Methods of Inspection and Quantitative Evaluation of Creep-Fatigue Small Cracks at Elevated Temperature

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1. Fot the purpose of investigating the crack propagation behavior in thermal fatigue, crack propagation tests of varying temperature fatigue and of creep under stress relaxation conditions were conducted.2. Two types of LCF life laws were derived from the two types of crack propagation equations based on nonlinear fracture mechanics at elevated temperature.3. The mechanism and mechanics of creep-fatigue interaction in the crack propagation process were verified using high-sensitivity electric potential methods.4. Relationships between surface crack dimension and electric potential drop were obtained using wood metal plates and glass-made cracks of several kinds of shape.5. Measurements of the number and the length of small surface cracks of 304 stainless steel were done by means of image analysis, and the behavior of small crack initiation and early growth were observed continuously during creep-fatigue tests with a thigh-temperature microscope.6. Based on the above results, stochastic models of crack initiaction adn propagation in creep and creep-fatigue were developed. In order to deal with the scatter of small crack initiation time and propagation rate caused by microstructural inhomogeneity, a random variable technic was used in the models. Numerical simulation was conducted by Monte Carlo method.