嵌块长度对胶接接头应力分布影响的数值分析

利用弹塑性有限元方法研究了钢嵌块长度对单搭接接头应力分布的影响,结果表明,采用钢嵌块形成混合连接接头后,接头上的应力流线分布发生了变化,钢嵌块中存在的压缩轴向应力和负值剥离应力有利于提高接头的承载能力;嵌块长度的增加使胶层和被粘物中界面处的峰值应力显著增大,而嵌块中的应力变化不大,嵌块长度超过4mm后,胶层中的应力峰值急剧上升;在需用嵌块增强胶接接头时应注意采用适宜的嵌块长度.     

基于理论解的三种胶接接头简化有限元单元

建立了非对称胶接接头位移理论,它能严格满足包括胶层端头剪应力为零在内的所有边界条件。由于上被粘物、下被粘物具有不同材料和厚度,研究非对称胶接接头更具一般性。以位移理论解为基础,通过一组特定位移约束条件确立了被粘物纵向和横向位移函数,获得了非对称T形、L形和单搭接接头简化有限元单元的单元刚度矩阵。这些胶接接头简化单元能大幅降低整体有限元模型的自由度数量,避免以往胶层单元胶层厚度估计过大的不足,并考虑了被粘物间连续传力的特性,更适于大型有限元模型中对胶接接头的简化建模。精细有限元模型的数值验证表明,三种胶接接头简化有限元单元精度很好,使其应用到诸如汽车白车身、飞机等大型有限元模型中成为可能。     

变形反射镜薄膜应力与元件变形有限元研究

残余应力是变形反射镜面临的一个重要问题。考虑薄膜中残余应力的起源,利用有限元软件建立了变形反射镜有限元模型。基于所建立的有限元模型,计算变形镜各部分受薄膜残余应力作用后的变形与应力分布。与试验测量结果进行对比,计算结果表明,建立的模型是合理的,薄膜产生的残余应力为张应力,氟化物薄膜产生的张应力大于ZnS薄膜产生的压应力,残余应力主要集中在变形反射镜镜面上,基本呈放射状分布,外圈驱动器受到应力较大。     

刚度非平衡胶接接头的二维应力分析

建立一种通用的分析刚度非平衡胶接接头应力的二维弹性理论。该理论通过使用完整的几何方程和完整的本构方程,不但考虑被粘体的弯曲、拉伸、剪切效应和胶层的剪切和撕扯行为,而且严格满足包括接头端头处胶层剪应力为零的所有边界条件,适用于被粘体/胶层厚度和材料属性的任何组合。与二维线弹性有限元分析对比表明:该理论解有很好的精度。最后对刚度不平衡程度对胶层应力分布的影响进行参数研究。     

SiC/Ti-6Al-4V复合材料在横向拉伸载荷下界面失效行为的数值模拟

采用有限元模拟了SiC/Ti-6Al-4V复合材料冷却过程和横向拉伸试验过程, 横向拉伸试样采用十字形试样。分别建立了平面应力和轴对称有限元模型, 采用平面应力有限元模型计算环绕纤维圆周的界面微区应力分布, 预测界面失效机制。采用轴对称有限元模型分析复合材料界面脱粘过程以及残余应力对界面径向应力分布的影响。结果表明: 对于SiC/Ti-6Al-4V复合材料十字形试样,在横向拉伸载荷下的界面失效由径向应力导致,界面失效模式为法向失效, 剪切失效模式未发生; 十字形试样在横向拉伸载荷下界面初始脱粘位置处于界面中间; 随横向拉伸应力增加, 十字形试样的界面脱粘对称向两边扩展; 界面径向应力随残余应力降低而升高。     

一种T形胶接接头的简化有限元单元

以胶接接头位移理论为基础,通过一组特定的位移约束条件确立了被粘物纵向和横向位移函数,建立了对称T形胶接接头的简化有限元单元,并得到单元刚度矩阵。这些接头单元能够克服胶层厚度估计过大的问题,降低了整体有限元模型的自由度数量,适用于大型有限元模型中对胶接接头的简化建模。精细有限元模型的数值验证表明,胶接接头简化有限元单元精度很好,使其应用到诸如汽车白车身、飞机等大型有限元模型中成为可能。     

