Optimization of adhesively-bonded single lap joints by adherend notching

The effect of adherend notching on the strength and deformation behavior of single lap joints was investigated. First, a parametric study was conducted using finite element analysis (FEA). This initial part of the research into the effect of notches on joint behavior involved determination of the optimum notch location and notch dimensions. This was done by using FEA in a series of models with different notch positions and geometries. The results of this parametric study were used to select the most promising lap geometries for further study. Next, more detailed FEA were conducted on the selected lap geometries. These data were compared with the experimental single-lap shear test results to assess the applicability of different failure criteria. Three different model adhesives were used: a rubber toughened film epoxy with nylon carrier, a styrene-butadiene-styrene block copolymer based deformable 'gel' adhesive, and a two-part, metal filled brittle epoxy adhesive. The FEA for single lap joints containing 'top notches' on the unbonded, top side of the adherends, at locations corresponding to the overlap ends, and bonded with the two-part metal filled epoxy provided the best agreement with the experimental results. The experimental results showed a 29% increase in joint strength with the introduction of the notches, which matched very well with the 27% decrease in the peak peel stress observed by the FEA results. For this brittle adhesive, the peel stress is almost certainly the governing failure stress. This was confirmed by matching of the FEA peak peel stress ratios with the experimental load ratios, for both the notched and unnotched specimens.     

FEM stress analysis and sealing performance improvement of box-shaped bolted flanged joints using silicone sealant under internal pressure and thermal conduction conditions

Interface stresses and sealing performance of thin-wall box-shaped bolted flanged joints using silicone sealant under internal pressure and thermal conduction conditions are analyzed by the thermo-elastic finite element method. The flexible flanges were fastened by M8 bolts and nuts with an initial clamping force (bolt preload) after being joined with the silicone sealant. In the elastic and thermo-elastic Finite Element Method (FEM) calculations, the effects of the bolt pitch distance, flange rigidity and flange thickness were examined on the interface stress distributions. In addition, the effects of the linear thermal expansion coefficient and Young's modulus of the silicone sealant in the steady temperature state were also examined from a design standpoint. In the experiments, leakage pressure was measured when the silicone sealant was applied between an aluminum flexible flange with 1 mm thickness and an aluminum flange (body) with 10 mm thickness. Measurements of sealing performance with the silicone sealant and a sheet gasket were also conducted. In addition, strains in the joint in steady temperature states were also measured by strain gauges to understand the effect of the operative temperature on the sealing performance. The experimental results were found to be in fairly good agreement with the calculated results. From the results, it was found that the effect of the thermal conduction condition was greater than that of the internal pressure on both the interface stress distributions and the sealing performance. In addition, the sealing performance was found to be better in the joint with the silicone sealant than that with the sheet gasket.     

The effect of surface properties on the adhesion of modified polychloroprene used as adhesive

The adhesion properties of polychloroprene can be improved by addition of such materials as piperylene–styrene co-polymer (PSC), VeoVa-10 polymer, VeoVa-11/methyl methacrylate/2ethylhexyl acrylate co-polymer (VeoVa-11/MMA/2EHA) and poly(vinyl acetate) waste (wPVAc). Here, the relationship between adhesion properties and surface tension of polychloroprene was investigated. Contact angle measurements have been used to study the effects of nature and content of polymeric additives on the adhesion and surface properties of polychloroprene. Low-surface-tension VeoVa-10 polymer has the tendency to migrate to the surface of polychloroprene; thus, adhesion is determined mainly by this additive property. Enrichment of polychloroprene film bottom layer by the additive was observed using high-surface-tension PSC and wPVAc. In this case, the adhesion properties of polychloroprene depend on the interactions at the interface. Adhesion properties of polychloroprene were found to depend not only on compatibility between adhesive components, but also on compatibility between the adherend and the adhesive.     

Diagnosis criterion for damage monitoring of adhesive joints by a piezoelectric method

Adhesive joints have been widely used for fastening thin adherends because they can distribute the load over a larger area than mechanical joints, require no holes, add very little weight to the structure and have superior fatigue resistance. Since the reliability of an adhesive joint is dependent on many parameters, such as the shape of joint, type of applied load and environment, an accurate prediction of the fatigue life of adhesive joints is seldom possible, which necessitates an in situ damage monitoring of the joints during their operation. Recently, a piezoelectric method using the piezoelectric characteristics of epoxy adhesives has been successfully developed for adhesive joints because it can continuously monitor the damage of adhesively bonded structures without producing any defects induced by inserting a sensor. Therefore, in this study, the damage of adhesive joints was monitored by the piezoelectric method during torsional fatigue tests in order to develop the diagnosis criterion for damage monitoring of adhesive joints by the piezoelectric method. The diagnosis criterion was developed by analyzing damage monitoring signals under various test conditions and adopting normalized parameters.     

