Investigation and analysis of the parameters affecting the performance of bolted structural joints

This master’s degree thesis was developed in collaboration with McLaren Automotive, a British luxury car company based at the McLaren Technology Centre in Woking, UK. The project focused on the investigation of the parameters affecting the performance of bolted structural joints, specifically a material stack including an adhesive layer, used in automotive component. The analysis and the experimental activity were carried out on CFRP/steel joints; the composite and the metallic material were joined using a bolt and an additional adhesive (epoxy glue). Particular attention has been paid to the definition of the most appropriate testing methods for the characterization of the joint preload. Exhaustive bibliographic research has been undertaken into techniques for measuring the preload of bolted joints and a screening of the most suitable method has been carried out. The selected method was able to monitor preload trends over time at low and high temperatures, i.e. -40°C and +70°C, according to in-service condition of the final component. Tests were carried out on several joints, varying the thickness of the adhesive layer. The test method was adapted to our case study and used to perform the tests in the laboratories of the Politecnico di Torino. The investigation included mechanical tests on joints (single-lap shear in compression), in order to test the bond strength and the fracture behaviour of the joints in four different configurations, i.e., similar joints (CFRP/CFRP joints) and dissimilar joints (CFRP/steel, 3 adhesive layer thicknesses). In addition, complete morphological analysis was performed, using scanning electron microscope (SEM) examination of the cross-section of the similar and dissimilar joints. Compositional analysis (EDS) was also carried out. This project will be used to increase McLaren automotive's knowledge in the area of mechanical joints and assembly characterisation, creating a basis for future studies for new applications and improvements.