内聚力模型参数对不同应力状态下粘接接头失效的影响

内聚力模型参数是影响粘接接头失效仿真分析的主要因素。本文基于内聚力单元建立碳纤维增强基复合材料(CFRP)/铝合金单搭接接头和对接接头的仿真分析模型,并通过试验验证其仿真精度。选取了内聚力单元的六个参数,即杨氏/剪切模量、模式Ⅰ/Ⅱ失效强度和模式Ⅰ/Ⅱ断裂韧性,采用单因素试验方法和正交试验方法设计试验方案,研究了内聚力单元参数对应力状态和失效载荷的影响规律。研究结果表明:内聚力模型单一参数对单搭接接头和对接接头失效载荷的影响呈近似线性关系;单搭接接头的失效载荷主要受模式Ⅰ断裂韧性和模式Ⅱ失效强度影响,对接接头的失效载荷主要受模式Ⅰ失效强度和模式Ⅰ断裂韧性影响;不同内聚力模型参数组合对失效载荷的影响存在一定的耦合作用。     

CRH3高速动车组空调通风口胶接结构设计

通过理论分析和计算确定了动车组空调通风口部件与铝合金车体胶接用胶粘剂的强度指标。介绍了胶粘剂的选择及胶接结构的设计原则,考查了搭接长度、搭接宽度、胶层厚度和被粘接材料厚度等对胶接件粘接强度的影响。结果表明:车体与空调通风口部件的胶接接头选择受剪切应力作用的搭接接头较适宜,并且搭接接头的承载能力随搭接长度或宽度增加呈先快速上升后趋于稳定态势;当搭接长度为10 mm、胶层厚度为6 mm、铝合金板厚度为5 mm且常温湿固化型单组分PU(聚氨酯)胶粘剂的剪切强度超过0.23 MPa时,搭接接头的承载能力相对最大。     

电热载荷对CFRP修理搭接接头剪切性能的影响

采用单层预浸料搭接简化构型,研究电热载荷对复合材料层板挖补修理搭接接头层间剪切性能的影响。通过自制的复合材料电热测试平台,测试了不同电流制度下,随搭接长度不同试样接头处温度分布。研究了不同电流强度下搭接接头温度变化规律,初步揭示了搭接接头长度-温度-电阻之间的关系,并对不同电流载荷下挖补搭接接头的层间剪切性能进行研究。研究结果表明,通电过程中,搭接区域温度升高较快,非搭接区域温度无明显变化。低电流时,电热促使接头树脂进一步固化,适当提高了接头搭接界面的粘接性能,其剪切强度略有上升;当通电电流过大时,接头温度急剧上升,对搭接区域产生过热损伤,降低了接头的剪切强度。     

单搭接接头承载能力与搭接长度关系定量描述

为了研究单搭接接头承载能力与搭接长度的定量关系,对不同搭接长度的单搭接接头进行了有限元分析,得到了单搭接接头承载能力与搭接长度的关系曲线图。并用曲线拟和的方法定量描述了接头承载能力与搭接长度关系。实验结果表明接头承载能力随搭接长度的增加而增长。接头承载能力与搭接长度为非线性关系,也验证了这种定量描述方法的合理性。     

粘接设计

负荷通过粘接接头,从一个元件往另一个元件传递是不均匀的。我们已知道造成这种粘接应力不均匀的各种因素。因而在设计粘接接头时,必需考虑各种可能的破裂方式。粘接技术最适宜于薄件的联接,粘接件越厚,粘接就越困难。粘接接头中传递剪切负荷的一个关键部位是弹性槽。这种弹性槽在搭接面中是个低负荷粘接区,它大致介予二个高应力粘接窄区的中心,高应力粘接窄区靠近搭接面的两缘,是负荷传递的有效区域。(见图1、2)     

CRH3型动车组侧窗的粘接结构设计

根据CRH3型高速动车组侧窗的结构,设计了侧窗粘接接头的形式及粘接密封结构;根据胶粘剂厂商提供的数据及粘接工艺,分析设计了侧窗粘接厚度和接缝宽度。研究结果表明:侧窗粘接接头应选择搭接接头,粘接厚度的理论设计值为5～10mm,接缝宽度的理论设计值为10～17mm;将动车组侧窗的受力情况进行模型化,通过理论计算,并引入衰减因子和安全因子,可推算出侧窗粘接用聚氨酯(PU)弹性胶粘剂的剪切强度需超过0.54MPa,而目前国内外PU结构胶的剪切强度均超过此数值。     

折曲单搭接胶接接头应力分布探讨

在同轴单搭接胶接接头的基础上,设计了一种新型折曲单搭接胶接接头。通过有限元方法研究了折曲胶接接头和同轴胶接接头的应力分布规律。研究结果表明:折曲单搭接胶接接头搭接区两端的应力峰值下降幅度均超过40%,并使搭接区出现应力峰值的位置从搭接区端部转移到中部,从而显著地提高接头的承载能力。当L1=7.5 mm时,胶层中应力分布较为理想,各应力呈对称分布,是一种优于普通同轴的胶接接头形式。     

