A non-contact technique for evaluation of elastic structures at large stand-off distances: Applications to classification of fluids in steel vessels

A novel technique for non-contact evaluation of structures in air at large stand-off distances (on the order of several meters) has been developed. It utilizes a recently constructed air-coupled, parametric acoustic array to excite the resonance vibrations of elastic, fluid-filled vessels. The parametric array is advantageous for NDE applications in that it is capable of producing a much narrower beamwidth and broader bandwidth than typical devices that operate under linear acoustic principles. In the present experiments, the array operates at a carrier frequency of 217 kHz, and the sound field several meters from the source is described spectrally by the envelope of the drive voltage. An operating bandwidth of more than 25 kHz at a center frequency of 15 kHz is demonstrated. For the present application, the array is used to excite vibrations of fluid-filled, steel containers at stand-off distances of greater than 3 m. The vibratory response of a container is detected with a laser vibrometer in a monostatic configuration with the acoustic source. By analyzing the change in the response of the lowest order, antisymmetric Lamb wave as the interior fluid loading conditions of the container are changed, the fluid contained within the steel vessel is classified.