Fractography - Physical basics

A mechanical component is an element designed to operate a machine assembly or metal structure, each element having a specific function, in parallel or in series with other components. Its design leads to its manufacture: material elaboration, transformation (hot or cold forming), volume or surface treatment...

The resistance — the "quality" — of this mechanical part, measured a posteriori by its longevity, is obviously a function of the properties of the material and the particular way in which it operates. In any case, it will one day perish through wear, corrosion or breakage. Even if it doesn't break, it can be replaced at an economically convenient time, and kept as a performance indicator.

As far as fracture is concerned, all crystallized materials, even non-metallic ones (ceramics, minerals), exhibit the same fracture characteristics. We shall confine ourselves here to crystallized metallic materials, in particular steels.

In this article, we'll be looking at the stresses experienced by mechanical components and the metal's ability to withstand them for as long as possible; in fact, a crack appears when the two factors - high stress and weak metal - are concomitant in the same place. This brings together the disciplines of mechanics and physical chemistry.

The peculiarities of fracture reliefs, which contain both indications of the actual state of operating stresses and of the local structure of the metal, will be examined in the articles "Fractography. Break morphology" and "Fractography. Macrographies and micrographies".

In fact, a fracture is the memory of the manufacture and use of the mechanical part. First and foremost, you need to know that this memory exists: it's directly visible; you need to decipher it: it's legible. Finally, the design and construction of the part or machine must be corrected: it is constructive.

The complete study of the subject includes the articles :

— M 4 120 - Fractography. Physical bases (this article) ;

—  - Fractography. Fracture morphology;

—  - Fractography. Macrography and micrography;

—  - Fractography.