Minimum variance guided wave imaging in a quasi-isotropic composite plate

Abstract

Ultrasonic guided waves are capable of rapidly interrogating large, plate-like structures forboth nondestructive evaluation and structural health monitoring (SHM) applications.Distributed sparse arrays of inexpensive piezoelectric transducers offer a cost-effective wayto automate the interrogation process. However, the sparse nature of the array limits theamount of information available for performing damage detection and localization.Minimum variance techniques have been incorporated into guided wave imaging to reducethe magnitude of imaging artifacts and improve the imaging performance for sparse arraySHM applications. The ability of these techniques to improve imaging performance isrelated to the accuracy of a priori model assumptions, such as scattering characteristics anddispersion. This paper reports the application of minimum variance imaging under slightlyinaccurate model assumptions, such as are expected in realistic environments.Specifically, the imaging algorithm assumes an isotropic, non-dispersive, single modepropagating environment with a scattering field independent of incident angle andfrequency. In actuality, the composite material considered here is not only slightlyanisotropic and dispersive but also supports multiple propagating modes, andadditionally, the scattering field is dependent on the incident angle, scattered angle,and frequency. An isotropic propagation velocity is estimated via calibrationprior to imaging to implement the non-dispersive model assumption. Imagingperformance is presented under these inaccurate assumptions to demonstrate therobustness of minimum variance imaging to common sources of imaging artifacts.