Introduction

Multiple element array transducers are extremely useful and popular in today’s inspection environment. The first applications were to carry out electronic B and C scans opposed to earlier developed mechanical scans. This was followed by phasing of the elements in a bulk wave problem where beam steering and focusing were possible during the electronic scanning process. Today, array transducers are being used in guided wave inspection. Linear comb and annular array sensors are two possibilities. Beyond electronic scanning and focusing it is also now possible to select time delay profiles for guided wave mode and frequency selection to optimize sensitivity to certain defects and penetration power in special situations. Thus, time delays for mode selection and electronic scanning can be superimposed for rapid and efficient NDT and SHM. As a consequence, such items as the excitation spectrum and the mode excitability function will be studied along with phasing principles for linear combs and annular arrays.

To employ guided waves for nondestructive evaluation (NDE) or structural health monitoring (SHM) purposes, these waves must first be generated in the structure of interest. Accordingly, to fully reap the benefits guided waves can offer, such wave generation should be performed in a well-designed, highly controlled manner, which is only possible through deliberate transducer design. Proper guided wave mode control can provide distinct advantages in terms of sensitivity to particular defects, sensitivity to environmental variables, penetration power, and other factors in guided wave inspection. Additionally, the suppression of spurious modes and/or the excitation of a particularly nondispersive mode can greatly enhance the potential signal analysis of gathered data by simplifying the waveforms.