Application of a two-mechanism model for environmentally-assisted crack growth

This report presents a short review of the proposed models for the prediction of crack growth rate during environmental fatigue tests on pressure vessel steel in LWR environments. The three component, crack-tip strain rate model proposed by CISE is discussed in detail and compared with the earlier models. The CISE model is more complete and able to predict both the maximum crack growth rate expected and the range of loading conditions where the maximum crack growth rate is to be found. Two environmentally based components of crack growth rate are assumed to be superposed on the purely mechanical fatigue, and two different mechanisms of enhancement are assumed to be active: one for the so-called ''true corrosion fatigue'' and another one for the stress corrosion cracking (SCC) component. Data sets obtained at MEA have been reduced using the proposed model, and the comparison with SCC from literature data has proven to be informative and worth further study. Finally, the temperature effect has been studied using MEA data. The results show that a change in temperature causes a change in the active mechanism, so that the SCC component is apparent only at higher temperatures (above 200/sup 0/C, approximately). At a temperature ofmore » about 90/sup 0/C, only the true corrosion component is present.« less