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  5. System of vibroacoustic diagnostics and crack formation,mechanism of nucleation of microcracks in the metal structures

System of vibroacoustic diagnostics and crack formation,mechanism of nucleation of microcracks in the metal structures

B.D. Khalmuradov, V.A. Gliva, V.M. Makarenko
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Investigation of the processes of crack formation is actual direction of materials science due to not fully clarified aspects of this issue.Particularly important is the link between structural changes in metal alloys and mechanical characteristics of these products. It is one of the promising directions of nondestructive testing of the physical state of the metal structures. It has been suggested that the presence of cracks change the vibration characteristics of structural materials. This was confirmed by tests on a model material. But the important is to determine the changes in the microstructure of materials at the stages of stress, far from the nucleation of macroscopic defects, which will establish a clear link with the deformation process of subcritical changes vibroacoustic characteristics of structures. Research was carried on samples of stainless steel, in which defects come to light by non-destructive difficult to control, especially acoustic emission, through most of the steel viscosity. We investigated the change of the dislocation structure of the material at different amounts of alternating loads. The observation carried by electron microscopy in the "transmission". We were are obtained typical dislocation structure in the initial state, and the different amounts of loading cycles. In particular orderly redistribution of dislocations were observed in the middle of the grain, the formation of slip bands. It was fixed changes in the structure due to the emergence and development of the twinning process. When large quantities load cycles (several hundreds of thousands) observed accumulation of dislocations at the grain boundaries and in the last stage - the formation of extremely large strips of their densities. It leads to a breakthrough of grain boundaries and appearance of microcracks. Getting those data by direct observation made it possible to establish accordance the microscopic changes (such as vibroacoustic) with the actual state of the microstructure. Thus, it can determine the initial stages of adverse changes in structural materials by mechanical stress, far from the boundary fluidity and solidity.
Keywords: a crack formation; diagnostics; dislocation structure; structural materials; vibroacoustic characteristics