Summary: A fatigue model based on a decrease in the carrier mass of a substance in the first quarter of a cycle. Also a fatigue model based on an increase in its density in the second quarter of a counterclockwise rotation cycle. As well as this model based a decrease in a carrier mass in a third quarter cycle and an increase in its density in a fourth quarter of a clockwise rotation cycle. The tangential stress and shear angle are related by the Hooke linear relationship. Depending on the initial physical and mechanical properties of the rod, its structural changes are controlled, which quantitatively reflect the changes in mass, density, stresses, shear modulus, which are calculated on each cycle. It is accepted that the brittle fracture of the rod occurs in a cycle in which the inequality of the initial fracture energies and the potential elastic energy pumped on this cycle is not fulfilled. The criterion for achieving the limit of fatigue is not to fulfill the inequality outside the accepted test base. The model algorithm is implemented in the software environment of computer algebra.