Helicopter manufacturers and airworthiness regulators worldwide have a strong interest in adopting the damage tolerance approach to replicate the success in the fixed-wing aircraft design. However, due to different issues, the well-established damage tolerance approach for fixed-wing aircraft has not yet been able to yield any major benefits for helicopters. The key challenges of applying damage tolerance methodologies to helicopters are the lack of understanding of load sequencing effects under helicopter loading conditions, inadequacy of our ability to predict fatigue crack growth in the near threshold regime, and the extremely large number of fatigue cycles experienced by a helicopter during service. This thesis aims to develop an improved fatigue life prediction technique for helicopter loading sequences, with two specific focal areas - fatigue crack growth models for near-threshold regime, and the application of damage tolerance methodologies to helicopter loading sequences. In order to achieve these goals, four related analyses were conducted to investigate the problems, with each analysis targeting a separate issue. The safe-life approach is still the dominant design method used for the helicopter components and the S-N curve is an important tool used for this approach. The high number of cycles in a helicopter service life could give one important region of the S-N curve – the runout region – additional importance. The first analysis was to examine the influence of the definition of runout on safe-life prediction. The results revealed that the endurance limit in the S-N curves is strongly dependent on the definition of the runout. Reducing the number of cycles associated with the runout (truncating tests) will not only shorten the S-N curves, but also alter the shape of the curves, and consequently lead to different life predictions. The second investigation focused on the influence of potential sequencing effects during the process of helicopter spectrum reconstitution. The results revealed that the reverse rainflow reconstitution method could provide reconstituted load sequences that would result in broadly similar fatigue lives, indicating that it is a robust method for reconstituting sequence for helicopter loading. The third investigation centred on the use of Effective Block Approach (EBA) to improve crack growth predictive capability for helicopter loading sequences. It was found that EBA is applicable under helicopter loading conditions and its use could provide a more robust alternative for predicting crack growth life without the need to calibrate the crack growth models. The final analysis addressed the role of roughness induced crack closure on fatigue crack growth in the near-threshold region. The results of experimental tests and modelling showed that the fracture surface roughness could be quantified over a wide range of stress ratios. A new modelling technique combining the plasticity-induced-crack-closure and roughness-induced-crack-closure mechanisms was developed. The model was demonstrated to provide predictions which showed good correlation with experimental data reported in the literature. The outcomes of the present research provide new insights and analytical tools to improve the fatigue design of helicopters, and particularly support the application of damage tolerance methodologies to helicopters.
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