PhD Thesis, University of Florida, 2006
Aircraft Structural Safety: Effects of Explicit and Implicit Safety Measures and Uncertainty Reduction Mechanisms

    Aircraft structural safety is achieved by using different safety measures such as safety and knockdown factors, tests and redundancy. Safety factors or knockdown factors can be either explicit (e.g., load safety factor of 1.5) or implicit (e.g., conservative design decisions). Safety measures protect against uncertainties in loading, material and geometry properties along with uncertainties in structural modeling and analysis. The two main objectives of this dissertation are: (i) Analyzing and comparing the effectiveness of structural safety measures and their interaction. (ii) Allocating the resources for reducing uncertainties, instead of living with the uncertainties and allocating the resources for heavier structures for the given uncertainties.

    Certification tests are found to be most effective when error is large and variability is small. Certification testing is more effective for improving safety than increased safety factors, but it cannot compete with even a small reduction in errors. Variability reduction is even more effective than error reduction for our examples.

    The effects of structural element tests on reducing uncertainty and the optimal choice of additional knockdown factors are explored. We find that instead of using implicit knockdown factors based on worst-case scenarios (current practice), using test-dependent explicit knockdown factors may lead weight savings. Surprisingly, we find that a more conservative knockdown factor should be used if the failure stresses measured in tests exceeds predicted failure stresses in order to reduce the variability in knockdown factors generated by variability in material properties.

    Finally, we perform probabilistic optimization of a wing and tail system under limited statistical data for the stress distribution and show that the ratio of the probabilities of failure of the probabilistic design and deterministic design is not sensitive to errors in statistical data. We find that the deviation of the probabilistic design and deterministic design is a small perturbation, which can be achieved by a small redistribution of knockdown factors.

Supervisor: Prof. Dr. Raphael T. Haftka
Co-supervisor: Prof. Dr. Bhavani V. Sankar
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MS Thesis, Middle East Technical University, 2002
Thermo-Mechanical Fatigue Analysis of a Stationary Jet Engine Component

    In this thesis, thermo-mechanical fatigue life of a stationary component of F110-GE-100 jet engine is assessed. Two-dimensional axisymmetric finite element model of the component itself, the neighbouring components and also the surrounding gas are generated by using a finite element package program, MARC, in order to perform thermal, stress and fracture mechanics analyses. Thermal analysis is performed to calculate temperature histories of engine components throughout a given mission. Stress analysis is performed to identify fracture-critical locations and to describe stress histories of the components. After determining the critical location, fracture mechanics calculations are performed by modeling a crack of various lengths at the critical location in order to describe crack propagation path and to calculate mode I and II stress intensity factors. Combining the outputs of thermal, stress and fracture mechanics analyses, fatigue lives and creep rupture times are calculated with a crack growth life prediction program, AFGROW. A linear damage summation method is used to assess thermo-mechanical fatigue life of the component of interest.

    Keywords: Thermo-mechanical Fatigue, Creep, Fracture Mechanics, Finite Element Method, High Temperature, Life Assessment

Supervisor: Prof. Dr. Mehmet A. Akgn
Co-Supervisor: Prof. Dr. Mustafa Doruk
Thesis
Thesis Presentation