Fatigue Properties of Reactor Pressure Vessel Steel and Damage Evaluation

Low-cycle fatigue tests were carried out on a reactor pressure vessel steel at 290°C in order to investigate an evaluation method for fatigue damage and fatigue straining histories. The initiation behavior of surface microcracks was examined, and the following results were obtained. (1) The initiation of surface cracks begins from the early stage of fatigue life and the crack density (number of cracks per unit surface area) continues to increase throughout the fatigue life. (2) The increasing behavior of the crack density can be correlated with the Coffin-Manson type parameter Δε_P · N^0.6 irrespective of an applied strain range (Δε_t). Therefore, fatigue straining histories represented by Δε_P · N^0.6 can be estimated from the measurement of surface crack density. (3) The grain boundary is one of the preferential sites of crack initiation, and micro-cracks which just initiated have a tendency to propagate along the grain boundary. The electrochemical etching was performed at a midplane of the fatigue damaged specimens where no geometrical damage could be expected. Etching properties of fatigue damaged specimens were also studied and were found to change with damage accumulation. The results obtained can be summarized as follows. (4) Many random-shape pits appeared only on the surface of the fatigue damaged material by the electrochemical etching. These random-shape pits initiated mainly at the triple point of grain boundaries and grew preferentially along the grain boundaries. These pits can be considered as a result from degradation of corrosion resistance of the demaged material caused by microstructural changes at or near grain boundaries during fatigue. (5) The area fraction of corroded region on the electrochemically etched surface shows a single correlation with Δε_P · N^0.6 and quantitative evaluation of fatigue damage accumulation can be made nondestructively by the electrochemical etching.