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

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3. Experimental results Loading variables and crack length, according to [5], are generalized by one parameter ΔK – the change in stress intensity per loading cycle. The dependency between crack growth rate and stress intensity factor is expressed by the equation: ⁄ [ ( ) ⁄ ( ) ⁄ ( ) ⁄ ( ) ⁄ ( ) ⁄ ] , (1) where ΔP is determined by the formula: (2) The experimental data were used to determine the crack increment da for a given number of cycles dN . The crack growth rate was determined by the ratio da/dN . Based on the test results of two specimens from the same material, the dependence of the crack growth rate da/dN on ΔK was determined. In double logarithmic coordinates, by processing the data using the least squares method, an equation of the type was obtained:

.

(3)

To calculate the coefficients of equation (3) is reduced to linear form: ,

(4)

where . Substituting the obtained values of the coefficients into the linear equation (4), we obtain: ,

(5)

or

(6)

After potentisation we obtain:

(7)

or

(8)

The obtained equation (8) characterises the growth rate of fatigue cracks depending on the change in the stress intensity factor during cyclic loading of steel 40 cooled after annealing in a heat-insulating mixture. The calculations according to the above formulae, similarly as it was done for steel 40, we have: (9) or