The effect of creep strain rate on damage accumulation in type 316H austenitic stainless steel

Abstract

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.

Date of Award	25 Jun 2019
Original language	English
Awarding Institution	University of Bristol
Supervisor	Christopher E Truman (Supervisor) & Mahmoud Mostafavi (Supervisor)