复合材料正交加筋层合圆柱壳结构阶梯式挖补修理的参数化研究

利用三维参数化有限元分析模型,对复合材料正交加筋层合圆柱壳结构的阶梯式挖补修理进行了参数分析.主要考虑了载荷情况、阶梯数、铺层顺序以及补片铺层错误等修理参数对阶梯式挖补修理结构胶层剥离应力和剪切应力分布情况的影响规律,并利用绝对值的平均值和均方差来描述胶层剪切应力剥离应力的分布情况,以确定最优的阶梯式挖补修理参数.基于该文建立的三维有限元模型可以深入地了解复合材料正交加筋层合圆柱壳结构阶梯式挖补修理区域胶层应力分布的详细情况,而且参数化分析结论也可为复合材料正交加筋层合板圆柱壳结构的阶梯式挖补修理方案的设计和分析提供参考.     

碳纳米管增强镁基复合材料热残余应力的有限元分析

为了探寻Ni层厚度对镀镍碳纳米管增强AZ91D镁基复合材料(Ni-CNTs/AZ91D)中热残余应力的影响, 在实验基础上, 建立不同Ni层厚度时Ni-CNTs/AZ91D复合材料的有限元模型, 模拟了Ni-CNTs/AZ91D复合材料中热残余应力的分布。研究发现: 在碳纳米管表面镀镍能够明显降低Ni-CNTs/AZ91D复合材料中的热残余应力。Ni-CNTs/AZ91D复合材料中, 热残余应力在Ni层厚度为6nm时最小; Ni层厚度由2nm增至6nm时, 热残余应力随着Ni层厚度的增加而减小; 当Ni层厚度超过6 nm时热残余应力随着Ni层厚度的增加而增大。复合材料中热残余应力的最大值随碳纳米管表面Ni层厚度的增加向Ni层与基体的界面移动。      

复合材料加筋板后屈曲特性研究

通过对复合材料加筋板进行轴向压缩实验和非线性有限元模拟,研究了其后屈曲阶段的损伤和破坏行为.加筋板在实验中被压缩至完全破坏,压溃的加筋板表现出复杂的后屈曲损伤,包括筋条脱粘、纤维断裂、分层、基体开裂等损伤模式.有限元模型中引入了Hashin准则和基于二次应力的胶层失效准则,失稳及破坏载荷的预测结果和实验值吻合较好.分析...     

6061-T6铝合金中厚板-节点套多层多道焊数值模拟

为研究铝合金中厚板-节点套接头在多层多道焊后的残余应力和变形分布,本文基于ABAQUS软件建立了该接头三维有限元模型,采用双椭球热源、生死单元法以及顺序耦合法,对6061-T6铝合金中厚板-节点套多层多道焊进行数值模拟,并分析了接头的温度场,以及在夹具约束下的焊接残余应力及变形的分布情况。研究结果表明:数值模拟与实际接头的熔池形状吻合度较高;摆动焊接过程中温度曲线呈多峰结构;焊件的升温速率明显大于冷却速率,且冷却速率随时间逐渐减小;焊接残余应力主要集中在焊缝及夹具区域,且小于6061-T6铝合金在室温下的屈服强度;接头的最大横向残余应力为129.9 MPa,中厚板上的横向残余应力大于节点套上的横向残余应力;接头的最大纵向残余应力为132.9 MPa,沿焊接方向,焊缝处的纵向残余应力呈山峰状分布;该接头在Y轴方向上的变形最大,为1.494 mm,该接头的最终变形结果为上凸变形。     

Finite Element Analysis of Mono- and Bi-Adhesively Bonded Functionally Graded Adherend

The stress concentration at the end of bonded lap joints is a major concern in the design and application of adhesive joints, and, therefore, many research works have been carried out to reduce the stress level in the bond line. Most of the proposed methods focus on changing adhesive geometries or properties to achieve an optimized model. In this paper, the stress and strain distribution for adherend with functionally graded properties was analyzed to investigate the effect of the adherend material properties and the type of joint on the stress distribution within bond line. The effect of ceramic volume fraction of the functionally graded materials (FGMs) on the stress concentration has been studied. Also, bi-adhesive joint is used as an alternative stress reduction technique for the joint. Results show that using bi-adhesively joint technique together with high-ceramic volume fraction FGMs can significantly reduce the shear and peel stress in the lap joint.     

