Static and fatigue characteristics of composite joint repair in aircraft structures

Composite materials are seeing increasing use in the engineering industry, in particular the aerospace sector. The move towards light weight high stiffness structures that have good durability and corrosion resistance has led to the move from metal structures to composite structures. With this brings the added concern of certifying new components as they must meet the same structural integrity, safety and durability requirements as those in metals and hence this is where the challenge now lies. When a metallic structure is damaged at some point during its life, there is a possibility it will be repaired however if the damage is significant then the component(s) will be replaced. With composite materials on the other hand, the flexibility in design and fabrication has led to larger single component pieces and thus repairing these structures is more economical than replacement. This puts further emphasis in creating new and efficient repair methods that can be certified whilst still withstanding the various stresses and loads inflicted on the aircraft during its remaining service life. In this study, a comparison is made between three fundamental joint structures - mechanically fastened joints, bonded joints and hybrid joints. The work is targeted towards the repair of aircrafts made from fibre reinforced polymer matrix composites known for their high strength, high stiffness, long fatigue life and low density. Stage 1 of the investigation focused on testing thin double lap joint repairs and Stage 2 focused on testing thick step lap joint repairs containing a total of five steps. The aim was to compare the static strength and fatigue resistance of a hybrid joint configuration consisting of both bonding and fastening, a purely fastened joint and a purely bonded joint. Rivets and countersunk bolts were selected as the fasteners for thin and thick joint repairs respectively. The effect of changing various parameters such as the mechanical fastener array, clamping pressure, bond strength, initial defects and curing conditions were also investigated. The experimental results for both Stage 1 and Stage 2 found there was no significant difference between the static strength of a bonded joint compared to a hybrid joint. However, the fatigue resistance of a hybrid joint was superior to a bonded joint configuration particularly where bondline defects such as initial cracks or a semi-cured adhesive was present. Finite element analysis (FEA) was also performed to verify the static and fatigue strength of the various configurations. Nonlinear adhesive material properties, fastener surface contacts and frictional forces were all included in the three-dimensional (3D) finite element (FE) models. The Multicontinuum Theory (MCT) is used to simulate the progressive failure process and determine the stress states in the various specimen configurations. The strain energy release rate (SERR) as a function of crack length for the bonded and hybrid specimens were also compared. Results found that it was vital to position fasteners closer towards the ends of the bondline to suppress rapid crack growth. As soon as a crack enters the fasteners’ clamping zone, there is a significant drop in the SERR which reduces the crack growth rate, leading to an improvement in fatigue resistance. The key advantage in this case, is being able to detect damage before catastrophic failure. A final extension to the investigation looked into optimising the individual step length, step heights and the number of steps for the thick joint repair cases. The previous step lap joint containing five steps with a 90mm long overlap was able to achieve only a 52% load recovery at best compared to a pristine undamaged parent structure. Through the parametric studies conducted using Abaqus CAE, two new step lap joint designs were numerically analysed and experimentally tested. One of the optimised step lap joint designs contained seven steps (with thin outer step heights and thicker inner step heights) and the second contained six steps (with an outer overlap). Overall, the static and fatigue resistance was improved in both of the new step lap joint designs. A bonded step lap joint with seven steps was now able to achieve a 70% load recovery with a 95% increase in fatigue resistance. Overall the work discussed as part of this dissertation provides detailed methods in optimising composite joint repair whilst providing benchmark comparisons between thin and thick fastened, bonded and hybrid joint cases. This is an area of research which has seen limited attention in the past but is crucial for the continual growth of composite material usage in both commercial and military sectors. Achieving composite joint repair certification is a stringent and costly process, through this research various methods have been presented to not only improve repair durability but to assist damage detection and prevent final catastrophic failure, all of which will greatly assist composite repair certification.