Ultrasonic technique to measure stiffness coefficients of CMC and its implications on characterizing material degradation

The main objectives of this research are to characterize the fiber content of composite materials using nondestructive methods, to characterize fiber, matrix and fiber-matrix interface degradation in a quantitative manner and to predict the elastic behavior of the composite material using ultrasonic techniques.

We can achieve the above objectives if we can experimentally compute the stiffness matrix that can be derived by either destructive or nondestructive methods. We focus on nondestructive methods to generate the stiffness matrix of ceramic matrix composites (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, Academic Press). If the density and elastic anisotropy of a solid are specified then the elastic moduli can be deduced from wave speed measurements of shear and longitudinal waves propagating in particular directions in the solid. The relations between wave speed and moduli follow from the theory of small amplitude elastic wave propagating in an anisotropic solid (Musgrave (1970). Crystal Acoustics, Holden-Day).

In this paper, we will discuss the experiments conducted on three CMC (CAS-Nicalon) 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 data acquisition are discussed along with the experimental results.