复合材料双搭接胶连接接头承载力计算中的胶层厚度问题

现有复合材料双搭接胶连接接头承载力计算公式只在有限的胶层厚度范围内适用,有修正必要。利用已有试验数据,解析分析了胶层厚度变化引起的接头承载力改变与其剪应力、剥离应力之间的关系,发现胶层增厚,接头承载力减小,胶层的剪应力和剥离应力均减小,剥离应力不是接头破坏的主要原因;同时,试件胶层没有明显缺陷,缺陷也不是接头破坏的主要原因。提出胶层增厚引起的胶层剪切性能改变是接头承载力减小的主要原因,可作为公式修正的思路。     

飞机单搭接接头胶接形式的优化

1前言飞机蒙皮结构的绝大部分为单面敞开式，这就决定了蒙皮破孔的原位胶接修理只能单面操作，接头形式只能采用单搭接。单搭接接头是诸多接头形式中的一种，通过考察分析，其接头形式的细节还能优化改进。本文优化研究重点是接头形式的优化，没有涉及参数优化。2接头形式优化计算结果在铝合金板与胶层组成的单搭接结构中，胶层是薄弱环节。胶层内部主要承受剪切应力，胶层两端，在垂直于胶层方向上还集中有正应力，是结构分析的危险部位。其胶层内部应力分布如图1所示。图1胶层中剪应力分布（不计塑性）本文拟采用线弹性有限元对四种胶接头…     

胶-螺混合连接承载力的参数影响研究

针对胶层与螺栓可以协调变形的胶-螺混合连接接头,通过理论分析与实验对比的方法,研究了胶层厚度、螺栓位置、螺栓刚度等参数对胶-螺混合接头承载力的影响规律与机理,分析了现有承载力计算公式的适用范围。研究结果表明,接头相关几何力学参数的变化将影响接头的破坏形式和承载力,现有承载力计算方法有一定的适用范围。     

不同胶厚对硬聚氯乙烯板搭接强度的影响

为验证不同胶层厚度对硬质聚氯乙烯(PVC)板材单搭接接头强度的影响,对不同胶层厚度的单搭接板材件进行拉伸-剪切载荷试验,确认胶层厚度会影响搭接接头的粘接效果,且胶层厚度由0.2 mm增加到2 mm时,搭接件能承受的最大应力由3.59 MPa减小至2.51 MPa;通过有限元仿真得出,当胶层的厚度由0.2 mm增加到2 mm时,搭接件能承受的最大应力由3.32 MPa减小到2.59 MPa;实验与仿真均得出了随着胶层厚度增加,搭接强度变弱的结果。对不同胶层厚度的搭接板材进行仿真,得出最大应力均出现在胶层的边缘位置,因此胶层边缘位置容易先发生破坏;与无胶瘤时相比,胶瘤的存在能减少胶层边缘处的应力集中现象。与无胶瘤的搭接板材件相比,胶瘤存在时的搭接板材件强度提升了20%,胶瘤的存在显著增加了搭接的强度性能。仿真结果与实验结果相比较,误差在10%内,仿真模型有较好的准确度。     

胶层Ⅰ/Ⅱ型断裂破坏内聚单元参数的确定和应用

建立了铝合金板单搭接胶接接头的三维弹性有限元模型,通过与G-R解析模型的胶层剪应力和剥离应力分布对比分析,验证了有限元模型的有效性。经有限元仿真拟合铝合金板单搭接胶接接头静拉伸试验的载荷-位移曲线,获得了胶层Ⅰ/Ⅱ型混合失效模式下内聚力单元的断裂参数值和胶层渐进破坏的过程,将胶层内聚力单元断裂参数值应用于含双裂纹复合材料胶接修补结构中,预测了修补结构的静拉伸强度。结果表明,有限元模型预测的剩余强度与实验值的相对误差为3.8%,因此运用内聚力单元仿真胶层的方法是有效的,并为复合材料胶接修补结构的承载能力分析提供了有力的依据。     

热-力载荷下复合材料/金属双面胶接接头界面力学模型

在温度和静拉伸载荷共同作用下,考虑胶层的材料非线性,建立了复合材料/金属双面胶接接头界面的力学分析模型,推导出弹性响应和塑性响应下胶层剪应力的分段表达式,使用胶层最大剪应变失效准则计算出胶层主导破坏的结构极限载荷,并与有限元数值结果进行对比和验证。分析表明,双面胶接接头应力分析理论模型与相关简化假设正确、合理。在此基础上,研究了复合材料/金属双面胶接接头在热-力载荷下的胶层剪应力分布特点、破坏模式和失效机理,为胶接结构的承载能力分析及结构改进设计提供理论依据。     

