Failure behavior of adhesively bonded joints with a rubber modified epoxy adhesive under tensile-shear combined cyclic loading

Rubber-modified epoxy adhesives are used widely as structural adhesive owing to their properties of high fracture toughness. In many cases, these adhesively bonded joints are exposed to cyclic loading. Generally, the rubber modification decreases the static and fatigue strength of bulk adhesive without flaw. Hence, it is necessary to investigate the effect of rubber-modification on the fatigue strength of adhesively bonded joints, where industrial adhesively bonded joints usually have combined stress condition of normal and shear stresses in the adhesive layer. Therefore, it is necessary to investigate the effect of rubber-modification on the fatigue strength under combined cyclic stress conditions. Adhesively bonded butt and scarf joints provide considerably uniform normal and shear stresses in the adhesive layer except in the vicinity of the free end, where normal to shear stress ratio of these joints can cover the stress combination ratio in the adhesive layers of most adhesively bonded joints in industrial applications.

In this study, to investigate the effect of rubber modification on fatigue strength with various combined stress conditions in the adhesive layers, fatigue tests were conducted for adhesively bonded butt and scarf joints bonded with rubber modified and unmodified epoxy adhesives, wherein damage evolution in the adhesive layer was evaluated by monitoring strain the adhesive layer and the stress triaxiality parameter was used for evaluating combined stress conditions in the adhesive layer. The main experimental results are as follows: S–N characteristics of these joints showed that the maximum principal stress at the endurance limit indicated nearly constant values independent of combined stress conditions, furthermore the maximum principal stress at the endurance limit for the unmodified adhesive were nearly equal to that for the rubber modified adhesive. From the damage evolution behavior, it was observed that the initiation of the damage evolution shifted to early stage of the fatigue life with decreasing stress triaxiality in the adhesive layer, and the rubber modification accelerated the damage evolution under low stress triaxiality conditions in the adhesive layer.     

Effect of the addition of aramid-CTBN block copolymer on adhesive properties of cured epoxy resins modified with CTBN

To improve the bond strength of carboxy-terminated butadiene acrylonitrile (CTBN)-modified epoxy resin, aramid-CTBN block copolymer was used as an additive to a bisphenol-A-type epoxy resin modified with CTBN. With the addition of aramid-CTBN block copolymer, the fracture energy values of the adhesive joints were more than twice that of the CTBN-modified system and about twelve times that of the unmodified system. Observations of the adhesive layers using an optical microscope revealed that the width of the damage zone near the crack-tip in the double cantilever specimen increased with the addition of the aramid-CTBN block copolymer. TEM micrographs showed that the diameter of the CTBN-dispersed phases decreased and a number of fine CTBN phases were dispersed in the epoxy matrix with the addition of the block copolymer. It is concluded that addition of the block copolymer leads to an increase in the dispersibility of the CTBN elastomer into the epoxy matrix and thus the bond strength of the block copolymer-added system is increased due to the increase in the area of the damage zone and the occurrence of shear deformation of the epoxy matrix.     

Determination of the impact tensile strength of structural adhesive butt joints with a modified split Hopkinson pressure bar

The impact tensile strength of structural adhesive butt joints was determined with a modified split Hopkinson pressure bar using hat-shaped specimens. A typical two-part structural epoxy adhesive (Scotch weld^® DP-460) and two different adherend materials (Al alloy 7075-T6 and commercially pure titanium) were used in the adhesion tests. The impact tensile strength of adhesive butt joints with similar adherends was evaluated from the peak value of the applied tensile stress history. The corresponding static tensile strengths were measured on an Instron testing machine using joint specimens of the same geometry as those used in the impact tests. An axisymmetric finite element analysis was performed to investigate the static elastic stress distributions in the adhesive layer of the joint specimens. The effects of loading rate, adherend material and adhesive thickness on the joint tensile strength were examined. The joint tensile strength was clearly observed to increase with the loading rate up to an order of 10⁶ MPa/s, and decrease gradually with the adhesive thickness up to nearly 180 μm, depending on the adherend materials used. The loading rate dependence of the tensile strength was herein discussed in terms of the dominant failure modes in the joint specimens after static and impact testing.     

