Mode excitation and imaging by the radiation force of ultrasound

Abstract

A method for noncontact excitation and displaying the vibration mode shapes in solids is introduced. This method utilizes the radiation force of amplitude-modulated focused ultrasound to remotely excite a resonant mode in the object. The ultrasound transducer is driven by a sinusoidally modulated continuous wave signal to produce an oscillatory radiation force on the object and drive it into one of its natural resonance modes. The acoustic field resulting from object vibration is detected by a hydrophone. A theoretical model has been developed that describes the relationship between the force, mode shape, and the acoustic field. It is shown that the acoustic field amplitude is proportional to the value of the mode shape at the focal point of the ultrasound beam. By scanning the ultrasound beam across the object it is possible to map the acoustic data into an image that represents the mode shape of the object. Experiments have been conducted on small steel, aluminum, and glass beams. The first five resonant modes were excited and imaged by the present method. Experimental results have shown remarkable agreement with the theory and computer simulation. Beam deflections in the order of tens of nanometers can be detected by this method.