Ultrasonic Examination – Ultrasonic Transducers
Ultrasonic waves for UT are created in a metal workpiece with a piezoelectric transducer that is coupled to the workpiece. The piezoelectric transducer converts high frequency electrical impulses into correspondingly high frequency mechanical vibrations that induce ultrasonic waves in the workpiece. For detection of reflected waves, the piezoelectric transducer operates opposite to the manner in which it creates ultrasonic waves, by converting the reflected stress (ultrasonic) waves to electrical impulses. The transducers are moved over the surface of the workpiece, usually manually, but automated scanning devices can be used.
UT with a longitudinal ultrasonic wave that is introduced into the workpiece perpendicular to the surface of the workpiece is referred to as “longitudinal wave UT.” A typical transducer for longitudinal wave UT is illustrated in Figure 700-13. Movement of the transducer over the surface of the workpiece would subject it to abrasion, and, therefore, a wear-resistant faceplate is normally provided. Except for this relatively thin faceplate, the piezoelectric element in the transducer directly induces the ultrasonic wave into the workpiece perpendicular to the surface without passing through another medium. Therefore, there is no mode conversion of the ultrasonic wave at the surface of the workpiece.
UT with an ultrasonic wave introduced into the workpiece at an angle is usually referred to as “shear wave UT.” A typical transducer for shear wave UT is illustrated in Figure 700-14. The piezoelectric element in the transducer is oriented at an angle to the surface of the work piece by interposing a wedge between the element and the workpiece. The piezoelectric element induces a longitudinal ultrasonic wave in the wedge, which is commonly manufactured from lucite. The lucite wedge has a significantly different velocity of sound than the metal workpiece, and, therefore, mode conversion of the longitudinal wave propagating through the wedge occurs at the interface with the workpiece. The angle of the wedge is most often selected to obtain a shear wave propagating through the workpiece at 45 degrees to the surface. However, shear waves at angles of 60 degrees and 70 degrees are also used, depending on orientation of the flaws to increase the intensity of reflected waves (echoes) to maximize the sensitivity for detecting small flaws. It is advisable to perform shear wave UT at two or more angles when the orientation of flaws is uncertain or random.
Categories: In-Service Inspection | Tags: Transducers, Ultrasonic | Leave a comment