Evaluating the strengths of thick aluminum-to-aluminum joints with different adhesive lengths and thicknesses

Composite joints are often the weakest elements in composite structures. In this paper, we propose a modified version of the damage zone theory based on the yield strain ratio. We use this framework to predict failure loads for various adhesive joints. Thick aluminum-to-aluminum joint specimens with eight different adhesive lengths and four adhesive thicknesses were manufactured and tested. The strengths of different adhesive lengths could be predicted to within 15.4% using the damage zone ratio method. In addition, the strengths of joints that feature different adhesive thicknesses were predicted to within 16.3% using the damage zone ratio method.     

Failure load evaluation and prediction of hybrid composite double lap joints

Hybrid joints are a combination of adhesive bonding and mechanical fastening and are known to combine the advantages of both joint types. In this paper, we evaluate and compare the strengths of mechanical joints, adhesive joints and hybrid composite joints. We manufactured and tested 10 hybrid joint specimens with different width-to-diameter (w/d) ratios, edge-to-diameter (e/d) ratios and adherend thicknesses. Additionally, the strengths of the hybrid joints were predicted using the Failure Area Index (FAI) method and the damage zone method, and we compared our theoretical predictions with our experimental results. From these data, we were able to predict hybrid joint strengths to within 23.0%.     

搭接长度和铺层方式对CFRP复合材料层合板胶接结构连接性能和损伤行为的影响

针对不同搭接长度和铺层方式的碳纤维增强树脂（CFRP）复合材料层合板单搭胶接结构进行了拉伸试验,观察了试件的受力过程和失效形态,获得了载荷-位移曲线;同时基于连续损伤力学模型和三维Hashin失效准则模拟了CFRP复合材料层合板的层内损伤形成和演化,并利用内聚力模型来模拟层间及胶层的失效损伤,对CFRP复合材料层合板单搭胶接结构在拉伸作用下的失效强度和损伤机制进行了预测,通过对比验证了该数值方法的有效性;通过数值试验比较不同搭接长度和铺层方式的单搭胶接结构及双搭胶接结构的连接强度和损伤行为,并提出了一种优化的CFRP复合材料层合板胶接结构。结果表明:CFRP复合材料层合板胶接结构的极限失效载荷随着搭接长度的增大逐渐增加并趋于稳定值,且结构的失效形式逐渐从胶层自身剪切失效过渡到邻近胶层的层合板层间分层失效;CFRP复合材料层合板胶接结构的连接强度和损伤行为随着铺层方式的不同而改变,通过对3种铺层方式的对比和分析,得到性能最好的铺层方式是[0₃/90₃]_2S;在搭接长度为5~20 mm时,通过对搭接长度进行优化,得到单搭胶接结构的最优搭接长度是17 mm,双搭胶接结构的最优搭接长度是19.3 mm,与搭接长度为20 mm相比,单搭胶接结构和双搭胶接结构的连接强度分别提高了13.26%和0.43%。      

基于临界应力强度因子的胶接对接接头抗拉强度简便预测方法

为了能够对胶接对接接头进行抗拉强度预测,该文以有限元分析方法为基础,通过利用参考问题的无量纲应力强度因子、接头模型与参考问题模型之间应力值的比值以及接头抗拉强度的试验数值,得到了胶接对接接头的临界应力强度因子数值,从而提出了一个基于临界应力强度因子的胶接对接接头抗拉强度的简便预测新方法。该方法能够在保证精确度的基础上快速计算出胶接对接接头的应力强度因子,通过使用任意某一胶层厚度时的抗拉强度试验数值预测出其他胶层厚度时对接接头的抗拉强度数值。通过三种胶接对接接头的算例对比分析,证明了该方法的预测结果与实际结果非常接近,预测精度良好,便于工程应用。     

基于粘聚区模型的含填充区复合材料接头失效数值模拟

建立了Ⅰ型与Ⅱ型失效模式耦合的粘聚单元本构模型, 并通过模拟双悬臂梁实验进行了验证。将粘聚单元插入填充区任何2 个实体单元之间, 预测填充区的随机裂纹, 模拟了接头在拉伸载荷下的失效。计算了复合材料基体、界面胶膜、填充物3 者不同强度、填充区半径、填充物刚度等多种情况下接头的拉伸失效。计算结果表明: 复合材料基体、界面胶膜、填充物3 者的强度显著影响接头的承载能力与失效模式; 随着填充区半径增大, 结构承载能力也随之提高。试验结果验证了模拟结果。      

An analytical solution for the analysis of symmetric composite adhesively bonded joints

Analytical solutions for adhesively bonded balanced composite and metallic joints are presented in this paper. The classical laminate plate theory and adhesive interface constitutive model are employed for this deduction. Both theoretical and numerical (finite element analysis) studies of the balanced joints are conducted to reveal the adhesive peel and shear stresses. The methodology can be extended to the application of various joint configurations, such as single-lap and single-strap joints to name a few. The methodology was used to evaluate stresses in several balanced adhesively bonded metallic and composite joints subjected to the tensile, moment and transverse shear loadings. The results showed good agreements with those obtained through FEM.     

