在拉伸过程中发生的断裂及界面破坏是制约薄膜组件性能和广泛应用的重要因素,其元件和结构稳定性决定了产品的质量和使用寿命,因此,研究固体薄膜在拉伸载荷下的变形和断裂失效至关重要.本文着重从理论、实验和数值模拟三方面综述了国内外对薄膜拉伸分叉行为的研究情况,并提出薄膜基底结构在拉伸载荷下尚需解决的力学问题.     

Predicting the failure of ultrasonic spot welds by pull-out from sheet metal

A methodology for determining the cohesive fracture parameters associated with pull-out of spot welds is presented. Since failure of a spot weld by pull-out occurs by mixed-mode fracture of the base metal, the cohesive parameters for ductile fracture of an aluminum alloy were determined and then used to predict the failure of two very different spot-welded geometries. The fracture parameters (characteristic strength and toughness) associated with the shear and normal modes of ductile fracture in thin aluminum alloy coupons were determined by comparing experimental observations to numerical simulations in which a cohesive-fracture zone was embedded within a continuum representation of the sheet metal. These parameters were then used to predict the load–displacement curves for ultrasonically spot-welded joints in T-peel and lap-shear configurations. The predictions were in excellent agreement with the experimental data. The results of the present work indicate that cohesive-zone models may be very useful for design purposes, since both the strength and the energy absorbed by plastic deformation during weld pull-out can be predicted quite accurately.     

Finite Fracture Mechanics model for mixed mode fracture in adhesive joints

Up to now the failure load assessment of bonded joints is still not fully understood. This work provides a new approach for assessing the crack initiation load of bonded joints. A failure model for single lap joints is proposed that is based on Finite Fracture Mechanics. Only two basic fracture parameters are required: the tensile strength and the fracture toughness of the adhesive. A coupled stress and energy criterion proposed in 2002 by Leguillon is used to model crack initiation in the adhesive layer. The theory of this criterion is outlined in detail, its relationship to other failure criteria is discussed and an overview of applications found in literature is given. An enhanced weak interface model that predicts a linear variation of the shear stresses in the adhesive layer is utilized to model the single lap joint. To compare joint designs and to reveal the limitations of the given approach a dimensionless brittleness number for mixed-mode loading is proposed. Along with a detailed discussion of the results for exemplary joint designs a comparison to experimental results from literature is performed. The two necessary fracture parameters are each taken from standard test results published in literature. A good agreement of the failure load predictions with the experimental results is observed. A remarkable outcome is that the presented failure model renders the adhesive thickness effect correctly. The paper concludes with a discussion of the limitations of the approach and the effect of material parameters.     

On flaw size and distribution in lap joints

By using adhesive as the bonding substance between metals or polymeric materials, simple structural joints can be made to bear relatively high loads. Applications have increasingly been made in substituting adhesive joints for conventional mechanical fastenings, especially in the aircraft and aerospace industries where weight is a predominant factor. In order to design a most effective adhesive-bonded joint, an understanding of the stress distribution along the joint is as important as the physical properties of the bonding agent. One of the most common and widely used adhesive joints is the single lap joint.Recent investigations using various analytical models have revealed that the cause of failure in an idealized ‘defect free’ lap joint is primarily due to the localized effect of high stress concentration at the lap ends. With the presence of flaw like defects in the adhesive layer, the load transfer from adherend to adhesive is expected to be different from the idealized joint. In addition, localized stress concentrations induced by irregular adhesive defects that may be found in practical engineering applications can further reduce fracture strength of such an imperfect joint.This paper is intended to describe an investigation into the effect of internal adhesive flaw size and distribution on the fracture behaviour of adhesive-bonded lap joints. The finite element method is used to gain a quantitative understanding of the localized shear stress distributions due to the presence of the internal flaws along the bonding layer. It is observed that the reduction in the fracture strength is relatively small when a flaw is located in the central portion of the bonding length. However, a flaw located near the lap ends of the adhesive joint can cause marked reduction in the fracture strength, due to its interaction with the high stress concentration at the lap ends.     

Very high cycle fatigue behavior of bridge steel welded joint

Very high cycle fatigue (VHCF) behaviors of bridge steel (Q345) welded joints were investigated using an ultrasonic fatigue test system at room temperature with a stress ratio R = ?1. The results show that the fatigue strength of welded joints is dropped by an average of 60% comparing to the base metal and the fatigue failure still occurred beyond 10⁷ cycles. The fatigue fracture of welded joints in the low cycle regime generally occurred at the solder while at the heat-affected zone (HAZ) in the very high cycle regime. The fatigue fracture surface was analyzed with scanning electron microscopy (SEM), showing welding defects such as pore, micro-crack and inclusion were the main factors on decreasing the fatigue properties of welded joints. The effect of welding defects on the fatigue behaviors of welded joints was discussed in terms of experimental results and finite element simulations.     

Development of a technology for laser welding of the 1424 aluminum alloy with a high strength of the welded joint

Results of an experimental study of properties of joints obtained by using different regimes of laser welding of the 1424 alloy (Al–Mg–Li) are reported. The strength and structure of the welded joints are determined. The influence of various types of welded joint straining on its strength is studied. It is demonstrated that the joint strength increases in the case of plastic straining.     

