PSI - Issue 13

Sergiy Kotrechko / Procedia Structural Integrity 13 (2018) 11–21 Sergiy Kotrechko/ Structural Integrity Procedia 00 (2018) 000–000

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In the first approximation, relation between the values of yield stress at critical temperatures, Y σ ( unirr c T ), Y σ ( irr c T ), and corresponding values of f σ is the following:

(9)

( ) Y c

j T σ ≈ σ

f

where j is the parameter, which equals the ratio n j 3 10 ≈ × , where n is the strain hardening exponent,

Y f σ σ . For the range IC Y J σ / =0.008÷0.050 mm, the value of

0 04 0 10 . . = ÷ n .

The value of threshold stress, th σ , up to a coefficient 0.8÷0.9, is equal to MC R - the minimum value of brittle fracture stress at uniaxial tension over the ductile-to-brittle transition temperature range [Kotrechko et al. (2007); Kotrechko et al. (2001)]. This stress is independent on fluence Φ at 22 100 10 Φ ≤ × m -2 [Balandin et al. (1984)]. Experimental determination of MC R for unirradiated steels is connected with the considerable methodological difficulties. This is due to high ductility of unirradiated RPV steels. As a result, the temperature of ductile-to-brittle transition at uniaxial tension is lower than the liquid nitrogen boiling point ( Т < 77 К ). Therefore, Kotrechko et al. (2016) have developed the quite simple technique to ascertain MC R by the value of nominal fracture stress at Т = 77 К for cylindrical specimens with notch of special geometry. Using above values, parameter λ is calculated. According to (7): ( ) σ − σ 2 The value of λ depends on fluence and is invariant to temperature. This enables to predict the effect of fluence on the level of brittle strength of metal, accounting for (7). 3.2. Ultimate levels of irradiation embrittlement of RPV steels Accounting for two constituents of radiation embrittlement, namely: ( i ) of radiation hardening and ( ii ) of reducing the level of brittle strength, enables to predict the maximum permissible levels c Φ of radiation embrittlement of RPV steels. For this purpose, it is advisable c R , as a measure of brittle strength, instead of the value of local IL K and a magnitude of the fracture probability P , c R is numerically equal ( ) Y c T σ at the critical temperature c T . According to (9), relation between c R and f σ       − τ × th Y λ = P th f 1 1 ln (10)

Fig. 4. The temperature dependences: fracture toughness CT T IC K 1 − of specimen СТ -1 Т at probabilities 5%, 50% and 95% (the Master curve method); fracture toughness ( %) 5 RPV IC K for the crack in reactor pressure vessel with the front line of length 150 mm at probability of fracture 5%; yield stress Y σ ; load IL K , corresponding to the constant value I Y J σ / ( I Y J σ / = 0.0365 mm). c T is the critical temperature of fracture at given loading level IL K and probability 5%; c R is the brittle strength level (RPV steel (15Cr2NiMoVA) at 22 2 40 10 m Ф − = × ) .

to use the ultimate strength of irradiated metal,

cleavage stress

f σ . For a given load

to the value of the yield stress

is the following:

σ

f

R

(11)

≈

c

j