PSI - Issue 36

Ihor Dmytrakh et al. / Procedia Structural Integrity 36 (2022) 298–305 Ihor Dmytrakh et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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These stress-strain diagrams (Fig. 4) served as a base for determining the yield stress values σ Y as the function of the parameter N H . The monotonic decrease of a yield stress value σ Y with increasing the charging-discharging cycles N H has been found (Fig. 5a). Here, it also may be pointed out on the full coincidence between the data received under conditions of the hydrogen charging-discharging of the specimens and in the case of presence of the hydrogen in the specimens as it was received in work of Dmytrakh et al. (2015).

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a

Fig. 5. (a) Dependence of the true values of yield stress σ Y on the number of cycles of hydrogen charging-discharging N H (The red solid triangle refers to the data presented in work of Dmytrakh et al. (2015); (b) Analytical description this plot by the exponential function (2).

Thus, it can be concluded that even a short-term presence of the low concentration of diffusible hydrogen in the steel leads to irreversible changes in the material structure that causes the changes in the macroscopic mechanical behaviour of the material. The above-received data can be satisfactorily described (Fig. 5b) by the exponential function at the standard deviations meaning R 2 = 0.98: ( ) H Y exp N   =    0 Y , (2) 0 Y  is the value σ Y under N H = 0 and α is some constant (σ Y = 254.4 MPa and α = – 0.0839). This description should not be considered as some modelling. Rather, it is an attempt to quantify the experimental findings in a continuum sense. The SEM images of the microstructure of the tes ted specimens (σ = σ f ) after the different number of hydrogen charging-discharging cycles N H showed the increased density of the void-like defects due to the applied stress (Fig. 6). Here, the defectiveness of material also increases with increase of the number of hydrogen charging-discharging cycles N H (Table 4). This facilitates the deforming ability of steel and formally reduces the value of the plasticity limit as the macroscopic characteristic of the material (Fig. 5). where

Table 4. Number defects and their total square in the specimens after the tensile test ( σ = σ f ) on the number of cycles of hydrogen charging-discharging N H . The area of observation was equal to 41300 μm 2 . N H , cycles 0 2 5 Number of defects 45 50 73 Total square of defects S , μm 2 110 131 161