Load ratio effects in the fatigue crack propagation of composite laminates and bonded joints

Fatigue cracks, if left unchecked, can lead to the catastrophic failure of the structure. Hence fatigue is an important design consideration. The development of a validated fatigue design methodology for bonded composite repairs is impeded by a lack of design criterion that is independent of load ratios, geometry and mode mixity. Consequently design methodology is largely empirical and involves large safety factors despite the recent research effort in this area. A critical review of the literature reveals that existing parameters correlating to the crack growths rates are proposed as a means of curve fitting and do not correctly account for the effects of mean loads. Therefore the objective of this Thesis is to develop a mechanistic fatigue model for bonded joints and composite laminates that is capable of quantifying load ratio effects. Firstly, by considering the theoretical similitude requirements in fatigue, a correlating parameter ΔGeq has been proposed. By maintaining similitude condition, this scaling parameter will be independent of specimen size. However the effects of load ratio can still be observed in the Mode I fatigue behaviour for both bonded joints and composite laminates. Therefore a new correlating parameter is needed to quantify these effects. In order to identify the mechanisms responsible for this behaviour, experimental tests and numerical simulations have been carried on two crack propagation mechanisms; cohesive debonding of bonded structures and delamination crack of composite laminates, under two crack propagation modes, Mode I and II.<br><br>Plasticity induced crack closure is identified as the primary cause of load ratio effects in the Mode I fatigue behaviour of bonded joints. Elastic-plastic analysis is performed on the bonded double cantilever beam model. The bonded joints are shown to experience significant level of crack closure and this phenomenon is quantified. A new correlating parameter, based on the crack closure model, is proposed and successfully eliminates the influence of mean loads. Load ratio also has a strong influence on the delamination growth along 0°//0° interface. Experimental evidence showed extensive fibre bridging in the wake of the crack. Hence this fatigue behaviour is hypothesised to be attributed to plasticity induced crack closure and/or fibre bridging. The Hill’s plastic potential is used to simulate the anisotropic yielding of the lamina. The numerical results eliminate plasticity induced crack closure to be the cause of the observed load ratio effects in the experimental data. However it is also found that crack closure can occur in delamination along 45º//0º and 90º//0º interfaces. Therefore this leaves fibre bridging as the only possible source for the strong influence of load ratio. <br><br>Rather than proposing new empirical relation to describe the observed fatigue behaviour, this Thesis develops a fundamental understanding of the effects of mean load and its mechanisms in bonded joints and composites. A scaling parameter is proposed which describes the crack tip conditions by considering the physical process. This will provide engineers a fatigue criterion which can be used to design a bonded composite repair for an arbitrary load ratio and mixed mode ratio.