The effect of the spew fillet on an adhesively bonded single-lap joint subjected to bending moment

The mechanical behaviors of an adhesively bonded single lap joint with a spew fillet under bending moment, where the widths of the upper-adherend and lower-adherend are not the same, were studied experimentally and numerically. Using AA2024-T3 aluminum alloy as adherend and DP460 as paste adhesive, four different types of single-lap joint samples (without spew fillet, with spew fillet at joint edges, with spew fillet at joint ends and with spew fillet at all edges) were produced for experimental studies. Stress analyses in the single-lap joint were performed with a three dimensional non-linear finite element method by considering the geometrical non-linearity and non-linear material behaviors of both adhesives (DP460) and adherend (AA2024-T3). The single lap joint with the spew fillet with different widths had a significant effect on the stress distribution. Additionally, the spew fillet increased the load carrying capacity of the joint and decreased the stress concentration of the joint.     

Coupled stress and energy criterion for composite failure: Pointwise versus averaged evaluation of the stress criterion

Abstract

In this work, failure loads and failure modes of single lap adhesive joints between composite laminates are investigated. To this aim, a coupled stress and energy criterion is applied and results are compared to numerical reference solutions using cohesive zone modeling and to experimental values from literature. Possible failure modes are adhesive failure along the adherend/adhesive interface, adherend failure as intralaminar failure in the first ply closest to the adhesive layer and interlaminar failure between the first and second ply. Suitable failure criteria adressing the different failure modes are implemented within the framework of the coupled criterion. The stress criterion is carried out in a pointwise or in an averaged manner, called point method or line method respectively. It is shown that two physically sound failure modes can only be predicted using the stress criterion in an averaged manner since the pointwise evaluation does not allow the formation of certain types of cracks.     

Application of two‐state M‐integral for analysis of adhesive lap joints

With the aid of the two‐state M‐integral and finite element analysis, the asymptotic solution in terms of the complete eigenfunction expansion is obtained for adhesive lap joints. The notch stress intensity is introduced to characterize the singular stress field near the notch vertex of adhesive lap joints. The proposed scheme enables us to extract the intensity of each eigenfunction term from the far field data without resort to special singular elements at the vertex. It turns out that a weak stress singularity is not negligible around the vertex when it exists in addition to the major singularity. For a thin adhesive layer, there exist two asymptotic solutions: one is the inner solution approaching the eigenfunction solution for the vertex at which the adherend meets with the adhesive and the other is intermediate solution represented by the eigenfunction series that would be obtained in the absence of the adhesive layer. An appropriate guideline for choosing the geometric parameters in designing the adhesive lap joints, particularly the overlap length or the size of the adhesive zone, is suggested from the viewpoint of minimizing the notch stress intensity. Copyright © 2001 John Wiley & Sons, Ltd.     

Thermal and geometrically non-linear stress analyses of an adhesively bonded composite tee joint

Adhesive bonding technique is used successfully for joining the carbon fibre reinforced plastics to metals or composite structures. A good design of adhesive joint with either simple or more complex geometry requires its stress and deformation states to be known for different boundary conditions. In case the adhesive joint is subjected to thermal loads, the thermal and mechanical mismatches of the adhesive and adherends cause thermal stresses. The plate-end conditions may also result in the adhesive joint to undergo large displacements and rotations whereas the adhesive and adherends deform elastically (small strain). In this study, the thermal and geometrically non-linear stress analyses of an adhesively bonded composite tee joint with single support plus an angled reinforcement made of unidirectional CFRPs were carried out using the non-linear finite element method. In the stress analysis, the effects of the large displacements were considered using the small displacement–large displacement theory. The stress states in the plates and the adhesive layer of the tee joint configurations bonded to a rigid base and a composite plate were investigated. An initial uniform temperature distribution was attributed to the adhesive joint for a stress free state, and then variable thermal boundary conditions, i.e. air flows with different velocity and temperature were specified along the outer surfaces of the tee joints. The thermal analysis showed that a non-uniform temperature distribution occurred in the tee joints, and high heat fluxes took place along the free surfaces of the adhesive fillets at the adhesive free ends. Later, the geometrical non-linear thermal-stress analysis of the tee joint was carried out for the final temperature distribution and two edge conditions applied to the edges of the vertical and horizontal plates (HP). High stress concentrations occurred around the rounded adherend corners inside the adhesive fillets at the adhesive free ends, and along the adhesive–composite adherend interfaces due to their thermal–mechanical mismatches. The most critical joint regions were adhesive fillets subjected to high thermal gradients, the middle region of HP, the region of the vertical plate corresponding to the free end of the vertical adhesive layer–left support interface. In addition, the support length had a small effect of reducing the peak stresses at the critical adherend and adhesive locations.     

