Physico–statistical approach: Procedures using the defect-free model of structural material - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 91

3.1 Determining the probability of failure at an arbitrary distribution
function of strength and load.

σs T , kgf/mm 2

3.2 Distribution curve of yield strength of steel grade St. 3.

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In the above approach, the safety factor should be calculated on the
basis of the mean expected yield strength and not the minimum yield
strength. The mean expected yield strength is determined on the basis of
the actual distribution curve of yield strength for grade St. 3 steel (Fig.
3.2). Processing this curve, the following is obtained for yield strength:

m = 2663 kgf/cm ,

2

D = 284 kgf/cm .

2

s s

T T

The coefficient of variation of yield strength is thus

r A =

284 / 2663 = 0.106. =

T

Safety factors in relation to the expected yield strength will be:
2663/1400 = 1.90 and 2663/1600 = 1.67, respectively.