Strength scaling of brittle graphitic foam

The effect of size on the compression strength of brittle graphitic foams is investigated. Experiments were carried out on cubic foam samples of varying side length. Load and load-point displacement were measured, from which expressions for the compressive strength of the foam were deduced and compared against predictions. The failure mechanism of the foams was found to be strut fracture and it was observed that this mechanism is related to the manner in which the load is introduced to the specimen. An expression for calculating the surface energy of the foam material was derived using an energy-balance approach. The value obtained for carbon was gamma(s) = 661 +/- 72 J m(-2). Using surface energy as an input, an expression for the compressive. strength, sigma(cr)*, was derived and quantified via the experimental results. This result was used to characterize the fracture behaviour of the foams. In addition, the fracture stress was compared with pre-existing theories available in the open literature. It is found that the mechanical behaviour of the foams can be characterized by three distinct representations: a discrete, continuum and a transition phase solid. It was found that linear elastic fracture mechanics and the surface-energy approach described the continuum mechanical behaviour of the foam accurately. The characteristic scaling behaviour of the foam was assigned to the variability of the load distribution and the imperfections of the struts. For failure to occur, at least 3%, and at most 12%, of damage accumulation was required.