PSI - Issue 65

14 4 S.A. Barannikova et al. / Procedia Structural Integrity 65 (2024) 11–16 S.A. Barannikova, A.M. Nikonova / Structural Integrity Procedia 00 (2024) 000–000  the acoustic properties degradation coefficient K V = (V H − V 0 ) / V 0, where V H and V 0 are the values of the propagation velocity of Rayleigh waves of the samples with hydrogen and without one. Fig. 1b demonstrates the relationship between the coefficients of change in mechanical properties and the propagation velocity of Rayleigh waves under hydrogen embrittlement conditions. The dependences of the coefficients of degradation of the ultimate strength (curve 1, Fig. 1b) and hydrogen embrittlement (curve 2, Fig. 1b) on the coefficient of degradation of acoustic properties are approximated by the sigmoidal Boltzmann function with correlation coefficients of 0.98. It should be noted that a sharp decrease in these parameters is also observed at a critical value of hydrogen concentration cr H C ≈ 5 ppm. The hydrogen storage in the metal leads to an increase in the velocity of Rayleigh waves. This phenomenon is associated with an increase in the stresses of the first kind due to the growth of the interphase surface of the carbide/matrix interface and the nucleation of microcracks in martensitic steels. The results of the analysis of the change in the ultrasound propagation velocity from the hydrogen concentration С Н showed (Fig. 2a) that in the considered range the V R (С Н ) dependencies are exponential, both in the initial state (curve 1) and in states with deformations corresponding to: the yield strength (curve 2), the limit state (curve 3) and the ultimate strength (curve 4)

H B       C

,

(1)

0 V V A    R R

exp

0 R V , A and B are constants depending on the hydrogen content

where

a

b

Fig. 2. (a) Effect of hydrogen on the velocity of Rayleigh waves; (b) variation in the velocity of Rayleigh waves V R (1) and derivative dV R /dε (2) with increasing of total deformation for the stages of linear hardening (II), parabolic hardening (III) and pre-failure (IV). As noted above (Fig. 1a), the dependence of the Rayleigh wave propagation velocity on the total strain V R (ε) has a sigmoidal shape (curve 1, Fig. 2b). The values of the Rayleigh wave velocities for the limit state * R V of all samples (curve 3, Fig. 2a) were determined by the magnitude of the strain corresponding to the extremum of the derivatives dV R /dε (curve 2, Fig. 2b), at which the maximum decline in the propagation velocity of Rayleigh waves occurs (curve 1, Fig. 2b). Fig. 2a can be interpreted as a map of deformation mechanisms during tension of AISI 420 steel under hydrogen embrittlement conditions based on Rayleigh wave velocity measurements. Thus, the region located above curve 2 refers to elastic deformation; the region located between curves 2 and 4 corresponds to the region of deformations above the yield point and below the ultimate strength and refers to plastic deformation; the region located below curve 4 refers to fracture. Curve 3 divides the region of plastic deformation into 2 sections, in which the maximum decline in the propagation velocity of Rayleigh waves occurs and corresponds to the stage of growth of microcracks. Based on the obtained data, an acoustic criterion for the limit state of a deformable material under hydrogen embrittlement conditions R was formulated: