Influence of strain range on fatigue life reduction of stainless steel in PWR primary water

Abstract

The environmental effect in the pressurized water reactor (PWR) water was investigated for various applied strain range using a type 316 stainless steel. The tests were conducted using cylindrical hollow specimens at 325°C. It was shown that the ratio of the fatigue life in the PWR water environment to that in air was about 0.3 to 0.4 regardless of the strain range when the applied strain ranges were 0.8% or more. Crack growth rates identified from spacing of striations observed on fractured surfaces were used to demonstrate that the fatigue life reduction in the PWR water environment could be attributed to the crack growth acceleration. The fractured surface observations revealed that crack initiation was enhanced by the PWR water environment. On the other hand, the reduction in the fatigue life was not significant when the strain ranges were 0.5% and 0.44%, and the specimens did not fail when the strain ranges were 0.38% or less. It was deduced that the crack initiation was not enhanced for the relatively small strain range, and the crack growth was not accelerated. Since the fatigue limit of the test material was 0.4% in the strain range in air, it was concluded that the fatigue limit was not reduced in the PWR water environment.

Environmental effect of PWR primary water on fatigue life of stainless steel (influence of loading rate on fatigue life reduction)

“…Meanwhile, the fatigue life in air was determined as the number of cycles at which the maximum peak stress dropped below 25% of the value in the stable state. It was shown that the fatigue life obtained using the hollow specimens was almost comparable with that obtained using solid bar specimens in air (25% load drop) 28,29 and high‐temperature water environment 29 . No significant difference was found in the fatigue life in air between the tests at 598 K and room temperature as noted by previous studies 3,30 .…”

“…The 0.2% proof and ultimate strengths at 296 K were 297 and 611 MPa, respectively 26 . The basic fatigue properties for this stainless steel were obtained in previous studies for air 27,28 and PWR water 28 environments.…”

“…Fatigue life was defined as the number of cycles at which the crack penetrated the wall thickness. Figure 5 shows, for each rise time, the fatigue life together with that obtained from tests in air of the same material 27,28 . The fatigue life is also summarized in Table 1.…”

“…It has been shown that the fatigue crack growth rate in the PWR water environment is closely correlated with the strain intensity factor range ΔK ε. 17,28,32 The following equation can be used for predicting the growth rate da / dN : where D p and m are material constants. Because fatigue life N f corresponds with the number of cycles necessary for a crack to reach the critical size for specimen failure, it can be predicted by integrating Equation 4 from initial depth a i to final depth a f .…”

Fatigue behavior of 316L austenitic stainless steel in air and LWR environment with and without mean stress

“…In JSME Code, no environmental effect is involved when the strain amplitude is equal or less than 0.11%. However, Kamaya [13] recently reported that the reduction of fatigue life of 316L steel in PWR water environment is not significant when the strain amplitudes are 0.25% and 0.22 %. He also observed that F en varies with strain range.…”