Estimation of the structural elements damage based on coercitive force measurements. Report 2. The results of experimental investigations of coercimetric control use for evaluation of paramagnetic austenitic steel damage degree under mechanical stress https://doi.org/10.33108/visnyk_tntu2019.02.007

Abstract

The results of experimental investigation of the coercive force response to mechanical loading
of laboratory samples made of paramagnetic (in the initial state) unstable austenitic steel AISI 304 (08X18N9)
associated with structural transformations of the original austenite into deformation martensite with finite ferriticperlite
decay, which causes the change in the magnetic properties of metal from paramagnetic into ferromagnetic
state are given in this paper. The experimental result of coercive forces response to different damaging effect load
type such as: tension and low cycle loading at voltage amplitude control («soft» load), which causes quasistatic
destruction, by implementing the initial plasticity of the metal (δ), and the low cycle load under control the
amplitude of deformation («hard» load), which causes the destruction of fatigue, due to the origin and development to the critical values of fatigue cracks are presented. The results of experimental studies of the response of the coercive force to mechanical stresses under stresses characteristic to multi-cyclic fatigue and at the presence of stress concentrators are also presented. According to the tests results, the staging of the damage accumulation processes during the «soft» and «hard» loads: the growth of the values of the coercive force corresponds to the
elastic-plastic deformation (the stage of origin of the cracks), and the decrease of their values associated with the
loss of solidity of the metal in the event of the appearance of pores or cracks (stages of cracks development) is determined. Establishing the damage accumulation stage by changing the direction of the kinetic curve of the
coercive force after a certain number of run cycles makes it possible to construct the curve of irreversible damage (according to French) and to evaluate the cyclic durability not according to the fatigue curve (destruction) of the metal, as adopted in engineering practice, and at the stage of cracks origin, which significantly reduces the risk
of destruction. Physically substantiated method of establishing the endurance limit for austenitic unstable steel, which is based on determining the growth rate of coercive force on short bases of cyclic loading is proposed. By
the magnitude of the growth rate of the coercive force, one can establish the values of the endurance limit at different durability bases. The possibility of using the structroscope to detect the most deformed zones, with the
determination of the main stresses direction, and the immutability of the metal in the form of pores and cracks of fatigue, surface and sub-surface defects and cracks is shown. The limitations in the possibility of assessing the degree of metal damage in multi-cyclic fatigue, including stress concentration zones, by measurements of the
coercive force values of due to the sensitivity of the device to the ratio of volumes of elastic and plastic-deformed metal in the zone of destruction (measurement)are defined. The use of the coercimetric control allows by means
of the results of measuring the changes in the coercive force in the most loaded structures sections during their operation to assess the level of damage received.