An Energy-Based Framework to Determine the Fatigue Strength and Fatigue Ductility Parameters for LCF/HCF Life Assessment of Turbine Materials

An energy-based framework is developed to determine the fatigue strength parameters of Basquin equation and the fatigue ductility parameters of Manson-Coffin equation to predict fatigue life of a steam turbine material. The proposed framework is based on assessing the complete energy necessary to cause fatigue failure of a material. This energy is considered as a fundamental material property and is known as fatigue toughness. As a first approximation, the fatigue toughness is equivalent to the monotonic tension energy of a material. This assumption was experimentally verified for weld metal of a weld joint constituent of a turbine rotor and for aluminum 6061-T6. However, in case of base metal of the weld joint constituent, the fatigue toughness was found to be higher than the monotonic energy. From the fatigue toughness and the experimentally determined fatigue life at two different stress levels, the cyclic parameters of Ramberg-Osgood constitutive relation that describes the hysteresis stress-strain loop were calculated. Next, the coefficients and exponents of Basquin and Manson-Coffin equations were expressed as functions of fatigue toughness and cyclic parameters of a material. The predicted fatigue life obtained from the present energy-based framework was found to be in a good agreement with the experimental data.