Multiple-site damage crack growth behaviour in Fibre Metal Laminate structures

Fibre metal laminates (FMLs)were developed and refined for their superior crack growth resistance and critical damage size that complimented the damage tolerance design philosophy utilized in the aerospace sector. Robust damage tolerance tools have been developed for FMLs. However, they tend to focus on the evolution of an isolated crack. There is also a risk that they will be invalidated overtime as a result of the occurrence of multiple cracks within one structure (one form of widespread fatigue damage). To combat another failure due to widespread fatigue damage, the airworthiness regulations were revised to include the concept of a Limit of Validity (LOV) of the damage tolerance analyses. Consequently, it is crucial to examine fatigue crack growth (FCG) in FMLs containing Multiple-site Damage (MSD) cracks despite their superior damage tolerance merits. The focus of this thesis therefore is to analyse MSD crack growth in FML structures. Mechanically fastened FML joints are potentially weak structural designs that are susceptible to MSD due to the stress rising contributors such as secondary bending, pin loading and open holes subjected to bypass loading. In this thesis, predictive models were developed to address several key mechanisms that affect FCG in FML joints containing MSD, and validated with corresponding experimental work. Then the predictive models were systematically integrated and implemented for FML joints. It was identified that the nature of fatigue in FMLs led to the load redistribution mechanism as the key factor to be modelled in predicting MSD growth in FMLs. The structural stiffness reductions caused by the presence of multiple cracks resulted in load redistribution from the other cracks to the single crack to be analysed, exacerbating the total stress intensity factor (SIF) experienced at the tips of the single crack, increasing the crack growth rate (CGR). The load redistribution mechanism was first substantiated by investigating FCG in FMLs containing discretely notched layers. The prediction model fairly captured the load redistribution mechanism by idealizing the notches in the metal layers as removals of metal strips. The crack acceleration over a major portion of the crack propagation was well predicted with the model; however, the surge in CGR over roughly 3 mm crack length prior to the link-up was underestimated since the plasticity interaction was not accounted for. The capability of modelling the load redistribution mechanism allows the states of multiple cracks to be analysed one by one. It was found that the load redistribution could not be symmetric for every crack and non-symmetric crack configurations therefore developed in FMLs with finite width. Hence, non-symmetric crack growth in FMLs was also investigated in this work. It was also found that both crack tip non-symmetry and delamination shape non-symmetry affected the crack growth in the metal layers. The model for non-symmetric crack growth in FMLs was validated with experimental data. Good correlation was observed. The model for MSD growth in FML panels sequentially analyses each crack state. The other cracks are idealized as removals of metal strips when analyzing the state of a single crack. This non-physical idealization of the cracks led to consistently conservative prediction results in comparison with the test data. Nevertheless, the prediction model provided good predictions of the evolution of MSD configurations. Additionally, it was proven that a very non-conservative predicted fatigue life could be obtained if the load redistribution mechanism was not considered. The effects of pin loading on FCG in FMLs were also investigated. The test data showed very rapid growth of the crack in the vicinity of the pin loading. The CGR decreased with increasing crack length. The model applied the principle of superposition to split the non-symmetric tension-pin loading into simpler tensile loading and a pair of point loads acting on the crack flanks. The SIFs for the simpler loading cases were derived and superposed to obtain the total SIF as a result of the tension-pin loading. The predicted CGR and equivalent delamination shape correlated with the measurements very well, but the model failed to predict the crack path and the measured delamination shape which were trivial issues for this work. The relevance and applicability of the developed models in this thesis for predicting the MSD behaviour in mechanically fastened FML joints was examined. The predicted results captured the trends of the measured CGR in FML joints containing MSD cracks, although there were some discrepancies. The discrepancies are mainly due to the two major shortcomings of the model which are neglecting the load redistribution over multiple fastener rows and neglecting the effects of secondary bending stresses.