Effect of low-cycle overloads on the fatigue strength of structural materials

Abstract

1. Preliminary low-cycle overloading changes the fatigue strength of materials and makes it necessary to correct the linear rule of additive damage. 2. Depending on the type of material, the load, and the total deformation, low-cycle overloading may have either a softening or hardening effect. 3. The effect of preliminary treatment is evident during subsequent fatigue loading as a softening effect if the structural condition of the material corresponds to stage II of the elastoplastic deformation curve, characterized by a destructive component. 4. For cyclically softened material (steel 14Kh2GMR) preliminary low-cycle loading in the range of strain proper (Npre=5 cycles) and in the region of destructive deformation (Npre=80 cycles) lowers the fatigue limit regardless of the strain amplitude (0.2 and 0.5%). 5. The rate of the softening process is determined by the structural condition of the material, which is characterized by the effective yield strength and varies with the preliminary heat treatment, preliminary deformation, and amplitude of deformation in the original half-cycle of loading. 6. In selecting technological operations associated with overloading it is necessary to establish the permissible level of the load (deformation) and the number of preliminary cycles, depending on the type of material, its structural condition, and the operating conditions.