Rubber-modified Adhesives Research Articles

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 Rubber Content on Fatigue Crack Growth Rate under Mode I Loading

Rubber-modified adhesives have attracted particular interest recently, owing to their property of dramatically increasing the fracture toughness of adhesively-bonded joints. In many cases, these joints are exposed to cyclic loads, and hence an understanding of the fatigue mechanisms is of paramount importance. This study was conducted to characterize the rubber content-dependent behaviour of rubber-modified epoxy adhesive under cyclic loading. Fatigue crack growth rates under mode I loading were measured using adhesively-bonded double cantilever beam specimens with rubber contents of 0, 5 and 14 wt.%. As a result, in the Paris region, an increase in rubber content reduced the fatigue crack growth rate. Similarly to the Paris region, the fatigue threshold for 5 wt.% was greater than that for 0 wt.%; however, the fatigue threshold of joints with 14 wt.% rubber content was nearly equal to those with 5 wt.%.

Damage behaviour of nano-modified epoxy adhesives subject to high stress constraint

ABSTRACTThis paper presents a combined experimental and numerical study on the damage behaviour of core–shell rubber (CSR)-modified epoxy adhesives subject to high stress constraints. The test method consists of a notched axisymmetric adhesive layer loaded in tension. The stress–displacement curves of the rubber-modified adhesives have been found to exhibit a sudden reduction in stiffness after an initial linear loading region. It has been demonstrated that this corresponds to the cavitation of the rubber particles. The stress of rubber cavitation remained essentially constant at a critical hydrostatic stress of approximately 21 MPa over different rubber contents and different stress constraints. It is important to note that the rubber cavitation stress is also dependent on the size of the rubber particles, and the diameter of the rubber core is approximately 170 nm in current work. The stress constraint had negligible effect on the failure strength of the adhesive joints for the studied systems.

Fracture Behaviour of Adhesives Modified with various Rubber Content under Combined Stress Conditions

Rubber-modified adhesives have attracted special interest recently due to their property of dramatically increase in fracture toughness of adhesively bonded joints, where the fracture toughness increases with the rubber content. However, an increase in rubber content decreases the joint strength without initial flaws. Besides, most adhesively-bonded joints have combined normal and shear stresses concentrated in the adhesive layer. Hence, the effect of rubber content on adhesively-bonded joints should be clarified under combined stress conditions. In this paper the aim is to characterize the rubber content dependent behaviour of the rubber-modified adhesive under combined stress conditions. Tensile tests were conducted for adhesively-bonded butt and scarf joints with various rubber contents, where these joints can cover the combined stress conditions in the adhesive layers of most joints in industrial applications. The influence of the rubber content on the joint strength is represented in the normal stress-shear stress diagram.

Yield Behaviour of Rubber-Modified Epoxy Adhesives under Multiaxial Stress Conditions: Effect of Plastic Deformability of the Matrix Phase

In a previous study, the yield stress in the adhesive layer of scarf and torsional butt joints with a rubber-modified adhesive was measured, and the results agreed with those calculated by a finite element method, where Gurson's constitutive equations were applied to the adhesive layer. In this study, to investigate the effect of plastic deformability of the matrix phase on the yielding behaviour of rubber-modified epoxy adhesives, the yield behaviour was investigated by using scarf and torsional butt joints, where a reactive diluent was used to vary the plastic deformability of the matrix phase. The applicability of Gurson's model was examined in terms of the yield stresses calculated by a finite element method with a variety of the model parameters. As a result, it is confirmed that the yield stresses of the rubber-modified adhesives with different matrix phases can be approximately estimated by Gurson's model, and the model parameter's agreement with the experimental data increases with increasing the plastic deformability of the matrix phase.

Yield behavior of rubber-modified epoxy adhesives under multiaxial stress conditions

In rubber-modified epoxy resins, a damage zone is generated in the vicinity of the crack tip due to the cavitation of rubber particles, which improves fracture toughness dramatically. Hence, in evaluating the stress distribution in adhesive joints with rubber-modified adhesives, the void formation and growth should be taken into account. In most studies, however, the adhesive layer is still considered as a continuum material governed by the von Mises yield criterion. For many ductile materials, Gurson's model is used for the stress analysis, in which the void formation and growth is taken into account. In a previous study, using adhesively bonded scarf and torsional butt joints, the effect of stress triaxiality on the yield stress in the adhesive layer was investigated. In this study, these experimentally-obtained yield stresses were compared with those obtained by a finite element method, where Gurson's constitutive equations were applied to the adhesive layer. As a result, the calculated yield stresses agreed well with the experimentally-obtained yield stresses. This indicates that Gurson's model is a useful tool for estimating stress distributions in adhesive joints with rubbermodified adhesives.

