Temperature Dependence of Dynamic Modulus and Damping in Continuous Fiber- Reinforced Al-(alloy) Matrix Composites at Elevated Temperatures

Mechanical damping (in the longitudinal vibration mode only) and temperature dependence of dynamic modulus in the longitudinal ( II ) and transverse ( ⊥ ) fiber direction were measured for several metal matrix composites. The piezoelectric ultrasonic oscillator technique was used to generate ultrasonic stress waves for longitudinal measurements at 80 and 150 kHz. Tests were conducted at room temperature and at elevated temperatures up to 450°C and strain amplitudes in the range of 10^−7 to10^−2 . The Metal Matrix Composites (MMC’s) studied include Alpure and Al(6061) alloy metal matrix reinforced continuous alumina (Al2O3), tungsten (w), high strength carbon (H.S.C), boron(B), and silicon carbide (SiC) fibers. The w-Al composite material showed a strain amplitude dependent damping at room temperature, while the Al2O3, B and SiC fiber reinforced Alalloy matrix composites exhibited essentially amplitude independent over a strain range of 10^−7 to 10^−4 and a slight nonlinear amplitude dependent damping at higher strain amplitudes. Increasing the area of fiber-matrix interface in H.S.C-Al matrix composites appeared to increase damping in such composites. The measured longitudinal vibration mode damping for H.S.C-Alpure matrix composite at temperature in the range of 25 to 350 °C has shown to exhibit strain amplitudedependent damping. The temperature dependence of dynamic modulus in the tested composites showed a linear, monotonic decrease in modulus with increasing temperature, except in the case of H.S.C fiber reinforced Al matrix composites which showed a non-monotonic decrease in modulus as temperature increased from room temperature to 450°C. It is suggested that this behavior is caused by residual stresses at the fiber-matrix interface. The flaws detected by ultrasonic flaw detectiontechniques in the tested composite plates did not significantly affect the modulus as the fibers carry the majority of load in the longitudinal fiber direction. Additionally, the weak fiber-matrix interfacial strength, matrix ductility, and the flaws, which are a potential source of sliding friction and energy absorbing mechanisms, did affect the damping in tested MMC’s specimens under longitudinal fiber vibration.