A MODEL TO PREDICT THE ULTRASONIC ECHO RESPONSES OF SMALL TARGETS IN SOLIDS

A novel model is developed to predict the echo response of a small tar get at arbitrary position in the field of a circular transducer direct ly coupled to a solid. The waves incident on the target are calculated using an earlier model, which describes the field radiated into solid s in terms of plane and edge waves and allows for the existence of bot h compression and shear waves. The new model treats the target as a po int-like radiator with motion given by the normal component of the inc ident particle velocity. An expression for the overall echo impulse re sponse is developed by invoking the principle of reciprocity, assuming a single transducer is used as both emitter and receiver. Simplificat ions inherent in the model keep computational times very short but all ow predictions to be made with an accuracy adequate to show all of the major features of the responses. Comparisons of the predictions of th e model with experimental measurements of echo responses from 2-mm-dia m round-bottomed holes in steel show good agreement. The complicated m ultipulse structure of the echo response from even a point-like target is explained in terms of the scattering, mode conversion, and recepti on of plane and edge waves. The implications of the results for quanti tative nondestructive evaluation using ultrasound are discussed.