Effect of aramid-CTBN block copolymer on phase structure and toughness of cured epoxy resins modified with carboxy-terminated butadiene acrylonitrile copolymer (CTBN)

An Aramid-CTBN block copolymer was added to a Bisphenol-A type epoxy resins modified with CTBN. With the addition of the copolymer, the impact strength of the CTBN-modified systems was about twelve times higher than that of the unmodified system. Transmission electron micrographs of the block copolymer-epoxy system showed that the diameter of CTBN-dispersed phase decreased and a number of fine CTBN phases were dispersed in the matrix. The deformability of matrix in these systems increased with a dispersion of the fine CTBN phase. It is concluded that the improvement of toughness of the cured resins is due to the increases in the ductility of epoxy matrix by the incorporating of the dispersion of the fine CTBN phase in the matrix.     

Comparison of fracture behavior between acrylic and epoxy adhesives

An experimental study was conducted on the strength of adhesively bonded steel joints, prepared epoxy and acrylic adhesives. At first, to obtain strength characteristics of these adhesives under uniform stress distributions in the adhesive layer, tensile tests for butt, scarf and torsional test for butt joints with thin-wall tube were conducted. Based on the above strength data, the fracture envelope in the normal stress-shear stress plane for the acrylic adhesive was compared with that for the epoxy adhesive. Furthermore, for the epoxy and acrylic adhesives, the effect of stress triaxiality parameter on the failure stress was also investigated. From those comparison, it was found that the effect of stress tri-axiality in the adhesive layer on the joint strength with the epoxy adhesive differed from that with the acrylic adhesive. Fracture toughness tests were then conducted under mode l loading using double cantilever beam (DCB) specimens with the epoxy and acrylic adhesives. The results of the fracture toughness tests revealed continuous crack propagation for the acrylic adhesive, whereas stick-slip type propagation for the epoxy one. Finally, lap shear tests were conducted using lap joints bonded by the epoxy and acrylic adhesives with several lap lengths. The results of the lap shear tests indicated that the shear strength with the epoxy adhesive rapidly decreases with increasing lap length, whereas the shear strength with the acrylic adhesive decreases gently with increasing the lap length.     

Butt joint tensile strength: interface corner stress intensity factor prediction

The asymptotic form of the interface corner stress field in a butt joint is discussed, and a failure analysis based on the stress intensity factor defining the magnitude of this asymptotic stress field is validated. A stress singularity of type Kr^δ(δ < 0) exists at an interface corner in a butt joint (i.e. where an interface intersects a stress-free edge). A simple relation defines the stress intensity factor K for an idealized butt joint composed of a thin elastic adhesive layer bonded between rigid adherends and subjected to transverse tension and uniform adhesive shrinkage. This stress intensity factor, referred to here as the free-edge stress intensity factor K_f, is applicable to both plane strain and axisymmetric geometries. The way that uniform adhesive shrinkage (thermal contraction) during cure alters interface corner stress fields is also discussed. When adhesive shrinkage is present, both constant and singular terms must be included in the asymptotic solution to attain good agreement with full field finite element results over a reasonably large interface corner region. Experiments have been carried out to investigate the applicability of a K_f-based failure criterion to butt joints. Butt joints were fabricated by bonding two stainless steel rods together with an epoxy adhesive (Epon 828/T-403). The measured joint strength increased by a factor of 2 as the bond thickness was reduced from 2.0 to 0.25 mm. The observed bond thickness effect is accurately predicted when failure is presumed to occur at a critical K_f value. This fracture criterion suggests that the butt joint tensile strength varies roughly as the reciprocal of the cube root of bond thickness when the adhesive's Poisson's ratio is between 0.3 and 0.4, residual stress levels at the interface corner are negligible, the adherends are essentially rigid relative to the adhesive, and small-scale yielding conditions hold at the interface corner.     