Evolution of Damage Zone in Adhesive Layer of Adhesively Bonded Butt Joints under Cyclic Loading

For rubber-modified adhesives, the damage zone plays an important role in governing adhesive strength. Recently, the damage to the adhesive layer has been investigated by in-situ observation under several load conditions. However, these observations are limited to the behaviour on the surface of the adhesive layer. If an adhesively bonded butt joint with a very thin adherend can be tested, damage in the interior of the adhesive layer will be observed in transmitted light, which facilitates clarification of the fracture mechanism of adhesive joints. In this study, to observe the damage in the interior of the adhesive layer, adhesively bonded butt joint specimens were made from 0.3 mm thick steel plates bonded by a rubber modified adhesive. These specimens were used for the fatigue tests, and the damage to the adhesive layer was examined by use of a microscopic video-camera under transmitted light. Furthermore, elastic-plastic finite element analysis of the butt joint was conducted to simulate the propagation behaviour of the damage zone. Modified Gurson's constitutive equations were applied to the adhesive layer and the rubber particles were considered to be the initial voids. The result for the analytical distributions of the void volume fraction was compared with the observed damage zone. It was found that the observed damage zone coincides with the high volume fraction zone.

Static and fatigue crack growth of epoxy adhesives and fractal dimensions

Fractured surfaces of epoxy adhesives under mode I static and fatigue (cyclic) loading have fractal characteristics. The effects of rubber modification, adhesive thickness and cross-head speed (static only) on both static and fatigue fracture surfaces of epoxy adhesives were examined using fractals. Under static loading, the fractal dimension becomes high due to rubber modification. It is related to the static crack growth properties for both unmodified and rubber-modified adhesives. The following equation gives the relationship between the observed crack extension resistance, G I and the fractal dimension, D: log G I =α 1 · D+β 1, where α 1 and β 1 are experimental constants. Regardless of whether adhesives contain rubber particles or not, the fractal dimension is not affected by adhesive thickness and cross-head speed. Under fatigue loading, the fractal dimension of the fractured surfaces becomes high due to rubber modification under the same energy release rate range, Δ G I when Δ G I is higher than 100 J/m 2. The fractal dimension decreases with an increase in the fatigue crack growth rate, d a/d N. A relationship is given by the following equation between a surface ‘fatigue working’ parameter for the crack, S(= da/ dN · ΔG I ) and the fractal dimension for unmodified and rubber modified adhesives: log S= log da dN · ΔG 1 =α 5 · D+β 5, where α 5 and β 5 are experimental constants. Whether the adhesives contain rubber particles or not, the fractal dimension as well as the d a/d N−Δ G I relation is little affected by adhesive thickness.

Fatigue-Fractured Surfaces of Epoxy Adhesives and Fractals under Mode I Cyclic Loading.

The effects of rubber modification and adhesive thickness on the fatigue-fractured surfaces of epoxy adhesives under mode I cyclic loading were examined using fractals. When the energy release rate range, ΔGI, is higher than 100 J/m2, the fractal dimension of fractured surfaces becomes high due to rubber modification. The fractal dimension decreases with increasing fatigue crack growth rate, da/dN. The following equation shows the relationship between parameter S(=da/dN·ΔGI)and fractal dimension D for unmodified and rubber-modified adhesives. log S=log(da/dN·ΔGI)=α4·D+β4 Here, α4 and β4 are experimental constants. Regardless of whether the adhesives contain rubber particles or not, the fractal dimension as well as the da/dN-ΔGI relation is little affected by adhesive thickness.

Fractured Surfaces of Epoxy Adhesives and Fractals under Mode I Loading. 2nd Report. Fatigue Crack Growth Properties and Fractal Dimensions.

The effects of rubber modification and adhesive thickness on the fatigue-fractured surfaces of epoxy adhesives under mode I cyclic loading were examined using fractals. When the energy release rate range, ΔG1 is higher than 100 J/m2, the fractal dimension of fractured surfaces becomes high due to rubber modification under the same ΔG1. The fractal dimension decreases with increasing fatigue crack growth rate, da/dN. A relationship is given by the following equation between the parameter, S (=da/dN×ΔG1) and the fractal dimension, D for unmodified and rubber-modified adhesives. log S-log((da)/(dN)·ΔG1)=α4·D+β4 where α4 and β4 are experimental constants. Whether the adhesives contain rubber particles or not, the fractal dimension as well as the da/dN-ΔG1 relation is little affected by adhesive thickness.

Effect of CNBR Modification on Mode I Static Fracture Characteristics of Epoxy Adhesives.

Epoxy adhesives were modified with CNBR (cross-linked acrylonitrile butadiene rubber) to increase their fracture toughness. Strengths of double cantilever beam specimens fabricated with unmodified and modified epoxy adhesives under Mode I loading were measured. The crack extension resistance given by strain energy release rate G was obtained from the test. Rubber-modified adhesives show higher fracture toughness (the maximum crack extension resistance) than unmodified adhesive. The crack initiates at a relatively low G and grows as G increases. For unmodified adhesive, the maximum crack extension resistance is almost the same as the crack extension resistance in steady-state crack growth. On the other hand, the maximum resistance appears before the crack grows steadily. The variation of crack extension resistance with respect to crack length was influenced by rubber content. Adhesive thickness also affects the crack extension resistance of rubber-modified epoxy adhesives. The fracture surface topology varies according to rubber content.