Damage progression in composite bolted joints
Creators: name: Chishti, Maajid
Publisher: name: RMIT University
Year: 2024

description: Despite the many advantages of adhesive bonding, bolted joints are still used to fasten composite aircraft structures because of the ease of assembly/disassembly, minimal surface preparations, use of common tools between metal and composite structures and airworthiness certification. Joining or repairing external aircraft structures inevitably involves the use of countersunk fasteners, which can induce complex three-dimensional stress fields near the bolt hole. Since bolted joints incur significant penalty of low strength compared to pristine composite laminates, it is important to understand the damage mechanisms and develop design tools to enable better design and optimisation of composite joints so as to take full advantage of composite structures. <br><br>This investigation focuses on single lap joints using countersunk fasteners, using both experimental testing and computational simulation techniques. Joints were tested in shear to failure at a range of bolt torques, hole clearances and countersunk depths to thickness ratio levels. To assist the development of predictive tools, straight-shank bearing tests were carried out to calibrate model parameters. Detailed microscopy analysis of failed specimens was conducted to characterise the through-thickness failure profile of countersunk bolted joints. Detailed finite element analyses using Abaqus/Explicit were conducted to gain insight into the behaviour of the single lap joints. The models accounted for in-plane and through-thickness composite damage, frictional contact, bolt torque and secondary bending effects in bolted joints under shear. <br><br>The experimental investigation and finite element analyses showed that the through-thickness damage contained mainly interlaminar and intralaminar shear cracks and delamination. The variations in selected parameters had marginal effects on ultimate failure load of the joints; however the bearing load was significantly affected. The variation in bolt-hole clearance and countersunk depth to thickness ratio can produce significant variation in the through-thickness damage profile. As the countersunk depth to thickness ratio increased, the damage to the bearing plane of the joint increased. Catastrophic bending failure occurred for the highest ratio of countersunk depth to thickness. The finite element investigation showed that stress concentration factor at the hole edge increases with hole clearance. A detailed analysis of initiation and progression of damage, in the plane and through the thickness of the laminate has been performed. A review of the literature indicates that the detailed investigation of damage mechanisms and joint parameters presented in this thesis appears to be the first for joints involving countersunk fasteners. <br><br>The present research also highlights a new method for determining the fracture energy associated with composite compression failure. The issues associated with the use of literature in determining material properties, friction coefficient and other modelling parameters are identified and discussed. The implications of capturing the overall effect of damage modes without a true mechanistic representation are also discussed. These new findings demonstrate that whilst capturing the overall behaviour and effect of joint parameters is possible, reliable predictive capability such as that required for aerospace design purposes remains a critical aspect for ongoing research.