Influence of Cavity Shape on Damage Parameter

The relation between cavity shape and damage parameter is analyzed for a cavitated material. The matrix consists of isotropic, incompressible power law creeping material. The cavities are assumed to be either cylindrical or spheroidal in shape and to be well separated. Damage is defined here from the creep rate equation postulated by Rabotnov. The result shows that the damage parameter is virtually independent of the creep exponent for penny-shaped cracks and for spherical cavities. It is also observed for elongated cavities aligned to the tensile direction, that the damage parameter approaches the Kachanov-Rabotnov damage measure, equal to the area fraction of cavities in a section of a specimen. However, for all other cases the Kachanov-Rabotnov damage measure underestimates the effect of cavities on strain rate. For a plane stress or strain problem with elliptical cavities, the effect of cavity orientation on the damage parameter is shown to be severe. This indicates the need for taking cavity orientation into account when formulating creep laws for multiaxial stress states.