Fatigue of metal: what is it and how can it be resisted?

Very often the cause of the failure of various techniques is metal fatigue. And this can happen not only after a long operation, but also at the very beginning of the operation of the equipment or car. The reason for this is the periodic dynamic loads experienced by a separate assembly or part not only during the performance of its main function, but also in the manufacturing process. As a result, the material begins to break down, even if the voltage value has not reached the strength limit.

In fact, fatigue is a change in the basic properties of 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 manufacturing the part.

To characterize the propensity of metal to this type of destruction, a term such as the limit of endurance is used. This parameter is numerically equal to the value of the maximum voltage that the material can resist for 10,000,000 or more cycles, that is, during the set loading time.

The weariness of the metal does not appear momentarily, and this is its main danger. It takes some time for the material to make the necessary changes, which, most likely, will not appear externally. Their character largely determines the initial properties of the metal, the stress state, the loading singularity and the influence of the external environment. Up to a certain point, all the phenomena that occur are reversible. However, over time, the resistance to destruction gradually begins to decline, and so-called fatigue damage appears.

At the very first stage, metal fatigue begins to manifest itself at the structural level, when microcracks begin to appear along grain boundaries and other components, which, when the structure is subsequently loaded, turns into macrocracks. This, in turn, becomes the main cause of the final destruction of the structural member during the work or sample during the mechanical testing.

Most clearly, metal fatigue characterizes the same name curve, which reflects the corresponding relationship between the number of loading cycles tested by the sample and the stages of damage, from the time of the crack to the final destruction of the object of investigation. Given that fatigue phenomena initially appear in places of structural imperfections, the distribution of which is of a probabilistic nature, then the characteristics of fatigue are subject to the same laws. Tests are most often carried out on a rotating specimen, to which a constant bending load is applied.

The weariness of metal largely determines the operating conditions of a particular design. The presence of an active medium and a sufficiently high temperature can greatly accelerate the negative processes occurring in the material. Resistance of the material is significantly reduced in the presence of various structural inhomogeneities, the presence of nonmetallic inclusions, uneven distribution of alloying elements, and also with insufficient surface purity. To prevent this, it is possible to 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 performed, hardening or surface quenching is carried out in various ways, for example, by laser hardening.