Amplitude-frequency dependence of damping properties of carbon foams

Wide dynamic and frequency range experimental measurements of dynamic shear modulus and specific damping capacity in carbon foams are presented. Samples have been placed in free vibration mounts, in vacuum. Electromagnetic excitation and optical detection of free vibration decay have been used. Elastic modulus and loss factor have been inverted from resonance frequencies and logarithmic decay measurement performed at several (up to ten) normal-mode resonances using a modified Demarest's theory of cube resonance. For the samples under investigation, the dynamic shear modulus was found to be proportional to sample density and equal to 1 GPa for 0.5 kg/m3. The specific damping capacity was found to be frequency independent in the 100 Hz-20 kHz frequency range and equal to 0.004 for 0.3 kg/m3 foam samples and 0.012 for 0.5 kg/m3 foam samples. Such behaviour cannot be explained by the difference in foam structure and must be caused by a difference in properties of the constitutive material. The specific damping capacity increases more than twice with increase in magnitude of material deformation. Technical problems, such as the contribution of parasitic damping to the loss factor, are discussed in detail.