Three-dimensional finite element stress analysis of single-lap adhesive joints of dissimilar adherends subjected to impact tensile loads

The stress-wave propagations and stress distributions in single-lap joints of dissimilar adherends were analyzed using an elastic three-dimensional finite-element method (DYNA3D). An impact tensile load was applied to the single-lap adhesive joint by dropping a weight. One end of the upper adherend in the single-lap adhesive joint was fixed and the other adherend (lower adherend) which was connected to a bar was impacted by the weight. The effects of Young's modulus and the thickness of each adherend on the stress wave propagations and stress distributions at the interfaces were examined. It was found that the maximum value of the maximum principal stress occurred near the edge of the interface of the fixed adherend. The maximum principal stress increased as Young's modulus of the fixed adherend increased. It was also observed that the maximum principal stress increased as the fixed adherend thickness decreased. In addition, strain responses in the single-lap adhesive joints of dissimilar adherends subjected to impact tensile loads were measured using strain gauges. Fairly good agreements were found between the FEM calculations and the experimental measurements.     

Paper properties affecting pressure-sensitive tape adhesion

The interaction between paper and pressure-sensitive adhesive was investigated by peel adhesion testing. The paper/adhesive peel curves were analyzed by plotting the logarithm of the peak peel force (i.e. the maximum value) against the logarithm of the peel rate, yielding two linear segments: a peel-rate-dependent interfacial failure domain and a rate-independent paper failure domain. Three independent parameters were extracted from these plots, namely, the interfacial peel force (F _in) at a low peel rate of 1 mm/min, the maximum peel force (F _c) and the slope (S _p) in the plot of log (interfacial peel force) versus log (peel rate). The linkages between paper and its peel responses were analyzed using the multivariate statistical methods. The paper properties influencing peel force in interfacial failure domain were found to be primarily the paper surface chemistry, characterized by oxygen/carbon ratio (determined by XPS), and secondarily paper surface roughness. The peel force increased with oxygen/carbon ratio and with the surface roughness. The log–log slope in the interfacial failure domain was found to be independent of paper properties; it is determined by the adhesive rheology. The governing paper property in the paper failure domain was found to be the paper internal bond strength as measured by an internal (Scott) bond tester.     

Viscoelastic and adhesion properties of EVA/tackifier/wax ternary blend systems as hot-melt adhesives

Two types of wax were added to a ethylene vinyl acetate (EVA) copolymer/aromatic hydrocarbon resin (tackifier) blend in the molten state and the miscibility, viscoelastic and adhesion properties of ternary blends as hot-melt adhesives (HMAs) were investigated. Miscibility and viscoelastic properties were studied using differential scanning calorimetry (DSC), Brookfield viscometry and dynamic mechanical thermal analysis (DMTA), and their adhesion strength was determined in terms of single lap shear strength. DSC thermograms of both types of waxes showed their melting peaks in a similar region to that of EVA/tackfier blend. It was difficult to evaluate the miscibility of ternary blends using DSC because the melting peaks of the waxes overlapped with those of the EVA/tackifier blend, although the glass transition temperature (T _g) of the ternary blend systems slightly increased with increasing wax concentration. However, their storage modulus (E′) increased slightly and loss tangent (tan δ) showed different peaks when two types of wax were added to the EVA/tackifier blend. Therefore, the miscibility of EVA/tackifier blend altered with addition of waxes. In addition, their melt viscosity decreased with increasing wax concentration. Furthermore, the adhesion strength of the ternary blends decreased with increasing wax concentration, despite the increment of storage modulus. These results suggested that the ternary blends of EVA/tackifier/wax were heterogeneous.     

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.     

Use of polymeric emeraldine salt for conductive adhesive applications

The wide range of electrical, electrochemical, and optical properties associated with Polyaniline (PANI) and its composites has made them attractive for many industrial applications. In this study, Emeraldine Salt (ES), which is a doped conducting form of PANI, was chemically prepared in situ using the oxidizing agent ammonium persulphate in the presence of aqueous HCl solution. In order to gain insight into the efficiency of electrical conduction in relation to the chemical and viscoelastic behaviors of ES in homogeneous powder and as filler in composite adhesive forms, the interrelationship between their electrical resistivity and morphology was studied. The pressure-dependent electrical conduction behavior of ES powder shows, among other factors, the dependence of electrical resistivity on the intrinsic chemical and viscoelastic properties of powders. In order to obtain electrically conductive composite adhesive forms, a nonconducting nitrocellulose solution based adhesive was filled with as-synthesized ES in the amount 30%, 40%, and 50% by volume, and the effects of filler concentration on the composite's electrical resistivity were investigated. The results of our investigation revealed a typical percolation threshold behavior with a critical concentration of approximately 30% by volume. Finally, single lap joints were made using aluminum and zinc (plated on copper) as well as silver substrates bonded using the ES filled nitrocellulose adhesive developed, and the corresponding electrical and mechanical properties of these bonded interconnections were investigated. A complex redox-reaction mechanism catalyzed by ES filler is thought to be occurring at the boundary layer between the adhesive and the substrate for conversion from semiconductor to insulator of the joints in the cases of aluminum and zinc (plated) substrates.     

