Detection of a Micro-crack on a Rotating Steel Shaft Using Noncontact Ultrasonic Modulation Measurement

Abstract

Detection of a micro-crack on a rotating structure is limited because conventional non-destructive testing (NDT) is developed based on contact-type transducers. By using air-coupled ultrasonic NDT system, micro-crack detection on a rotating steel shaft is studied. Through air-coupled ultrasonic system, ultrasonic-guided waves at two distinctive frequencies are generated. Then, their ultrasonic modulation at the sum and difference of input frequencies are generated due to micro-crack. Finally, the spectral correlation (SC) of ultrasonic modulation is computed for diagnosis of micro-crack. This is the first study for the detection of a micro-level fatigue crack on a rotating steel shaft using noncontact ultrasonic NDT system. The developed system is experimentally validated using the steel shaft through cyclic loading tests. At each cyclic loading case, the result was compared with conventional penetrant test and metallographic analysis.