Browsing by Author "Snowden, KU"

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    Crack initiation and crack growth assessment of a high pressure steam chest
    (Elsevier, 2011-01) Payten, WM; Wei, T; Snowden, KU; Bendeich, PJ; Law, M; Charman, D
    Extensive cracking had occurred in a number of high pressure steam chests. An assessment was undertaken based on the R5 British Energy methodology to assess the components for both creep–fatigue damage initiation and crack growth analysis to determine fitness for purpose. The analysis determined that the remaining base rupture endurance life of the component was greater then 1 million hours, however, due to the start-up and shutdown ramp rates, creep–fatigue damage greater then unity has occurred leading to crack initiation in a number of locations. These cracks were confirmed during internal inspection of the steam chest. A subsequent crack growth analysis determined that the component could safely be returned to service for the expected future life of the station. © 2011, Elsevier Ltd.
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    Creep-fatigue prediction of low alloy ferritic steels using a strain energy based methodology
    (American Society of Mechanical Engineers (ASME), 2009-07-26) Payten, WM; Dean, DW; Snowden, KU
    The accumulation of creep-fatigue damage over time is the principal damage mechanism which will eventually lead to crack initiation in critical high temperature equipment. A model that calculates the creep damage under conditions of strain control has been developed that assumes on a macroscopic level that the energy dissipated in the material may be taken as a measure of the creep damage induced in the material. This then assumes that the creep damage is directly proportional to absorbed internal energy density. The model developed is derived from considerations of mechanistic cavity growth. The model makes use of already existing creep data and relatively easily determined fatigue data for estimation of life under non-steady state conditions. The predictions of the energy-density exhaustion approach are compared with the results of creep-fatigue tests on a low alloy ferritic steel 1/2Cr-1/2Mo-1/4V (CMV) and with creep-fatigue calculations using a number of current models. The predicted results of the energy-density model are found to have good correlation with the measured creep-fatigue lives.
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    Effect of repeated loadings on the stress relaxation properties of 2.25Cr-1Mo steel at 550°C and the influence on the Feltham 'a' and 'b' parameters.
    (Elsevier, 2010-05-25) Humphries, SR; Snowden, KU; Yeung, WY
    This paper describes studies undertaken to evaluate the stress relaxation behaviour of 2.25Cr–1Mo steel, a material used extensively in high temperature steam components. Repeated stress relaxation loadings were conducted on normalised and tempered 2.25Cr–1Mo steel at 550°C, under constant strain, in a closed loop testing system. Detailed analysis of stress relaxation curves is presented. Application of the data to the Feltham equation for subsequent use in life assessment procedures is discussed and analysed. © 2010, Elsevier Ltd.
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    Effects of prior stress relaxation on the prediction of creep life using time and strain based methods
    (ASME International, 2010-07-18) Payten, WM; Snowden, KU; Dean, DW; Humphries, SR; Edwards, L
    A critical requirement for both next generation conventional and nuclear plants is the development of simplified inelastic design and fitness for purposes procedures that give a reasonably accurate prediction of the complex multi-axial time dependent stress strain behavior. The accumulation of this inelastic strain in the form of coupled creep-fatigue damage over time is one of the principal damage mechanisms which will eventually lead to crack initiation in critical high temperature equipment. Two main procedures that address creep-fatigue loading are generally used, either a time fraction or a ductility exhaustion approach. It is generally accepted that these methods enable conservative predictions within a factor of 2 to 3 and hence are reliable methods for code based design and fitness for purpose type assessments. However, for complex cycles, this may not be the case, for example prior relaxation cycles are found to accelerate the creep rupture of the material with the result that a significant reduction in creep life can be observed. An investigation was undertaken into the influence of prior relaxation on resultant failure using a typical low alloy ferritic power station steel. Both time based and strain based methods were used to predict the damage caused by the stress relaxation cycles followed by operation at steady state. The predictions found that while ductility exhaustion methodologies based on mean properties where adequate in predicting the failure life, time fraction methods were found to be extremely non-conservative for mean properties and only lower bound solutions provided an estimate of remaining creep life. The Monkman-Grant approach resulted in predictions that erred on the conservative side. The results have implications for both current and future conventional and nuclear power stations as it may be difficult for time based approaches to accurately account for complex cycling, shakedown conditions or stress relaxation at welds. © 2013 by ASME
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    High temperature remaining life cost assessment
    (9th International Conference and Exhibition 2009 Operating Pressure Equipment Incorporating the AINDT Biennial Conference, 2009-08-26) Payten, WM; Bendeich, PJ; Snowden, KU
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    A strain energy based methodology for the prediction of creep fatigue damage in high temperature components
    (9th International Conference and Exhibition 2009 Operating Pressure Equipment Incorporating the AINDT Biennial Conference, 2009-08-26) Payten, WM; Dean, DW; Snowden, KU
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    A strain energy density method for the prediction of creep-fatigue damage in high temperature components
    (Elsevier, 2010-03-25) Payten, WM; Dean, DW; Snowden, KU
    The accumulation of creep–fatigue damage over time is the principal damage mechanism which will eventually lead to crack initiation in critical high temperature equipment. A model has been developed that assumes on a macroscopic level that the energy dissipated in the material may be taken as a measure of the creep damage induced in the material and hence the creep damage is directly proportional to absorbed internal energy density. The model developed is derived from considerations of mechanistic cavity growth and is based on rupture elongation to failure data using true strain. The predictions of the energy density exhaustion approach are compared with the results of creep–fatigue tests on low alloy ferritic steels. The predicted results of the energy density model are found to have good correlation with the measured creep–fatigue lives. © 2010, Elsevier Ltd.
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    Use of a simplified analytical expression for metastable thermal stress analysis and its application to creep-fatigue damage of a 2.25Cr 1Mo thick walled component
    (Elsevier, 2010-02) Payten, WM; Snowden, KU; Bendeich, PJ
    Thick walled pressure vessels are of considerable importance in a wide range of industries. The evaluation of stresses is necessary not only from a design point of view but also for fitness for service analysis of ageing infrastructure. The accumulation of creep–fatigue damage over time is the principal damage mechanism which will eventually lead to crack initiation in critical high temperature fossil plants. Many power stations are being subjected to two-shift operation due to changes in demand and competition from cheaper energy sources, and in the future from added carbon taxes. To assess high temperature components for creep–fatigue damage for example, under faster ramp rates and additional cycles, as a first pass it would be useful to explore the feasible operational envelope using simplified calculations. These are, however, generally not available and more complex finite element analysis is necessary. This paper uses a simplified closed form solution for metastable thermal stresses in thick walled pressure vessels. This form of solution can if necessary be used with either stress concentration factors or superposition of polynomials for more complex components derived from FEA analysis, such that the closed form solution can be used to estimate any ramp rate on the unit. In this case the ramp rates are considered to provide sufficient time to become metastable. Many existing units rely on heavy section 2.25Cr 1Mo steel (P22) pipe-work and tubing, and hence for two shifting can be subjected to high levels of cyclic strain. Based on the simplified expression developed, an operational envelope is explored for thick walled cylinders constructed using P22 steel. Creep–fatigue damage is calculated based on the R5 methodology. The analysis shows that for thick walled components with minimal stress concentrations, creep will dominate the life of the component. However, complex interaction between base rupture, onset of significant cycling, creep, and fatigue dictates the upper bound on feasible ramp rates, as a result it is possible to construct screening curves based on the effective elastic stress intensity range. © 2010, Elsevier Ltd.