Beam and Mode Analysis of Weak Bonding Flaws in a Layered Aluminum Plate

Location and identification of faults in multilayer elastic materials by ultrasound is aided by a physically based parametrization of the input, scattered and detected fields. When the transducer input is beam-shaped, the beam-to-mode conversion in the unflawed layered environment suggests a “good” parametrization in terms of a self-consistent hybrid beam-mode format. The scattered field produced by interaction of this beam-mode field with a fault zone should then be parametrized in a similar manner. This strategy guides the present investigation of a weak bonding flaw in a multilayer aluminum plate. The horizontal and vertical displacements excited by a high frequency two-dimensional dilatational (P) Gaussian input beam have previously been tracked through successive cross sections in the perfectly bonded material. The resulting displacement profiles reveal clearly the beam-like character near the source, the deterioration of the successively reflected beam due to P-SV coupling at the boundaries, and the eventual evolution of oscillatory mode-like patterns. This input is now allowed to interact with an elongated weak bond zone. The induced equivalent forcing terms are modeled in the Born approximation, and the scattered field is evaluated accordingly. Depending on the flaw size, its location relative to the input and output transducers, and other variables, the detected response at the plate surface may contain beam-like or mode-like features. The beam-like phenomena are explored here with a view toward finding conditions through which the physical observables that should facilitate flaw location and identification are enhanced. Although, for convenience, the numerical data have been generated by normal mode summation, the results reveal clearly that the hybrid beam-mode format, to be developed next, furnishes the proper parametrization.