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Title: The effect of creep strain rate on damage accumulation in type 316H austenitic stainless steel
Author: Hares, Edward
ISNI:       0000 0004 7968 2306
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2019
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With the constant growth in technology, there is an ever-growing demand on the power generation industry. This has made extending the life of conventional and nuclear power plants critical. Components operating at high temperatures can be subject to a variety of different loading conditions which include constant load creep, stress relaxation and creep-fatigue. This research highlights the impact these conditions have on the structural integrity of Type 316H stainless steel, which is a material commonly used to fabricate components in nuclear power plants e.g. pipes and other vessels. Laboratory experiments exploring different creep modes are typically conducted on uniaxial specimens. However, since service components in plants tend to experience multiaxial states of stress, the experimental and computational work reported within this thesis are predominantly on notched bar specimens. Adding a notch to standard specimens allows a multiaxial state of stress to be applied within standard uniaxial test rigs. It also allows creep failure data to be obtained more rapidly because of the increased stress concentration. The material used in this research was an ex-service austenitic stainless-steel Type 316H. Constant load creep, stress relaxation and creep-fatigue all result in an increase in creep strain within components. It has been postulated that the more slowly creep strain is accumulated the more damaging it can be to service components. This rather unintuitive postulation has been made due to failures occurring within components with low levels of creep strain that have been in operation for several decades. The damaging effect of creep strain can be assessed by conducting constant load creep tests comparing the creep strain on failure and time to rupture for a variety of different applied net section stresses. A range of net section stresses were tested and it was subsequently found that the greater the net section stress, the greater the creep strain on failure and the shorter the time to rupture. This showed that equal amounts of creep strain accumulated more slowly were more damaging to notched specimens under constant load creep conditions. The damaging effects of creep strain rate can be assessed in repeat relaxation tests with varying dwell lengths as short-term relaxation tests can isolate the effects of the rapid accumulation of creep strain and the longer-term dwell tests can isolate the effect of creep strain accumulated more II slowly. Creep fatigue experiments allow the effect of reverse plasticity on subsequent creep to be explored. Two different creep damage models were validated and assessed using the experimental data obtained in this work. The models were the "Spindler damage model" and the "Stress Modified Ductility Exhaustion" damage model. The Spindler damage model is a ductility exhaustion model where failure is deemed to have occurred when a finite limit to ductility is reached. The stress modified model is similar, but the ductility of the material is a function of the strain rate and applied stress. Once these models had been validated they were used to understand what was happening locally at the notch as this could not be monitored during testing for all the experiments. One significant aspect of the results obtained from this research is that they can contribute to decisions whether nuclear power plants' stainless-steel components service lives can be prolonged, and they allow for accurate predictions of when components will fail based on their creep strain history. The experimental results show that the material being tested has a strain rate dependent ductility (a given amount of creep strain is less damaging the faster it is accumulated). The results add further characterisation to a commonly used service material and validate existing creep damage models for use on this material. The novelty of this work is that results from laboratory and finite element experiments showed this material to exhibit a clear strain rate dependent ductility. All experiments conducted showed this material had an increased creep ductility at increased strain rates. The experimental methods used for conducting repeat relaxation and creep fatigue experiments of type 316H stainless steel were also novel.
Supervisor: Truman, Christopher ; Mostafavi, Mahmoud Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available