Improving the fatigue lifetime prediction of screws for a large range of mean stresses

Abstract

When he tries to predict the fatigue lifetime of a screw for highly loaded assemblage, the designer is in front of two types of difficulties. The first one is how to take into account the effect of the mean stress on the damage ; the second one, related to the geometry of the screw, is how to know the influence of the notch effect on the damage in the thread root. The Haigh’s or Goodman’s diagram help us to answer the first question ; the second question is partially answered by Neuber’s or gradient’s methods. Here, the method is based on an elasto-viscoplastic analysis leading to the knowledge of the local stress and strain components. Then, a multiaxial failure criterion obtained from the damage rules is locally applied. This leads to a method of building the Haigh’s local diagram. Thus, the way of processing includes three principal stages : ‐ determination for the considered steel of the elastoplastic behaviour rules under uniaxial cyclic loadings with or without mean stress ; ‐ determination of the damage rules leading to the failure criteria as well as the cumulative calculations ;