Fracture and yield behavior of adhesively bonded joints under triaxial stress conditions

Most of adhesively bonded joints are under complicatedly distributed triaxial stress in the adhesive layer. For the estimating of the strength of adhesively bonded joints, it is crucial to clarify behavior of yield and failure of the adhesives layer under triaxial stress conditions. Two types of the adhesively bonded joints were used in this study: One is the scarf joint which is under considerably uniform normal and shear stresses in the adhesive layer, where their combination ratio can be varied with scarf angle. The other is the butt joint with thin wall tube in which considerably uniform pure shear can be realized in the adhesive layer under torsional load conditions. These joints can cover the stress triaxiality in adhesive layers of most joints in industrial application. The effect of stress triaxiality on the yield and fracture stresses in the adhesive layer were investigated using the joints bonded by three kinds of adhesives in heterogeneous and homogeneous systems. The results showed that both the yield and failure criterion depend on the stress triaxiality and that the fracture mechanism of the homogeneous adhesive is different from that of the heterogeneous one. From these experimental results, a method of estimating the yield and failure stresses was proposed in terms of a stress triaxiality parameter.     

基于遗传算法的碳纤维增强树脂复合材料层合板单搭胶接结构的多目标优化

基于遗传算法对碳纤维增强树脂复合材料(CFRP)层合板单搭胶接结构进行了多目标优化,以提高其结构性能。首先,通过三维Hashin准则和三角形内聚力模型建立三维有限元模型来预测CFRP层内损伤过程、层间失效和胶层损伤过程,并通过试验验证其有效性。其次,利用拉丁超立方抽样(LHS)方法和二次多项式响应面法(RSM),基于搭接长度、胶层厚度和被胶接件宽度等胶接参数建立以拉伸强度和剪切强度为目标函数的多目标优化代理模型。最后,基于遗传算法(GA)对拉伸强度和剪切强度代理模型进行优化,得出一组Pareto解集,并基于理想解排序方法(TOPSIS)对Pareto非劣解集进行折中处理,得到最好的胶接参数设计方案。结果表明:CFRP层合板单搭胶接结构的数值模拟结果与试验结果相比具有很高的吻合度,验证了有限元方法的可靠性;CFRP层合板单搭胶接结构的拉伸强度和剪切强度与搭接长度、胶层厚度和被胶接件宽度具有显著的关联性;二次响应面代理模型结果与数值模拟结果相比误差均小于2.3%;与常规的单搭胶接结构方案进行对比,搭接拉伸强度和剪切强度分别提高了2.65%和17.24%。      

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。      

Experimental study on CFRP-to-steel bonded interfaces

This paper presents an experimental study on the behaviour of CFRP-to-steel bonded interfaces through the testing of a series of single-lap bonded joints. The parameters examined include the material properties and the thickness of the adhesive layer and the axial rigidity of the CFRP plate. The test results demonstrate that the bond strength of such bonded joints depends strongly on the interfacial fracture energy among other factors. Nonlinear adhesives with a lower elastic modulus but a larger strain capacity are shown to possess a much higher interfacial fracture energy than linear adhesives with a similar or even a higher tensile strength. The variation of the interfacial shear stress distribution in a bonded joint as the applied load increases clearly illustrates the existence of an effective bond length. The bond–slip curve is shown to have an approximately triangular shape for a linear adhesive but to have an approximately trapezoidal shape for a nonlinear adhesive, indicating the necessity of developing different forms of bond–slip models for different adhesives.     

Fatigue strength and fatigue fracture mechanism of three-sheet spot weld-bonded joints under tensile–shear loading

This paper deals with fatigue behavior of three-sheet spot weld-bonded (SWB) joints to investigate the influence of adhesive bonding on fatigue strength and fracture mechanism. From the results of tensile–shear fatigue tests, the fatigue strength of the SWB joint was higher than that of the spot welded (SW) joint. For fatigue behavior in the SWB joint, debonding between a steel sheet and adhesive propagated from an edge of bonded area to a nugget edge, and a fatigue crack initiated at the nugget edge. It was concluded that the delayed fatigue crack initiation at the nugget edge in the SWB joint resulted in the fatigue strength improvement of SW joint.     

