Mechanical Damping and Dynamic Modulus Measurements in Continuous Fiber-Reinforced Composite Materials: Al/Al2O3 and Al/W

Measurements of mechanical damping, or internal friction (Q−1), and dynamic Young's modulus (E) have been made near 80 kHz and at strain amplitudes (ε) in the range of 10−8 to 10−4 on small specimens of the following two continuous fiber-reinforced metal matrix composites (MMCs): 6061 aluminum reinforced with alumina (Al/A12O3) and 6061 aluminum reinforced with tungsten (AI/W). Baseline experiments were also performed on 99.999% aluminum (pure Al) for comparison puposes. The temperature (T) dependence of modulus up to 475°C was determined for AI/A12O3 and pure Al. The rate of modulus decrease with increasing temperature for AI/A12O3 and Al was the same, that is, dE/dT was essentially the same for both materials. Thus, the reduction in modulus observed for the Al/Al2O3 was attributed to the reduction in modulus of the Al matrix and not that of the Al2O3 fibers. The strain amplitude dependence of damping was examined for all three materials. The pure Al exhibited classical dislocation damping behavior with strain amplitude dependent damping starting at a strain of 2 × 10−5. The Al/Al2O3 specimens showed only mild dependence of damping on strain amplitude starting at strains near 10−5. The AI/W exhibited significant amplitude dependence of damping starting at strains of 2 × 10−6 with the fiber diameter being a major factor in determining the damping behavior. The Q−1 versus ε data for Al/Al2O3, when analyzed in terms of the Granato-Lücke (GL) theory for dislocation damping, yielded minor pinning lengths of dislocations near 10−8 m and mobile dislocation densities near 1011 m−2. The same analysis for the Al/W data gave values near 10−8 m for the minor pinning lengths and 1012 M−2 for the dislocation density. Relative to the results for pure Al, the minor pinning lengths for Al/Al2O3 and AL/W are comparable (10−8 m for pure Al), but the dislocation densities are much higher (109 m−2 found in the pure Al). The relatively high dislocation densities calculated for these aluminum matrix MMCs agree with previous findings of other researchers and may be associated with the fiber/matrix interface.