It has been shown that the degree of fatigue life reduction due to the pressurized water reactor environment depends on strain rate. It was reported that there was an insensitive strain zone, at which the strain rate had little influence on the fatigue life. The insensitive strain zone existed from the minimum peak strain to a threshold strain, which was determined empirically. This study aimed at determining the insensitive strain zone that corresponds to the crack mouth closure. Fatigue test runs were conducted using cylindrical hollow specimens of Type 316 stainless steel. The cold worked material was also used for the fatigue test. The strain rate was changed at a pre-determined strain, at which the crack mouth was deduced to be opened. It was shown that the fatigue life was almost the same regardless of the strain rate at the insensitive strain zone. Also, the insensitive strain zone could be determined by assuming the crack mouth opened when the stress reached zero in the strain rising phase.

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