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

This 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.     

Fatigue Failure of Rubber-to-Rubber Joints

This paper presents an experimental investigation on fatigue failure of rubber-to-rubber similar and dissimilar joints made from natural rubber and EPDM. The effects of interlinking density, relative proportion of one matrix in the two-component joints, filler loading in the filled part of the joint and strain level on the fatigue life have been studied. Adhesion (peel strength) between similar and dissimilar rubber-to-rubber joints has also been investigated to attempt to determine a correlation between adhesion and fatigue failure.     

Fatigue Failure of Rubber-to-Rubber Joints

This paper presents an experimental investigation on fatigue failure of rubber-to-rubber similar and dissimilar joints made from natural rubber and EPDM. The effects of interlinking density, relative proportion of one matrix in the two-component joints, filler loading in the filled part of the joint and strain level on the fatigue life have been studied. Adhesion (peel strength) between similar and dissimilar rubber-to-rubber joints has also been investigated to attempt to determine a correlation between adhesion and fatigue failure.     

Investigations of Stresses in Glass Bottles Under Internal Hydrostatic Pressure

The behavior of glass bottles under internal hydrostatic pressure has been studied. Three commercial beer bottle designs were investigated by four different methods of experimental stress analysis, namely, photoelastic, electric strain gauge, brittle coating, and brittle model methods. The "fixation" or "stress-freezing" method of three-dimensional photoelastic stress analysis employing Fosterite models has been applied to the study of glass bottles apparently for the first time. By this method extensive measurements of the surface stress distributions have been made and are presented in graphical form. Other tests corroborate these results and illustrate the characteristics of bottles under internal pressure, including the effects of certain elements of bottle design. Results obtained by all four methods correlate well and should find valuable and practical applications in similar studies on other types of glass containers     

填充环氧树脂界面残余应力研究 Ⅴ.界面应力集中及残余应力的影响

本文针对橡胶和玻璃珠填充环氧树脂体系,通过显微光弹法测定填充粒子的界面应力集中系数,发现橡胶填充体系的应力集中发生在粒子赤道处,而玻璃珠体系发生在粒子两极。在低拉伸时,界面残余应力对应力集中的影响较大,但在高拉伸,特别是达到材料伸长率时,界面残余应力的作用可忽略。     

Elastic Behaviour of Ternary Rubber-filled Vulcanizates

Near-equilibrium stress–strain measurements have been carried out on ternary rubber vulcanizates. The effect of variation of the butyl rubber content on the elastic behaviour of the ternary rubber vulcanizates has been studied. It has been found that butyl rubber (IIR) is less sensitive to the vulcanization system used than either natural rubber (NR) or styrene–butadiene rubber (SBR). One can obtain a partially crosslinked system with an IIR phase embedded in the crosslinked matrix of NR and SBR. The role played by carbon black during mixing of the ternary blend has been investigated. The Mooney–Rivlin relationship was used to describe the behaviour of the ternary rubber matrix. The constants 2C₁ and 2C₂ have been calculated by use of the strain-amplification factor and the total crosslink density of the ternary rubber–carbon black systems has been investigated. The data have been evaluated in terms of the molecular theories of rubber elasticity. The elastic behaviour was found to be intermediate between the affine and phantom limits of the theory. © of SCI.     

Strength prediction of adhesively bonded carbon/epoxy joints

A finite element approach has been used to obtain the stress distribution in some adhesive joints. In the past, a strength prediction method has not been established. Therefore in this study, a strength prediction method for adhesive joints has been examined. First, the critical stress distribution of single-lap adhesive joints, with six different adherend thicknesses, was examined to obtain the failure criteria. It was thought that the point stress criterion, which has been previously used for an FRP tensile specimen with a hole, was effective. The proposed method using the point stress criterion was applied to adhesive joints, such as single-lap joints with short non-lap lengths and bending specimens of single-lap joints. Good agreement was obtained between the predicted and experimental joint strengths.     

Carbon nanotubes as Raman sensors of vulcanization in natural rubber

A method has been developed to synthesize composites with cross-linked natural rubber (NR) and dispersed single-wall carbon nanotubes (SWNT). The mechanical response of the NR samples was observed to change as a function of the amount of sulfur used for cross-linking, based on the number density of cross-links resulting from the vulcanization process. The relationship between SWNTs D^∗ wavenumber shift and the amount of sulfur have been obtained by means of Raman spectroscopy. The cross-link densities of the NR and SWNT/NR samples have been calculated from uniaxial stress-strain measurements, and plotted as the function of the amount of added sulfur. Comparison of the results from mechanical measurements and Raman spectroscopic measurements showed that SWNT Raman sensors are sensitive to the cross-link density in natural rubber and can be used to evaluate the cross-linking process of rubbery materials.     