Corona discharge treatment of EVAs with different vinyl acetate contents

Four ethylene vinyl acetate (EVA) co-polymers with different vinyl acetate (VA) contents (9–20 wt%) were treated with corona discharge to improve their adhesion to polychloroprene (PCP) adhesive. The thermal properties of the EVAs decreased as their VA content increased, caused by a decrease in crystallinity. The elastic and viscous moduli of the EVAs decreased and the temperature and modulus at the cross-over between these moduli decreased with increasing VA content. Contact-angle measurements (water), infrared spectroscopy (ATR-IR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to analyse the surface modifications produced in the corona-discharge-treated EVAs. The corona discharge treatment produced improved wettability and created roughness and oxygen moieties on the EVA surfaces. The higher the VA content and the higher the corona energy, the more significant modifications were produced on the EVA surface. The VA content also affected the T-peel strength values of treated EVA/polychloroprene + isocyanate adhesive joints, as the values increased with increasing VA content. Mixed failure modes (interfacial + cohesive failure in the EVA) were obtained in the adhesive joints produced with corona discharge treated EVAs containing more than 9 wt% VA. The accelerated ageing of the joints did not affect the T-peel strength values, but the locus of failure in most cases became fully cohesive in the EVA, likely due to the higher extent of curing of the adhesive.     

Stress and failure analyses of scarf repaired CFRP laminates using a cohesive damage model

This study describes stress and failure analyses of tensile loaded repaired Carbon Fibre Reinforced Composite (CFRP) laminates, using scarf configuration. A numerical model including interface finite elements was used to obtain peel and shear-stress distributions in the directions tangent and normal to the scarf. These stresses were evaluated at several locations in the repair, namely in the middle of the adhesive, at interfaces between adhesive and patch, and between adhesive and parent material. Several scarf angle values were considered in the analysis. A cohesive mixed-mode damage model was also used to carry out the failure analysis, in order to assess the efficiency of the repairs, for different stacking sequences. A study was performed to evaluate the influence of the mechanical properties of the adhesive and parent laminate/adhesive and adhesive/patch interfaces on the strength and failure modes of the joint. It was concluded that the strengths of the adhesive and interfaces are more important than the fracture properties in the failure process of the repair. It was also verified that the strength of the repair increased exponentially with the scarf angle reduction.     

Tensile Strength of Joints Bonded With a Nano-particle-Reinforced Adhesive

In order to improve the tensile lap shear strength of adhesively bonded joints, nano-particles were dispersed in the adhesive using a 3-roll mill. The dispersion states of nano-particles in the epoxy adhesive were observed with TEM (Transmission Electron Microscopy) with respect to the mixing conditions, and the effect of nano-particles on the mechanical properties of the adhesive was measured with respect to dispersion state and weight content of nano-particles. Also the static tensile load capability of the adhesively bonded double lap joints composed of uni-directional glass/epoxy composite and nano-particle-reinforced epoxy adhesive was investigated to assess the effect of nano-particles on the lap shear strength of the joint. From the experimental and FE analysis results, it was found that the nano-particles in the adhesive improved the mechanical properties of the adhesive. Also the increased failure strain and the reduced CTE (coefficient of thermal expansion) of the nano-particle-reinforced adhesive improved the lap shear strength of adhesively bonded joints.     

Effect of Adhesive Compliance in the Assessment of Soft Adhesives with the Wedge Test

Wedge tests are usually analysed assuming that the free, unbonded members may be treated as encastré cantilever beams. However, if the adhesive layer is sufficiently flexible (e. g., due to low elastic modulus), then significant strain in the bonded region may occur and lead to modification of the behaviour outside this region. Using in conjunction a sensitive strain gauge method on asymmetric wedge tests and a mathematical analysis developed from the work of Winkler, we conclude that the standard, simple beam theory approach significantly overestimates crack length for a supple adhesive layer. The present contribution mainly considers strain effects in the intact, bonded zone, rather than fracture per se. However, it is concluded that, if in fracture tests, the incorrect values of crack length obtained from the encastré beam assumption are employed to calculate fracture energy using the simpler model, there will be some self-compensation and little error in estimates of the latter will result (at least in the cases presently studied).     

Tissue adhesive using synthetic model adhesive proteins inspired by the marine mussel

The surface free energy and its dispersion and polar components of pigskin were determined by wettability measurements. The contact angles and work of adhesion of solutions of the synthetic model adhesive sequence poly(Gly-Tyr-Lys) inspired by marine adhesive proteins were measured on the epidermis and the dermis of pigskin. Also the surface free energy of pigskin was determined using contact angles of certain probe liquids. When a poly(Gly-Tyr-Lys) buffer solution containing tyrosinase as a bioadhesion formulation was used to close an incision of a living pig, a good incision adhesion and reduced immunological response after 1 week were observed from photographs using an optical microscope and the amount of macrophages by image analysis.     

Molecular dynamics study of a nano-particle joint for potential lead-free anisotropic conductive adhesives applications

Molecular dynamics (MD) simulation was performed to simulate the application of an anisotropic conductive adhesive (ACA). The dynamics of a copper sphere (radius 2 nm) between two copper substrates was investigated, using the embedded atom method (EAM). The structure evolution of the sphere was analyzed by x–z plane projection, pair-correlation function, and potential energy curve. x–z plane projection and potential energy curve showed that with applied strain the copper sphere went through an order–disorder–order type of phase transition. Pair-correlation function results showed a more ordered sphere structure with applied compressive strain as compared to the simulation without strain.