A model allowing for the influence of geometry and stress in the assessment of fatigue data

Abstract

Usually, the Wohler field of a material is obtained from fatigue lifetime data resulting from testing specimens of reduced size in the laboratory. This basic information finds subsequent application in lifetime prediction of larger structural and mechanical components. Thus, an important question arises: how can the S-N field be transformed into an ideal one referred to a characteristic size (length, area or volume) subjected to a constant stress distribution in order to achieve a safe structural integrity design? In this work, the influence of specimen geometry and variable stress state on the fatigue lifetime distribution for constant amplitude fatigue tests is investigated. An experimental program has been carried out with unnotched specimens of nominally the same material but differing in length, diameter, and shape. The experimental data is fitted to a newly developed fatigue model, capable of describing the S-N-field in a probabilistic manner accounting for both the specimen geometry and the variable stress state of the specimens. As the estimated Wohler field is referred to an elemental surface, loaded by a constant stress level Ds, the extrapolation of the fatigue resistance to different specimen geometries is possible. Additionally, problems encountered due to scatter of the material properties are discussed.