Life-span of mechanical systems - Study of the impact of random solicitation

Prediction of the service life of a mechanical system or component, operating under real conditions of use, is based on the fatigue state of the materials making up the components studied.

Material fatigue occurs whenever stresses and strains vary over time. These random stresses take on very different forms (see [BM 5 030] and [BM 5 031]). Fracture can occur at relatively low stresses, sometimes below a conventional limit known as the "endurance limit" S_D.

Material fatigue is approached in two ways.

The first is based on a global approach in which the material is considered as a homogeneous medium on a macroscopic scale. The mechanical characteristics of the material are presented by fatigue curves, the best-known of which is the "Wöhler curve". The critical points of components are defined by the most damaging stress points, and service life calculations are made at these points.

The second approach to material fatigue is based on a local approach, where potential characteristic defects in the material (cracks) are considered. In these zones, stresses lead to the definition of a cracking rate, and failure occurs when a crack length limit is reached.

These two approaches to calculating the fatigue life of systems or mechanical components use the same random load distributions.

The presentation of this dossier will, however, be limited to the global approach, as this way of approaching strength of materials is very widespread and based on extensive expertise in industry. A significant improvement in the quality of predictions is expected when random loads are taken into account. It should be noted, however, that the second approach is not excluded and that all the developments presented here can be extrapolated.