Multiaxial static and fatigue behaviour of elastic and structural adhesives for railway applications

Adhesive bonding has become a fundamental manufacturing technology in the railway industry by allowing lightweight design of structures, which is beneficial in terms of energy, environmental and cost efficiency. Due to the polymeric nature of adhesive materials, the presence of stress multiaxiality has a strong effect on their mechanical behaviour under static and fatigue conditions. The present work deals with an investigation of the multiaxial behaviour of an elastic (polyurethane-based) and a structural (epoxy-based) adhesive for railway applications. Samples with different stress multiaxialities (butt joint, scarf joint, and thick adherend shear test joint) were tested under static and fatigue conditions. The stress multiaxiality was defined as the ratio between the hydrostatic stress (p) and the von Mises equivalent stress (q). Finite Element Analysis showed that the mechanical properties of adhesives have a strong influence of the stress multiaxiality distribution of joints with elastic adhesive reaching higher levels of stress multiaxiality. Static tests revealed that elastic adhesive joints are more sensitive to multiaxiality (i.e. higher hydrostatic stresses) than their structural counterparts, especially in samples with larger hindering of lateral contraction. From fatigue test results of both adhesives, namely SN curves, it was demonstrated how multiaxial p-q fatigue diagrams can be constructed. One of the main advantages of this approach is the possibility of predicting the fatigue lifetime of joints regardless of their multiaxial stress state.