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Reliability of Components Operating under Creep- Fatigue Conditions

Vojdani, Ali | 2019

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 52382 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Farrahi, Gholamhossein; Wang, Bin; Mehmanparast, Ali
  7. Abstract:
  8. The probabilistic behavior and the reliability of high-temperature defective components under creep-fatigue conditions have been assessed, considering different sources of uncertainty in “material behavior”, “geometric parameters” and “operation conditions”. For this purpose, deterministic calculations of creep-fatigue crack growth have been derived based on the R5 procedure, and then these relationships have been used at the heart of the probabilistic assessments. One of the aims of this dissertation was to present a procedure for assessing the reliability of defective components operating under creep-fatigue conditions. This procedure has been implemented on a benchmark problem, i.e. pressurized cracked pipes. In this benchmark problem, by using FORM and sampling methods, different issues such as component reliability, basic system reliability (consisting of two components) and sensitivity analysis of random variables have been studied and discussed. Also, the effects of decreasing the scatter of random variables as well as the effect of changes in geometrical parameters on the reliability results have been investigated. Sensitivity analysis showed that the initial crack depth, a0 had a high importance measure in reliability analysis. It was also observed that the implemented importance sampling method in parallel system reliability analysis significantly reduced the computational costs compared to typical Monte-Carlo sampling method. In addition to the aforementioned problem, in the context of another benchmark problem (i.e. cracked plates), the influence of input parameters’ randomness on the uncertainty of creep-fatigue crack growth response has been studied. In this problem, probability distribution of various outputs such as stress intensity factors, propagated crack lengths (at surface and depth), and creep rupture life have been evaluated, using sampling method; and some results (crack growth) were compared with analytical and experimental results. The probability distribution of propagated cracks indicated that the uncertainty (standard deviation) at the cracks’ surface, ci was much greater than the uncertainty in the
    depth of the cracks, ai
  9. Keywords:
  10. Creep-fatigue Interaction ; Crack Growth ; Uncertainty ; Reliability ; Probabilistic Assessment ; Sensitivity Analysis

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