Masao Sakamoto

High-Temperature Ultrasonic Fatigue Testing at 1000°C

A high-temperature ultrasonic fatigue testing system was developed to evaluate the gigacycle fatigue properties of single-crystal superalloys used in aircraft engine turbine blades. In this development, a commercial ultrasonic fatigue testing machine was considerably modified to achieve high-temperature fatigue testing. The developed system took account of temperature dependency of Youngs modulus, and also had a function to evaluate the Youngs modulus. In order to protect the testing system from the heat of a specimen, straight and round rods were inserted between the testing system and the specimen. Other modifications achieved accurate control of temperature, edge displacement and resonance frequency, which were necessary for accurate control of stress amplitude. The testing system was first applied to a heat-resistant steel at 650 °C to check its accuracy, and next to SC superalloy samples at 1000 °C. In the conventional fatigue tests on the heat-resistant steel, the results were coincident in a frequency range from 1 Hz to 800 Hz, suggesting that comparable results would be obtained in ultrasonic fatigue testing at 20 kHz. In case of the SC superalloy samples, conventional fatigue tests were conducted at only 10 Hz, so the frequency effects were not clarified. In both cases, ultrasonic fatigue testing showed good agreement with conventional fatigue testing. The accuracy of the developed system is therefore high, even at 1000 °C. In these results, the SC superalloys showed no fatigue limit, indicating gigacycle fatigue tests to be necessary.