Minimum-Variance Imaging in Plates Using Guided-Wave-Mode Beamforming

This article presents improvements to ultrasonic imaging of solid platelike structures using the minimum-variance distortionless response (MVDR) beamforming processor. The primary application of this work is in the nondestructive testing (NDT) of platelike components that are widely used in aerospace, marine, and civil structures. The study proposes a new set of weights, or MVDR replica vectors, that are based on the physics of the propagating Lamb modes, including the symmetric mode S0, the antisymmetric mode A0, and the shear horizontal mode SH0. Numerical results show that these wave-mode weights, combined with geometrical spreading, improve the focus of the array by increasing the dynamic range and the spatial resolution of the image. In addition, quite dramatic improvements in image quality are achieved by combining, or compounding, the multiple Lamb modes naturally present in the plate in both transmission and reflection. As shown in recent work applied to bulk waves in 3-D solids, the compounding of Lamb modes in plates increases the array gain without increasing its physical aperture.