PSI - Issue 22

Yaroslav Dubyk / Procedia Structural Integrity 22 (2019) 275–282 Yaroslav Dubyk / Structural Integrity Procedia 00 (2019) 000 – 000

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3

Fig. 1. Mechanical properties for 20ºC and 350ºC, statistical data is taken from Zaporizhzhya NPP: (a) Yield strength; (b) Ultimate strength.

The general shape of the fracture toughness curves was assumed to express as exponential function with four parameters:



 K T T

    

IC K A B e

(1)

Here: A is the lower shelf asymptote; B and α are parameters defining the shape of the exponential curve. Generally, for deterministic integrity assessment of reactor piping and equipment is defined as the lower envelope of a large number of experimental data, thus deterministic evaluation of the brittle strength by (1) will always contain certain conservatism. Fracture toughness test are not found in piping documents, as only Charpy V-notched impact tests experiments were conducted for each piping, to prove the critical temperature shift. According to Charpy impact experimental data, and methodology taken from PNAE G-7-002-86 (1989), a hyperbolic tangent function was used to determine T K0 . In accordance with PNAE G-7-002-86 (1989), the critical temperature of brittleness (CTB) is calculated by formula:

(2)

0 K K N F T T T T T     T

Here , , T N F T T T     are shifts of CTB due to the thermal aging, fatigue and neutron fluence respectively. Obviously for our piping’s 0 F T   , as they do not experience loadings of neutron or gamma irradiation. , T N T T   were taken in a conservative way, and finally a 30 K T C   was obtained. Experimental data for fracture toughness (see Fig.2a) is taken from Timofeev at al. (2000), we used data only for piping specimens. Median curve was fitted using least square method, and the standard deviation for normal distribution is obtained, see Eq. 3 and Fig. 2 (left):





0.01689 36.9 44.46 e   

K T T 

22.93 Standard Deviation C  

IC K

(3)

As a rule of tomb for crack critical length estimations Failure Assessment Diagrams (FAD) are used. They have different according to the national standard recommendations or steel type (ferrite, austenitic and so on). For our