Study of the Effect of Strain Amplitude Ratio at Two-Frequency Cyclic Loading

Abstract

The operating modes of loading elements of machines and structures exhibit, as a rule, more complicated character of their loading cycles compared to sinusoidal used in the practice of calculations and experiments. It is noted that, in a number of cases, the actual conditions of load changing can be idealized by dual-frequency loading modes with superposition of the high-frequency component of the main workload attributed to the effects of vibrations, aero- and hydrodynamic impacts, regulation of the working process, etc. Testing of three steel samples with different cyclic properties has shown that such two-frequency regimes lead to a decrease in the durability in comparison with single-frequency loading equal in the amplitude of maximum stresses. This reduction depends on the parameters of the basic low-frequency and imposed high-frequency loads. Evaluation of this reduction can be performed both using the laws of summation of the damage expressed in the strain terms and using an analytical expression considered below, which includes the calculated or experimentally determined durability for single-frequency loading with the maximum (total) amplitude of the effective stress and the durability coefficient characteristic of each type of material and determined by the ratio of amplitudes and hours of low and high stresses. A computational-experimental analysis of the effect of the amplitude of low-frequency and superimposed high-frequency loading under two-frequency modes of stress change on the cyclic durability has shown that the imposition of the high-frequency component of cyclic deformation on the main low-cycle loading process leads to a significant decrease in the cyclic durability; the level of the decrease correlates with the level of ratios of amplitudes and frequencies of the summarized harmonic processes of load application.