Elastic constants and internal friction of an SiC-fiber-reinforced Ti-alloy-matrix crossply composite: Measurement and theory

The complete elastic-constant tenser C-ij and the corresponding internal friction Q(ij)(-1) for a cross-ply composite are measured. It consists of silicon-carbide fibers in a Ti-6Al-4V matrix; it contained 35 vol.% fibers. With tetragonal symmetry, this composite possesses six independent C-ij and Q(ij)(-1). Principally, two measurement methods were used: noncontacting and point-contact resonance-ultrasound spectroscopy. Specimens were rectangular parallelepipeds measuring a few to several millimeters. By changing the magnetic-field direction, which is needed for the Lorentz-force coupling, a particular macroscopic-vibration-mode group among the eight groups for free vibration of a rectangular parallelepiped could be independently excited and detected. This provides a big advantage in identifying the vibration modes of the observed resonance peaks, whose frequencies yield the C-ij by solving an inverse problem. Contactless coupling allowed the measurement of intrinsic internal friction, avoiding the energy loss associated with contact coupling. The internal-friction tenser was determined by a free-decay method. Theoretical calculations were made using an extended Mori-Tanaka mean-field theory and good theory-measurement agreement was found. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.