发电机护环在高转速、高应力状态下运行。发电机轴颈与护环属紧配合结构，由于发电机轴颈硬度低，发电机护环硬度高，频繁拆装使发电机轴颈与护环结合部位产生永久变形使预紧应力降低，从而产生松动，导致易产生疲劳裂纹，超声波检测时护环两端头产生的裂纹回波与轴颈结合面产生固有回波处在相同位置，且互相混淆不易检出。采用双晶收发探头检验方法提取了发电机护环两端头产生的裂纹动态波。在试块上试验可发现0.5 mm线切割槽的动态波形；经现场测试发现了甘肃某厂3号机两只护环上的裂纹，证明了此方法的可行性。

Generator retaining rings operate under the condition of high speed and stress. Due to the different hardness of generator journal and generator retaining rings as well as permanent deformation caused by frequent disassembling, the pre-tightened stress of the originally close-connected structure between generator journal and generator retaining ring is reduced dramatically. The reduction will lead to the looseness of connection. Moreover, the looseness makes it easy to produce fatigue cracks. The cracks are difficult to detect because the echoes by them are in the almost same position with the echoes by the joint surface. This article adopts the double-crystal transceiver probe detection method to collect the dynamic echoes of cracks produced at both ends of the generator retaining rings. The method has detected the dynamic echoes of the 0.5 mm Linear cutting groove and two cracks on the No.3 generator retaining ring in a factory province, which has proved the feasibility of the method.