Observation of cracking and measurement of fracture toughness in graphite

The dominant mechanical failure mechanism in brittle or quasi-brittle materials is cracking, and the parameters that characterize the fracture process of a given material are the extension of cracked region as a function of externally applied load and the resistance of the deforming material to the advance of cracking, also known as fracture toughness. In the present investigation we develop and demonstrate a technique for observing and studying the process of macroscopic crack initiation and propagation, and for determining the fracture toughness of the brittle or quasi-brittle materials. We will address issues such as specimen design using only small amount of material, loading configuration that can generate stable crack growth in brittle solids, diagnostics for identifying crack initiation and quantifying the extent of crack growth, and scheme for extracting the stress intensity factor at the tip of a growing crack. The technique described is applied to the brittle material graphite.