Automatic Ultrasonic Testing of Non-metallic Inclusions Detectable with Size of Several Tens of Micrometers Using a Double Probe Technique along the Longitudinal Axis of a Small-diameter Bar

An improvement of automatic ultrasonic testing through a double probe technique along the longitudinal bar axis used in a round bar, with a diameter of several millimeters, is proposed. Non-metallic inclusions of several tens of micrometers in the cross-sectional length can be detected using this novel technique, whereas the detectability in a conventional normal beam technique is limited to 100-150 mu m. The main advantages of this technique are an increased working sensitivity owing to a decreased surface echo width and the use of a shear wave with a shorter wavelength as compared with a conventional normal beam technique. As another advantage of this technique, malfunctions caused by air bubbles in the coupling medium can be eliminated. Further, the beam paths of the surface echo and the bottom echo are discussed herein using the propagation time difference between both echoes in Appendix.