CFRP与铝板胶螺混合连接结构拉伸性能研究

基于三维渐进损伤理论,建立了复合材料层合板-铝板双搭接胶接连接、螺栓连接、胶螺混合连接结构拉伸强度预测模型,数值仿真结果与试验高度吻合,验证了所建模型的可行性。在此基础上,探究了搭接宽度、搭接长度、胶层厚度、接触面摩擦系数和螺栓个数等参数对胶螺混合连接结构拉伸性能的影响。结果表明:随着搭接宽度和搭接长度的增加,接头失效载荷先逐渐增加后趋于稳定,最优搭接宽度和搭接长度为30 mm和35 mm;胶层厚度对混合连接结构的拉伸失效载荷基本没有影响;胶螺混合连接结构中螺栓接头和层合板之间、螺栓与孔之间的摩擦系数越大,连接结构的拉伸失效载荷越大;在搭接区域相同的情况下,双钉混合连接结构的拉伸失效载荷比单钉的拉伸失效载荷提高了69%。     

冲击载荷作用下胶层厚度对金属双搭接胶接接头应力分布的影响

采用LY12铝合金材料在Ansys12.0软件平台上建立环氧树脂胶粘剂双搭接胶接接头有限元模型进行分析,着重考察了在冲击载荷作用下胶层厚度的改变对胶接接头应力分布的影响。研究结果表明:在一定范围内,随着胶层厚度的增加,胶接接头中最大应力值逐渐下降,而最小应力值开始先下降,在胶层厚度为0.4 mm时最小,之后应力值又继续增大,且最大应力值总是出现在胶层界面的边缘处;在冲击速率为3.4 m/s时,胶接接头在0.4、0.6 mm的胶层中节点的轴向应力S_x、剥离应力S_y、剪切应力S_(xy)、第一主应力S_1、等效应力S_(eqv)随着胶层厚度的增加,波动范围变窄,应力峰值也变小,而胶层厚度为0.2 mm时应力值波动异常剧烈,峰值很大。     

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。     

折曲胶接接头应力分布的数值分析

运用有限元法研究了偏轴、同轴和折曲型等三种单搭接胶接接头的工作应力分布规律。结果表明：与其他两种接头相比,折曲型单搭接胶接接头可有效降低搭接区端部的剥离应力和剪切应力峰值;在数值分析所用参数及条件下,Ⅰ型折曲接头的剥离应力降幅超过了75％,其他峰值应力的降幅也超过了45％～50％,并且应力分布趋于均匀;Ⅰ型折曲接头使出现应力峰值的位置从搭接区的端部转移至中部,从而显著提高了接头的承载能力,是一种优于普通同轴接头的胶接接头形式;对于受剪切载荷作用的接头而言,采用Ⅱ型折曲接头更为合理,可进一步提高接头承受剪切载荷的能力。     

Transverse cracking of a simple lap joint under axial load

The failure mode of axially loaded simple, single lap joints formed between thin adherends which are flexible in bending is conventionally described as one of axial peeling. We have observed – using high-speed photography – that it is also possible for failure to be preceded by the separation front, or crack, moving in a transverse direction, i.e. perpendicular to the direction of the axial load. A simple energy balance analysis suggests that the critical load for transverse failure is the same as that for axial separation for both flexible lap joints, where the bulk of the stored elastic energy lies in the adhesive, and structural lap joints in which the energy stored in the adherends dominates. The initiation of the failure is dependent on a local increases in either stress or strain energy to some critical values. In the case of a flexible joint, this will occur within the adhesive layer and the critical site will be close to one of the corners of the joint overlap from which the separation front can proceed either axially or transversely. These conclusions are supported by a finite element analysis of a joint formed between adherends of finite width by a low modulus adhesive.     

Nonlinear Analysis of theTorque Transmission Capability of Adhesively Bonded Tubular Lap Joints

Calculated torque transmission capability of adhesively bonded tubular lap joints using linear elastic material properties is usually much less than the experimentally-determined one because the majority of the load transfer of the adhesively bonded joints is accomplished by the nonlinear behavior of rubber-toughened epoxy adhesives.

Although the adhesively bonded tubular double lap joint has better torque transmission capability and reliability than the single lap joint, the nonlinear analytic or numerical analysis for the adhesively bonded tubular double lap joint has not been performed because of numerical complications.

An iterative solution that includes the nonlinear shear behavior of the adhesive was derived using the analytic solution. Since the iterative solution can be obtained very quickly due to the simplicity of the algorithm, it is an attractive method of designing adhesively bonded tubular single and double lap joints.     

Tensile Strength of Joints Bonded With a Nano-particle-Reinforced Adhesive

In order to improve the tensile lap shear strength of adhesively bonded joints, nano-particles were dispersed in the adhesive using a 3-roll mill. The dispersion states of nano-particles in the epoxy adhesive were observed with TEM (Transmission Electron Microscopy) with respect to the mixing conditions, and the effect of nano-particles on the mechanical properties of the adhesive was measured with respect to dispersion state and weight content of nano-particles. Also the static tensile load capability of the adhesively bonded double lap joints composed of uni-directional glass/epoxy composite and nano-particle-reinforced epoxy adhesive was investigated to assess the effect of nano-particles on the lap shear strength of the joint. From the experimental and FE analysis results, it was found that the nano-particles in the adhesive improved the mechanical properties of the adhesive. Also the increased failure strain and the reduced CTE (coefficient of thermal expansion) of the nano-particle-reinforced adhesive improved the lap shear strength of adhesively bonded joints.     