粘接理论研究的一些新进展

近年来本文作者在粘接理论方面作了一些初步的研究,本文主要介绍粘接界面形成配位键的初步证实及用边界元法计算粘接搭接接头应力结果的分析。     

异质被粘物新型单搭接接头应力分析

利用有限元分析软件ANSYS建立了异质被粘物用2种胶分段粘接的单搭接接头的力学模型,分析了胶层中剪切应力的分布情况。结果表明,合理地布置高弹性模量胶粘剂及低弹性模量胶粘剂的位置,可以有效地降低刚度不对称接头胶层中应力的峰值,避免胶层因小刚度侧应力过高而导致接头过早地失效。     

湿热环境对铝合金接头性能的影响

选择单搭接接头,研究高铁车窗使用胶粘剂的耐候性,分别进行不同循环老化周期耐候性试验,研究不同老化周期对粘接性能的影响。通过对老化前后粘接接头准静态拉伸试验,确定其衰减比率和失效模式,并使用扫描电镜对失效接头断面进行表征。最后使用最小二乘法拟合出剩余强度与老化周期之间的关系曲线。研究结果表明:随着老化时间的增加,粘接强度退化明显,说明环境因素对粘接强度产生巨大影响。     

Optimal Design of the Adhesively-Bonded Tubular Single Lap Joint

In this paper, a method for the optimal design of the adhesively-bonded tubular single lap joint was proposed based on the failure model of the adhesively-bonded tubular single lap joint. The failure model incorporated the nonlinear mechanical behavior of the adhesive as well as the different failure modes in which the adhesive failure mode changed from bulk shear failure, via transient failure, to interfacial failure between the adhesive and the adherend, according to the magnitudes of the residual thermal stresses induced by fabrication.

The effects of the design parameters for the adhesively-bonded tubular single lap joint, such as the thicknesses of adhesive layer and adherends, the bonding length, and the scarfs of adherends, on the torque transmission capability and the efficiency of the adhesive joint were investigated.     

Optimal Design of the Adhesively-Bonded Tubular Single Lap Joint

In this paper, a method for the optimal design of the adhesively-bonded tubular single lap joint was proposed based on the failure model of the adhesively-bonded tubular single lap joint. The failure model incorporated the nonlinear mechanical behavior of the adhesive as well as the different failure modes in which the adhesive failure mode changed from bulk shear failure, via transient failure, to interfacial failure between the adhesive and the adherend, according to the magnitudes of the residual thermal stresses induced by fabrication.

The effects of the design parameters for the adhesively-bonded tubular single lap joint, such as the thicknesses of adhesive layer and adherends, the bonding length, and the scarfs of adherends, on the torque transmission capability and the efficiency of the adhesive joint were investigated.     

Reliability analysis of bonded joints with variations in adhesive thickness

Bonded joints are used in several industrial applications as a surrogate of more expensive repairs, but their reliability must be ascertained. Failure in a bonded joint mainly occurs in the adhesive due to stress concentrations that directly depend on the adhesive thickness. In practice, it is difficult to ensure a good accuracy of the final adhesive thickness, leading to uncertainty to its spatial variability. This uncertainty greatly influences the strength of the bonded joint. This work deals with one of the main key issues in bonded joints: the influence of the spatial variations in the adhesive thickness on the reliability of the joint and an excessive shear stress level caused by the adhesive thickness variations may lead to failure. This paper provides reliability analysis by considering the adhesive thickness as a stochastic field. The experimental thickness field is obtained so as to identify the stochastic parameters. These parameters are then introduced in a structural reliability model to evaluate the failure probability. Results show the influence of adhesive thickness uncertainty on bonded joint failure.     

On the use of fracture mechanics in designing a single lap adhesive joint

The design of adhesively bonded joints is a quite difficult task, due to the stress singularity that arises at the edges of the adhesive adjacent to the loaded substrate. This stress singularity makes any design approach based on elastic stress analysis inconvenient. A more convenient design tool for an adhesive joint should be based on its mode of failure. Most of the adhesive joints fail at the adhesive/adherend interface or very close to it in the adhesive layer. Therefore, a fracture theory such as linear elastic fracture mechanics (LEFM) can be used to analyse the failure of an adhesive joint. In this paper, the design of a single lap joint using a fracture mechanics parameter, i.e. the strain energy release rate (SERR), is discussed. The SERR is extracted from a finite element model using Irwin's virtual crack closure integral. A design equation relating the lap length to the adherend thickness through some design parameters is derived.     

