Adhesive modelling in multi-material structures : Evaluating the strength and fatigue life of adhesive joints

Abstract: Advancements in material science and manufacturing techniques are enabling the use of lightweight metal alloys and polymer composites in several combinations and shapes for producing more efficient and lightweight structures for automotive applications without compromising strength, stiffness and/or durability. When evaluating the strength of the structure, the joints are of importance. For multi-material structures adhesives are often the best type of joints. However, traditional finite element methods using stress criteria cannot accurately predict the failure of these adhesive joints under static loading. In this thesis work a strength and fatigue model, formulated using energy release rate theory, is implemented in a post processing tool. Given a finite element model of an adhesive joint and a list of boundary elements and nodes this tool calculates the energy release rates in mode I and mode II, and if the fracture toughness of the adhesive is known, a prescribed mixed-mode failure index is calculated. To evaluate its predictions joint strength results are correlated to experiments. Specimens with combined shear and normal load forms the underlying experimental setup with change in strain rate and adhesive thickness as varying parameters. Methods for implementing the model for a car body structure with multiple adhesive joints is investigated, the tool proves to be scalable, however, the required finite element setup at the adhesive boundaries may not be present in a car body model and thus further work needs to carried out to accommodate irregularities like non-matching mesh in the car body finite element model. This model may be used for assessing the strength and durability of a car body structure comprising different materials joined together using adhesives.