The implication of modelling concrete fracture with a fictitious crack of zero fracture process zone (FPZ) height is addressed because FPZ height, in reality, is not zero and is bound to vary during crack growth. The ligament effect on fracture energy G(F) is explained by the nonuniform distribution of a local fracture energy gf showing the influence of specimen boundary and variation of FPZ height. The nonuniform g(f) distribution is then used to determine the size-independent G(F). The recent boundary-effect model based on a bilinear g(f) function is confirmed by the essential work of fracture (EWF) model for the yielding of deeply notched polymer and metal specimens. The EWF model provides a theoretical basis for the bilinear g(f) distribution. The principal rationale of the boundary-effect model, the influence of FPZ height on fracture energy, is supported by experimental observations of thickness effect on fracture toughness of thin polymeric adhesives between metals. (C) 2004 Elsevier Ltd. All rights reserved.