Ultrasonic nondestructive evaluation of thin viscoelastic plates : the inverse problem

Vita.The acoustic response of a think, linear-viscoelastic plate is completely described by four of its properties: thickness, wavespeed, density and attenuation. The qualifer "thin" is used to describe a situation when the duration of the incident pulse (or the length of the incident pulse) exceeds the round-trip time (or the twice the specimen thickness), i.e any two successive echoes, due to the incident pulse, from the front and the back surfaces of the plate are inseparable. In this dissertation, a fequency-domain technique for the ultrasonic nondestructive evaluation of thin plates is presented. The technique utilizes plane-longitudinal waves normally incident upon a viscoelastic plate immersed in an elastic fluid (water). The through-transmission transfer function of a plate is examined in detail for a variety of engineering materials. The theory and the experiment are compared for engineering materials over a wide range of frequencies. The comparison is found to be excellent except at certain frequencies; the discrepancy is attrbuted to (conjectured) excitation of the spurious leaky-Lamb waves. A systmeatic analysis of the sensitivity of the transfer function to each of the acoustical properties is presented. The inverse problem, defined as the determination of the acoustical properties from the comparison of the measured and predicted transfer functions, is solved for the following cases: (a) measurment of any one of the four acoustical properties, given the remaining three; the inversion algorithm utilizes the well-known secant method. (b) simultaneous measurements of two, three or all of the four acoustical properties; the inversion algorithm utilizes the Gauss method in conjunction with the Incremental search method. The technique is used to measure the acoustical properties of two classes of materials: thin elastic plates made up aluminum, stainless steel, glass, tungsten, and leaded-brass; and thin visocelastic plates made up of Plexiglas, graphite/epoxy composite and bulk aerospace adhesive. The errors are found to be frequency dependent and become progressively worse as frequency decreases. Finally, a new technique is developed for the simultaneous measurment of thickness and wavespeed in thin adhesive layers bonding a pair fo thick adherends. This technique is automated and computer-controlled and can be easily adapted for in-situ applications. Results are presented for aluminum-bonded joints and the properties of the cured bond are compared with that of the bulk adhesive