Damage relaxation in cyclic strained stainless steel

Improved endurance in fatigue tests with intermediate holds and annealing to roughly simulate steady state normal operation between fatigue transients in nuclear power plant (NPP) components has been reported [1]. Similar effect is expected in plant components - e.g. in a pressurized water reactor (PWR) surge line or in pressurizer spray lines. Quantification of this effect is a topic of continued research on fatigue performance of niobium stabilized austenitic stainless steel. Holds affect cyclic stress strain response. Stress amplitude, tensile mean stress and apparent elastic modulus are increased immediately after a hold, while decreased by cycles in between. A decrease of volume is measured during hot holds at zero external stress and a stress is generated, when applying a constant temperature hold in strain control. This all suggest cyclic accumulation of lattice defects and recovery during holds. Recovery may occur through thermally activated dislocation migration together with diffusion, grouping and annihilation of lattice defects. A mechanism informed explanation and thermodynamic model is sought for to improve transferability of laboratory data to real plant components and accuracy of fatigue usage assessment. Anticipated mechanisms behind gradual changes in material responses are discussed in relation to quantitative effects of holds.