聚酰亚胺改性环氧树脂胶黏剂的研究

环氧树脂和聚酰亚胺的性能具有一定的互补性,用聚酰亚胺对环氧树脂进行改性可以综合两者的优点,得到具有良好机械性能和粘结强度的耐高温环氧胶黏剂。用聚酰亚胺中间体聚酰胺酸(PAA)对环氧树脂(EP)进行改性,加入一定量的端羧基丁腈橡胶(CTBN),用4,4’-二氨基二苯砜(DDS)做固化剂,先在一定温度下进行预反应,然后在一定的工艺条件下固化,通过调节不同的配比,得到具有较高耐热性的环氧树脂胶黏剂。具体研究了PAA用量、DDS用量、CTBN用量对胶黏剂力学性能的影响,筛选较好的配方。采用热重分析仪(TG)和差热扫描量热仪(DSC)等研究胶黏剂的耐热性能,并利用傅立叶变换红外光谱(FTIR)对各树脂进行结构表征,采用扫描电镜(SEM)对固化后胶黏剂的断面形貌进行了分析。     

Mechanical and tribological properties of epoxy modified by liquid carboxyl terminated poly(butadiene-co-acrylonitrile) rubber

Diglycidyl ether of bisphenol A (DGEBA)-based epoxy resin was modified using liquid carboxyl-terminated poly(butadiene-co-acrylonitrile) (CTBN) rubber. The liquid CTBN contents used ranged from 2.5 to 20 parts per hundred parts of resin (phr). Mechanical properties of the modified resins were evaluated and the microstructures of the fracture surfaces were examined using SEM technique. The changes in storage modulus and the glass transition temperature were also evaluated using dynamic mechanical analysis (DMA). The tribological tests were performed using a ball-on-disc tribometer. The worn surfaces and the ball counter-mates after tribological tests were investigated using optical microscope technique. The results revealed the influence of liquid CTBN content on mechanical and tribological properties, and also microstructure of the modified epoxy resins. Impact resistance increased whereas the storage modulus and the hardness decreased when the CTBN rubber was introduced to the epoxy network. The coefficient of friction of the CTBN-modified epoxy was lower than that of the neat epoxy. The CTBN content of lower than 10 phr was recommended for improving the wear resistance of epoxy resin. Changes in tribological properties of the CTBN-modified epoxy correspond well to those in mechanical changes, especially the toughness properties. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Analysis and design of adhesively bonded joints for fatigue and fracture loading: a fracture-mechanics approach

An experimental–computational fracture-mechanics approach for the analysis and design of structural adhesive joints under static loading is demonstrated by predicting the ultimate fracture load of cracked lap shear and single lap shear aluminum and steel joints bonded using a highly toughened epoxy adhesive. The predictions are then compared with measured values. The effects of spew fillet, adhesive thickness, and surface roughness on the quasi-static strength of the joints are also discussed. This fracture-mechanics approach is extended to characterize the fatigue threshold and crack growth behavior of a toughened epoxy adhesive system for design purposes. The effects of the mode ratio of loading, adhesive thickness, substrate modulus, spew fillet, and surface roughness on the fatigue threshold and crack growth rates are considered. A finite element model is developed to both explain the experimental results and to predict how a change in an adhesive system affects the fatigue performance of the bonded joint.     

Investigating the influence of the chamfer and fillet on the high-cyclic fatigue strength of adhesive joints of steel parts

This paper presents the results of research undertaken to determine the possibility of improving the fatigue properties of peel-loaded adhesive-bonded joints by the constructional modification of the adherend. Fatigue strength tests were carried out on the electromagnetic inductor at the resonance frequency of the adhesive-bonded joint specimens. The tests were carried out on the specimens bonded by means of Bison Epoxy and Epidian 57 epoxy compositions with poly-aluminium chloride hardener. The joined elements were modified by making the chamfer or fillet to enlarge the thickness of the adhesive layer with the aim of reducing the stress concentration in the frontal part of the joint. This modification is the result of a research that confirms the existence of a stress concentration on the short section of the frontal part of an adhesive joint. This phenomenon can lead to the rapid initiation of adhesive joint destruction. The fatigue strength tests revealed a significant improvement in fatigue endurance.     

Crack-growth behavior of epoxy adhesives modified with liquid rubber and cross-linked rubber particles under mode I loading

The crack-growth resistance (R-curve) of bulk single-edge notch bend (SENB) and adhesively bonded double cantilever beam (DCB) specimens was investigated under mode I loading conditions using two types of rubber-modified epoxy adhesive: one was a liquid rubber (CTBN)-modified adhesive and the other was a cross-linked rubber particle (DCS)-modified adhesive. As a result, for both the SENB and DCB specimens, the gradient of the R-curve for the DCS-modified adhesive was steeper than that for the CTBN-modified one, however, the difference in fracture toughness between DCS- and CTBN-modified adhesives is smaller for DCB than for SENB specimens. To elucidate such behavior, crack-growth simulation based on Gurson's model was conducted, where the DCS- and CTBN-modified adhesives were characterized by both the initial void fraction and nucleation. The difference in the behavior of R-curves was also observed in simulations. Moreover, it was found that the difference in fracture surface roughness observed by SEM for both adhesives correspond to the variation in R-curves.     

