PSI - Issue 59

Viktor Kovalov et al. / Procedia Structural Integrity 59 (2024) 771–778 V. Kovalov et al. / Structural Integrity Procedia 00 (2019) 000 – 000

777

7

After potentisation we obtain:

(19)

or

.

(20)

Using a similar calculation technique, data were obtained for samples from 40 and 40X steels, which underwent heat treatment with cooling in air (Table 2).

Table 2. Experimental data and elements of fatigue crack growth rate calculation № sample ΔK da , mm dN , cycle. mm/cycle lg (ΔK)

l i , mm

For model specimens made of steel 40 cooled from the annealing temperature in air

26.564848 24.659494 88.67684 30.23422 58.087932 48.462428 42.5684 39.6875

0.5 0.7 2.3 0.4

20000 38000 20000 8000 22000 9000 42000 26000

0.000025 0.0000018 0.000115 0.00005

1.42430734 1.39198416 1.94781021 1.48049877 1.76408592 1.68540517 1.62908733 1.59865374

-4.602 -4.735 -3.939 -4.30103 -4.0414 -3.9542 -4.623 -4.415

23.6 24.3 25.3 25.7

1

2

For 40X steel model specimens cooled from the annealing temperature in air

2 1 1 1

0.00009090 0.0001111 0.000002381 0.000038

30 31 30 31

1

2

Experimental data indicate a higher rate of crack growth in 40X steel model specimens than in 40 steel specimens. Moreover, with increasing intensity of applied stresses, the curve da/dN=f( ΔK) grows faster for 40X steel model specimens than for 40 steel specimens (Fig. 4).

Fig. 4. Dependence of fatigue crack growth rate on the change of stress intensity factor during cooling of specimens in air (1) and during cooling in a heat-insulating mixture (2).

Cooling the specimens in a heat insulating mixture reduces the crack growth rate in both steel 40 and steel 40X specimens. According to the obtained data, a greater volume of pearlite is observed in 40X steel after heat treatment than in 40 steels. Moreover, the pearlite content decreases depending on the cooling method after annealing: when cooling in a heat - insulating mixture, the amount of ferrite is higher than when cooling in air. Thus, the growth rate of fatigue cracks depends on the volume content of excess plastic phases in the structure: the more plastic ferrite in the structure, the slower the cracks grow. On the contrary, an increase in the volume content of pearlite increases the crack growth rate. The observed difference in the growth rate of fatigue cracks in steels with ferrite-perlite structure with different pearlite content is explained by the general regularities concerning the resistance to crack initiation and sensitivity to stress concentration. Fatigue (endurance)