Thesis (PhD)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: This thesis presents a failure evaluation methodology for nuclear graphite
components used in high-temperature gas reactors. The failure methodology is
aimed at predicting the failure of real parts based on the mechanical testing
results of material specimens. The method is a statistical failure methodology for
calculating the probability of failure of graphite components, and has been
developed and implemented numerically in conjunction with a finite element
analysis. Therefore, it can be used on any geometry and load configuration that
can be modelled using finite element analysis.
The methodology is demonstrated by mechanical testing of NBG-18 nuclear
grade graphite specimens with varying geometries under various loading
conditions. Some tests were developed as an extension of the material
characterisation, specifically engineered to assess the effect of stress
concentrations on the failure of NBG-18 components.
Two relevant statistical distribution functions, a normal distribution and a twoparameter
Weibull distribution are fitted to the experimental material strength
data for NBG-18 nuclear graphite. Furthermore, the experimental data are
normalised for ease of comparison and combined into one representative data
set. The combined data set passes a goodness-of-fit test which implies the
mechanism of failure is similar between data sets.
A three-parameter Weibull fit to the tensile strength data is only used in order to
predict the failure of independent problems according to the statistical failure
methodology. The analysis of the experimental results and a discussion of the
accuracy of the failure prediction methodology are presented. The data is
analysed at median failure load prediction as well as at lower probabilities of
failure.
This methodology is based on the existence of a “link volume”, a volume of
material in a weakest link methodology defined in terms of two grouping criteria.
The process for approximating the optimal size of a link volume required for the
weakest link failure calculation in NBG-18 nuclear graphite is demonstrated. The
influence of the two grouping criteria on the failure load prediction is evaluated. A
detailed evaluation of the failure prediction for each test case is performed for all
proposed link volumes. From the investigation, recommended link volumes for
NBG-18 are given for an accurate or conservative failure prediction.
Furthermore, failure prediction of a full-sized specimen test is designed to
simulate the failure condition which would be encountered if the reactor is
evaluated independently. Three specimens are tested and evaluated against the
predicted failure. Failure of the full-size component is predicted realistically but
conservatively. The predicted failure using link volume values for the test rig
design is 20% conservative. The methodology is based on the Weibull weakest
link method which is inherently volume dependent. Consequently, the
conservatism shows that the methodology has volume dependency as
experienced in the classic Weibull theory but to a far lesser extent. / AFRIKAANSE OPSOMMING: Hierdie tesis beskryf ‘n metode wat gebruik kan word om falings in kern grafiet
komponente te voorspel. Hierdie komponente word in hoë temperatuur gas
reaktore gebruik. Die falings metodologie beoog om die falings van regte
komponente te voorspel wat gebaseer is op meganiese toets resultate van
materiaal monsters. Dit is ‘n statistiese falings metodologie wat die
waarskynlikheid van faling vir grafiet komponente bereken. Die metode is
numeries ontwikkel en geïmplementeer deur middel van die eindige element
metode, dus kan die metodologie toegepas word op enige geometrie en
belastingsgeval wat dan gemodelleer kan word deur gebruik te maak van eindige
element metodes.
Die metodologie word gedemonstreer deur gebruik te maak van NBG-18 kern
grafiet toets monsters. Sommige van hierdie toetse is ontwikkel as ‘n uitbreiding
van die materiaal karakterisering wat spesifiek ontwerp is om die effek van die
spannings konsentrasies op die faling van die NBG-18 komponente te evalueer.
Twee relevante statistiese verspreiding funksies word gekoppel aan die
eksperimentele sterkte data van die NBG-18 kern grafiet, naamlik ‘n normale
verspreiding en ‘n twee-parameter Weibull verspreiding. Die data stelle word ook
genormaliseer vir gemak van vergelyking en gekombineer in een
verteenwoordigende data stel. Die gekombineerde data stel slaag ‘n korrelasie
toets wat impliseer dat die meganisme van faling soortgelyk is tussen die data
stelle.
‘n Drie-parameter Weibull korrelasie op die trek toets monsters word gebruik vir
die statistiese falings metodologie. Die analise van die eksperimentele resultate
sowel as ‘n bespreking van die akkuraatheid van die faling voorspelling
metodologie word voorgelê. Die data word geanaliseer by gemiddelde faling
voorspelling asook by laer voorspellings van falings. Hierdie metode is gebaseer
op die bestaan van ‘n “ketting volume” wat die volume van ‘n materiaal wat
gebruik word in die swakste ketting voorstel en koppel aan die metodologie.
‘n Metode vir die benadering van die ketting volume word voorgestel en
daaropeenvolgend gebruik om die ketting volume te bereken vir NBG-18. ‘n
Gedetailleerde evaluasie van die falings voorspelling vir elke toets geval word
uitgevoer vir die voorgestelde ketting volumes. Gebaseer op hierdie ondersoek is
voorgestelde ketting volumes vir NBG-18 gegee vir beide akkurate en
konserwatiewe falings voorspellings.
Verder was ‘n volgrootte strukturele toets ontwikkel om dieselfde falings
omstandighede te simuleer wat verwag is gedurende normale werking van die
reaktor. Drie monsters word getoets en geëvalueer teen die voorspelde faling vir
beide die berekende ketting volume groottes. Faling van die volgrootte
komponente word realisties asook konserwatief voorspel. Die voorpselling is 20%
konserwatief. Die metodologie is gebaseer op die Weibull metode wat inherent
volume afhanklik is; gevolglik dui die konserwatisme aan dat die metodologie oor volume afhanklikheid beskik soos ondervind word in die klassieke Weibull teorie,
maar tot ‘n baie kleiner mate.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/97071 |
| Date | 03 1900 |
| Creators | Hindley, Michael Philip |
| Contributors | Blaine, D. C., Groenwold, A. A., Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 158 pages : illustrations |
| Rights | Stellenbosch University |
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