Assessment of damage under combined low and high cycle fatigue loading at high temperatures

The present study aims at investigating the response of a type 316LN austenitic stainless steel (the primary structural material for the core components of SFRs) to simultaneous application of LCF and HCF cycles. All the tests were carried out over a wide range of temperature from ambient to 923 K, at three different strain amplitudes (±0.25%, ±0.4 and ±0.6%). One LCF (major) cycle, together with a specific number of superimposed HCF (minor) cycles, is referred to as a 'block'. Cyclic life was found to be governed by LCF strain amplitude, block size and temperature. For a given strain amplitude, the cyclic life decreased when the block size is increased, due to an increase in the HCF damage. This phenomenon was more prominent with the increase in temperature from 300 to 923 K as multiple damage modes like ratcheting and creep get activated at higher temperatures, apart from LCF. An anomalous behavior in terms of saturation in cyclic life with block size as well as temperature was observed at 823-873 K which is attributed to the strong presence of dynamic strain aging (DSA) in the above temperature range. DSA suppressed stress-dependent damage (ratcheting) originating from the HCF cycling within the block, enabling LCF to be the major damage contributor in this case. (author)