The fatigue degradation in Nuclear Power Plants (NPPs) is evaluated by a design fatigue curve that is published in ASME code which is based on the results of uniaxial fatigue test data. On the other hand, stress that occurs at actual piping elements, such as elbows, is not uniaxial stress but biaxial stress, with different stress ratios. Stress ratio is defined as the ratio of stress range in one axis to that in the other axis. In this study, a pressurized disc fatigue test machine is developed in order to conduct biaxial fatigue test under different stress ratio more easily and low-cost. A newly designed test specimen is adopted to generate biaxial stress under different stress ratios near the center of the specimen. This test machine can generate a specific biaxial stress with different stress ratios at specimen by adjusting air pressure and shape of the specimen.

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