Trajectories of principal stresses after plastic deformation

Improving research efficiency of mechanical stresses in metals can be achieved by studying and taking into account the fields of the trajectories of the main stresses (isostats), which qualitatively characterize the stress state. The purpose of the work was to determine the possibility of detecting the isostates in a St3 steel sample, following its plastic deformation. The methodology is based on the magnetoelastic method, manifested by the dependence of magnetic permeability of ferromagnetic materials upon mechanical stresses, acting in them. The method is implemented using the IMN-4M monophase magnetoelastic meter of mechanical stress: the base of the device gage is 5 mm, the angle meter error is ±2°. The sample is a plate with dimensions of 150 x 150 x 4 mm. Coordinate grids with 10 x 10 mm cells were applied to its front and back surfaces. Using the IMN-4M device, we measured the values of tilt angles for tangents to principal stress trajectories in all nodes, then we created isostates by consecutive adjustment of the direction and radius of curvature for a raised curve. The plastic deformation of a plate was performed by shooting small lead pellets with kinetic energy of 1916 J. The maximum plate curvature in the impact zone of main charge equaled 7 mm. After a shot, the measurements were repeated in the same nodes of coordinate grids, and the isostates were plotted again. The experiment has shown that: the magnetoelastic method makes it possible to detect the isostates even after plastic deformation; the trajectories are present both on front and back surfaces of the deformed plate; isotropic points and parallel isostates are available in fields, formed by the shot. Izotropic zones were formed on front and back surfaces, in which isostates can not be plotted due to random orientation of tangents. The "disturbances" of fields with disruption of regular trajectory flows were detected. The results of the work may be useful for researchers and technical engineers, engaged in the study of elastic and plastic deformations.

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