The fracture behaviour of adhesively-bonded composite joints: Effects of rate of test and mode of loading

The present paper discusses the results of an investigation into the effects of test rate and the mode of loading on the fracture energy, G_c, of adhesively-bonded fibre-composite joints. Various carbon-fibre reinforced-polymer (CFRP) matrix composite substrates have been bonded using two different types of automotive structural epoxy-adhesives. They have been tested via loading the bonded joints in mode I (tensile), mode II (in-plane shear) and mixed-mode I/II from slow rates (i.e., of about 10^?5 m/s) up to relatively high rates of test of about 15 m/s. The high-rate tests were photographed using a high-speed digital video camera to record the deformation of the joint and the fracture behaviour. An analysis strategy has been developed for the various modes of loading (i) to account for the observed fracture behaviour, (ii) to circumvent the problems posed by oscillations in the load traces due to the presence of dynamic effects in the faster tests, and (iii) to account for the kinetic energy associated with the moving specimen arms in the faster tests. Based on the analysis strategy developed, the effect of the test rate on the fracture energy, G_c, for the different loading modes for the joints has been ascertained. Furthermore, various different fracture paths were observed in the tests. They were either cohesive, in the adhesive layer, or interlaminar in the composite substrates. The exact fracture path observed was a function of (i) the type of composite substrate, (ii) the type of adhesive, and (iii) the mode of loading employed. However, the nature of the fracture path was found to be quite insensitive to the test rate. Essentially, it was found that joints subjected to mixed-mode I/II loading were more likely to exhibit an interlaminar fracture path in the composite substrates than when loaded in either pure modes I or II. The propensity for a given joint to exhibit such a fracture path via delamination of the composite substrate has been explained by calculating the transverse tensile stresses induced in the loaded composite arms, and comparing this value to the measured transverse tensile strength of the composite. Following this approach, the underlying reasons for the observed fracture path were identified and could be predicted. Also, the proposed scheme provides a route to design against delamination failure occurring in adhesively-bonded fibre-composite test specimens.     

Embedded piezoelectric sensors to measure peel stresses in adhesive joints

Although peel stresses are believed to be responsible for failure in many adhesive joint geometries, the measurement of these peel stresses has been elusive. In this work, embedded poly(vinylidene fluoride) piezoelectric sensors were used to measure peel stresses in adhesively bonded joints. Piezoelectric KYNAR^® film was etched to produce multi-area stress sensors which were bonded into adhesive joints. Calibration results and results for single-lap and elastomeric butt joints are presented. The elastomeric butt joint was compared to an analytical solution for the bond-normal stresses, and the single-lap joint results were compared to finite-element analysis. Promising features and liminations of this technique are discussed.     

粘接剂对钛合金异质自冲铆接头力学性能的影响

为了研究粘接剂对自冲铆接头性能的影响,分别以TA1-5052和TA1-Q215的单搭自冲铆接头及粘接-自冲铆复合接头为研究对象,采用剖面直观检测法分析接头成形质量,基于静力学试验研究各组接头的静力学性能及失效形式。结果表明:与自冲铆接头相比,复合接头的钉脚张开度和残余底厚值减小;粘接剂对TA1-5052单搭自冲铆接头失效形式无影响,但改变了TA1-Q215单搭自冲铆接头的失效形式;复合接头的静力学性能比自冲铆接头更优;相对于自冲铆接头,TA1-5052复合接头静失效载荷增加了17.5%,能量吸收值增加了13.3%;TA1-Q215复合接头静失效载荷增加了6.8%,能量吸收值增加了25.8%。     

冲击载荷下镁铝合金裂纹动态扩展过程的数值模拟

采用基于黏聚裂纹模型的扩展有限元方法,开展了镁铝合金结构冲击破坏过程的数值模拟研究。通过镁铝合金三点弯曲试样冲击实验,获得了不同子弹撞击速度下试样的冲击破坏模式。在此基础上,建立了实验结构的扩展有限元模型,并采用最大主应力准则,以及含损伤型的本构关系模拟材料的冲击断裂行为。对于裂纹尖端附近区域,采用黏聚裂纹模型模拟裂纹的断裂过程。对子弹速度分别为12.2、15.1、26.3 m/s的3种工况下镁铝合金试样的动态破坏过程进行了数值模拟研究,获得了与实验相一致的断裂模式。计算结果表明,试样以Ⅰ型断裂模式为主,裂纹沿初始预制裂纹方向扩展。当裂纹扩展到一定程度后,在试样韧带区域被撞击端附近,由于应力波及边界效应导致该区域处于复杂应力状态,试样出现复合型断裂模式,裂纹偏离原扩展路径,与本文实验结果相吻合。     

The effects of geometry and material properties on the fracture of single lap-shear joints

A review of the mechanics of lap-shear joints is followed by a detailed analysis of the problem using a cohesive-zone approach. The cohesive-zone model allows not only the influence of geometry to be considered, but also allows the cohesive properties of the interface and plastic deformation of the adherends to be included in the analysis. The first part of the paper examines the strength of elastic joints, with an emphasis on the effects of geometry, the cohesive strength of the adhesive, and mode-mixedness. The cohesive-zone models show a transition to the predictions of linear-elastic fracture mechanics under conditions where these are expected to apply. The second part of the paper examines the effect of plasticity in the adherends, and looks at the transition between the elastic and plastic regimes. The final part of the paper makes comparisons between the predictions of the numerical calculations and experimental observations for a model system consisting of a commercial adhesive used to bond an aluminum alloy. Using cohesive-zone parameters previously determined for this particular combination of materials, the numerical predictions show excellent agreement with the experimental observations.