Evaluation of fatigue life data by normalising procedures

Abstract

Structures and mechanical components are frequently submitted to loads of variable amplitudes and of a random nature. The corresponding fatigue life prediction has to be analysed by means of damage accumulation models which utilise as basic information the S-N field of the material, determined from fatigue life tests conducted at various different constant stress ranges. Thus, the reliability of the life prediction under variable amplitude loading depends to a great extent on the quality of the estimation of the parameters related to the S-N field. Accordingly, a statistical non-linear regression analysis of the fatigue results is needed on account of the limited number of fatigue results spread over several stress ranges and of the considerable scatter of the results within each stress range. Since two random variables have to be considered − the stress range ∆σ or the stress level σ, depending on the material tested, and the number of cycles to failure N − two different statistical distributions, F(N; ∆σ), representing the number of cycles to failure given the stress range ∆σ, or else E(∆σ; N), representing the stress range given the number of cycles to failure, could be envisaged. Both distributions must fulfil physical and statistical conditions for the statistical model to be valid. In this paper, a consistent statistical model for analysing the S-N field is presented as well as methods for estimating the model parameters, based on normalising test data. Additionally, damage indices identified as the normalised variables are defined and their interpretation discussed