Fracture toughness and shear behavior of composite bonded joints based on a novel aerospace adhesive

In this paper, the mechanical performance of the newly developed LMB aerospace structural adhesive has been characterized experimentally. To this end, a comparison has been performed with the Epibond 1590 A/B adhesive on the basis of the effects of thermal aging, wet aging and adhesive thickness on the fracture toughness and shear behavior (static and fatigue) of bonded joints between CFRP laminates. Mode-I and -II fracture toughness was measured through tensile and 3-point bending tests on double-cantilever beam specimen, respectively, while the shear behavior of the joints was assessed through static and fatigue double-lap shear tests. A fractographic analysis of the fracture surfaces was performed in order to detect the failure mode of the bonded joint. The experimental results show a higher fracture toughness and improved fatigue behavior for the LMB adhesive.     

Torsional fatigue resistance of plasma sprayed HA coating on Ti–6Al–4V

The torsional strength of plasma sprayed hydroxyapatite (HA) coatings was studied under static and cyclic loading. The torsional shear tests were conducted in a frustum test device developed in this laboratory, which adapted to various coating thicknesses. The interfacial fatigue resistance was measured in terms of interfacial fatigue strength defined as the average maximum stress (_fmax). A staircase fatigue method was employed to determine the interfacial fatigue strength; this method resolved the uncertainty in detecting coating failure during torsion fatigue. The values for coating shear strength and shear fatigue strength obtained from the torsional tests did not differ from those obtained by previous tensional shear tests in this laboratory. The fatigue strength of one million cycles was about 35% lower than static shear strength. This finding might be used for estimating fatigue life span without cyclic loading tests.     

Torsional fatigue with axial constant stress of oligo‐crystalline 316L stainless steel thin wire

A series of symmetric torsional fatigue with axial constant stress tests, a kind of multiaxial fatigue test, was conducted on oligo‐crystalline 316L stainless steel thin wire, which was less than 3.5 grains across diameter of 200 μm. The material presents significant cyclic hardening under symmetric torsion cycling, and hardening is more obvious with the increasing shear strain amplitude. However, symmetric torsional cycle with constant axial stresses tests characterize rapid initial hardening and then gradually softening until fatigue failure. The axial stress has a great effect on torsional fatigue life. Fractography observation shows a mixed failure mode combined torsional fatigue with tensile strain because of axial tensile stress. A newly proposed model with axial stress damage parameter is used to predict the torsional fatigue life with constant axial stress of small scale thin wire.     

复杂面内应力状态下平面编织高铝纤维增强氧化铝基复合材料强度及疲劳寿命预测方法

针对平面编织氧化铝基复合材料提出了一种复杂面内应力状态下的强度准则和疲劳寿命预测方法。通过拉伸、压缩及纯剪切试验,分别获得了材料的静强度指标。考虑材料拉、压性能的差异和面内拉-剪联合作用对材料强度的影响机制,提出了修正的Hoffman强度理论。采用该强度理论预测得到的偏轴拉伸强度与试验结果基本一致,偏差不超过10%。开展了偏轴角θ=0°、15°、30°、45°,应力比R=0.1,频率f=10 Hz的拉伸疲劳试验,试验结果表明随着偏轴角的增加,相同轴向拉伸载荷下的疲劳寿命逐渐降低。由于面内剪切应力分量的作用,疲劳失效由纤维主导逐渐过渡到纤维和基体共同主导的模式。基于单轴疲劳寿命曲线,采用Broutman-Sahu剩余强度模型表征剩余强度随疲劳循环次数的变化规律,结合剩余强度演化模型和修正的Hoffman强度理论,提出了一种面内复杂载荷条件下的疲劳寿命预测模型,并引入疲劳剪切损伤影响因子表征拉-剪应力联合作用对材料疲劳行为的影响。采用本文提出的疲劳寿命预测模型,预测不同偏轴角拉伸疲劳寿命,预测结果与试验结果基本一致,偏差在1倍寿命范围内。比较结果表明在给定应力比、温度和疲劳载荷频率条件下,该疲劳寿命预测模型可以用来预测平面编织氧化铝基复合材料拉-剪复杂面内载荷条件下疲劳寿命。      

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.     

Fatigue behaviour of tensile steel/CFRP joints

In this paper the fatigue performance of tensile steel/CFRP (Carbon Fibre Reinforced Polymer) double shear lap joints is discussed. Joints were realized with two steel plates and two CFRP strips bonded using epoxy adhesive. Fatigue tests were performed on 16 specimens under constant stress range loading cycles. Two stress ratios (R = 0.1 and R = 0.4) were considered to investigate their influence on the fatigue lifetime. Debonding was observed to occur at stress concentration zones and propagate along the CFRP/adhesive interfaces. The stiffness degradation of the steel joint due to progressive debonding of the adhesive represents an index for the subsequent and progressive global failure. S–N curves are defined and compared to the fatigue resistance of welded detail categories of the Eurocode 3. The tests showed that the stress ratio, R, has a marginal influence on the fatigue lifetime of the steel/CFRP double shear lap joints. Finally, a fatigue limit corresponding to a stress range in the steel plate equal to 75 MPa was conservatively estimated during the tests. The fatigue limit seems to be insensitive to the stress ratio R.     

