Assessment of damage in ceramics and ceramic matrix composites using ultrasonic techniques

This paper addresses the application of ultrasonic sensing to damage assessment in ceramics and ceramic matrix composites. It focuses on damage caused by thermal shock or oxidation at elevated temperatures, which often results in elastic anisotropy. This damaged-induced anisotropy is determined by measuring the velocities of ultrasonic waves in different propagation directions. Thermal shock damage is assessed in ceramic samples of reaction bonded silicon nitride (RBSN). Thermal shock treatment from different temperatures up to 1000 C is applied to produce the microcracks. Results indicate that most microcracks produced by thermal shock are located near sample surfaces. Ultrasonic measurements using the surface wave method are found to correlate well with measurements of degradation of mechanical properties obtained independently by other authors using destructive methods. Oxidation damage is assessed in silicon carbide fiber/reaction bonded silicon nitride matrix (SCS-6/RBSN) composites. The oxidation is done by exposing the samples in a flowing oxygen environment at elevated temperatures, up to 1400 C, for 100 hr. The Youngs' modulus in the fiber direction as obtained from ultrasonic measurements decreases significantly at 600 C but retains its original value at temperatures above 1200 C. This agrees well with the results of destructive tests by other authors. On the other hand, the transverse moduli obtained from ultrasonic measurements decrease continually until 1200 C. Measurements on the shear stiffnesses show behavior similar to the transverse moduli. The results of this work show that the damage-induced anisotropy in both ceramics and ceramic matrix composites can be determined successfully by ultrasonic methods. This suggests the possibility of assessing damage severity using ultrasonic techniques

RECENT ADVANCES IN WAVES IN LAYERED MEDIA

The topic of wave propagation in layered media has for a long time been of major interest in underwater and geophysical acoustics. Later on interest in this field arose due to needs in acousto-microelectronics, mainly surface acoustic wave devices, nondestructive evaluation of composite and layered materials and oil exploration. New applications have presented new challenging theoretical and experimental problems. One trend is to describe more complicated media including anisotropy, nonhomogeneity, imperfect interfaces, and wave diffraction on inhomogeneities ; a second trend is related to solutions of the inverse problem, in which properties of the media should be determined from the wave signature. In this paper we review recent developments in theoretical and experimental studies of elastic wave propagation in layered media with applications to ultrasonic evaluation of materials. We will focus mainly on anisotropy and media property reconstruction. We will also discuss the boundary conditioned for the imperfect interface, which may involve partial contact or other interface inhomogeneities. Such generalized boundary conditions can be systematically incorporated in the transfer matrix approach.La propagation d'ondes en milieu stratifié est depuis longtemps un sujet d'étude majeure dans les domaines d'acoustique sous-marine et géophysique. L'intérêt dans ce champs d'étude s'est depuis lors étendu aux domaines de l'acousto-microélectronique, principalement relié aux dispositifs générateur d'ondes acoustiques de surface, de l'évaluation non-destructive des matériaux composites et stratifiés, et de l'exploration des champs pétrolifères. Ces nouvelles applications ont introduit de nouveaux problèmes tant théoriques qu'expérimentaux. Une solution possible est de décrire ce milieu complexe incluant anisotropie, non-homogénéité, interfaces imparfaites, et la diffraction d'ondes sur les inhomogénéités ; une seconde solution est reliée aux solutions du problème inverse dans lequel les propriétés du milieu étudié sont déterminées à partir des données recueillies. Dans cette étude, nous révisons les plus récents développements expérimentaux et théoriques dans le domaine de la propagation d'ondes acoustiques en milieu stratifié appliqués à l'évaluation des matériaux. Le focus de notre étude porte sur la reconstruction des propriétés et de l'anisotropie du milieu. Nous allons de plus discuter les conditions de limite pour une interface imparfaite, ceci peut impliquer un contact partiel ou autre inhomogénéités à l'interface. Ces conditions de limite généralisées peuvent être incorporées systématiquement dans l'approche de matrice de transfert