where c f ≈ ≈ half of the FPZ length, α α 0 = a / D = relative initial crack length (which is constant for geometrically similar scaling); g ( α α 0 ) = k 2 ( α α 0 ) = dimensionless energy release function of linear elastic fracture mechanics (LEFM), which brings about the effect of structure geometry; k ( α α 0 ) = K ( α α 0 ) b D / P , and K = stress intensity factor. Fitting Eq. 8 to σ σ N data from tests of geometrically similar notched specimens of very different sizes is a good way to identify the G f and c f of the material.