Estimation of parameter for predicting time to failure using three stress corrosion cracking methods - Constant load, constant strain, and slow strain rate technique

The stress corrosion cracking (SCC) behavior of the commercial austenitic stainless steels, Type 304 and Type 316 (UNS S30400 and UNS S31600), has been investigated in acidic solutions to verify whether or not a parameter for prediction of time to failure can be identified for the three different SCC methods (constant load, constant strain, and slow strain rate technique [SSRT]), so that the parameter is linear vs time to failure. The parameter for the constant load method has been already found to be the steady-state elongation rate ((l)over dot(ss)). For the constant strain method, the slope of a linear part in a stress relaxation curve (stress vs time), which is termed a stress relaxation rate ((sigma)over dot = dsigma/dt), was identified as the parameter for predicting time to failure. For SSRT, the parameter was the ratio of a maximum stress in a corrosive environment to that in an inert one from stress-strain curve. Both (l)ovr dot(ss) and (sigma)over dot could be obtained at a time within 10% to 20% of time to failure, while the ratio of the maximum stress was obtained at a time of 20% to 80% of time to failure, depending upon strain rate. Furthermore, with regard to the evaluation of SCC susceptibility for Type 304 and Type 316 stainless steels, the three SCC methods showed the same result, that is, Type 304 was more susceptible to SCC than Type 316. On the basis of the results thus obtained, the most plant-relevant parameter was concluded to be the steady-state elongation rate in constant load tests.