Development of New System for Delayed Fracture Testing of Coil Springs by Using Acoustic Emission.

Delayed fracture sensitivity is one of the most important problems for strengthening the spring steels. Recently, some papers are reported to improve the delayed fracture life by adding the alloy elements such as Vanadium and Boron or by refining the grain size. But, in usual case, delayed fracture properties are only evaluated by the total life for fracture and there are few studies on initiation of crack and its propagation. AE (Acoustic Emission) method is already well known to detect the initiation of (initial) crack of structural materials. Hence, we developed a new system for testing the delayed fracture properties of coil springs by AE method. In this study, we clarified the superiority of this new method by the fact that the location of initial crack and its propagation of coil springs are detected in detail by analyzing AE signals. Using this method, we researched the effects of hardness distribution on the behavior of delayed fracture. The incubation time of initial crack occurrence and its frequency of events vary depending on the hardness distribution of coil spring, especially on surface hardness. Many events of AE signals in lower hardness of coil springs were considered to be related with the depth of quasi-cleavage fracture region detected by scanning electron microscope.