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

在20世纪建造的许多大型钢结构中，老化问题日益严重。在21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在安全性和适用性方面的完整性，金属疲劳是一个关键课题。特别是近十年来，超高周疲劳现象已被阐明，断裂机理和预防技术需要更多的研究活动。本项目的目标是利用我们自主研发的适用于低温条件下的疲劳试验机，建立适用于多种材料的超声疲劳试验方法，以及开发利用声发射(AE)进行超高周疲劳试验中亚表面裂纹的检测方法。对于前者，已配备了超声波疲劳试验机，并对奥氏体不锈钢进行了疲劳试验。评估各种材料的高周疲劳特性是一个很有前途的课题。由于后者是一项非常困难的任务，目前还没有实现亚表面裂纹的检测技术。然而，除了亚表面裂纹的检测外，光纤传感器的声发射检测技术被广泛应用于各个学科。光纤声发射技术是解决电厂给水管道堵水的有效方法。除了这些研究工作外，还研究了从裂纹萌生到最终断裂的疲劳裂纹监测技术，因为这对完成项目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.