Effect of pre-strain history on small crack growth under Low Cycle fatigue for JIS SFVQ1A steel

Low cycle fatigue tests were performed for a low alloy steel, JIS SFVQ1A, used for pressure vessels of nuclear power plants. The effect of pre-strain history on the small crack initiation and growth was investigated in detail using cellulose acetate replicas. Under the tests in which the total strain range, Δ ε, is constant, surface crack length, 2c, was smaller for the tests with larger Δ ε due to the different numbers of small crack initiation and coalescence. The pre-strain histories were applied at Δ ε of 8 or 16% with its fatigue usage factor, UF, of less than 0.2, followed by fatigue loading at Δ ε = 2% until fracture. In these tests, the relationships between 2c and UF agreed with each other unless crack coalescence occurred. The scatter in fatigue life was attributed to the coalescences of small cracks. Fracture mechanics approach was applied to predict the fatigue lives and to characterize the growth behavior of small fatigue cracks.