Fatigue of ferrous alloys - Influencing factors

Fatigue is a complex process. The number of influencing factors is very large. While it is possible to test materials under certain conditions, it will be difficult to carry out all the tests with a representative load and the actual geometry of the mechanical components. However, it is interesting to quickly analyze the role and effect of each parameter that can change the assessment elements. It is not possible to consider the effects in an integrated way. Each parameter is therefore analyzed separately. A distinction is made between different types of parameters: load and environmental factors, geometry and materials. The first category of parameters includes: type of loading (uniaxial, multiaxial with or without phase shift), average stress, loading and overloading spectrum, loading frequency, temperature, corrosion and fretting. The second category of parameters includes surface finish, notch effect and scale effect. The third category of parameters includes material characteristics, material microstructure, residual stresses and surface treatments. A designer needs to analyze all these parameters in order to draw up as exhaustive a list as possible, and quantify them in order to calculate the fatigue strength of his part. The aim of this article is to provide the data and methodology needed to quantify these factors in a predictive calculation, and to indicate the limits of their use.

Either by analyzing bibliographical results, or (and this is preferable) from actual dynamic tests, we can find out the endurance limit of a material under very specific conditions (shape of the specimen or part, test conditions, etc.). However, at the manufacturing stage or during use, other factors will come into play which, ultimately, will have a primordial influence, whether beneficial or detrimental, on the service life of the finished product. The designer's art consists precisely in drawing up as exhaustive a list as possible of these various influencing factors. He must then quantify each of them to finally calculate the endurance of his part.

This article is the sequel to Fatigue of ferrous alloys. Definitions and diagrams . The following article gives examples of fatigue calculations.