Criteria for the endurance of structural materials

Periodically recurring cyclic loads affecting structural elements can be classified as dynamic, despite the fact that there are no significant inertial forces. This type of loading is typical for many mechanical engineering structures, such as shafts, axles, connecting rods, springs and rods. Practical observations indicate that loads varying over time in magnitude or in magnitude and direction can cause structural failure at stress levels well below the yield strength or strength of the material. This type of fracture is called fatigue failure, because the material "gets tired" under the influence of repeated repeated loads. Fatigue failure is the process of material destruction that occurs under the influence of repeatedly varying stresses. Material fatigue implies a gradual accumulation of internal damage, which leads to cracks and, eventually, to complete destruction. The endurance of a material is defined as its ability to withstand fatigue damage and destruction. The physical nature of this phenomenon is quite complex and has not yet been fully disclosed, however, the process of crack formation and growth is considered to be the main cause of fatigue failure. An approach to determining the endurance of materials has been developed using the new energy theory of strength. Based on this theory, formulas were derived that make it possible to estimate the service life of structural elements under various loading conditions, both symmetrical and asymmetric.