Defects Inspection in Wires by Nonlinear Ultrasonic-Guided Wave Generated by Electromagnetic Sensors

Featured Application The second harmonic measurement method using EMAT proposed in this paper has established a system that can detect microdefects in wires without destroying objects before wire processing. Steel wires are widely used as raw materials for spring valves in engines. Considering the quality and safety issues of their structure, there is a demand to develop nondestructive inspection approaches to detect initial damages in steel. In this study, nonlinear ultrasonic-guided waves generated by an electromagnetic acoustic transducer (EMAT) were used to inspect the defects in steel wires. As one of the noncontact testing methods, the use of EMAT has significant advantages to decrease the nonlinearity induced by instruments and transducer contact condition. The principles of design and manufacturing of EMAT are first introduced. The fundamental theory of nonlinear guided waves is also briefly discussed in this investigation. Phase-matched guided wave modes were generated and measured by using EMAT. Variations of acoustic nonlinearity corresponding to existing defects in specimens were obtained. A scanning electron microscope (SEM) was used to check the existence of microdefects in specimen. The results indicate that the use of EMAT can be an effective means to generate and measure nonlinear ultrasonic-guided waves for inspection of microdefects.