Carboxyl‐terminated butadiene acrylonitrile rubber/epoxy polymer alloys as damping adhesives and energy absorbable resins

Carboxyl‐terminated butadiene acrylonitrile (CTBN) liquid rubber/epoxy (diglycidyl ether of bisphenol‐A: DGEBA) / diamino diphenyl methane (DDM) resins, in which CTBN was 60 wt % as the major component, were formulated to evaluate the damping and adhesive properties. In cases where acrylonitrile (AN) was 10～18 mol % as copolymerization ratio in CTBN, the blend resins showed micro‐phase separated morphologies with rubber‐rich continuous phases and epoxy‐rich dispersed phases. The composite loss factors (η) for steel laminates, which consisted of two steel plates with a resin layer in between, depended highly on the environmental temperature and the resonant frequencies. On the other hand, in the case where AN was 26 mol % in CTBN, the cured resin did not show clear micro‐phase separation, which means the components achieve good compatibility in nano‐scale. This polymer alloy had a broad glass‐transition temperature range, which resulted in the high loss factor (η > 0.1) for the steel laminates and excellent energy absorbability as the bulk resin in a broad temperature range. Also the resin indicated high adhesive strengths to aluminum substrates under both shear and peel stress modes. The high adhesive strengths of the CTBN/epoxy polymer alloy originated in the high strength and the high strain energy to failure of the bulk resin. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

中温固化丁腈橡胶改性环氧树脂胶黏剂的研究

制备了四种环氧树脂(E-51、AG-80、AFG-90、TDE-85)及其复合树脂体系的液体丁腈橡胶CTBN改性胶黏剂,研究了这些胶黏剂及加入不同质量的TDE-85和AFG-90后的粘接性能。研究结果表明,TDE-85对改性胶的室温剪切和剥离强度贡献较大,AFG-90对高温剪切强度贡献较大。在TDE-85改性胶(TC-23)和TDE-85/E-51复合树脂改性胶(TEC-23)中,添加10%的AFG-90,可使它们的室温、100℃剪切及室温剥离强度分别达到37.4MPa、16.7MPa,65.4N·cm-1和33.7MPa、18.2MPa,60.0N·cm-1,具有较好的综合性能。     

Experimental Investigation of Epoxy Bonded Polymethylmethacrylate Joints

This paper presents an experimental investigation into various aspects of epoxy-bonded polymethylmethacrylate (PMMA) and PMMA-to-aluminium joints. The effects of adhesive thickness, overlap area, surface roughness, and environmental exposure on the joint strength were studied. Results indicated that the joint strength was not directly proportional to the overlap area, while sanding had a positive effect on the joint strength. A negative effect was observed when adhesive thickness was increased. The fatigue behaviour of adhesively-bonded joints under dynamic loading was found to be independent of frequency, for the range of values tested; however, it was dependent on the test temperature with greater reduction in fatigue life observed in PMMA-to-aluminium joints at higher temperature. Empirical equations from which the fatigue life of joints can be predicted were obtained by regression analysis. Intermittent fatigue testing of the joints was also performed. The epoxy adhesive tested proved to be a satisfactory choice for outdoor exposure. The rate of degradation of the adhesive was slow with the adherend itself degrading at a faster rate than the adhesive or the bondline.     