Strength prediction of adhesive joints after cyclic moisture conditioning using a cohesive zone model

This paper presents a methodology to predict the strength of adhesive joints under variable moisture conditions. The moisture uptake in adhesive joints was determined using a history dependent moisture prediction methodology where diffusion coefficients were based on experimental cyclic moisture uptake of bulk adhesive samples. The predicted moisture concentrations and moisture diffusion history were used in a structural analysis with a cohesive zone model to predict damage and failure of the joints. A moisture concentration and moisture history dependent bilinear cohesive zone law was used. The methodology was used to determine the damage and failure in aluminium alloy – epoxy adhesive single lap joints, conditioned at 50 °C and good predictions of failure load were observed. The damage in the adhesive joints decreased the load carrying capacity before reaching the failure load and a nonlinear relationship between the load and displacement was observed. Changes in crack initiation and crack propagation were also observed between different types of joints. The presented methodology is generic and may be applied to different types of adhesive joint and adhesive.     

三维编织复合材料与金属胶接结构的力学性能及优化

通过试验和理论研究了三维编织纤维增强树脂基复合材料（FRP）与钢板的单搭粘结性能。建立了三维编织复合材料和钢板的单搭粘结有限元模型,并基于内聚区模型和Hashin破坏准则来模拟胶接层及三维编织复合材料的破坏,分别从极限载荷、破坏位移和破坏模式三方面将计算结果与试验结果进行对比,验证了有限元模型的有效性。在此模型基础上,研究了胶层厚度、粘结长度对单搭粘结极限载荷和破坏位移的影响,并提出针对不同工况下的优化方案选择的建议。最后建立了一种新型连接结构,相比较于原有单搭接连接方式,其拉伸强度和弯曲强度均有了较大提升。     

复合材料夹芯管胶接连接结构力学特性分析

针对在航空结构中广泛应用的复合材料蜂窝夹芯圆管中的接头这一最脆弱的部分,发展了一种分析复合材料蜂窝夹芯圆管胶粘接头力学特性的解析模型.该模型根据Gibson修正公式得到了蜂窝芯子的等效弹性参数,再运用经典的复合材料壳理论和线弹性理论得到管接头的控制方程,并通过状态空间法进行求解.运用本文模型,计算了管接头在扭矩和弯矩作用下胶层内的剪应力和剥离应力;同时采用有限元法对模型进行了数值模拟,并将模拟结果与模型计算结果进行了对比,最后分析了搭接长度对胶层内应力的影响.     

Damage modelling of adhesively bonded joints

A cohesive zone model (CZM) has been used in conjunction with both elastic and elasto– plastic continuum behaviour to predict the response of a mixed mode flexure and three different lap shear joints, all manufactured with the same adhesive. It was found that, for a specific dissipated CZM energy (Γ₀) there was a range of CZM tripping tractions (σ_u) that gave a fairly constant failure load. A value of σ_u below this range gave rise to global damage throughout the bonded region before any crack propagation initiated. A value above this range gave rise to a discontinuous process zone, which resulted in failure loads that were strongly dependent on σ_u. A discontinuous process zone gives rise to mesh dependent results. The CZM parameters used in the predictions were determined from the experimental fracture mechanics specimen test data. When damage initiated, a deviation from the linear load–displacement curve was observed. The value for σ _uwas determined by identifying the magnitude that gave rise to the experimentally observed deviation. The CZM energy (Γ ₀) was then obtained by correlating the simulated load-crack length response with corresponding experimental data. The R-curve behaviour seen with increasing crack length was successfully simulated when adhesive plasticity was included in the constitutive model of the adhesive layer. This was also seen to enhance the prediction of the lap shear specimens. Excellent correlation was found between the experimental and predicted joint strengths.     

