PSI - Issue 26

V.N. Kytopoulos et al. / Procedia Structural Integrity 26 (2020) 113–119 V.N. Kytopoulos / Structural Integrity Procedia 00 (2020) 000 – 000

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response. Primary, elastic tensile stresses favor an increase in the magnetic energy signal (Stefanita et al. 2000; Sulliran et al. 2004; Blaow et al. 2007). At the same time, however, plastic strain-induced increase in the pinning sites in form of dislocation pile-ups and tangles, exhibit a pronounced suppressive effect on the wall motion and hence in the magnetic energy signal (Sulliran et al. 2004). Thus, the dominating combined influence of the applied elastic and effective flow stress over the plastic strain effects increasingly the J-parameter within the precursor magnetic stage I* results.

J

J

MPa J

MPa J

Figure 3: Mechanical stress-st rain and magnet ic J response curves for 200 hours of corrosion t ime .

Figure 4: Mechanical stress-st rain and magnet ic J-response curves for 400 hours of corrosion time.

J

J

MPa J

MPa J

Figure 5: Mechanical st ress-strain andmagnet ic J-response curves for 800hours of corrosion t ime

Figure 6: Mechanical st ress-strain andmagnetic J-response curves for 1000 hours of corrosion t ime.

Beyond the characteristic magnetic- strain point ε* c and within the posterior magnetic stage II*, the micromagnetic behavior is characterized by an evident overall decrease in the J-parameter of specific ME-response. It seems, this time, that plastic strain-induced effects dominate over the applied stress effects in decreasing the specific ME-response. This is because, the produced volumetric micromechanical damage or inhomogeneities in form of microcracks, voids cavities and cell-structured dislocation networks may act as additional strong pinning sites, by reducing the specific ME-response. In other words, the above volumetric inhomogeneities under applied true stress conditions would act as a dense-spaced local triaxial stress raisers which may lead to a significant increase in the magnetoelastic interaction energy with domain walls and hence in the pinning strength. This afterwards, results in a decrease in the effective wall velocity and mobility and consequently in the rate of magnetic volume change. This in turn, leads to a decrease in the induced V rms energy signal which in combination with the increase in the pinning sites density (N) results in a net decrease in the ratio of J-parameter. Furthermore, the interesting shift of the maximum of physicomechanical J-curves with corrosion time towards lower strains may be seen as indicative of a combined occurrence of hydrogen-assisted environmental embrittlement as well as magnetic hardening processes. This shift is consistent and correlated with the total-overall shift of the maximum of the associated (mechanical) stress-strain curves towards lower strains with time of