Minimum variance imaging in plates using guided wave mode beamforming

This paper presents improvements to ultrasonic imaging of solid plate-like structures using the minimum variance distortionless response (MVDR) beamforming processor. The primary application of this work is the nondestructive testing of plate-like 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 dynamic range and spatial resolution of the image. Additionally, 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 3D solids, the compounding of Lamb modes in plates increases the array gain without increasing its physical aperture.