The Effect of Humidity on the Adhesion Behavior of AC Anodised Aluminium

The effect of humidity on the surface forces of hot AC anodised AA6060 alloy was studied. The surface adhesion forces were measured in various humidity environments using an atomic force microscope (AFM). The environmental durability of the joints bonded at different humidity conditions was investigated using wedge test experiments. The results from AFM and durability tests indicate that there is a strong capillary effect at around 70–80% relative humidity. This effect was attributed to the change in the adsorption behavior of water on the anodised oxide surface forming bulk liquid and hence dissociating the bonds across the interface. At the lower humidity levels below 60% RH, no capillary effect was observed and the total adhesion forces followed the dry case values.     

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.     

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.     

The Static Failure of Adhesively Bonded Metal Laminate Structures: A Cohesive Zone Approach

Adhesively bonded metal laminates are used in aerospace applications to achieve low cost, light weight structures in the aerospace industry. Advanced structural adhesives are used to bond metal laminae to manufacture laminates, and to bond stringers to metal laminate skins. Understanding the failure behaviour of such bonded structures is important in order to provide optimal aircraft designs. In this paper, the static failure behaviour of adhesively bonded metal laminate joints is presented. A cohesive zone model was developed to predict their static failure behaviour. A traction–separation response was used for the adhesive material. Three joint configurations were considered: a doubler in bending, a doubler in tension and a laminated single lap. The backface strains and static failure loads obtained from experimental tests were used to validate the results from finite element modelling. The models were found to be in good agreement with experiments.     

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.     

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).     

Investigation of Electrically Conductive Structural Adhesives using Nickel Nanostrands

Several conductive nanomaterials are investigated for structural electrically conductive adhesive applications, including carbon nanofibers and nickel nanostrands. The suitability of nanostrands as a conductive filler is reviewed. Adhesive formulations based on Hysol 9396 epoxy are tested for electrical and structural properties. Several formulations are found to be capable of providing enhanced adhesive strength while affording excellent electrical conductivity. The development of full strength structural conductive adhesives can enable a wide range of applications where the strength of current commercially available electrically conductive adhesive systems is a limiting factor. Superior conductivity results are obtained by the nickel nanomaterials, with milliohm gap resistance and resistivity on the order of 10^?2 Ω cm possible at loading of 5 vol%. Initial results indicate that these systems present good survivability in thermal cycling conditions.     

An experimental investigation of the effect of interface adhesion on the fracture characteristics of a brittle-ductile layered material

The effect of interface adhesion on the failure characteristics of brittle-ductile layered material was experimentally investigated. Single-edge-notched fracture specimens were prepared by bonding two Homalite-100 layers to a thin aluminum layer using three different types of adhesives. The specimens were loaded under three-point bending and photoelasticity was used for full-field observation of the failure process. Fracture tests revealed two competing modes of failure: delamination along the Homalite-aluminum interface, and crack re-initiation in the Homalite layer across the reinforcing aluminum layer. The failure modes were directly influenced by the characteristics of the adhesive bond. Maximum load retention and energy dissipation capability during the fracture process was observed for a urethane based adhesive that formed an interfacial bond that was resistant to delamination, and additionally exhibited low modulus and large strain-to-failure, thereby suppressing crack re-initiation.     

Influence of surface morphology on the adhesion strength of epoxy–aluminum interfaces

Adhesively bonded aluminum joints have been increasingly used in the automotive industry because of their structural and functional advantages. Interfacial debonding in these joints has become a major concern limiting their performance. The present work is focused on experimental investigation of the influence of surface morphology on the interfacial fracture behavior of the epoxy- aluminum interface. The specimens used in this experimental study were made of an epoxy- aluminum bimaterial strip in the form of a layered double cantilever beam (LDCB). The LDCB specimens were debonded by peeling off the epoxy layer from the aluminum substrate using a steel wedge. Interfacial fracture energy was extracted from the debonding length using a solution for the specimen geometry based on a model of a beam on an elastic foundation. This model was validated by direct finite element analysis. The experimental results establish a direct correlation between the surface roughness of aluminum substrate and the fracture resistance of the epoxy-aluminum interface. The results emphasize the importance of choosing surface features at an appropriate length scale in studying their effects on interfacial fracture resistance.