A finite element constitutive modeling of the adhesive bonding between wood and steel plates

The present work discusses the use of a finite element to model the behavior of adhesive in bonding structures. The main objective of the work is to develop an appropriate constitutive model for a flexible adhesive material in a composite structure of wood joined with steel plates. Nowadays, in many cases, modern adhesives offer an high strength solution to realize composite structures. Structural adhesive bonding is widely used as joining technique in composite structures, thanks to its favorable manufacturing costs and its suited mechanical characteristic of stress distribution, which can be is competitive if compared with fastening joint which, instead, causes a concentration of stress around holes. However, the structural effectiveness of the adhesive material needs to be proven in different and specific operative conditions of structure life. This rises some questions, one of these is to know, with an high reliability, the mechanical behavior of the adhesive under some loading conditions. On this matter the scientific literature has produced a certain number of interesting works. Concerning this, the actual scientific answers, above all, deal with the stress distribution along the joint and the interface between adhesive and each adherents; and deal with the concentrations of stress on the overlap ends. An adequate study of the mechanical behavior of the adhesive material is important to give effectiveness and safety to the composite structure. This task, however, is difficult when the problem is affected by various factors such as the geometry of the joint, the stiffness of the adhesive and the surface of overlap between the adherents and the adhesive. These aspects are also influenced by loading and fatigue and by durability under environmental conditions. This implies that a large amount of experimental tests are need to correctly know the overall behavior of a specific composite structure and particularly of the adhesive material. However, today a powerful tool of discretized method can be used, such as the finite element method (FEM), to numerical analyze a structure. Following this approach, in this paper an hyperelastic constitutive model has been adopted for a polyurethane flexible adhesive, by considering geometrical and material non-linearity, and an orthotropic elastic constitutive law has been adopted for wood, within the framework of the FEM. By means of a FEM code a series of composite joints, under static loading conditions, has been numerical analyzed to found the stress distribution around the adhesive overlap. The model has been evaluated by comparing the numerical results, retrieved from the analysis, with experimental tests to check its ability to represent the effective mechanical behavior of the structural assemblage. The good results have demonstrated the usefulness of the proposed model.