PSI - Issue 16

Ihor Dmytrakh et al. / Procedia Structural Integrity 16 (2019) 113–120 Ihor Dmytrakh et al. / Structural Integrity Procedia 00 (2018) 000 – 000

117 5

  n da dN C K    ,

(2)

where C and n are constants of the material and testing conditions. Their values and standard deviations r 2 are presented in Table 2. High values of r 2 confirm the correctness of this analytic description.

Table 2. Values of the parameters C and n in equation (1) depending on hydrogen concentrations C H in the specimens. C H , ppm n C r 2 Test series A 0.001 4.71 10 1 10   0.96 0.209 4.86 11 5 10   0.97 0.456 4.29 10 5 10   0.96 Test series B 0.001 5.55 11 1 10   0.96 0.209 5.16 11 2 10   0.96 0.514 5.92 12 4 10   0.98 1.231 5.66 12 9 10   0.99

The analytic representations of the diagrams of fatigue crack-growth rate by the Paris dependence for different initial hydrogen concentrations in the specimens are shown in Fig. 4. It was discovered that the influence of the concentrate ion C H on the fatigue crack growth in 20 steel was ambiguous for both series of tests. For this concentration, there exists a characteristic value C H = 0.209 ppm for which the fatigue crack growth rate decreases and, hence, the cyclic crack-growth resistance increases. This is especially well visible in the section of the diagrams for 20 MPa m  ΔK (Fig. 5) and da / dN = 10 --4 mm/cycle (Fig. 6). The indicated effect can be explained by the interaction of hydrogen with dislocations. Indeed, hydrogen may affect dislocations in two different ways: either blocking their motion, or increasing their mobility. Just the competition of these mechanisms determines the resulting effect, positive or negative, on the resistance of materials to fatigue crack propagation as it was state by Murakami et al. (2010). It is worth noting that these results are restricted to the Paris domain and, hence, their analysis is reasonable solely for the indicated part of the diagram of cyclic crack-growth resistance of steel. To estimate the near threshold domain and the domain preceding the final catastrophic failure of the material, it is necessary to perform additional investigations.

Fig. 4. Analytic description of the diagrams of fatigue crack-growth rate for the specimens with different hydrogen concentration according to the Paris relation: (а) test series A (curves 1 – 3 correspond to C H = 0.001 ppm, 0.209, 0.456); (b) test series B (curves 1 – 4 correspond to C H = 0.001 ppm, 0.209, 0.514, 1.231).