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A Thermo-Mechanical Fatigue Life Prediction Model for Aluminum Alloy (A356.0) with Thermal Barrier Coating

Azadi, Mohammad | 2013

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 44398 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Farrahi, Gholamhossein
  7. Abstract:
  8. Thermal barrier coating (TBC) systems have been widely used in gas turbines and diesel engines. Their advantages include reducing the fuel consumption, improving the performance and increasing the fatigue lifetime at high temperatures. In the present thesis, the fatigue behavior of an aluminum alloy (the A356.0 alloy), with and without a typical TBC system is studied. The application of this research can be introduced as coating the cylinder head in diesel engines. Initially, coating process parameters (including the feed rate and the nozzle distance) of the plasma spraying, with the objective of bending strength are optimized. The optimum value of the TBC thickness is obtained based on thermal shock fatigue (TSF) tests and the stress distribution by numerical simulations. Various models for the lifetime prediction of the A356.0 alloy, with and without coating layers are presented under low cycle fatigue (LCF) loadings at different temperatures and also out-of-phase thermo-mechanical fatigue (OP-TMF) loadings. A new fatigue lifetime prediction model, based on the plastic strain energy with a stress correction factor, a temperature effect term and a strain effect factor, is presented. To evaluate the accuracy, results (calculated lifetimes) of the present model are compared to other classical and energy criteria. According to this objective, room temperature (RT) and high temperature (HT) LCF and OP-TMF tests are carried out. Effects of the maximum temperature, the dwell time (or the hold time at the maximum temperature) and the thermo-mechanical loading factor (or the ratio of the mechanical strain to the thermal strain) on the OP-TMF lifetime of A356.0 alloy specimens, with and without coating layers, are investigated. Also, influences of the mechanical strain amplitude and the temperature on the LCF lifetime of A356.0 alloy specimens, with and without coating layers, are studied.
    Based on obtained results in this thesis, the value of the bending strength was maximized at 30 gr/min of the feed rate and 80 mm of the nozzle distance. The top coat layer with 300 µm thickness showed higher TSF lifetime. The LCF lifetime of the A356.0 aluminum alloy at 200°C, unlike the expectation, was more tham that one at the room temperature due to micro-strutural changes of the materail. The cycle number of the TMF lifetime in the aluminum alloy (with and without coating layers) was almost without changes by increasing the dwell time. Results of the fatigue lifetime prediction by the present energy model demonstrated that this new model had higher accuracy and lower scatter-band in comparison to other existed models. In addition, the number of material constants in the present model is lower that that one in Sehitoglu’s model
  9. Keywords:
  10. Aluminum Alloy ; Low Cycle Fatigue ; Thermomechanical Fatigue ; Thermal Barrier Coating ; Plastic Strain

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