端羧基液体丁腈橡胶改性环氧结构胶的研究

采用端羧基液体丁腈橡胶(CTBN)增韧环氧树脂,制备了双组分室温固化环氧结构胶。利用傅里叶变换红外光谱仪(FTIR)、微机控制万能材料试验机及扫描电镜(SEM)对固化过程、固化产物剪切强度及固化产物微观形态进行了表征。该胶树脂甲组分的最佳制备条件如下:环氧树脂与CTBN的质量比8∶1,反应温度200℃,保温时间2.5 h。该胶在室温下固化24 h,室温剪切强度可达29.24 MPa,耐介质性能良好,CTBN改性环氧树脂增韧效果显著。     

F-992抗蠕变光学结构胶

以改性环氧树脂为主粘料,用反应性端羧基丁腈橡胶（CTBN）为增韧剂,与改性多元胺端氨基聚醚固化剂配制成具有较高交联密度的光学结构胶。因较高的交联密度和使用耐热改性剂,提高了耐热性;CTBN与环氧树脂进行预反应,并加入气相法白炭黑,提高了胶液的贮存稳定性和粘接强度。制备的光学结构胶拉伸剪切强度15．7MPa,压缩剪切强度25．4MPa,常温条件下蠕变值小于2英寸,表现出良好的粘接强度和耐热、耐冲击振动及抗蠕变性。     

Experimental Investigation of Epoxy Bonded Polymethylmethacrylate Joints

This paper presents an experimental investigation into various aspects of epoxy-bonded polymethylmethacrylate (PMMA) and PMMA-to-aluminium joints. The effects of adhesive thickness, overlap area, surface roughness, and environmental exposure on the joint strength were studied. Results indicated that the joint strength was not directly proportional to the overlap area, while sanding had a positive effect on the joint strength. A negative effect was observed when adhesive thickness was increased. The fatigue behaviour of adhesively-bonded joints under dynamic loading was found to be independent of frequency, for the range of values tested; however, it was dependent on the test temperature with greater reduction in fatigue life observed in PMMA-to-aluminium joints at higher temperature. Empirical equations from which the fatigue life of joints can be predicted were obtained by regression analysis. Intermittent fatigue testing of the joints was also performed. The epoxy adhesive tested proved to be a satisfactory choice for outdoor exposure. The rate of degradation of the adhesive was slow with the adherend itself degrading at a faster rate than the adhesive or the bondline.     

The effect of adhesive thickness on tensile and shear strength of polyimide adhesive

The effect of adhesive thickness on tensile and shear strength of a polyimide adhesive has been investigated. Tensile and shear tests were carried out using butt and single lap joints. Commercially available polyimide (Skybond 703) was used as adhesive and aluminum alloy (5052-H34) was used as adherends. The tensile strength of the butt joints decreased with increasing adhesive thickness. In contrast, adhesive thickness did not seem to affect the shear strength of single lap joints. The fabricated joints using the polyimide adhesive failed in an interfacial manner regardless of adhesive thickness. The linear elastic stress analysis using a finite element method (FEM) indicates that the normal stress concentrated at the interface between the adherend and the adhesive. The FEM analysis considering the interfacial stress well explains the effect of adhesive thickness on the joint strength.     

Strength of Adhesively-Bonded Butt Joints of Tubes Subjected to Combined High-Rate Loads

The dynamic strength of adhesively-bonded joints was investigated experimentally. The strength of the bonded joints under combined high rate loading was measured using the clamped Hopkinson bar method. Tubular butt joints bonded by epoxy resin were used for the experiment. Combined stress waves of tension and torsion were applied to the specimens. The strength of the adhesively-bonded joint was determined by measuring the stress waves propagated in the load output tube of the specimen. It was found that the dynamic strength of the adhesive joints was greater than the static strength under tensile and shear load.     

Rubber-toughening of glass fiber-epoxy filament wound composites

An attempt was made to improve the properties of filament wound glass fiber-epoxy composites by addition of carboxyl-terminated-butadiene-acrylonitrile (CTBN)-rubber to the matrix. The interlaminar G_Ic of unidirectional glass fiber-epoxy increased significantly with CTBN addition whereas the flexural strength decreased. The weepage stress in hoop loading of ±50 degree angle-ply pipes was higher for unmodified as compared with 10 phr CTBN-modified epoxy matrix pipes. However, the strain at weepage and the stress and strain at nonlinearity were higher for the pipes based on toughened epoxy. The reason for the lower weepage stress is that, although the toughened matrix pipes show higher resistance to cracking, their stiffness is lower. The addition of small amounts of CTBN-rubber (about 5 percent by weight is suggested) to the present epoxy matrix for filament winding can be done with only a minor increase in viscosity. This is a way of toughness enhancement for applications where the matrix stiffness reduction and increased moisture absorption are of minor importance.