REMOTE MEASUREMENT OF THE ELASTIC PARAMETERS BY ULTRASOUND- STIMULATED VIBRO-ACOUSTIC SPECTROGRAPHY M. Fatemi and J. F. Greenleaf Ultrasound Research Laboratory Department of Physiology and Biophysics Mayo Clinic and Mayo Foundation Rochester, MN 55905 INTRODUCTION Estimation of the Young's modulus of metals based on the resonance frequency of a given structure has been reported previously. In some of these methods, an electromagnetic or piezoelectric actuator has been used to induce resonance in the structure [1]. The resulting response could be measured by different means, for example by a piezoelectric device. In most cases either the excitation and/or detection require some form of contact with the structure. In many applications such a contact is not desirable, either because of its loading effect or limitation in accessing the object, especially when the object under test is too small for such measurements. Here, we present a remote and contactless method for measuring the Young's modulus of a metallic rod. This method, called Ultrasound-Stimulated Vibro-Acoustic Spectrography (USVAS) [2], utilizes a highly localized radiation force to excite the object and measures the frequency response of this object via a remote microphone. The resonance frequency is determined from the frequency response data. This method provides high spatial definition because it is capable of exerting an oscillatory force in a very small region of the object. METHOD AND MATERIALS Ultrasound-Stimulated Vibro-Acoustic Spectrography (USVAS) When a collimated ultrasound beam impinges normally on the surface of an object, it will produce a radiation force on the object in the beam direction. This force is proportional to the time-average energy density of the incident wave [3]. USVAS employs Review o/Progress in Quantitative Nondestructive Evaluation. Vol. 18 Edited by Thompson and Chimenti, Kluwer Academic/Plenum Publishers, 1999 1703