ULTRASONIC TECHNIQUE TO MEASURE STIFFNESS COEFFICIENTS OF CMC AND ITSIMPLICATIONS ON CHARACTERIZING MATERIAL DEGRADATION

The main objectives of this research are to characterize the fiber con tent of composite materials using nondestructive methods, to character ize fiber, matrix and fiber-matrix interface degradation in a quantita tive manner and to predict the elastic behavior of the composite mater ial using ultrasonic techniques.We can achieve the above objectives if we can experimentally compute the stiffness matrix that can be derive d by either destructive or nondestructive methods. We focus on nondest ructive methods to generate the stiffness matrix of ceramic matrix com posites (CMC) using ultrasonic techniques. The use of ultrasonic waves in measurement of the dynamic elastic moduli of solids is well known (Truell et al. (1969). Ultrasonic Methods in Solid State Physics, Acad emic Press). If the density and elastic anisotropy of a solid are spec ified then the elastic moduli can be deduced from wave speed measureme nts of shear and longitudinal waves propagating in particular directio ns in the solid. The relations between wave speed and moduli follow fr om the theory of small-amplitude elastic wave propagating in an anisot ropic solid (Musgrave (1970). Crystal Acoustics, Holden-Day). In this paper, we will discuss the experiments conducted on three CMC (CAS-Nic alon) unidirectional blocks with varying fiber fractions estimated at 31, 42 and 51%, using ultrasonic longitudinal, transverse and surface acoustic wave (SAW) velocities. Techniques to improve and automate dat a acquisition are discussed along with the experimental results.