Experiments and Modelling of Composite–Aluminium Bolted Joints

Abstract: The aeronautical industry has always strived towards weight-efficient structures since they improve aircraft performance, enable more fuel or payload, reduce cost of manufacturing and operating the aircraft, and reduce the fuel consumption and emissions. Modern airframes are often built in materials with high strength-toweight and stiffness-to-weight ratio, such as aluminium or fibre composites. Fibre composite materials are becoming increasingly common, requiring bolted joints between composite and aluminium parts. Bolted joints are often weak points in the structure, and understanding their mechanical properties in terms of stiffness and strength is of great importance for designing aircraft that are safe to fly. The aim of the research presented in this licentiate thesis is to improve the understanding of shear-loaded, composite–aluminium, bolted joints in terms of structural compliance, deformation, load distribution, strength and failure, by performing experiments and developing simulation models that describe those experiments. Experiments are performed where the optical digital image correlation (DIC) technique is used to measure the deformation of the test specimen surface during quasistatic load application until specimen failure. The DIC method enables more detailed deformation measurements compared to traditional methods. Data processing algorithms for noise reduction of DIC measurement data are developed and the results are used in a novel way to detect beginning bearing damage within the material by observing the specimen surface. The experimental DIC data is used for adapting models, where the goal is to create a structural element that represents a fastener. A structural element is computationally efficient and suitable for implementation in large scale models of airframe structures in an industrial context. A model is proposed and the selection of parameters for this model are investigated. The parameters are identified by fitting to experimental data. This licentiate thesis is divided into two parts, where the first part gives an introduction and background to the research, while the second part consists of two appended papers.