Browsing by Author "Hindley, Michael Philip"

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    Next generation high temperature gas reactors : a failure methodology for the design of nuclear graphite components
    (Stellenbosch : Stellenbosch University, 2015-03) Hindley, Michael Philip; Blaine, Deborah; Groenwold, A. A.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    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.