An investigation on the semi-empirical and computational high cycle fatigue-life determination methodologies under spectral loading

Mehmet Ali Makbuloglu

doi:10.17918/00010481

This thesis investigates the high cycle fatigue (HCF) life prediction of materials under variable amplitude loading conditions, essential for aerospace and engineering applications. The study compares two common methodologies: the Basquin-type stress-life relations, and a Stress Intensity Factor variant, Aérospatiale's Internal Quality Factor (IQF) technique. Basquin-type methods, grounded in empirical S-N data, use fatigue strength coefficients and exponents to predict life based on stress amplitudes but often lack precision under non-standard conditions. SIF methods offer a nuanced analysis by incorporating geometric and material-specific factors, enhancing predictive accuracy for complex structures such as notched and bolted assemblies. The IQF method, an extension of SIF methodologies, integrates various loading conditions and material responses, providing a thorough fatigue life prediction framework. The research begins by identifying the limitations of traditional Basquin-type stress-life relations, particularly their inability to accurately predict fatigue life under variable amplitude loading due to their reliance on empirical data and the omission of mean stress effects. The methodology involves adjusting the S-N curve based on modification factors, such as notch factors and stress concentration factors, to refine predictions. The IQF methodology, developed by Aérospatiale, integrates parameter descriptions, examples, and a systematic procedure for evaluating fatigue life under complex loading conditions. By applying this methodology to notched and bolted assemblies, the study assesses its effectiveness in predicting fatigue life, comparing its performance to the Basquin-type approach. The thesis details the assumptions, procedures, and steps involved in each methodology, emphasizing the importance of geometric parameters and load spectra in fatigue analysis. The research utilizes various assumptions, such as uniform load distribution and simplified geometric models, to streamline the analysis while maintaining relevancy to industrial standards. For example, CAD designs and S-N curves for different simplified geometries common in aerospace applications are combined with Ground-Air-Ground load spectrums with mean stress adjustment capabilities to create an open-source dataset for future research. Results from the fatigue tests indicate that the IQF methodology provides the most reasonable life predictions among other models, which is natural because the stress concentration factor in Basquin-type relations are integrated by IQF, and in an experimental procedure like this, it is inevitable to have incompatible cross-data such as kt affecting the results in this manner. Regardless, the requirement of sensitivity in correction parameters' calculations is primarily recognized. Future work should focus on refining the interchangeability of parameters across methodologies, incorporating more complex load conditions, and expanding the empirical data available to validate theoretical models further.

An investigation on the semi-empirical and computational high cycle fatigue-life determination methodologies under spectral loading

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An investigation on the semi-empirical and computational high cycle fatigue-life determination methodologies under spectral loading

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