Mode mixing in shear horizontal ultrasonic guided waves

SH guided waves are used increasingly for non-destructive testing (NDT) applications, particularly for pipes and pipe supports using circumferentially guided wave modes. In practical implementations, it is not always straightforward to ensure single-mode operation and this requires consideration when interpreting results. During shear horizontal (SH) wave generation or SH guided wave interaction with geometrical changes or defects, multiple SH guided wave modes may be produced, depending on the shear wave speed, the frequency of operation, the thickness of the sample and the transducer characteristics. This paper discusses the interference patterns created as the multiple SH modes mix (for both continuous tone generation and short bursts), and the problems caused by the interference patterns on applications such as NDT. In particular, the patterns can lead to defects being missed during an NDT inspection using SH waves, and a way to circumvent this problem is suggested.