Stress corrosion crack growth rate measurement in high temperature water using small precracked bend specimens
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Doctoral thesis (monograph)
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VTT publications, 531
AbstractThe applicability of elastic-plastic fracture mechanics to stress corrosion crack growth rate measurements was studied. Several test series were performed on small elastic-plastically loaded SEN(B) specimens in high temperature water. One test was performed on a 25 mm C(T) specimen under linear-elastic loading. The tests on the SEN(B) specimens were performed using either rising displacement or a combination of rising and constant displacement loading. The test on the 25 mm C(T) specimen was performed using a combination of constant load and constant displacement. The studied materials were AISI 304 steel in sensitized, mill-annealed and irradiated conditions, AISI 316 in cold-worked condition, Inconel 82 and 182 weld metals in as-welded and thermally aged conditions and ferritic low activation steel F82H in tempered condition. The crack growth rate tests were performed in simulated pure BWR water and simulated BWR water with 10-100 ppb SO42- at 230-290°C. It was shown that intergranular stress corrosion cracking susceptibility can be determined using an elastic-plastic fracture mechanics approach. Fracture surface morphology in sensitized AISI 304 and welded AISI 321 steels depends on the applied loading rate in BWR water. The fracture surface morphology changes from transgranular to intergranular, when J-integral increase rate is decreased. However, extremely slow displacement rate is needed for the fracture surface morphology to be fully intergranular. Rising J results in transgranular stress corrosion cracking (or strain-induced corrosion cracking) also in the mill-annealed AISI 304 and 321 steels. Tests on irradiated AISI 304 steel showed that welding together with exposure to low neutron fluence in the BWR operating conditions results in a higher susceptibility to stress corrosion cracking than welding or irradiation alone. Ferritic low activation steel F82H (in tempered condition) is not susceptible to stress corrosion cracking under static loading conditions in high temperature water. However, its fracture resistance is clearly lower in water than in inert environment. Even as low an amount as 10 ppb SO42- in the otherwise pure BWR water results in one order of magnitude higher stress corrosion crack growth rate than the crack growth rate is in the pure water. Higher sulphate concentrations do not increase the crack growth rate further. Inconel 82 weld metal is much more resistant to stress corrosion cracking than Inconel 182 weld metal. No relevant crack growth rates could be measured for Inconel 82 weld metal in this work. Cold-worked (20%) AISI 316 steel is susceptible to intergranular stress corrosion cracking in BWR water. Sulphate in BWR water increases the crack growth rate also for AISI 316 steel. However, the observed effect was not as pronounced as for Inconel 182 weld metal. The results indicate that the same crack growth rates can be obtained using small SEN(B) specimens under elastic-plastic loading conditions and large specimens predominantly under linear-elastic loading conditions. The crack growth rates of the studied IGSCC susceptible materials were independent of the stress intensity factor level, KJ or KI, in the studied stress intensity ranges. At very low J-integral increase rates, the crack growth rate is linearly dependent on the loading rate. Loading rate may be a better parameter to correlate with the crack growth rate than the load, e.g., stress intensity factor. The observed dependence between crack growth rate and dJ/dt implicates that the specimen size has an effect on the crack growth rate in constant load tests. In constant load tests the specimen size, dJ/dt and crack growth rate are interconnected.
stress corrosion cracking, stress corrosion testing, linear-elastic fracture mechanics, elastic-plastic fracture mechanics, boiling water reactor, stainless steel, nickel-base weld metal, crack growth rate, fracture surface morphology