Determination of the Region of Stabilization of Low-Cycle Fatigue HSLE Steel from Test Data

Abstract

Steel, NIONIKRAL 70 (NN-70), selected in this study to investigate the experimental behaviour affected by fatigue loading, among other things, is used in shipbuilding and for manufacture of pressure vessels as well. The experiment was conducted using smooth round specimens made of steel NN-70 as parent material (PM). When selecting stabilized hysteresis as a representative of all of stabilized hysteresis for one strain level, and for the further processing of low-cycle fatigue test results, the recommendations of standards have been used as well as the methodology based on which linearity of the stabilization regions of low-cycle fatigue was numerically determined. In present paper, the behaviour of high-strength low-alloy (HSLA) steel under conditions of low-cycle fatigue (LCF) has been experimentally tested and analyzed. Based on the experimental results obtained in the programme EXCEL, characteristic regions of low-cycle fatigue of steel NN-70 have been determined, the most important being the region of stable behaviour of materials, so-called “the region of stabilization”. From this region, on the basis of pre-defined requirements, characteristic stabilized hysteresis have been isolated for each strain level, based on which the indicators of low-cycle fatigue of steel NN-70 have been identified. For selecting typical stabilized hysteresis and for further processing of the low-cycle fatigue test results, new methodology with two new methods were used: “Method of the middle stabilization (ms)” and “threshold NDT method (tNDT)”. The comparisons of those two methods are shown in this paper.