An Iterative Solution for the Torque Transmission Capability of Adhesively-Bonded Tubular Single Lap Joints with Nonlinear Shear Properties

The adhesively-bonded tubular single lap joint shows large nonlinear behavior in the load-displacement relationship, because structural adhesives for the joint are usually rubber-toughened, which endows adhesives with nonlinear shear properties. Since the majority of load transfer of the adhesively-bonded tubular single lap joint is accomplished by the nonlinear behavior of the adhesive, its torque transmission capability should be calculated using nonlinear shear properties. However, both the analytic and numerical analyses become complicated if the nonlinear shear properties of the adhesive are included during the calculation of torque transmission capabilities.

In this paper, in order to obtain the torque transmission capabilities easily, an iterative solution which includes the nonlinear shear properties of the adhesive was derived using the analytic solution with the linear shear properties of the adhesive. Since the iterative solution can be obtained very quickly due to its simplicity, it has been found that it can be used in the design of the adhesively-bonded tubular single lap joint.     

Three dimensional finite element analysis of bi-adhesively bonded double lap joint

Hybrid-adhesive joints are an alternative stress reduction technique for adhesively bonded joints. The joints have two types of adhesives in the overlap region. The stiff adhesive should be located in the middle and flexible adhesive at the ends. In this study, the effect of the hybrid-adhesive bondline on the shear and peeling stresses of a double lap joint were investigated using a three-dimensional finite element model. We developed a three dimensional model of the double lap joint based on solid and contact elements. Contact problem is considered to model the interface as two surfaces belonging to adherend and adhesive. Finite element analyses were performed for four different bond-length ratios (0.2,0.4,0.7 and 1.3). The results show that the stress components can be optimized using appropriate bond-length ratios. To validate the finite element analysis results, comparisons were made with available closed-form solutions. The numerical results were found in good agreement with the analytical solutions.     

Non Linear Fracture Mechanics for Adhesive Lap Joints

The Rice Cherepanov J is calculated for a lap joint in pure shear. By choosing as a condition for fracture a critical value J_c of this quantity the fracture load of the joint is calculated for linear elastic, perfectly plastic and linear hardening behavior of the adhesive. Comparisons are given with experiments with various adhesives and overlap lengths.     

Development of a Fatigue Failure Model for the Adhesively Bonded Tubular Single Lap Joint under Dynamic Torsional Loading

The adhesively bonded tubular single lap joint shows nonlinear torque transmission capability and deformation characteristics under static torsional loading because of nonlinear properties of the adhesive. However, the dynamic or fatigue torque transmission capability can be calculated with linear, analysis because the stress-strain relation under torsional fatigue loading is linear, due to the small dynamic transmission capability compared with the static torque transmission capability.

In this paper, a failure model for the adhesively bonded tubular single lap joint under torsional fatigue loading was developed with respect to the adhesive thickness, which is the critical factor for the static torque transmission capability. Also, a design method for the adhesively bonded tubular single lap joint under torsional fatigue loading was proposed.     

Measurement of adhesive shear properties by short beam shear test based on higher order beam theory

This paper refers to the measurement of the shear properties of adhesive bonding by a new beam theory using the short beam shear (SBS) test configuration. A novel higher-order sandwich beam theory has been developed to analyze the adhesive bonded beam that consists of two adhered laminates and a single layer of adhesive in between. The closed form analytical solution for the SBS test model of the adhesively bonded beam is obtained in terms of deflection and stress distribution. The present theory has been used for calculating the adhesive shear modulus from the structural compliance. The initiation of stiffness degradation for the short beam shear test model was used as the critical load value for deriving the adhesive shear strength. A finite element model is built for validating the present model, and to evaluate its suitability for measuring adhesive shear properties. The present theory shows better accuracy for measuring the shear modulus than existing theories for both thin and thick adhesive layers. The measured strength values are more accurate than those obtained from the single lap joint shear test model. This theory can be used for adhesive materials with linear elastic deformation behavior.     

Development of a Fatigue Failure Model for the Adhesively Bonded Tubular Single Lap Joint under Dynamic Torsional Loading

The adhesively bonded tubular single lap joint shows nonlinear torque transmission capability and deformation characteristics under static torsional loading because of nonlinear properties of the adhesive. However, the dynamic or fatigue torque transmission capability can be calculated with linear, analysis because the stress-strain relation under torsional fatigue loading is linear, due to the small dynamic transmission capability compared with the static torque transmission capability.

In this paper, a failure model for the adhesively bonded tubular single lap joint under torsional fatigue loading was developed with respect to the adhesive thickness, which is the critical factor for the static torque transmission capability. Also, a design method for the adhesively bonded tubular single lap joint under torsional fatigue loading was proposed.     

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.