Adhesive bonding of coated steels

A lack of knowledge about the strengths of adhesively bonded metallic and organic coated sheet steel has been hindering proper application of these metallic materials for practical purpose in Australia. To address this problem, first an extensive database on the initial joint strengths were compiled. Lap shear test and Width Tapered Cantilever Beam test geometries were employed for evaluating the average stress at failure and the fracture energy of adhesive joints respectively. By observing the mechanism of failure and the data on joint strengths, reasons explaining why some adhesives formed a stronger joint than the others were determined. Extensive surface characterisation techniques by SEM, AFM and contact angle goniometer were utilised for these investigations. It was evident from these studies that a correlation existed between the joint fracture energy 'Gc' and total thermodynamic work of adhesion 'Wa'. The acid - base interaction between adhesive and adherend was found to be influencing the joint strength. 'Gc' was affected by the bulk fracture energy of the adhesive 'Ga'. It was evident that a joint strength model could be developed on the basis of some correlation between 'Gc' and 'Wa' and 'Ga'. For the estabhshment of the model, polished steels and unfilled adhesives were selected. A statistical software package was utilised for correlating 'Gc' with 'Wa' and 'Ga', thereby establishing a model for predicting initial joint strengths. The durability of adhesive joints on exposure to a harsh environment were observed by exposing lap shear and 'wedge test' samples to a proper environment. The mechanism of failure were also studied by unaided eye. Moisture absorption studies were also carried out using epoxy and polyurethane adhesives. It was observed that although a decrease in joint strength of epoxy bonded joints every month may be related to moisture absorption, the overall strength retention may not depend on this factor alone. For polyurethane adhesive, joint strength decay was influenced to a greater extent by the adhesion mechanisms rather than moisture absorption.