Fatigue performance of metallic reverse-bent joints

Adhesively bonded lap shear joints have been investigated widely and several ideas have been proposed for improving joint strength by reducing bondline stress concentrations. These include application of adhesive fillets at the overlap ends and use of adhesive with graded properties in the overlap area. Another, less common, approach is to deform the substrates in the overlap area in order to obtain a more desirable bondline stress distribution.
Previous work carried out by the authors on a number of different substrate materials indicated that a reverse-bent joint geometry is useful for increasing joint strength. Results from static stress analysis and experimental testing demonstrated that significant improvements could be achieved. This paper presents results of further work carried out to assess the fatigue performance of reverse-bent joints. Substrates with different yield and plastic deformation characteristics were used and the effects of different overlap lengths on strength were examined. The results of this research show that the improvements obtained under static tests conditions translate to even higher benefits in fatigue. The paper also explains the failure mechanism of the joints under fatigue loading.     

63Sn-37Pb和Sn-0.7Cu钎料的力学性能

对63Sn-37Pb和Sn-0.7Cu两种钎料进行了单轴拉伸试验、纯扭试验和纯扭疲劳试验.结果表明,钎料拉伸和纯扭时的应力应变关系有很强的应变速率相关性.弹性模量和剪切模量受应变速率的影响很小.钎料的屈服强度、剪切屈服强度、抗拉强度和抗剪强度也受应变速率的影响.钎料是剪切型破坏材料,并且发生循环软化.     

Composite-to-metal tubular lap joints: strength and fatigue resistance

The axial strength and fatigue resistance of thick-walled, adhesively bonded E-glass composite-to-aluminum tubular lap joints have been measured for tensile and compressive loadings. The joint specimen bonds a 63 mm OD aluminium tube within each end of a 300 mm long, 6 mm thick E-glass/epoxy tube. Untapered, 12.5 mm thick aluminium adherends were used in all but four of the joint specimens. The aluminum adherends in the remaining four specimens were tapered to a thickness of 1 mm at the inner bond end (the bond end where the aluminum adherend terminates). For all loadings, joint failure initiates at the inner bond end as a crack grows in the adhesive adjacent to the interface. Test results for a tension-tension fatigue loading indicate that fatigue can severely degrade joint performance. Interestingly, measured tensile strength and fatigue resistance for joints with untapered adherends is substantially greater than compressive strength and fatigue resistance.The joint specimen has been analyzed in two different ways: one approach models the adhesive as an uncracked, elastic-perfectly plastic material, while the other approach uses a linear elastic fracture mechanics methodology. Results for the uncracked, elastic-plastic adhesive model indicate that observed bond failure occurs in the region of highest calculated stresses, extensive bond yielding occurs at load levels well below that required to fail the joint, and a tensile peel stress is generated by a compressive joint loading when the aluminum adherends are untapered. This latter result is consistent with the observed joint tensile-compressive strength differential. Results of the linear elastic fracture mechanics analysis of a joint with untapered aluminum adherends are also consistent with the observed differential strength effect since a mode I crack loading is predicted for a compressive joint loading. Calculations and a limited number of tests suggest that it may be possible to selectively control the differential strength effect by tapering the aluminum adherends. The effect of adherend material and thickness on fracture mechanics parameters is also investigated. The paper concludes by examining the applicability of linear elastic fracture mechanics to the joints tested.     

粘接剂对压印连接强度的影响及数理统计分析

作为替代连接技术,压印和粘接表现出独特的优势。以压印和压印-粘接复合接头为对象,采用数理统计分析检验试验数据的合理性,进而对比不同材料组合及连接方式来分析粘接剂对压印连接接头强度的影响。试验表明,压印连接的剪切强度比剥离强度高,异种材料组合、压印-粘接复合能获得更有效的连接结构;胶层厚度为0.1mm时获得较好效果。     

