Mode I Fracture Energy of One-Component Polyurethane Adhesive Joints as Function of Bond Thickness for the Automotive Industry

The interfacial fracture of adhesively bonded structures is a serious issue for the widespread application of a variety of modern industries. For many manufacturing applications such as automotive structures, bond line thickness can vary considerably. This factor influences adhesively bonded joints performance. Therefore, its effect has to be examined experimentally and should be taken into consideration in the design of adhesive joints. Most of the results from the literature are for usual structural epoxy adhesives, which are usually formulated to perform in thin sections. However, polyurethane adhesives are designed to execute in thicker sections and might have a different behaviour as a function of adhesive thickness. In this study, an experimental procedure is undertaken to characterise the effect of different adhesive thicknesses between aluminium substrates on the mechanical behaviour of a one-component polyurethane adhesive. The mode I fracture toughness of the adhesive was measured using double cantilever beam (DCB) tests with various thicknesses of the adhesive layer ranging from 0.3 to 2 mm. The fracture energy, G_IC, was found to be directed by the thickness of the adhesive layer. It increased linearly up to 1 mm adhesive thickness and then it decreases.