Development of AE system for detecting sub-surface crack due to very high cycle fatigue at reduced temperature

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Aging problems are becoming serious for many of the large scale steel structures constructed in the 20th century. In the 21st century, it is required to use these old structures as long as possible based on high quality maintenance. In order to evaluate the integrity of structures and devices with respect to safety and fitness for service, metal fatigue is a critical subject. Especially, phenomenon of very high cycle fatigue has been clarified in this decade, and thus more research activities are required for fracture mechanisms and prevention techniques.The objectives of this project are the establishment of ultrasonic fatigue testing method available for a wide range of materials using our originally developed fatigue testing machine, which is applicable for low temperature regime, as well as the development of detecting method for sub-surface crack during very high cycle fatigue tests using acoustic emission (AE). For the former, ultrasonic fatigue testing machine had been ready and the results were obtained for austenitic stainless steel. It is a future projects to evaluate very high cycle fatigue characteristics for wide variety of materials. Since the latter is a very difficult task, detecting technology of sub-surface crack has not been achieved yet. However, the AE detecting technology using optical fiber sensor was applied for various subjects except the sub-surface crack detection. The optical fiber AE technique was very effective for water blockage of water pipes of power generation plant. Besides these research works, fatigue crack monitoring technique from the crack initiation to final fracture was investigated, because it is important to complete the original objectives of the project.