Effect of higher strain range cycling on near fatigue-limit fatigue life

The objective of this study is to clarify the effects of pre-cycling above the fatigue limit on near the fatigue-limit fatigue behaviors of SA508, low alloy steel for nuclear pressure vessels, correlating fatigue lifetime wit h microstructural changes in the bulk. It has been observed that dislocatio n cell structure is well developed in an as-received SA508 sample and the m isorientation among cells increases with fatigue accumulation during fatigu e tests. The cell to cell misorientation was measured and statistically qua ntified by the Selected Area electron beam Diffraction (SAD). It was shown that a fatigue crack started growing abruptly when the SAD value (the mean misorientation among cells) exceeded a critical angle, 4-5 degrees. Fatigue tests were performed for SA508 samples, first at a high total strain range (0.62%) to 10, 100, and 6000 cycles followed by cycling at near the fatigu e-limit stain range (0.40%). The sample with pre-cycling for 100 and 6000 c ycles failed while ones with pre-cycling for 10 cycles and without pre-cycl ing did not rupture till about 10(6) cycles where the tests were terminated . Small surface cracks were found all samples pre-cycled at the high strain range but the cracks were arrested for a long time at the near fatigue lim it cycling followed by an abrupt growth at the failure. It was found by the SAD that samples failed at the fatigue limit when the average misorientati on among cells exceeds the critical angle regardless of pre-cycling histori es. The SAD value changed even during cycling below the fatigue limit, and the change in SAD value was strongly influenced by the number of pre-cyclin g above the fatigue limit. These changes well agreed with fatigue lifetime of the samples.