PSI - Issue 81

Bezhenov Sergiy et al. / Procedia Structural Integrity 81 (2026) 260–263

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theoretically possible, which is typical for the region of the HCF. The closest relationship exists between stress σ р and the specific activation energy of destruction of the main element of the alloys under study (correlation coefficient is equal to 0.9977). The abscissa of the poles of different classes of steel was determined by searching for optimal values of the main statistical parameters (maximum correlation coefficient with minimum standard deviation). We used samples of fatigue test results for samples of various materials with different technological heredity with a given constant pole ordinate and a variable abscissa value. The results are given in the Fig. 2.

Fig. 2. Determination of the abscissa of the pole of HCF curves: (1) carbon steel, (2) low-alloy steel, (3) Ti-based alloy, (4) Ni-based alloy.

The studies have shown that the optimal value of the pole abscissa for medium carbon steel approximately corresponds to 100 loading cycles, for low-alloy steel – 555 cycles, for Ni-based alloy – 1025 cycles and for Ti-based alloy – 3200 cycles. A rather significant difference between the optimal values of the abscissas of the poles can be explained by differences in the kinetics of stabilization deformation processes in materials of different classes, which depends on the chemical composition of the alloy. An explicit connection between the abscissa of the pole and the mechanical and physical properties of the alloys under study has not been established. Fatigue curves constructed taking into account the coordinates of the poles are shown in the Fig. 3 for carbon and low-alloy steels with different technological inheritance as well as for Ni- and Ti-based alloys. As a result of the correlation analysis of fatigue test data taking into account the coordinates of the poles for all the materials studied, the values of the slope of the fatigue curves (the values of the parameter m) were determined. This made it possible to establish the values of critical stresses σ cr limiting the area of high-cycle fatigue. AE monitoring of fatigue tests of samples with registration of signals simultaneously in three different frequency ranges, as in previously conducted studies of nickel alloy Bezhenov (2024), revealed the identity of AE radiation activity for all studied materials with different technological heredity. The appearance of AE signals in the mid-frequency range ( 0.5 … 1.0 MHz ) during cyclic loading of the sample correlates well with the moment of initiation and propagation of microcracks, which corresponds to the French line on the generalized fatigue diagram according to Ivanova. This point can be used as the criterion for definition of crack resistance of the material on the prescribed level of stresses. 4. Conclusions It is found that the ordinates of the poles of the fatigue curves correspond to the activation energy of destruction of the volume of one mole of the metal that forms the basis of the alloy. It is revealed that the abscissas of the poles of the fatigue curves are related to the duration of the structure stabilization period, and therefore can differ significantly for different materials. An explicit connection between the pole abscissa and the mechanical and physical properties of the alloys studied has not been established. It is shown that ultrasonic hardening of the surface of products has a positive effect on the fatigue resistance characteristics of all materials investigated. It is suggested that the specific changes of the AE count rate in various frequency bands determine the different stages of the fatigue fracture, therefore, they can be used to detect cracks of microscopic size.