Investigations into the fatigue behaviour of nuclear grades of austenitic stainless steel

A combination of fractography, microstructural analysis and finite element modelling was used to investigate several topics relating to the fatigue of nuclear grades of austenitic stainless steel operating in both air and simulated PWR water environments. The work is broadly separated into four main categories. The first two involved analysing specimens from standard fatigue endurance tests using a wide range of microscopic techniques. The relevance and uses of a modern laser scanning confocal microscope are presented and the benefits of using such a technique are discussed. Methods for the automation of both striation counting procedures and hysteresis data analysis are described and the results are demonstrated. Finite element analyses were performed in order to develop the understanding of fatigue crack growth within standard cylindrical endurance specimens. A variety of different crack tip parameters were used in order to develop expressions for crack growth rates in terms of the strain intensity factor and the J-integral. The derived expressions were compared to the results of striation spacing measurements from multiple endurance specimens that were tested in both air and water environments. The expressions were used to perform back-fitting calculations on standard endurance curves in order to produce alternative curves representing the number of loading cycles to cause the initiation of short cracks with depths in the range of 0.25-0.5 mm. The effects of hold-times on the fatigue life of stainless steel endurance specimens were explored as part of the international AdFaM research programme. Results from the programme partners are presented which demonstrate the beneficial effects of static hold-times on extending the fatigue lifetime of specimens. A range of microstructural analyses were performed on test specimens and results are presented. No significant effects of hold-times on microstructure, crack growth rates or material hardness were found. Analysis of hysteresis data demonstrated an increase in the cyclic hardening and a decrease in the plastic strain range after a hold. From an analysis of the fatigue test results, it was concluded that hold-times affect the earliest stages of fatigue (nucleation and initiation), most likely due to the effects of strain ageing. Several possible explanations for the observed phenomenon of specimen shrinkage during static holds are presented and discussed, however no conclusive explanation was identified. Further work is identified that could lead to future improvements in the understanding of all areas of investigation that have been reported. Overall, the work reported here has helped to develop the understanding of fatigue behaviour and mechanisms in the materials of interest. This was done through investigations using a synergistic combination of microscopy and numerical modelling techniques.