PSI - Issue 50

Svetlana Barannikova et al. / Procedia Structural Integrity 50 (2023) 33–39 S. Barannikova, M. Nadezhkin, P. Iskhakova / Structural Integrity Procedia 00 (2023) 000 – 000

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with increasing attenuation coefficient α. At the last (pre-failure) stage IV, the growth rate of the martensitic α' phase reaches saturation, the velocity V decreases to a minimum value at the ultimate strength, and the attenuation coefficient α rises owing to the accumulation of defects and the development of cracks. The decline in the deforming force at the end of the stretching process, associated with the formation of a macroscopic neck, corresponds to a slight increase in V . An increase in the propagation velocity within this deformation region indicates the pre-critical state of the deformable material. Figure 2 displays the damage parameter D v , determined from the measured velocities of ultrasonic waves V in the deformed and initial state, as a function of relative deformation  /  f of Fe-18% Cr - 10% Ni alloy at different temperatures. In turn, Figure 3 depicts the D v (  /  f ) and D v (σ/σ B ) curves of the alloy at a test temperature of 180 K, revealing the differences in the relationships between the ultrasound velocity and strain/stress in the relevant plastic deformation regions.

Fig. 1. Strain-stress diagram σ(  ) and corresponding changes of martensite volume fraction f α′ , velocity V , and attenuation α of Rayleigh acoustic waves vs. relative deformation  /  f at T = 211 K

With an increase in the relative deformation  /  f , the values of D v in the entire temperature range increase according to the sigmoidal law. Herein, fluctuations in the room-temperature D v (  /  f ) dependence in the interval of 0.4-0.9 (Fig. 2, Curve 1) are associated with the Portevin – Le Chatelier effect, accompanied by the movement of single localized deformation fronts (Barannikova et al. (2004). Meanwhile, with a further decrease in temperature, this effect was not observed under the specified loading rate conditions.