ABSTRACT
KiNmax = aNmax D1/2 S(ao/D) (1) where aN is a nominal stress defining the load level (typically a force divided by thickness and size), and S(a/D) is the geometrical shape factor, which, according to our definition of positive geometry, must have positive first derivative for a = a0. The fracture energy GF is then defined (assuming plane stress), as
GF := lim D__90. [KiNmaj2/E (2) This is an ideal definition because the limit indicated in (2) cannot be performed
experimentally. In practice, experiments are performed on a finite range of sizes, a model is postulated, the parameters of the model are adjusted so that experimental results are fitted, and then the model is used to extrapolate to larger sizes. The trouble is that, as will be shown later, different models may fit the experimental results within experimental scatter, while leading to different large size extrapolations.
