A COMPARISON OF DIFFERENT LOCAL ANALYSIS METHODOLOGIES FOR FATIGUE STRENGTH ASSESSMENT

This thesis work, developed in cooperation with the Czech Technical University in Prague, systematically analyses and compares different approaches for the local fatigue strength assessment of notched specimens. The paper collects experimental data from the FABEST analysis campaign. After the experimental tests using the Amsler testing machine were performed, the data were efficiently collected and processed using statistical techniques to determine the classical S-N curves, i.e. stress versus number of cycles to failure. Subsequently, the theory of critical distances, the relative stress gradient method and the strain energy density were applied through finite element analysis and other computer tools in order to compare the theoretical approaches with experiment. To achieve this goal, the thesis has been divided into eight chapters: - In the first chapter, the classical fatigue strength analysis, i.e. the one that takes a nominal stress as a reference, was introduced by means of some historical references. In the final paragraphs, a particular criticality in the use of this approach was highlighted. - The second chapter is based on Neuber's innovative idea from which multiple approaches for the evaluation of fatigue strength will arise. In particular, the theory of critical distances - TCD - is described and it brings together two methods: the first is based on an integral average determined up to a certain length ϵ and the second is based on the extraction of a certain stress at a distance x_c. Both of them use the material curve to determine an effective stress. - The third chapter presented the theory of the relative stress gradient - RSG - which defines a local component S-N curve to determine fatigue strength. Various techniques from different authors and also from the FKM guidelines were studied. - In the fourth chapter, the point of view was changed, i.e. a new physical quantity was considered: the strain energy density - SED. Again, this approach rests its foundation on Neuber's idea of structural volume. - In the fifth chapter, the experimental apparatus adopted to obtain the necessary data was described. Furthermore, not only the statistical methods for the determination of fatigue curves were described, but further results of fatigue tests performed on other types of material and geometries were added. - In Chapter Six, the fatigue crack initiation points and the fatigue failure surface of the specimens under analysis were described by means of microscopic analysis. - In the seventh chapter, FEM analyses were conducted to determine the physical quantities required to apply the theoretical approaches described in the first chapters of the thesis. In the second part, all useful parameters were determined using the Matlab programming language in order to establish a comparison between TCD, RSG and SED with the experimental tests in order to determine the deviations between experimental reality and the analytical approach. - In the eighth and final chapter, an accuracy analysis between the different approaches was performed with the aim of finding the best method to predict the fatigue strength. Finally, the various appendices describe the static tensile tests for determining certain parameters useful for the analyses and the programming codes used.