Fatigue of ferrous alloys - Definitions and diagrams

Fatigue is the modification of material properties under repeated, variable stress or strain. In general, the term describes defail-lances that lead to cracking and material failure. Most critical mechanical components fail under alternating mechanical stresses. Examples include crankshafts, connecting rods and gears in the automotive industry, and landing gear, turbine and compressor blades in the aerospace industry. As a degradation mode, fatigue is associated with 50-90% of mechanical system failures. It is therefore important to take this phenomenon into account in mechanical design.

Unfortunately, since the beginning of the 19th century, a number of structures and mechanical components have often failed, sometimes catastrophically, when subjected to dynamic loads that are considered modest compared to the static capacities of materials commonly defined by their breaking strength and yield strength. The culprit is fatigue damage.

Fatigue failure can occur under various types of stress, the simplest of which is failure induced by alternating external stress. If residual stresses exist in the material, these will be superimposed on external stresses. These stresses can also be combined with other stresses, such as creep or thermal fatigue caused by temperature variation. Stressing the material in a corrosive or brittle environment accelerates failure. Another example is fretting fatigue.

Right from the design phase, it is essential to be able to quantify this risk of failure, taking into account the stresses imposed. However, a number of terms and concepts need to be clarified beforehand, and we need to know how to use an endurance diagram. With the introduction of structural analysis, it is now possible to assess the stress level in complex structures, which requires further development of multiaxial fatigue criteria.

In this first article, we will describe the different approaches available to engineers. Next, we will study the influence of a number of factors (tensile strength of the material, material microstructure, type of loading, dimension, surface finish, residual stresses, temperature, corrosion, fretting, frequency, etc.), indicating as far as we can the limits of their use in the article [BM 5 043]. Finally, examples of fatigue calculations are given in [BM 5 044] and [BM 5 045].