This chapter explores the design, testing and analysis of composite bolted joints and begins with a review of literature relating to joint mechanical behavior. The fundamental influence of joint geometry and stacking sequence on the joint response is discussed along with the significant effects of bolt–hole clearance, lateral constraint and loading velocity. The large number of studies featured in the literature review is an indication of the complexity involved in optimizing composite bolted joint design. In order to handle this complexity, accurate 3D FE modeling is considered an invaluable aid in the design of composite bolted joints and is a key focus of the chapter. The use of 3D FE modeling in the design of composite bolted joints is examined in the form of two case studies. The first case study outlines the prediction of bolt–hole clearance effects in single-bolt, protruding-head and countersunk joints, using linear elastic implicit FE models. The models are shown to accurately capture the delay in load take-up and reduced joint stiffness associated with bolt–hole clearance, while layer-by-layer stress distributions provide a detailed insight into the ply loading at the bolt hole. The nature of the ply loading in countersunk joints is shown to be very different to that of protruding-head joints and in all cases, ply stresses are shown to be highly dependent on the level of clearance. The second case study focuses on the prediction of bearing failure in a single-bolt countersunk joint, using a 3D explicit FE model. A physically based damage model is implemented in a VUMAT in order to predict bearing damage and includes Puck failure criteria, a nonlinear shear law and a crack band model to mitigate mesh sensitivity. The resulting progressive damage analysis is both predictive and robust, affirming the benefits of using explicit finite element analysis which are discussed earlier in the chapter.

This paper studies the progression of damage in carbon fibre-reinforced polymer (CFRP) countersunk composite bolted joints (CBJs) with neat-fit clearance, subjected to quasi-static loading. Damage mechanisms, comprising of fibre buckling and breakage, matrix damage, shear damage and inter-laminar delamination within the CFRP composite parts of the joints have been studied. Load-displacement curves, X-ray and optical microscopic images in single- and three-bolt CBJs were used to investigate damage and deformation characteristics. The observations were then employed to further investigate the type of failure and the extent of damage. The evolution of damage within the composite parts was correlated to the failure characteristics of the joints: It was found that the type and extension of damage is strongly correlated with the ultimate failure load point of the joint in single-bolt CBJs. A combined inter/intra-laminar damage consisting of fibre cluster breakage, extensive fibre buckling, debonding and delamination was observed at the ultimate failure load. This study was then extended to three-bolt CBJ where damage surrounding each bolt and its corresponding failure load was strongly correlated: The final study showed that the ultimate failure point in single-bolt CBJ and the first-bolt-failure point in three-bolt CBJ correspond to the composite plies undergoing intra-laminar damage with the size reaching to the edge of the countersunk head. This damage developed extensively through the thickness of the composite parts underneath the countersink, and in the direction opposite to the loading direction. Outside the countersunk head, debonding and delamination were found to be the dominant damage driving mechanisms. Finally, a new design rule has been proposed to predict the response of multi-bolt joints (damage area and failure load) by using the response in single-bolt CBJ as an initial baseline.