Axisymmetric Waves in Layered Anisotropic Fibers and Composites

The complicated morphology of the new generation of advanced fibrous composites gave further impetus to the study of the interaction of ultrasonic waves with multilayered concentric cylindrical systems. Typically, the fiber consists of a cylindrical core embedded in a cladding region followed by a distinct interface zone separating the fiber system from the host (matrix) region. In addition, the cladding region itself often consists of subregions which can be identified as distinct layers. Each individual layer can posses certain degree of microscopic anisotropy adding to the macroscopic anisotropy produced by the presence of layering and imperfect interfaces. Relatively few efforts have been spent upon the study of free and immersed homogeneous anisotropic rods [1–5]. These works are insufficient to model real situations encountered in materials characterization of advanced fibrous composites. In order to better model advanced fibrous composites at least three major effects need to be accounted for. These are the inhomogeneous nature of the structure as reflected in its multilayering, the inherent microscopic anisotropy of some of the constituents and finally the quality of the interfaces. In this paper we briefly describe a unified analytical treatment of wave propagation along the fiber direction of multilayered coaxial fibrous systems embedded in a host material. A more detailed discussion of this general treatment will be presented elsewhere [6]. Figure 1 shows typical geometric situations including (a) a single multilayered fiber, (b) a single multilayered fiber either immersed in an infinite fluid or embedded in an infinite solid, and an infinite composite material with periodically distributed multilayered fiber

Continuum Modeling of Ultrasonic Behavior in Fluid-Loaded Fibrous Composite Media with Applications to Ceramic and Metal Matrix Composites

Elastic wave propagation in fibrous composite materials has been the subject of numerous investigations in recent years. However, the morphology of fiber-reinforced composites can seriously complicate the calculation of their wave propagation properties. Since it is clearly not practical to attempt a solution of the completely general elastic-wave problem, most prior work [1–4] has employed various approximations to render the calculations tractable. Our own approach [5,6] to interacting continua offers an alternative procedure for modeling the response of composites, where in particular, a rational construction of the mixture momentum and constitutive-relation interaction terms is given. This theory leads to simple wave propagation equations which potentially contain the full influence of the microstructure, that is, the distribution of displacements and stresses within individual constituents of the composite

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In high temperature composites interphases are added between the fibers and the matrix to improve their thermal and mechanical properties. Chu and Rokhlin have determined interphasial elastic properties and interphasial damage for ceramic and intermetallic matrix composites by measuring bulk wave phase velocities along different directions in the composites [1, 2, 3, 4]. Effective interphasial moduli were found from velocity data using micromechanical models. More recently, fatigue damage in the fiber-matrix interphases in SiC/Ti-15-3 composites has been assessed using both the ultrasonic wave velocity and scattering (attenuation) measurements as functions of the stages of the fatigue life cycle [5, 6]. It has been found that the velocity and attenuation (scattering) of ultrasonic waves in these composites are very dependent on the interphase properties due to the large diameters of the fibers. On the other hand the frequency dependences of the wave velocity and attenuation have not been fully studied experimentally

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Ultrasonic techniques show great promise for nondestructive characterization of high temperature composites which are usually manufactured with specially designed fiber-matrix interphases. Ultrasonically measured composite elastic moduli are important mechanical characteristics, and the problem of interphase characterization is also often related to the measured composite moduli. The characterization of interphase is critical since the interphase transfers load from the fiber to the matrix and its properties significantly affects the overall mechanical performance of the composite. Chu and Rokhlin have recently developed methods to determine fiber-matrix interphase elastic properties from ultrasonically measured composite moduli using static micromechanical models [1, 2, 3, 4]. Since their method is based on measurements of ultrasonic wave velocities in different directions in the composite and on relating them to the static composite elastic moduli, error may be introduced in determining the static composite moduli from wave velocity data if dispersion is not negligible. We have performed experimental measurements and theoretical studies of dispersion and attenuation for waves propagating along and normal to fibers in a SiC/Ti unidirectional metal matrix composite [5, 6, 7]. In this paper we focus on the effect of fiber-induced dispersion on determination of the composite and fiber-matrix interphase moduli