Very often, the cause of failure of various equipment is metal fatigue. Moreover, this can happen not only after prolonged use, but also at the very beginning of the operation of equipment or a car. The reason for this is the periodic dynamic loads that a single unit or part experiences not only during the performance of its main function, but also in the manufacturing process. As a result, the material begins to collapse, even if the voltage value has not reached the ultimate strength.
In fact, fatigue is a change in the basic properties of a metal (mechanical and physical) under the influence of cyclic stresses and deformations. This entails structural changes in the material that occur at the macro and micro levels, which are largely determined by the initial properties and the condition for the manufacture of the part.
To characterize the tendency of a metal to this type of destruction, a term such as endurance is used. This parameter is numerically equal to the value of the maximum stress that the material can resist for 10,000,000 or more cycles, that is, during the set loading time.
The fatigue of the metal does not appear momentarily, and this is its main danger. It takes some time for the necessary changes to take place in the material, which, most likely, will not appear externally. Their nature is largely determined by the initial properties of the metal, the stress state, the peculiarity of loading and the influence of the environment. Until a certain point, all occurring phenomena are reversible. However, over time, the resistance to fracture gradually begins to decline, and so-called fatigue injuries appear.
At the very first stage, metal fatigue begins to manifest itself at the structural level, when microcracks begin to appear along the boundaries of grains and other components, which upon subsequent loading of the structure turn into macrocracks. This, in turn, becomes the main cause of the final destruction of the structural element during operation or of the sample during mechanical testing.
The metal fatigue is most vividly characterized by the curve of the same name, which reflects the corresponding relationship between the number of loading cycles tested by the specimen and the stages of damage, starting from the moment a crack occurred and ending with the final destruction of the object of study. Considering that fatigue phenomena initially appear in places of structural imperfections, the distribution of which is probabilistic, the characteristics of fatigue obey the same laws. Tests are most often carried out on a rotating sample, to which a constant bending load is applied.
Fatigue of the metal is largely determined by the operating conditions of a particular design. The presence of an active medium and a sufficiently high temperature can significantly accelerate the negative processes taking place in the material. The resistance of the material is significantly reduced in the presence of various structural inhomogeneities, the presence of non-metallic inclusions, uneven distribution of alloying elements, and also with insufficient surface cleanliness. To prevent this, one can resort to various surface treatments, which can create residual compressive stresses in the upper layer of the material. Most often, for this purpose, diffusion saturation is carried out, hardening or surface hardening is carried out in various ways, for example, by laser hardening.