R-curve determination of carbon refractories

Over the last few years it has become apparent that a useful way of characterising the fracture behaviour of both coarse grained refractories and fine grained engineering ceramics is by way of the R-curve. As a crack propagates the resistance of the material to propagation is not always a constant as it is, for example, for glass. It is common that this resistance increases and a plot of this resistance with crack extension is known as an R-curve. This paper describes a new test configuration which can be used with a soft testing machine, but provides a long length of straight stable crack growth for the size of test-piece. This method is particularly straightforward for carbon refractories as a potential drop method can be used to measure the crack extension. Results are described for a number of materials using different sizes of test-piece. The largest gives about 150mm of stable crack growth and even after this long length of growth the resistance to propagation can still be rising. It was also found that if a photograph of the test-piece before fracture was compared in a stereoscopic viewer with a series of photographs taken during the test, then the cracks in even very coarse-grained carbons could be seen clearly.