A study of environmental exposure on structural adhesively bonded austenitic stainless steel.

Structural adhesive bonding is becoming a popular alternative to the more traditional joining methods, such as spot and laser welding, for joining metallic substrates intended for structural applications. Structural adhesive bonding offers many advantages, for example enhancing fatigue resistance, the ability to join dissimilar materials and providing cost effective joining solutions. The work presented in this thesis studies the effectiveness of bonding austenitic stainless steel using a two-part structural epoxy adhesive.A comprehensive review of literature has been carried out covering the mechanisms of adhesion, the importance of surface pre-treatments and surface analytical techniques used to evaluate the chemical and physical attributes of substrates prior to bonding and the failure analysis of fracture surfaces. In addition techniques used to study the environmental durability and fatigue performance of adhesive joints has being appraised. The first experimental phase evaluated the effect of commercially available stainless steel finishes on adhesive joint durability using the standard overlap specimen. Environmental exposure included natural outdoor weathering and a high humidity environment coupled with the application of an applied load. It was noted that to further appraise more durable pre-treatments used prior to bonding different testing configurations was required. Perforated single overlap joints and wedge test specimens were used to assess eleven different pre-treatments, ranging from relatively simply to more complex electro-chemical techniques. All pre-treatments included in the research were physically and chemically characterised using scanning electron microscopy (SEM), surface profilometry and X-Ray Photoelectron Spectroscopy (XPS). The complementary data from the perforated lap shear and wedge crack extension testing ranked the pre-treatment used in a similar fashion. The two most durable treatment were the Accomet C proprietary coating and the sulphuric acid sodium dichromate anodisation process. A pattern emerged revealing the more durable pre-treatments produced higher Cr:Fe ratios on the surface. Post failure analysis of fractured specimens was carried out using SEM and XPS. It was shown that after the environmental exposure of specimens to a high humidity hydro-thermal stress regime the most durable pre-treatments initially failed cohesively within the adhesive and then interfacially between the adhesive and associated oxide layer.The second stage of the experimental work evaluated the fatigue performance of single overlap joints. The fatigue performance of joints was increased using cost-effective surface conditioning techniques. It was also shown the fatigue response of the adhesive joints is dependant upon test frequency, the effect of which being more prominent at low frequencies. The effect of mean load has also been evaluated, and revealed a reduction in load amplitude seriously diminishes the fatigue lifetime of specimens. The effects of aqueous ageing in distilled water at ambient temperatures was assessed. It was shown that continuous immersion for up to 72 weeks caused total delamination of the adhesive from the substrate. It was postulated that moisture ingression into the interfacial region fills air voids and caused the adhesive to displace from the adherend.An elastic model based on beam theory has being developed to determine the elastic rotation of the overlap region of single lap joints under the application of a tensile load. The model was validated by experimental results, and evaluated the effects of adherend thickness and overlap length. The model has the potential to aid engineers when designing structures using adhesive bonding, especially concerning thin gauge applications where plastic deformation can occur under comparatively small loads.