Fatigue Design Loads

Fatigue is damage that occurs in structures subjected to time-varying loads. This damage is characterized by the development of cracks, leading to loss of functionality or sudden failure of the part. Faced with this phenomenon, design offices need to dimension structures to ensure their operation for a given duration and under given conditions of use. Two approaches are considered, depending on the performance and safety requirements of the part in the structure, and the context of use (system with or without regular maintenance).

The first imposes a guaranteed or safe service life without intermediate testing, which requires that cracks are not initiated.

The second is part of a more general approach known as damage tolerance, and aims to control the existence and development of cracks by guaranteeing non-rupture in service between two inspections.

The models associated with these approaches are based on experimental databases that incorporate more or less influential parameters. The basic database for the first approach is based on specimens subjected to cyclic loading at constant amplitude and frequency during the test. The number of cycles to failure or crack detection is the main experimental input. By collecting the results of tests carried out at various load amplitudes, it is possible to construct Wöhler curves describing the number of cycles to failure as a function of the variation in load amplitude. The second approach is based on tests on pre-cracked specimens subjected to loads of constant amplitude. They provide the cracking rate as a function of the variation in the stress intensity factor ΔK. The curve describing the variation in cracking speed as a function of this factor has a linear part in a logarithmic diagram that can be described by a power law known as the Paris law.

The study of the Wöhler and Paris curves highlights two major points from the point of view of this article:

• high sensitivity of the number of cycles to failure to load amplitude. A small variation in load amplitude results in a large variation in the number of cycles to failure, or in a large variation in the rate of propagation of an existing crack. In fact, the variation is exponential;

• a wide dispersion in the number of cycles to failure at a given load amplitude.

Most design models seek to establish equivalences between the situation of the structure to be designed and the test parameters associated with the database. This obviously requires knowledge of the situation. In many fields, however, this knowledge is very imperfect. It is at this stage that the concept of representative loading or that of the safety...