超声强化氨基化多壁碳纳米管改性环氧黏接接头性能研究

目的 探究超声处理对氨基化多壁碳纳米管(MWCNTs–NH₂)改性环氧黏接接头黏接性能和热稳定性的影响,为强化MWCNTs–NH₂改性环氧胶黏剂与铝合金的黏接提供参考。方法 通过机械搅拌与声波破碎的方法将质量分数为0.75%的MWCNTs–NH₂添加到环氧胶黏剂基体中,使用MWCNTs–NH₂改性环氧胶黏剂制备铝合金黏接接头,基于超声辅助黏接工艺在铝合金黏接过程中进行超声处理。通过傅里叶变换红外光谱仪(FTIR)分析MWCNTs–NH₂改性环氧胶黏剂基体官能团的变化情况。采用单搭接剪切强度试验测定黏接接头的拉伸剪切强度。通过扫描电子显微镜(Scanning Electron Microscope,SEM)观察黏接接头拉伸失效断面以及铝合金与胶黏剂间的黏接界面。通过热失重分析仪(TGA)测试并记录胶黏剂试样质量随温度变化的曲线。结果 经超声处理后,MWCNTs–NH₂与树脂基体间的化学反应增强。与纯环氧黏接接头相比,超声处理后的MWCNTs–NH_2...     

Analysis of composite-to-metal joints with bondline flaws

Stress analysis techniques have been developed for load transfer in metal-to-composite adhesively bonded joints with bondline flaws such as variable bondline thickness and debond in the adhesive layer. Two joint configurations, namely, single-step-lap bonded joint and smoothly tapered scarf joint, have been investigated. The problem is formulated on the basis of the assumptions that both the metal and the composite are under generalized plane stress conditions and that the adhesive acts as a shear spring. Differential equations are obtained for the load transfered from the metallic layer to the composite layer. Numerical results are obtained for the force and stress in the composite layer, the stress in the metal and the stress in the adhesive. The influence of bondline flaw location on the stresses in the adhesive and the adherends has been investigated.     

Fatigue damage modelling of adhesively bonded joints under variable amplitude loading using a cohesive zone model

In this paper, the fatigue response of adhesively bonded joints under variable amplitude (VA) cyclic loading was predicted using a numerical model. The adhesive layer was modelled using the cohesive zone model with a bi-linear traction-separation response. A damage model, incorporating fatigue load ratio effects, was utilised in conjunction with the cohesive zone model to simulate the detrimental influence of VA fatigue loading. This model was validated against published experimental results obtained from fatigue tests of adhesively bonded single lap joints subjected to various types of VA fatigue loading spectra. This model successfully predicted the damaging effect of VA fatigue loading on the adhesively bonded joints and was generally found to be a significant improvement on the other damage models considered.     

Experimental and numerical investigation of the influence of imperfect bonding on the strength of NCF double-lap shear joints

The influence of imperfect bonding, owing to partial lack of adhesive, on the strength of composite non-crimp fabric (NCF) double-lap shear (DLS) joints was experimentally and numerically investigated. Fabrics were layered and compacted using a thermoplastic veil while infiltration of the preforms was done using the vacuum assisted process. Paste adhesive bonding was carried out by implementing the novel insertion squeeze flow process. Quality of adhesive bonding was tested using X-ray imaging and ultrasonic C-scan inspection. The tensile lap shear strength of the DLS joints was determined experimentally. Digital macrographs revealed that the specimens failed due to shear failure of the adhesive (debonding) and fracture of the composite boundary layer. As a second approach, a mesomechanical model based on the FE method and the (homogenized) progressive failure analysis method was developed. In the model, the areas without adhesion, as detected by the C-scans, were included. Numerical simulations of failure initiation and progression at the NCF joint and the adhesive indicated that it is possible to predict the strength and failure mechanisms of the imperfect bonded DLS joints.     

Experimental deformation analysis of an adhesively bonded multi-material joint for marine applications

The strength and deformation of full-scale adhesively bonded multi-material joints is studied in this paper. Four joints with a thick layer of methyl methacrylate adhesive (MMA) have been manufactured in shipyard conditions. In two specimens, cracks have been introduced at steel–adhesive and composite–adhesive interfaces. One cracked and one un-cracked specimen were subjected to quasi-static tensile testing; the two remaining specimens were stepwise loaded/unloaded with increasing load until failure. The strain in the adhesive layers was measured with digital image correlation (DIC). This showed a predominant shear deformation and dissimilar shear strain patterns for different bond lines. Fibre Bragg (FBG) sensors were used to monitor strains at steel and composite constituents and to detect the onset and evolution of damage in the un-cracked specimen. Strains measured by FBG sensors correspond well with DIC results at nearby regions. All specimens failed by delamination of the composite panel near the composite–adhesive interface.