Study of the behaviour of adhesive joints of steel with CFRP for its application in bus structures

In recent years, the use of adhesives in structural applications is growing, achieving a great current implementation in the industry, due to the benefits that this technology is capable of providing to complex-shaped structures, both in aerospace and automotive applications. Adhesive joints show many advantages in comparison with other traditional joints such as welded joints, because they offer a continuous joint with homogeneous stress distribution, they are able to joint dissimilar materials (such metals and composite materials) and they do not require large investments. Current bus steel structures present fatigue problems due to the rigidity of the commonly used welded joints. Crack problems due to fatigue are evident in the areas of the bus structure closest to the rear door, being the joint between the side vertical pillars and the waist rail the most critical. A finite element model (FEM) of a bus steel structure is developed, in order to obtain the forces that work on the reference node. From the obtained force values, the value and type of stress at the reference node are calculated. A new carbon fiber reinforced polymer (CFRP) node is developed, replacing the existing welded joint by steel-CFRP adhesive joint. The new node design allows to obtain mainly shear stress. Because of that, single lap joint specimens are developed to carry out the experimental procedure. This work is focused on the study of a structural adhesive for its application in this new type of joint taking also into consideration manufacturing criteria (mounting periods, costs, etc.). This new adhesive joint shows strength values an order of magnitude higher than the requests at the node, and higher than 30% of strain values, minimizing fatigue problems.