1 ANISOTROPIC DAMAGE MODELLING OF COMPOSITE MATERIALS USING ULTRASONIC STIFFNESS MATRIX MEASUREMENTS A. Paipetis 1,2 , Y. Z. Pappas 1 , D.E. Vlachos 1 and V. Kostopoulos 1 1 Applied Mechanics Laboratory, Dept of Mechanical & Aeronautical Engineering, University of Patras 2 Marine Materials Technology Dept, Hellenic Naval Academy ABSTRACT The monitoring of the elastic properties of Al 2 O 3 /Al 2 O 3 composite material during the exposure at high temperature environment that simulates the working conditions of a gas turbine has been performed non- destructively using ultrasonics. The method is based on velocity measurements of the elastic waves that propagate in an orthotropic medium. These were estimated experimentally using a custom pulser-receiver setup which allows control of the angle of the incident pulse on the sample, while the latter is immersed in a water bath. The derivation of the elastic constants in order to reproduce the stiffness matrix of the composite is an inverse wave propagation problem described by the Christoffel equation. The damage initiation and propagation as depicted by the deterioration of the moduli of the material was described using deterministic and stochastic approaches. Finally, the damage accumulation process was simulated as a Markov process. 1.INTRODUCTION The unique properties of continuous fibre ceramic matrix composites are attributed to the combination of enhanced structural integrity and high temperature (HT) stability [1,2]. In this way, they offer the best alternative to other exotic materials such as superalloys, which are now very close to their physical limits. The increased high temperature performance, the modular reinforcement design as the tailoring of their microstructure allows their use in high-end applications [3], [4-8]. In this work the performance of Oxide-Oxide (Al 2 O 3 /Al 2 O 3 ) composites is studied. Oxide-Oxide (Al 2 O 3 /Al 2 O 3 ) composites offer high temperature stability in oxidising environments up to 1200 ˚C [9]. The study involves the non-destructive evaluation of the stiffness of the composite during thermal loading, simulating the service environment of the composite. The stiffness matrix was constructed using the ultrasonic technique [10, 11]. The procedure is based on the measurement of the time difference for the longitudinal (QL) and one or both transverse waves (QT), and is only valid when the QL and the QT waves are appropriately separated [12]. The subsequent numerical solution of the inverse scattering problem yields the stiffness values [11]. Stiffness properties were monitored in all principal directions in order to monitor anisotropic damage development. This is crucial in materials possessing limited degree of symmetry, such as fibrous composites, as the material anisotropy is expected to change according to the material loading history [13]. Exponential anisotropic damage functions were developed to describe the deterioration of the composite properties [14]. Additionally a stochastic damage accumulation model is employed using Weibull statistics [15] and discrete time Markov chain models to yield service life probability distributions [16]. A Markov process is defined as a process that depends on its current state and its transition to future state and is independent of its past. Finally, a simulation of the degradation path is performed assuming a discrete Markov process with deterministic or stochastic parameters [17].