Damage modelling procedure and fastener positioning optimization of adhesively reinforced frictional interfaces

Abstract

This thesis is concerned with the development of the shear damage simulation procedure for combined mechanically clamped and adhesively reinforced frictional joint interfaces. The first step has been to experimentally measure the shear fracture behaviour of high strength steel interfaces using annular ring specimens subject to constant normal pre-stress. The experimental programme included variations of the pre-stress, surface roughness and epoxy curing temperature. Results showed that these factors significantly influence both the maximum attainable peak shear loads and the critical fracture energy release rate of the interfaces. For the selected structural adhesive, the strength contributions of the reinforcing adhesive and friction due to the pre-stress conformed well to the principle of superposition. Comparisons of adhesively reinforced and non-reinforced interfaces with identical surface conditions and pre-stress demonstrate a substantial contribution due to the epoxy reinforcement. Test results are exploited for characterization of the non-local shear stress vs. displacement responses. A damage evolution model for the abraded interface is formulated for room temperature- and heat-cured adhesives. Model parameters are fitted using regression analysis. Decohesion finite elements involving the cohesive zone model are adapted to model progressive degradation of the reinforced interfaces. A non-local friction law is also implemented when modelling the surface interaction using finite element contact. Based on the principle of superposition, the fracture potential and steady frictional dissipation of the interface can, therefore, be independently characterized. An applicability of the presented damage simulation process is shown by finite element analyses on the structural multi-fastener connections. Corresponding experiments on full-scale joints were performed. The measured peak shear loads show a good correlation with the computed results. To further strengthen multi-fastener connections, a geometric optimization procedure is formulated for equalizing fastener loads due to an applied eccentric load. In summary, the objective of this thesis is to develop a damage simulation procedure to analyse decohesion initiation of adhesively reinforced frictional interfaces under quasi-static shear loading. Both the numerical modelling procedure and new material property data for a design of more optimal structures involving multi-fastener connections are presented.