Microhardness of structural materials under the influence of cyclic stresses

Conclusions

• 1. As the result of cyclic loading of annealed materials the microhardness first increases, then decreases below the original level, followed by failure. It can be assumed that the initial stresses cause an increase of the lattice defects but no damage. Further cyclic loading leads to discontinuities and accumulated damage.

• 2. In cold worked plastic materials one finds a monotonic reduction of the microhardness with increasing numbers of loading cycles. Evidently because of the high initial dislocation density, the increase in the number of lattice defects with the number of loading cycles leads to loosening of the lattice and damage of the sample.

• 3. During cyclic loading of thermally hardened structural steel the microhardness first decreases and then increases, failure occurring as the microhardness increases. Probably the drop of the microhardness is due to resorption of the areas of lattice defects because of favorable diffusion conditions at the beginning of cyclic loading [3], while the later increase is due to cold working. The reduction of internal stresses in the first stage must have a favorable effect on the fatigue resistance of thermally hardened steel.