On estimation of the structural elements damage according to coercive force. Message 3. Results of experimental studies of the possibility of using a coercive-metric control to estimate the degree of damage of ferromagnetic steels under mechanical loading https://doi.org/10.33108/visnyk_tntu2019.03.007

Abstract

This paper presents the results of experimental studies on the response of the coercive force
to the mechanical loads of laboratory samples from ferromagnetic steels of 05kp, 17G1C and steel 20. It is shown
that the differences in the response of the coercive force to the mechanical loads of ferromagnetic and austenitic
steels are in the physical nature of these phenomena. For ferromagnetic steels, changes in the values of coercive
force in static and cyclic deformation are associated with the ordering of the domain structure of the metal (from
chaotic to directed). For paramagnetic (non-magnetic in the initial state), changes in the values of coercive force
in static and cyclic deformation are associated with structural transformations of the original austenite into a
deformation martensitic with a finite ferrito-perlite decay, which causes a change in the magnetic properties of
the metal from the paramagnetic into the ferromagnetic state. The device proposed by the author with the modified
design of the sensor allowed to obtain new data on the response of the coercive force to mechanical loads and to
establish the basic laws of deformation and destruction of the metal in the most loaded and damaged local zones,
including areas of stress concentration, elements of structures made of ferromagnetic steels. It has been
experimentally established that when measured in mechanical load, the measured values of the coercive force
depend on the orientation of the poles of the sensor magnets relative to the direction of loading. At static or cyclic
loading to stresses that exceed the boundary of fluidity, after unloading, the value of coercive force in 1,5–2 times
exceeds the same values in the loaded state. At the same time, in the case of static or cyclic loading with excess of
the existing stresses of the boundary of flux during unloading, there is a reversible turn in 900 directions of the
maximum values of the coercive force that causes the corresponding «jump» of these values. The detection of a
sharp change in the values of the coercive force, in excess of the stresses of the boundary of fluidity, during the
monitoring of the VAT of structural elements allows to establish the boundaries of plastic and elastically deformed
metal and may be the background for the use of other non-destructive research methods, for example, measuring
the thickness of the metal and finding defects. The use of a coercimetry control allows separating the damage that
is obtained in the metal under conditions of elastic deformation and damage caused by elastic-plastic and plastic
deformation, as well as destruction. The dependence of the values of the coercive force on the orientation of the
direction of measurement relatively the direction of loading allows us to determine the direction of the main
stresses. The present results can be useful in diagnosing the quality of modern technologies, for example, the local heat treatment of constructive elements of the type «thermoshocks» and in developing a method for evaluating the
residual stresses from welding. The experimental results obtained give an optimistic prediction of the possibility
of using a coercimetric control to monitor the accumulation of damage in ferromagnetic metals by changing the
values of the coercive force.