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

世纪建造的许多大型钢结构，其老化问题日益严重。进入世纪，人们要求在高质量维护的基础上尽可能长时间地使用这些旧结构。为了评估结构和设备的安全性和适用性，金属疲劳是一个关键的问题。特别是，超高周疲劳现象在近十年来已经被阐明，因此需要更多的研究活动来研究断裂机制和预防技术。本项目的目标是建立超声波疲劳试验方法，该方法适用于广泛的材料，使用我们最初开发的疲劳试验机，适用于低温区，以及超高周疲劳亚表面裂纹声发射检测方法的发展。对于前者，超声疲劳试验机已经准备就绪，并获得了奥氏体不锈钢的结果。评估各种材料的超高周疲劳特性是一个未来的项目。由于后者是一个非常困难的任务，尚未实现的表面下裂纹的检测技术。然而，使用光纤传感器的AE检测技术被应用于除亚表面裂纹检测之外的各种主题。光纤声发射技术在解决电厂水管堵水问题中取得了很好的效果。在此基础上，开展了从裂纹萌生到最终断裂的疲劳裂纹监测技术研究，这对完成本课题的原定目标具有重要意义。

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.