ULTRASONIC MEASUREMENT OF THE THICKNESS, PHASE-VELOCITY, DENSITY OR ATTENUATION OF A THIN-VISCOELASTIC PLATE .1. THE FORWARD PROBLEM

The purpose of this two-part paper is to present a new technique for ultrasonic non-destructive evaluation (NDE) of any one of the four acoustical properties of a 'thin plate' (thickness, wave-speed, density or attenuation) assuming the other three are known. The technique utilizes plane longitudinal waves that are normally incident upon a viscoelastic plate immersed in an elastic fluid (water). By a 'thin plate' we mean a situation where two successive echoes from the specimen cannot be separated in the time domain. Part I is concerned with the forward problem. The through-transmission transfer function of a plate is examined in detail for a variety of engineering materials. The comparison between theory and experiment is found to be excellent except at a few frequencies; here, the discrepancy may be attributed to the (conjectured) excitation of the spurious leaky-lamb waves.