复合材料π接头拉伸力学性能的试验和计算研究

采用试验和数值模拟的方法对整体化复合材料π接头在拉伸载荷作用下的力学特性进行研究。在Instron 8803电液伺服材料试验机上进行了π接头试验件的拉伸试验,记录试验过程中损伤产生及破坏过程,记录初始失效载荷和最终失效载荷。试验结果表明,填料是π接头破坏的关键部位,需要进行深入研究。提出了复合材料π接头力学性能数值模拟的基本假设和方法,基于通用有限元商用软件,建立π接头三维力学分析模型,获得π接头各部位应力分布情况;基于基本假设,对最大应力失效准则进行修正,并给出π接头各部位损伤载荷的预测值。计算预测π接头的初始损伤部位与试验吻合,初始失效载荷计算值与5个试件试验数据均值相比误差为0.53%,表明了数值分析方法的可行性。     

Three-dimensionally printed designable joint for carbon fibre reinforced plastics

Three-dimensional (3D) printing of carbon fibre reinforced plastic (CFRP) components shows promise for building light-weight strong components for industry. Many structures require multiple components to be joined to form the completed parts. However, tightening of metal bolts used to join CFRP can damage the composite and the bonding strength of adhesive bonding is variable. Here, we propose a new joint system, based on all-3D printed parts. Snap-in joints and Ajax-pins induce a mechanical joining force at the interface between parts, together with a bearing force. This joint system offers comparable strength to that of adhesive and metal bolt joints, while adding negligible weight to the parts. Furthermore, 3D printing of the surface structures, holes and pins enables unprecedented control over the joint design, enabling strength optimization through the thickness and in-place directions as required. The Snap-in joint system shows great potential for the fabrication of strong light-weight 3D printed structures.     

制造公差对复合材料螺栓连接结构强度分散性的影响

为解决由于螺栓和复合材料板制孔制造过程中不可避免的制造公差引起的配合间隙的不同,而使复合材料螺栓连接钉载分配不同,从而导致螺栓连接强度分散性的问题,以英制高锁螺栓和孔板制孔的制造公差为例,研究不同配合间隙下双剪四钉螺栓连接钉载分配情况,并进一步采用改进的特征曲线法、改进的强度包线法和渐进损伤模型对螺栓连接结构的强度进行预测,得到四钉连接失效强度的分散区间分别为[-3.87%,2.16%]、[-4.01%,3.95%]和[-3.16%,5.14%]。结果说明如果紧固件及螺栓孔均符合制造标准,则其对强度的影响范围不超过6%。     

Failure of carbon composite-to-aluminum joints with combined mechanical fastening and adhesive bonding

Composite-to-aluminum double lap joints were tested to obtain the failure loads and modes for three types of joints: adhesive bonding, bolt fastening and adhesive-bolt hybrid joining. A film type adhesive FM73 and a paste type adhesive EA9394S were used for aluminum and composite bonding. A digital microscope camcorder was used to monitor the failure of the joints. It was found that hybrid joining improves joint strength when the mechanical fastening is stronger than the bonding, as when the paste type adhesive is used. On the other hand, when the strength of the bolted joint is lower than that of the bonded joint, as when the film type adhesive is used, bolt joining contributes little to the strength of the hybrid joint.     

A parametric study on the failure of bonded single-lap joints of carbon composite and aluminum

A parametric study on adhesively bonded carbon composite-to-aluminum single-lap joints was experimentally conducted. FM73m, a high strength adhesive produced by Cytec, was used for bonding. The primary objective of this study is to investigate the effects of various parameters, such as bonding pressure, overlap length, adherend thickness, and material type, on the failure load and failure mode of joints with dissimilar materials. While metal bonded joints generally fail at the adhesive, the final failure mode of all the tested bonded joints with dissimilar materials was delamination of the composite adherend. Bonding strengths of the tested joints were lower than the metal-to-metal bonded joint strength. The specimens bonded under pressure of 4 and 6 atm yielded higher failure loads than under pressure of 3 atm, which is within the range of the manufacturer-recommended bonding pressure. Failure loads of the joint increased slightly at an overlap length larger than 30 mm. Increasing adherend thickness resulted in an increase of the failure load, but was not linearly proportional to the failure load.     

