Installation and Testing Methods for Frequency Stability under Cyclic Loading of Metals and Alloys

For manufacturing parts operating under complex conditions of cyclic loading, as well as products with stable dimensions, materials with minimal manifestations of inelastic properties are required. To study them, it is necessary to conduct specialized narrow-focus tests using machines and installations with appropriate experimental techniques. This paper presents the design of an electromagnetic installation for fatigue and frequency stability testing operating in a self-oscillating mode, in which the cyclic loading frequency is always equal to the natural frequency of oscillations of a sample. The installation control system contains two closed loops—for excitation of self-oscillations and for the oscillation amplitude stabilization. The sample is loaded by electromagnetic force and unloaded owing to the elastic forces of the material. This work presents a technique and algorithms for calculating the stresses of samples of various geometric shapes to assess changes in the amplitude–frequency characteristics. A calculated relationship is established between the force applied to the sample and its displacement at the point of force application, followed by determining the stress by the known force. The results of calibration tests for the static loading mode of samples are presented and the forces acting on the sample (external, inertia, elasticity) are estimated taking into account the maximum stress and maximum deformation amplitude. Static and cyclic loading modes are compared. The frequency characteristics are obtained when testing steel samples according to the proposed method. An analysis of the experimental results of tests with breaks during cyclic loading and continuous tests is carried out. It is shown that breaks in cyclic tests lead to an abrupt increase in frequency, while such jumps are absent during continuous tests. At the same time, a comparative analysis of the results of the considered types of tests showed that the total frequency deviation for the entire operating cycle is approximately the same in both cases. It is shown that the increase in frequency after rest is random and does not depend on the number of operating cycles.