Fracture toughness - a measurable materials parameter

The paper presents a brief history of the development of fracture mechanics toughness measurement before makings the suggestion that tearing resistance is best indexed by energy dissipation rate, D, rather than by currently standardised J integral theory. A number of new insights are presented to justify this position. It is first explained that, even in small scale yielding (SSY), a material has an inherent ductility which immediately elevates the energy dissipation rate for SSY, D-SSY, above the value of J at initiation, J(i). The ratio of D-SSY to J(i) is suggested as a new way of indexing resistance to tearing instability. The term Crack Stability Index (CSI) is introduced to describe this ratio. For a hypothetical material following Griffith theory the CSI is 1. However for most structural materials the CSI exceeds 5, and may be as high as 20 in some cases. The CSI may be thought of as the inherent safety factor between tearing initiation and tearing instability. The energy dissipation rate method does not require the assumption that toughness increases with crack growth (crack growth resistance curve). A method of estimating D-SSY from a fully yielded test piece is described. Another parameter D* which is D divided by current ligament length in a fully yielded test piece, is identified as having a direct link to critical crack tip strain. An example is given from the numerical analysis literature where J integral theory fails to predict correctly the behaviour of a large component. The energy dissipation rate approach described here makes a much more successful extrapolation from small-scale behaviour.