Experimental investigation of the failure of steel AISI 316 by the methods of structural analyses

The failure of reactor steel AISI 316 under tension was investigated by structural analysis and scanning contact potentiometry (SCP). In real time surface potentiograms were plotted by the change of which the growth of crack nucleus was tracked from its initiation till the stage of cup fracture formation. The nucleus of the microscopic crack on the potentiogram was first detected at the end of the next to the last cycle of testing at a load of 525 MPa in the yield region and then was persistently reproduced on potentiograms in subsequent tests up to the failure. The most noticeable changes in the parameters of dynamic waviness and roughness occurred at the moments of sharp crack growth. Using the results of the SCP method, three main stages of crack development were identified. In the first one, under loads corresponding to the conditional yield point, a nucleus was formed in the region of the maximum tangential stresses. This region of localization arises as a result of macroscopic loss of stability due to the significant mechanical energy accumulation by the system, which leads to an increase in the magnitudes of the plastic strain fluctuations. The second stage, the one of a progressive growth of the crack nucleus, is the longest in time and lasts from the moment of detection of the nucleus to the sample failure. The amplitude of the electrical potentials in the central part of the sample increased with the rise in intensity of the applied load which is associated with an increase in the inhomogeneity of the internal stress field around the nucleus, as well as the accelerating creep process and the growth of plastic strain in this region. At the pre-failure stage in the hardened surface layer, a macro-groove appears in the form of a standing deformation wave along the fracture line. Under the microscope, shear strain bands on the sample surface are also visible.