Effects of Strain Holding and Continuously Changing Strain Rate on Fatigue Life Reduction of Structural Materials in Simulated LWR Water

Abstract

The fatigue life reduces remarkably with reduction in strain rate in simulated light water reactor (LWR) water but the effects of strain wave form on this reduction are still not clear. This paper provides fatigue life data obtained from stepwise strain rate change tests, sine wave tests and strain holding tests. The effects of varying strain rate on fatigue life reduction can be estimated very well by the modified rate approach (MRA) method in the case of the step wise strain rate changing as shown in authors’ previous papers [1, 2, 3, 4, 5]. In the case of sine wave, however, the fatigue life reduction is much less compared to that predicted by the MRA method. The mechanism of such difference is not clear and the quantitative assessment of the fatigue life reduction caused by irregular strain wave form in actual transient seems impossible. The current MRA method gives always conservative assessment for sine wave straining and thus it is judged that this method need not be revised any more. The fatigue life reduction caused by strain holding at the peak of straining cycle in simulated BWR water had been reported in the previous paper [6]. In actual thermal transients, however, strain is not usually held at the peak of straining cycle but at the point somewhat reduced from the peak after the stabilization of temperature. In considering this phenomenon, additional fatigue tests in which the strain was held at the point somewhat reduced from the peak were carried out. In such conditions, the fatigue life reduction caused by strain holding disappeared. The similar fatigue tests with peak strain holding were also carried out in simulated PWR water and no fatigue life reduction can be observed. Considering the effects of strain holding on fatigue, the model for evaluating fatigue life reduction in LWR water was revised.

Influence of Strain Rate and Waveshape on Environmentally-Assisted Cracking during Low-Cycle Fatigue of a 304L Austenitic Stainless Steel in a PWR Water Environment

“…The DSA phenomenon, in term of enhanced cyclic hardening, is thus particularly effective when the positive slow strain rate is applied in tension, i.e., with a positive deformation. This result is, however, not in agreement with the findings reported in [3] which indicate no difference in peak hardening in light water reactor (LWR) environment. Samples from interrupted tests were fatigued to failure under load control at high frequency.…”

“…In this context, among the various factors controlling the fatigue life in PWR medium, strain rate is extremely important. Indeed, most of the environmentally-assisted cracking mechanisms are time-dependent and, therefore, depend to some extent on the exposure duration [1][2][3][4]. Nevertheless, one cannot exclude complex interactions between the environmental exposure duration and the load signal waveform to account for damage mechanisms at the crack tip.…”

“…Typically, strain-control triangular signals are commonly applied to establish the data required in design codes. However, especially when investigating the very low strain rate domain, saw-tooth signals with a slow loading rate and a fast unloading rate are also considered in order to reduce the test duration, by assessing that the fatigue lives are only dependent of the rising load part duration [2][3][4]. However, this type of loading does not fully account for the actual loading conditions in industrial components.…”

Effect of material heat treatment on fatigue crack initiation in austenitic stainless steels in LWR environments.

“…The relevant fatigue ε-N data for austenitic SSs in air include the data compiled by Jaske and O'Donnell 16 for developing fatigue design criteria for pressure vessel alloys, the JNUFAD * database from Japan, and the results of Conway et al 17 and Keller. 18 In water, the existing fatigue ε-N data include the tests performed by General Electric Co. (GE) in a test loop at the Dresden 1 reactor, 19 the JNUFAD database, studies at Mitsubishi Heavy Industries, Ltd. (MHI), [20][21][22][23][24][25] Ishikawajima-Harima Heavy Industries Co. (IHI), 26,27 and Hitachi 28,29 in Japan, and the present work at ANL. [4][5][6][7][30][31][32]…”

Strain Rate Change From 0.04 to 0.004%/S in an Environmental Fatigue Test of Cf8m Cast Stainless Steel

“…In pressurized water reactors (PWRs), the life of pressure boundary components is significantly affected by environmental fatigue, and some findings confirm that their life can be remarkably decreased as the strain rate of fatigue loading decreases [1][2][3][4]. To estimate the effect of strain rate variation of cast stainless steel CF8M on fatigue life in the PWR environment, low-cycle environmental fatigue test were conducted by KEPCO Research Institute (KEPRI) [5].…”