Size effects on mixed-mode fracture behavior of polygranular graphite

The aim of this paper is to present a combined experimental-theoretical approach for studying the effects of specimen size on the mixed mode fracture resistance of a polygranular graphite. The edge cracked semi-circular bend (SCB) specimens under three-point flexural loading with different sizes are used for conducting the fracture tests on graphite. The experimental results revealed that the fracture resistance of graphite increases significantly with specimen size. For instance, the effective fracture resistance in pure mode I loading enhances from 0.585 MPa√m to 1.115 MPa√m when the sample radius increases from 5 mm to 30 mm. To justify the size effect on the fracture resistance of graphite, the modified maximum tangential stress (MMTS) criterion is employed. The criterion makes use of a more accurate mathematical model for calculating the stress field around the crack tip. The MMTS criterion also assumes that the extent of fracture process zone around the crack tip depends on the size of specimen. It is shown that the MMTS criterion can provide good estimates for the fracture resistance of graphite samples and hence it can be used for predicting the fracture loads of real graphite components with different sizes.