I. Introduction
With the development of rail transport, more and more high-speed trains are being put into service. On high-speed trains, bolts are much widely applied to transfer, connect and fasten many kinds of parts. but because of the complex running environment, bolt defects occur from time to time, threatening the safety and stability of high-speed trains. Figure 1 shows the normal bolts and missing bolts on high-speed train. In addition, the scattered distribution and tiny size of these parts makes detection of defects difficult.1