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Algorithm for determination of S-N curves of the structural elements subjected to cyclic loading

Stojković Nenad (College of Applied Technical Sciences, Niš)
Stojić Dragoslav (Faculty of Civil Engineering and Architecture, Niš)
Živković Srđan (Faculty of Civil Engineering and Architecture, Niš)
Topličić-Ćurčić Gordana (Faculty of Civil Engineering and Architecture, Niš)

Fatigue life prediction of structural elements subjected to cyclic loading is usually performed using S-N curves, obtained from the experimental data from fatigue tests. However, in some cases the samples do not exhibit failure, due to reaching the predetermined number of cycles, failure of a non-relevant segment or terminating the test because of some other reason. These samples are usually referred to as runouts, and the data obtained from them could be used for determination of S-N curves as well. In this paper, the algorithm based on Maximum Likelihood method is proposed for the determination of S-N curves from experimental data that contain runouts. Following the algorithm, a MATLAB code was written and the verification was performed using the experimental data from the literature. The results showed that it could be successfully used for taking into account the runouts in the process of determination of S-N model parameters. It was concluded that the inclusion of runouts could significantly influence the predicted fatigue life, especially at the lower stress levels.

Keywords: fatigue, S-N curve, algorithm, maximum likelihood method