A two-dimensional stress analysis of single-lap adhesive joints subjected to external bending moments

The stress distributions in single-lap adhesive joints of similar adherends subjected to external bending moments have been analyzed as a three-body contact problem using a two-dimensional theory of elasticity (plain strain state). In the analysis, both adherends and the adhesive were replaced by finite strips. In the numerical calculations, the effects of the ratio of Young's moduli of the adherends to that of the adhesive and the adhesive thickness on the stress distribution at the interfaces were examined. It was found that the stress singularity occurs at the edges of the interfaces and that the peel stress at the edges of the interfaces increases with decreasing Young's modulus of the adherends. It was noticed that the singular stress decreases at the edges of the interfaces as the adherend thickness increases. In addition, photoelastic experiments and FEM (finite element method) calculations were carried out and fairly good agreement was found between the analytical and the experimental results.     

Development of correlation between potassium hydroxide number and conductivity of concentrated natural rubber latex

Increase in the ion concentration in the medium was found to increase conductivity and potassium hydroxide number (KOH No) in natural rubber latex (NRL). Addition of long chain fatty acids can increase the ion concentration in the medium and stability of NRL. A series of concentrated natural rubber latex samples from three different areas with different soils and climatic conditions were tested for the parameters such as KOH No and conductivity. They have been measured over a period of 62 days, upon addition of soap to natural rubber latex concentrate. The result showed that there was a strong positive linear correlation between conductivity and KOH No. The regression equation to express the relationship between the variables has also been found. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Interfacial stress concentration and the effect of residual microstress on tensile fracture in filled epoxy resins

Interfacial stress concentrations in rubber-filled epoxy resin and glass bead–filled epoxy resin systems were studied by a photoelastic method. We found that the stress concentration is independent of the applied stress and the particle size, but decreases with increasing chemical bonding at the interface between rubber and matrix. For the rubber-filled system, stress concentration is present at the equator of particles, while for either the untreated or treated glass bead-filled system, it is on the two poles. The effect of interfacial residual microstress on tensile fracture was studied, and results showed that the function of residual microstress is limited as tensile elongation increases.     

Water-based adhesive formulations for rubber to metal bonding developed by statistical design of experiments

Waterborne adhesives for rubber to metal bonding have been available since 1990. However, published information about their formulation has been limited, as proprietary restrictions are exercised by companies. As a consequence, the way these adhesives interact with substrates has not been studied extensively. With the aim of investigating the effect the components of a waterborne adhesive have on rubber to metal bonding, fractional factorial and surface response methodologies of design of experiments were employed in this study. Twenty six formulations were prepared with a polychloroprene latex as the adhesive polymer. Viscosity, wettability and non-volatile solids content were measured with each liquid adhesive, while the mechanical strength was evaluated by applying a tensile mechanical stress over cured solid adhesive films. Adhesion properties were evaluated by using a single lap-shear test on metal to metal joints and a pull-out test on rubber to metal joints. The results showed that the components with the largest relative influence on cohesive and adhesives forces were tackifier resin, silicon dioxide and polychloroprene latex type. In order to better understand the contributions of these variables, mathematical models correlating them with the response variables were obtained. This study is valuable in explaining how, through statistical methods, a waterborne adhesive for rubber to metal bonding can be formulated with a reasonably low number of experiments.     

Elastomeric joint performance of PVC,VC, and FRC Pipes

For rubber ring joints to resist plant root penetration in drier countries, it has been reported and promulgated into some national standards that a sufficiently high contact pressure between the rubber and the pipe must be maintained. The evaluation of the appropriate interfacial pressure to be used has been under discussion for some time and different values have been stated in different standards. This matter has been the subject of considerable controversy, and decisions on the interface pressure used will affect the performance and ease of insertion of a rubber ring joint. This paper reports and discusses previous and current research on root penetration past elastomeric joints, the existing test requirements of elastomeric pipe joints for resisting root penetration given in standards, and the inconsistencies in performance requirements across the different pipe material types. The initial interface sealing pressures for commonly used PVC. VC and FRC joints were assessed and these are discussed in light of the accelerated root intrusion tests performed in Black Hill Flora Centre in Adelaide, South Australia.     

