PSI - Issue 26

116 V.N. Kytopoulos et al. / Procedia Structural Integrity 26 (2020) 113–119 V. N. Kytopoulos / Structural Integrity Procedia 00 (2020) 000 – 000 unloaded state of the material. Thereafter, as it will be shown in the present study, this behavior of the J-parameter may change substantially under certain conditions for a stressed material. Moreover, it was observed in this study that the specific J-parameter may be a robust and characteristic experimental variable for the tested material in the sense that for the same stress conditions, it may remain almost constant by changing the measurement locations on the specimen surface and especially by including the mostly inevitable lift-off effect simulated by varying the probe-surface inclination angle. Further, it was also observed that this parameter is almost independent of the electronic setting of the apparatus such as signal gain and/or voltage threshold levels. In general, because this parameter has a more specific physical meaning and may alter under influence of certain mechanical and physical factors of this study such as cumulative hydrogen as well as stress-strain induced microstructural changes, this parameter in the following will be seen to describe the physicomechanical behavior of the used steel under these conditions. Furthermore, in a more specific magnetic aspect, the proposed J-parameter could be an indicator of the current states of the ME-spectrum changing from numerous, low energy wall jumps (small pulses) described by a decreasing J-parameter to view, high energy wall jumps (large pulses), described by an increasing J-parameter. In the context of the above-mentioned it is suggested to presume that an increase (decrease) in the J-parameter may indicate an associated decreasing (increasing) magnetic hardening tendency of the steel (Sulliran et al. 2004). As such, the magnetic hardening of steel seems to be an important micromagnetic property which, as it will be shown later, can be correlated with its hydrogen-assisted embrittlement process. In the indicative series of Figs.(2-6), the measured physicomechanical behavior expressed through the change with strain of J-parameter of the corrosion-free and corrosion-related material is presented. For convenience of the following discussion in each figure the corresponding engineering stress-stain curve of the material was inserted. At this place it is pointed out that all the magnetic measurements were preformed, as much as possible, within the forming necking region, where true stress conditions prevail. Otherwise elastic stress unloading effects taking place beyond this region would markedly alter the magnetic data. Thus, it is interesting to observe the formation of a maximum in the J-parameter curve, similar to that in the stress-strain curve. This fact implies to assume that certain characteristic microstructural changes should occur for both type of curves at their maximum point. For the stress-strain curve, at this critical maximum point, the well-known phenomenon of plastic instability in from of a localized necking sets on, which is a precursor of ductile fracture. At the same time, in the necking region, a state of high triaxial true stress develops by producing various internal volumetric mechanical damage in form of microcracks and microcavities which by coalescence lead to the final fracture of the material (Hertzberg 2007). Because all of these, it seems logical to try to analyze, for example as shown indicatively in Fig.2, the mechanical response behavior by two major characteristic stages, a prenecking stage I, and a post necking stage II, separated by the critical- strain point ε c, corresponding to the ultimate tensile strength value.

Figure 2: Mechanical stress-strain and magnetic J – response curves for virgin (as received)

Consequently, it would be helpful and of valuable practice to try to analyze, as shown in the same figure, the stress induced ME-response behavior on the above similar basis of the mechanical response stages. This can be made by intro ducing a prior magnetic stage I* and a posterior one II*, separated by the corresponding critical magnetic-strain point ε* c of maximum magnetic J-signal. As such, an evident overall continuous increase in the J-parameter within the stage I* is observed. This fact is attributed to a commending influence of applied stress and plastic strain effects on the magnetic