胶层厚度对碳纤维/双马来酰亚胺树脂复合材料平-折-平混合连接接头力学性能的影响

以碳纤维/双马来酰亚胺(BMI)树脂复合材料平-折-平(FJF)连接接头为对象,通过试验对比分析了特定胶层厚度下碳纤维/BMI树脂复合材料FJF连接接头的静强度和疲劳性能,并探究了胶层厚度对碳纤维/BMI树脂复合材料FJF混合接头力学性能的影响。利用背面应变技术对碳纤维/BMI树脂复合材料FJF混合接头搭接区端部胶层开裂进行监测。利用有限元软件ABAQUS对不同胶层厚度下碳纤维/BMI树脂复合材料FJF混合接头搭接区胶层应力分布进行了分析。结果表明,碳纤维/BMI树脂复合材料FJF混合接头的平均拉伸极限载荷、搭接区端部胶层开裂平均循环次数和平均疲劳寿命均随着胶层厚度在0.1~0.3 mm范围内增加而增大。不同胶层厚度的碳纤维/BMI树脂复合材料FJF混合接头均经历相同的失效阶段,即搭接区胶层端部开裂,胶层沿搭接区断裂扩展,最终靠近加载端孔边拉伸断裂,呈±45°断口。随着胶层厚度在0.1~0.3 mm范围的增加,搭接区端部胶层剥离应力、剪切应力及孔边胶层压缩应力均减小。在胶层厚度为0.1~0.3 mm范围内,剪应力是胶层破坏的控制因素。      

Failure load prediction of laminates repaired with a scarf-bonded patch using the damage zone method

The damage zone method (DZM) is an efficient way to predict the failure of composite structures with a minimum of real testing. Particularly, it is useful when the failure mechanism is too complicated to be accurately analyzed by a merely numerical method. The aim of this study was to use the damage zone model to predict the failure load of repaired laminates, in which scarf-bonded joints were used for repair. The model uses a test-based critical damage zone and stress-based failure criteria. A total of 45 carbon-epoxy composite (USN) laminate scarf-repaired specimens were first tested with two different defect sizes, four scarf angles, and three overlap layer sizes. The Tsai-Wu and Tsai-Hill criteria were used for the laminate, and the maximum shear stress criterion for the adhesive was adopted to predict failure onset. The predicted failure loads were compared to test results and a good agreement was obtained with a 9.2% maximum deviation for almost all parameters with the exception of a case with an unrealistically large scarf angle. To verify the feasibility of the DZM for different material, additional 30 repair specimens using other unidirectional carbon-epoxy laminate were then also tested and the predictions were confirmed by the results of the experiment.     

交变载荷对CFRP复合材料-铝合金粘接接头剩余强度的影响

为了给碳纤维增强聚合物（CFRP）复合材料粘接结构的安全设计及应用提供参考,针对CFRP复合材料-铝合金对接接头,研究了拉-拉交变载荷作用下的疲劳寿命特性及剩余强度变化规律。设计专用夹具,完成接头的制作及固化,并测试其拉伸、剪切准静态失效强度,在此基础上进行不同载荷水平下的疲劳寿命测试。选取特定载荷水平,测试不同循环次数后的接头剩余强度,并对失效形式进行观察分析。结果表明:CFRP复合材料-铝合金对接接头强度-寿命（S-N）曲线在单对数坐标上符合线性函数规律;随着交变载荷循环周期的增加,接头剩余强度呈先慢后快的下降趋势,而且在较大的载荷水平下,下降幅度更为明显;经历交变载荷循环前、后接头失效形式发生改变,由局部CFRP复合材料表层撕裂转变为局部界面破坏。结合试验测试所获得的初始失效准则,并引入疲劳退化因子,建立内聚力模型对交变载荷作用下的粘接接头强度衰减进行数值模拟,结果表明所建立模型能够有效预测交变载荷作用下的接头剩余强度。      

Strength prediction of tubular composite adhesive joints under torsion

This paper addresses prediction of the strength of tubular adhesive joints with composite adherends by combining thermal and mechanical analyses. A finite element analysis was used to calculate the residual thermal stresses generated by cooling down from the adhesive cure temperature, and a nonlinear analysis incorporating the nonlinear adhesive behavior was performed to accurately estimate the mechanical stresses in the adhesive. Joint failure was estimated by three failure criteria: interfacial failure, adhesive bulk failure, and adherend failure. The distributions of residual thermal stresses were investigated for various stacking angles. The effect of residual thermal stresses on joint strength was also taken into consideration. The results indicate that the residual thermal stresses, depending on the stacking angle, have a significant influence on the failure mode and strength of adhesive joints when a subsequent mechanical load is applied. Good agreement is also obtained between the predicted joint strength and the available experimental data.     

焊点间距对胶焊接头应力应变分布和强度的影响

采用数值分析和试验研究方法考察了焊点间距对两种胶焊中应力应变分布和接头强度的影响。结果表明,高弹性模量胶粘剂胶焊接头中,增大焊点间距,焊点就三小耐的妆区边缘胶层中的应力应变略有增大,低弹性模量释粘剂胶焊接头中,焊点应力集中程度和搭接区边缘胶层中的剪应变值都随焊点间距的增加而增大,增加焊点间距,两种接头的胶层中度都将降低。试验所得接头断裂时的名义应力随焊点间距的增加而略有减小,与计算结果吻合良好。