聚合物熔体窄缝脉动挤出行为的光散射研究

利用聚合物熔体的双折射性质,对振动力场作用下窄缝挤出模头中的聚合物熔体脉动挤出行为进行光散射研究,用光弹性图像和光强度矩阵来表征聚合物熔体受到的剪切应力。振动力场的引入使光弹性图像和光强度矩阵发生了规律性变化。结果表明,振动力场使聚合物熔体剪切应力减小、表观粘度降低,最终使得挤出压力降低、产量提高。     

A feasibility study of scattered-light photoelasticity in the determination of the side pressure distribution of the pressed powder bed

A method for the determination of the side pressure distribution of the pressed powder bed by the scattered-light photoelastic technique has been proposed. A cylindrical container made of photoelastic-sensitive material was used as a die, the powder in which was pressed uniaxially. The stress distribution in the wall of the container was measured by the scattered-light photoelastic method. The stress distribution at the inner surface of the container wall was obtained by extrapolation, and the pressure components of the powder bed at the powder—container interface were evaluated from the stress. The existence of a maximum of the side pressure near the surface of the powder bed is suggested.     

Molecular weight between physical entanglements in natural rubber: A critical parameter during strain-induced crystallization

The purpose of this paper is to clarify the role of crosslink density of natural rubber on its strain-induced crystallization. A series of new in situ synchrotron X-ray diffraction experiments were performed during the stretching process of weakly and highly vulcanized natural rubber samples. The experimental data have been analysed in terms of both crystallite size and crystallization rate. Moreover, a careful treatment of previously published data that might appear contradictory has been done. The comparison between all these data, coming from NR of different origins and with different crosslinking states, demonstrates that the molecular weight between physical entanglements in natural rubber appears as a key parameter for strain-induced crystallization (SIC).     

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.     

Prediction of crack length and crack growth rate of adhesive joints by a piezoelectric method

As adhesive joints have been widely used for fastening thin adherends, the damage tolerance design of adhesive joints has become important, and the estimation of initiation and propagation of a fatigue crack in the adhesive has become necessary. However, the measurement of crack length of tubular joints has been difficult because the observation of crack initiation and growth in the adhesive layer by conventional methods is not easy. In this work, a prediction method for the fatigue crack length in the adhesive layer of tubular single-lap adhesive joints was developed by the piezoelectric method. In order to obtain the relationship between the fatigue crack length and the piezoelectric signal, finite element analysis was conducted and verified by experiments. The damage of the adhesive joints was monitored by the piezoelectric method during torsional fatigue tests on tubular single-lap adhesive joints. Using the damage monitoring signals and the relationship between the fatigue crack length and the piezoelectric signal, a method for predicting fatigue crack growth in the adhesive layer of tubular single-lap adhesive joints was developed.     

Mechanism of Toughening in Rubber Toughened Polyolefin—A Review

The recent advances and theories in the studies of the toughening mechanism have been reviewed to explain the effect of rubber particles in different rubber modified Polyolefin materials. To elucidate toughening effect, major theories e.g., critical particle distance, particle size, micro deformation by stress field of rubber, shear yielding and crazing phenomena has been reviewed. Based on these theories, variety of blends of rubber modified Polyolefin materials has been compared but no one of these provided adequate information to be considered as total theory of toughening. To achieve the objective of toughening, it is important to maintain critical particle size, uniform particle distribution and good interfacial adhesion by inclusion of suitable compatibilizer in the matrix. Particular attention has been paid to study the type of morphology and bimodal distribution of rubber particles to elucidated toughening effect. Rubber particle cavitation, which comes from micro-voids and rubber phase interface are then further discussed.     

Natural rubber–cassava starch foam by compression moulding

Abstract

The foaming processes of mixtures of cassava starch–water and cassava starch–natural rubber latex blends have been carried out by compression moulding. The appropriate conditions under which to produce expanded foam are as follows: a temperature of 150°C, 10·8 MPa pressure, and a 2 min moulding time. For the foam from the cassava starch with water as a blowing agent, it was found that water levels in the range of 150–200% by weight of the dry starch gave good conditions for foaming. The resulting foamed material has a uniform closed cell structure. Regarding blending of cassava starch with natural rubber, the natural rubber could not be dispersed in the gelatinised starch when blended at a temperature of 70°C. To stabilise and prevent the coagulation of natural rubber in the blending process, Nonidet P40, a nonionic surfactant, was used. A suitable amount of Nonidet P40 was 1·5% by weight of natural rubber latex. The compressive stress and the storage modulus of the foam obtained increased (42–233%) with increasing natural rubber content owing to the high elasticity of the natural rubber and its promotion of more elasticity to the foams. When 2–5% of benzoyl peroxide by weight of natural rubber was added to the rubber latex, the compressive stress of the foam was further increased (20–118%) owing to vulcanisation of the natural rubber. Furthermore, an addition of 15–30% of calcium carbonate by weight of the dry starch of the blends was found to increase the compressive stress and storage modulus of the foams (69–148%) and the hardness and brittleness of the foams.