Free Vibration Analysis of an Adhesively Bonded Functionally Graded Tubular Single Lap Joint

In this study, the three-dimensional free vibration analysis of an adhesively bonded functionally graded tubular single lap joint was carried out using the finite element method. The functionally graded tubes of the adhesive joint are composed of ceramic (Al₂O₃) and metal (Ni) phases varying through the tube thickness. The adhesive material properties, such as modulus of elasticity, Poisson's ratio, and density were found to have negligible effect on the first ten natural frequencies and mode shapes of the adhesive joint. The optimal design parameters of the adhesive joint, such as overlap length, inner radius of the inner tube, outer and inner tube thicknesses, and the through-the-thickness material composition variation were searched using both the artificial neural networks (ANNs) and the genetic algorithms (GAs). For this purpose, the natural frequencies and modal strain energy values were calculated for an adhesive joint with random geometrical properties and material compositions through the tube thicknesses, and were used for training the proposed artificial neural network models. The outer tube thickness, the inner tube-inner radius, and the compositional gradient exponent had considerable effect on the natural frequencies, mode shapes, and modal strain energies of the functionally graded tubular single lap joint, whereas the overlap length and the inner tube thickness had a minor effect. The GAs integrated with ANNs was employed to determine optimal design parameters satisfying both maximum natural frequency and minimum modal strain energy conditions for each natural mode of the tubular adhesive joint.     

Geometric Factors Impacting Adhesive Lap Joint Strength and Design

Too often adhesive thickness, adherend thickness and other geometric factors are not explicitly considered in adhesive joint design. This study includes experimental and computational research exploring the means of enhancing the engineering design process for adhesive lap joints to include such effects. It clearly demon-strated that both the cleavage stresses and the shear stresses, near the bond termini, play important roles in lap 'shear' joint failure. Finite Element and Fracture Mechanics analyses were used to examine the energy release rate applied to growth of cracks in adhesive lap joints. Lap joints with similar geometries to those analyzed were designed, fabricated and tested. In a separate set of experiments the bond termini were constrained in the direction normal to the uniaxial loading. If the strength of lap shear joints is dominated by the adhesive shear strength, then constraining the lateral motion of the bond termini should have little or no effect on the overall shear strength of the adhesive joint. This work clearly demonstrates that this is not the case. If cleavage stresses are important in lap joints then constraining the bond termini, in a direction normal to the bond area, should have a commensurate effect on the overall strength of the lap joint. None of the ASTM standardized 'lap shear tests' provide any insight into this premise. This paper also presents analyses and experimental results for lap joints to which several methods of lateral constraint were applied near the bond termini. The analytical and numerical methods described and used for explaining and predicting such effects might be a useful adhesive joint design tool.     

Debond and failure characteristics of a double-lap adhesively bonded joint

A novel double-lap joint design was used to bond steel adherends using a structural epoxy adhesive. Different levels of debond were built into the joint using a mold release agent during fabrication. The damping capacity measurements of the debonded specimens were obtained using a FFTbased impulse-frequency response vibration technique. The joint strengths were obtained by loading the specimens to failure in a servo-hydraulic MTS 850 test system. It was observed that the failure strength of the joint correlated well with the loss factor (a measure of damping). Empirical equations for predicting the strength of the joint in terms of the loss factor or resonant frequency are presented. A torsional vibration test rig was also used to evaluate the damping properties and to predict the mechanical properties of the bulk adhesive used in the fabrication of the adhesive joint. SEM fractographs of both the bulk adhesive specimen and the debonded joints are examined and the modes of failure presented.     

A numerical study of adhesively bonded lap joints

The two types of joint discussed in this paper are a thick adherend symmetrical lap joint, and a symmetrical double lap joint. The effect of varying adherend and adhesive thicknesses on the stress distribution in the thin adhesive layer is discussed. These analyses were used in the design on a lap shear test to characterize certain aerospace adhesives used in bonded repair of structural components. An alternative analytical approach for the estimation of the load-carrying capacity of the double lap joint is also presented.     

Stress analysis and strength evaluation of single-lap band adhesive joints of dissimilar adherends subjected to external bending moments

Single-lap band adhesive joints of dissimilar adherends subjected to external bending moments are analyzed as a four-body contact problem using a two-dimensional theory of elasticity (plane strain state). In the analysis, the upper and lower adherends and the adhesive which are bonded in two regions are replaced by finite strips. In the numerical calculations, the effects of the ratio of Young's moduli of the adherends, the ratio of the adherend thicknesses, and the ratio of the band length to the half lap length on the stress distributions at the interfaces are examined. A method for estimating the joint strength is proposed using the interface stress and strain obtained by the analysis. An elasto-plastic finite element analysis (EP-FEA) was conducted for predicting the joint strength more exactly. Experiments to measure strains and the joint strength were also carried out. The results show that the strength of a single-lap band adhesive joint is almost the same as that of a single-lap adhesive joint in which the two adherends are completely bonded at the interfaces. Thus, the single-lap band adhesive joints are useful in the design of single-lap joints.