A simple intuitive justification of this size effect may be given by considering the flexural failure of an unnotched simply supported beam under a concentrated load P at midspan (Fig. 2d). Due to material heterogeneity, what decides the maximum load P is not the elastically calculated stress σ σ 1 = M c / I = 3 P L / 2 b D 2 at the tensile face, where M = P L / 4 = bending moment, D = beam depth, c = D / 2 , I = b h 3 / 12 and b = beam width. Rather, what decides is the stress value σ σ ¯ ¯ roughly at distance l 0 / 2 from the tensile face, which is at the middle of FPZ (2c). Noting that σ σ ¯ ¯ = σ σ 1 − − σ σ n ′ l 0 / 2 , where σ σ n ′ = stress gradient = 2 σ σ 1 / D and σ σ ¯ ¯ = f t ′ = intrinsic tensile strength of the material, and considering the failure condition σ σ ¯ ¯ = f t ′ , one gets P / b D = σ σ N = σ σ 0 / ( 1 − − D b / D ) where σ σ 0 = ( 2 D / 3 L ) f t ′ , which is a constant because for geometrically similar beams L / D = constant. This expression is valid only for small enough l 0 / D , and so (according to the first two terms of the binomial expansion) one may approximate it as