Significance of loading mode on stress corrosion cracking

It is important to predict the service life of pressure boundary components such as pressure vessel and piping operating in high temperature water environments. In order to establish the common predictive model of environmentally assisted cracking behavior is one of the essential step at present particularly in LWR life evaluation and prediction. In this paper, based upon a dissolution model, crack growth behavior of stress corrosion cracking of reactor pressure vessel steel, SA533B in the simulated LWR environment under monotonic rising load (in other words Slow Strain Rate Test) and constant load was quantitatively simulated by the combination of FEM stress-strain analysis on growing crack and metal dissolution at a crack tip. Some experiments such as constant load test by use of trapezoidal wave form and Slow Strain Rate Test were conducted to compare the results with simulation ones. From simulation results the mechanism of SCC crack growth under constant load and SSRT is discussed based on the dissolution model from the view point of current transient curve, and the correlation between crack propagation rate and crack tip strain rate is discussed and evaluated quantitatively. The latter is expressed as following equation, (1) da/dt = A×(dε_ct/dt)/ε_f)^B where A and B are constants, and ε_f is oxide film rupture strain. It is observed that the parameter which controls this relationship is S anion concentration at a crack tip and steady state oxide film dissolution rate I_ss. It is considered that the mechanism which controls a crack tip strain rate is different among various loading modes. There are four mechanisms which control a crack tip strain rate: (1) Mechanical loading, (2) Mechanical crack growth, (3) Crack growth due to dissolution and (4) Creep. The relative contribution of each of these on crack tip strain rate is considered to be different for each loading mode.