Tensile creep damage of 2D-C/SiC composites evaluated using the fractal dimension and elastic modulus

Tensile creep tests of 2D-C/SiC composites were carried out at 1300℃ or 1500℃ under vacuum. When the creep time reached 0h, 0.5h, 2h, 10h, 25h, 50h, the testing was stopped and the specimen was removed from the furnace to examine the surface morphology by SEM, and the elastic modulus by a vibration device. The fractal dimension of the surface micro-cracks was evaluated from SEM micrographs. It was found that during creep many micro-cracks are produced on the sample surface and the fractal dimension of the cracks increased from 0 to 1. Both the fractal dimension and elastic modulus changes can be used to evaluate the creep damage of the composites, and they give similar damage curves. Both the damage parameter D vs. creep time and the fractal dimension vs. creep time curves include two stages: a fast developing stage of creep damage followed by a slower changing stage. In the slow stage, the damage curves evaluated from elastic modulus show an initial decline and then rise again. However, those evaluated from the fractal dimension monotonically increase with creep time. Considering matrix cracks as the main form of creep damage, the variation of fractal dimension mainly reflects the local damage of crept specimens while the variation of elastic modulus reflects the damage in the whole specimen.