A method for studying the frequency stability of materials during tests for multi-cycle fatigue of steel

Abstract

For trouble-free operation without loss of elastic and inelastic properties of particularly critical elements of electrical-to-mechanical vibration converters during a long period of cyclic operation, it is necessary, in addition to studying the fatigue characteristics of materials used for their manufacture, to study these alloys for frequency stability, since minor deviations in the frequency of natural oscillations lead to unacceptable errors in the operation of such high-precision products. To carry out such studies, we developed and constructed an original installation, in which sinusoidal loading is carried out according to the “soft” scheme of flat samples cantilever bending operating in self-oscillation mode. The frequency of cyclic loading in this installation is generated by current pulses, which are a response to the frequency of the test sample natural oscillations converted using electronics.  As a result, frequency equality is achieved in the test process. An algorithm for calculating stresses depending on the loading amplitude of steel samples of different geometric shapes was developed. It is shown that the stress on the sample calculated by the deformation amplitude in all cases is 8 – 10 % higher than the stress calculated by the force, regardless of the shape of the proposed samples. To verify the proposed research method, martensitic-aging steel was tested at loads close to the fatigue limit, since frequency stability in this range is of great interest. We obtained the frequency characteristics in the multi-cycle test area. It was determined that with an operating time of 50 million loading cycles, the frequency change was 0.75 Hz. The dynamics of frequency stability was revealed: the frequency changed most intensively during the first 10 million loading cycles, during this time the frequency changed by 0.54 Hz.