Very often the cause of failure of the various equipment is metal fatigue.Moreover, it may occur not only after prolonged use, but at the beginning of the equipment or vehicle.The reason for that are the periodic dynamic loads experienced by an individual node or item not only during the execution of its primary functions, but also in the manufacturing process.As a result, the material begins to break down, even if the voltage does not reach the value of the tensile strength.
In fact, fatigue - this change in the fundamental properties of metal (mechanical and physical) under cyclic stresses and strains.This involves structural changes in the material occurring at the macro and micro levels, which are largely determined by the properties of the source and condition of manufacturing parts.
To characterize the metal tendency to this kind of destruction of such a term is used, the limit of endurance.This parameter is numerically equal to the value of the maximum voltage that can resist material for 10 million cycles or more, that is, within the time of loading.
metal fatigue does not appear minutely, and this is its main threat.It takes some time to get the material necessary changes have occurred that are likely not to be outwardly manifest.They largely determine the nature of the original properties of the metal, the state of stress, a feature of loading and environmental effects.Up to a certain point, all occurring phenomena are reversible.However, over time, resistance to degradation gradually begins to decline, and there are the so-called fatigue damage.
At the very first stage of metal fatigue begins to appear at the structural level, when the grain boundaries and other components of micro-cracks are beginning to appear that the subsequent loading of the structure are converted into macro-cracks.This in turn becomes a main cause of the final destruction of the structural element in the process or sample during mechanical testing.
most clearly characterizes the fatigue curve of the same name, which reflects the appropriate relationship between the number of samples tested loading cycles and stages of damage, starting from the moment of the cracks and ending with the final destruction of the object of study.Given that fatigue phenomena originally occur in areas of structural imperfections, the distribution of which is probabilistic in nature, then the characteristics of fatigue are subject to the same laws.Tests are carried out more often on a rotating sample, attached to a constant bending load.
fatigue largely determine the operating conditions of a particular design.The presence of the active medium and a sufficiently high temperature can significantly speed occurring in the material negative processes.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, as well as the lack of cleanliness of the surface.To avoid this, it is possible to resort to various surface treatments, which can create in the upper material layer residual compressive stress.In most cases, this purpose is performed diffusion saturation, performed work hardening or surface hardening in various ways, such as by a laser hardening.