Enhanced layerwise model for laminates with imperfect interfaces - Part 2: Experimental validation and failure prediction

In this paper, the layerwise model for laminates with imperfect interfaces and enhanced in Part 1 of this work is confronted with experimental results. The model calculations are validated by comparing its sliding predictions in double lap adhesive joints to the sliding measurements performed in a previous paper. The model predictions agree with the experimental results. Finally, the model is applied to the failure analysis in double lap joints and T-peel joints exhibiting adherend, adhesive and cohesive failures. The model calculations and pertinent failure criteria provide accurate predictions and may become a helpful tool suitable to the design of adhesive joints.     

Evaluation of fatigue strength of adhesively bonded single and single step double lap joints based on stress singularity parameters

Most adhesively bonded joints have stress singularity points at the corners of the adhesive/adherend interface. Recently, stress singularity parameters, i.e. the intensity of stress singularity, K, and the order of stress singularity, λ, have been used to evaluate the strength of adhesively bonded joints. However, in many cases, stress singularity fields of adhesive joints cannot be formulated strictly by using constant values of K and λ. To apply these parameters to evaluate the strength of an adhesive joint, it is necessary to determine a key stress component and characteristic range for calculating the apparent singularity parameters K_app and λ_app. In this study, the endurance limits of adhesively bonded single lap, cracked single lap and single step double lap joints are evaluated using the stress singularity parameters. The results indicate that fatigue failure criterion for these joints can be obtained by using the apparent singularity parameters K_app and λ_app which are calculated by the least square method for the maximum principal stress distributions in the range from 0.05 to 0.5 mm from the singularity point.     

New designs of adhesive joints for thick composite laminates

New joint designs are proposed for adhesive bonding of thick multilayered composite adherends. The objective is to reduce or eliminate the failure modes associated with delamination and tensile and/or shear failure of the surface plies that are often observed in lap joints, and provide for a better stress distribution in the adhesive. In contrast to lap-joint designs, which transfer in-plane tensile stresses and other loads from the adherends to doubler plates by out-of-plane shearing of the surface plies, the new joint configurations transfer most of the load by in-plane shear and normal stresses, through bonded inserts or interlocking interfaces which have the same thickness as the laminate adherends. Doublers will transfer a calculated percentage of the load. Finite-element evaluations of the internal stresses in laminates, joined in both the conventional lap method and the new manner, suggest that the proposed load-transfer mechanism may improve joint efficiency by substantially increasing the size of adhesively bonded areas, and by making the stresses in the adherends nearly uniform through the thickness of the laminate. Some of the designs allow for selected ratios of shear to normal stresses in the adhesive layers. The stress concentrations often found in conventional designs, in the adherend surface plies and the adhesive layer at the leading edges of the doublers, are substantially reduced.     

Ultrasonic Study of Environmental Damage Initiation and Evolution in Adhesive Joints

This article reports on an experimental study of environmental degradation of adhesive joints by an ultrasonic angle—beam technique. The technique is based on measurements of the frequency response of the reflection of obliquely incident ultrasonic signals from a joint bondline. Ultrasonic measurements were performed using a special ultrasonic goniometer with only one ultrasonic transducer. By this method, the degradation of single lap adhesive joints was studied as a function of exposure in NaCl solutions at 68°C under static tensile load. It was found that joint degradation is accompanied by a shift of the ultrasonic reflection spectrum minimum to a lower frequency. Two stages of adhesive joint environmental degradation can be distinguished: a) a relatively slow adhesive joint degradation dominated by adhesive creep, and b) delamination along the adhesive/adherend interface, leading to failure. Several degradation mechanisms are found in the first stage to affect the position of the spectral minimum. The first is adhesive creep caused by normal-to-bond-plane stress concentration at the joint overlap edges. This mechanism is found to have the dominant effect on the ultrasonic signature. Second, changes of the effective density and elastic moduli of the adhesive layer also affect the spectrum of the reflected signal. The third mechanism is the degradation of the adhesive—adherend interface. In the second stage of the joint degradation process, delamination along the adhesive/adherend interface occurs and is followed by joint failure. While the time span of the first stage changes significantly from joint to joint, the time span of the second stage (failure by delamination) in our conditions is about 30—40 h. The delamination results in a significant additional spectral minimum shift to a lower frequency that can be used as an indicator of failure initiation.