The damping behavior of unidirectional fiber-reinforced metal-matrix composites (MMCs), such as boron/aluminum, graphite/aluminum, and graphite/magnesium is reviewed to examine the effect of microstructure on material damping. Although it is quite difficult to isolate the operative damping mechanisms because of several possible energy dissipation sources, the key microstructural features that do play an important role in composite damping behavior have been identified.
Damping measurements of as-fabricated unidirectional MMCs exhibit a transition from strain amplitude-independent to amplitude dependent damping behavior at strain levels of about 10-5. At low strain amplitudes (<10-5), measured damping is consistent with the damping values predicted by the Hashin theory. Vibrational energy dissipation processes include contributions from different operative mechanisms in the constituent phases of the MMC, namely, fiber, matrix, and fiber-matrix interface. The strain amplitude-independent damping behavior of composites is influenced by various anelastic relaxation mechanisms in the fiber, interphase, and matrix.
The strain amplitude-dependent damping response in these MMCs could involve non-relaxational mechanisms such as microplastic deformation, friction, and dislocation motion. Near the fiber-matrix interface, each composite exhibits a highly dense dislocation substructure associated with the residual stress state generated during the composite fabrication. Damping test data and interface structure analyzed in terms of Granato-Lucke (G-L) theory of dislocation damping indicate that the G-L theory describes the operative energy dissipation in the strain amplitude damping region, Thus, knowing the peak damping contribution due to different microstructural features, the MMC could be potentially designed to provide damping under different operational conditions.