Strength and Fracture Characterization of a Novel Polyurethane Adhesive for the Automotive Industry

Adhesive bonding by structural adhesives has been used for several decades, helping to solve various problems related to the conventional joining techniques, such as welding, riveting, or bolting. Adhesive joints present less structural weight, lower manufacturing cost, the possibility to join different materials, and high fatigue strength. The increasing use of composite materials also helped to the growing use of adhesive joints, since these do not break the reinforcing fibers' continuity. An adhesive joint is mainly subjected to peel and shear loads. However, the knowledge of the tensile (E) and shear (G) moduli of the adhesive, and its tensile (sigma(f)) and shear failure strengths (tau(f)), is not enough to predict the joint behavior. In fact, the critical strain energy release rate in tension (G(Ic)) and shear (G(IIc)) are equally necessary for advanced modelling techniques such as cohesive zone modelling (CZM). This work aimed to study a novel structural polyurethane adhesive to obtain material property data that can be further used for the strength prediction of bonded structures. With this purpose, 4 tests were performed: tensile testing to bulk specimens, shear testing with thick adherend shear tests (TAST), double-cantilever beam (DCB), and end-notched flexure (ENF) tests. These tests will allow values to be determined for the mechanical and fracture properties of the adhesive in tension and shear. The parameters to predict the strength of adhesive joints with this adhesive by various methods were provided, ranging from the easy to apply analytical methods to the most advanced numerical methods available nowadays. A detailed comparison is also undertaken with an adhesive of the same family. The obtained results were in close agreement with the few data provided by the manufacturer (E and rf), while fracture data was also provided with a good agreement between data reduction schemes.