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

The complete elastic-constant tensor Cij and the corresponding internal Qij-1 for a crossply 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 Cij and Qij-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 Cij 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 tensor 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