Fatigue tests - Low-cycle fatigue, crack propagation and complex loadings

The degradation of structures and parts subjected to cyclic loading is a progressive phenomenon that needs to be characterized by carefully selected tests, in particular to provide material data for design models. The aim of the articles devoted to fatigue testing is to present the main fatigue test methods that can be used to compare materials and manufacturing processes, or to obtain the data needed to predict fatigue life.

This article deals with the life expectancy of structures and components in service. Fatigue cracking tests are described, and the way in which the presence of defects can be taken into account in determining the residual life of a part in service is discussed.

Next, we look at fatigue testing in the oligocyclic domain, which enables us to consider the plastic deformation generated by a high stress amplitude for design and dimensioning purposes, and to study in greater detail the behavior of materials subjected to local cyclic plasticization.

Complex loads are the subject of the last two sections of this article. Estimating the fatigue behavior of components or structures in service is generally not based on constant loading amplitudes over time, nor on simple, uniaxial loads.

Various experimental approaches can be used to estimate service life under variable amplitude loading. These are presented in the third section. The fourth and final section deals with multiaxial loading. In this case, fatigue behavior can be dimensioned using specific criteria, the aim of which is to calculate an equivalent uniaxial stress in terms of fatigue strength in the multiaxial state.