The internal damping of Al-Al2O3(f) composites during thermal cycling: The effect of fibre content and matrix strength

A simple model is proposed to describe the observed effect of the volume fraction of fibres on the internal friction, elastic modulus and deformation at zero applied stress during the cooling portion of a thermal cycle between 450 K and 100 K. The sample is considered as composed by three phases, each one showing different intrinsic damping: the reinforcing fibre, the aluminium matrix far from the fibre and the zone around the fibre, close to the matrix-fibre interface, called the plastic zone. It is assumed that the main contribution to the internal friction arises in these plastic zones, which grow during cooling in a non strain hardening matrix. A uniform dislocation density in the zone is considered. The internal damping of the whole sample is predicted, as a function of fibre content, for the limit case of uniform strain. The model predicts satisfactorily the characteristic temperatures defined in the elastic modulus curve if they are identified as the temperatures for the beginning of growth of the plastic zone, and the temperature for overlapping of fully developed zones, respectively.