Effects of Outdoor Exposures and Boron Compounds on Bonding Strength of Desmodur-VTKA Adhesive

This study was carried out to determine the shear strength of an adhesive on weathered wood, impregnated with a boron compound, using a long term dipping method. The shear strength of D-VTKA adhesive on two types of wood species each containing one of two types of the impregnated material, under different weathering conditions, was measured. The results showed that the highest shear strength (11.01 N/mm²) was obtained with the control samples of untreated, oriental beech wood, and the lowest was obtained for Scots pine impregnated with boric acid after two seasons of exposure (six months). Weather conditions affected the shear strength in a negative way. Finally the results showed that if the boron compounds were supported with non-leaching chemicals, they could be recommended as fire-retardant additives and would extend the life of wood bonded with D-VTKA adhesive.     

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.     

Effect of overlap length on durability of joints bonded with a pressure-sensitive adhesive

In this study, the effect of overlap length on durability of a film type adhesive, Structural Bonding Tape (SBT) 9244, which possesses pressure-sensitive and visco-elastic properties, was investigated. Single-lap joints with 1.62 and 3.2 mm adherend thicknesses and at 12.5, 25 and 50 mm overlap lengths consisting of AA2024-T3 alloy as the adherend were exposed to two environmental conditions for exposure times of up to 90 days. The exposure environments were 100% relative humidity (RH) and 3.5% NaCl solution. At the end of exposure times, the failure surfaces were examined by Scanning Electron Microscopy (SEM) after the strength of joints was determined with the lap shear test. It was observed that with increasing overlap length, not only the failure load increased, but also the degradation rate decreased. In addition, as the metal adherends do not absorb any water and moisture from the environments, the metal adherend thickness had no effect on durability of the adhesively bonded joints.     

Influence of water immersion on the single-lap shear strength of aluminum joints bonded with aluminum-powder-filled epoxy adhesive

Durability of adhesively-bonded aluminum joints was investigated by measuring the joint strength using the single-lap shear test before and after exposure to distilled water and seawater. Fractured specimens were examined by photography and scanning electron microscopy to determine the failure modes. Addition of Al particles as much as 50 wt% did not cause any significant decrease in adhesive joint strength. Moreover, varying the Al filler content in the adhesive did not have a significant effect on adhesive behavior in either of the two environments studied. The unexposed adhesive joints failed almost completely in a cohesive (in the adhesive) failure mode. Some decrease in strength was observed in adhesive joints after exposure to both distilled water and seawater for 6 months. The decrease in adhesive joint strength was more significant for specimens immersed in distilled water than those immersed in seawater, probably due to the higher amount of moisture in the adhesive in distilled water than in seawater, as observed in a related moisture diffusion study. The joints exposed to distilled water or sea water failed in more than one mode. The interior part of the adhesive lap area failed in a cohesive mode while an adhesion failure mode was observed near the edges of the adhesive lap area, which is believed to be a result of moisture diffusion through the edges.     

Preparation of Highly Adhesive and Superhydrophobic Epoxy-Based Thin Film by Sol–Gel Process

In this study the preparation of a superhydrophobic epoxy-based thin film with excellent high adhesion properties was carried out by mixing epoxy polymer solution, MTMOS solution, and silica powder. It was found that the adhesion between the film and a substrate could be improved by adding PET fibers to the above solution. The characteristic properties of the as-prepared films were analyzed by contact angle measurements, scanning electron microscopy (SEM) and by atomic force microscopy (AFM). The adhesion between the film and the substrate was estimated by the cross-cut method. The experimental parameters were: the mole fraction of MTMOS, weight fraction of silica powder, weight fraction of PET fibers, and the curing temperature. The result indicated that the contact angle and the sliding angle of the hybrid film were 153° and 4°, respectively, and the adhesion test result was very good (5B degree), while the mole fraction of MTMOS was 0.4, the silica powder concentration was 30 wt%, and the PET fibers concentration was 0.15 wt%, with the curing temperature in the range of 130–180°C.