This paper deals with the determination of a stress-crack opening (σ-w) curve for the characterization of the fracture behavior of plain and fiber reinforced concretes. The objective is to provide a constitutive model for the application of cohesive crack models in the analysis of structural members. Several researchers have proposed the use of beam tests to obtain the σ-w relation indirectly using inverse analysis. In the strategy described here, an error function is defined as the integral of the squared differences between a numericallyor analytically-obtained load-crack opening (P-CMOD) curve and the corresponding curve resulting from three-point bend tests of notched prismatic specimens. Optimization techniques are employed to find the set of σ-w parameters that leads to the minimum error. The strategy has been implemented in an object-oriented code, which is capable of handling any σ-w curve description. The current implementation uses a third party analytical code for computing the P-CMOD curve corresponding to a trial σ-w set of parameters. Implementation of new σ-w models or different schemes for computation of P-CMOD curves, e.g., using the finite element method, is straightforward. Example applications are presented.1 665670Hillerborg, A., Modeer, M., Petersson, P.-E., Analysis of crack formation and crack growth in concrete by means of fracture mechanics and finite elements (1976) Cem. Concr. Res., 6, pp. 773-782Guinea, G., Planas, J., Elices, M., A general bilinear fit for the softening curve of concrete (1994) Mater. Struct., 27, pp. 99-105Kitsutaka, Y., Oh-Oka, T., Fracture parameters of high-strength fiber reinforced concrete based on poly-linear tension softening analysis (1998) Fracture Mechanics of Concrete Structures, pp. 455-464. , Eds H.Mihashi & K.Rokugo, Aedificatio Publ., Freiburg, GermanyKooiman, A.G., Van Der Veen, C., Walraven, J.C., Modelling the postcracking behaviour of steel fibre reinforced concrete for structural design purposes (2000) Heron, 45 (4), pp. 275-307Roelfstra, P.E., Wittmann, F.H., Numerical method to link strain softening with failure of concrete (1986) Fracture Toughness and Fracture Energy of Concrete, pp. 163-175. , Ed. F.H.Wittmann, Elsevier ScienceStang, H., Olesen, J.F., On the interpretation of bending tests on FRC materials (1998) Fracture Mechanics of Concrete Structures, pp. 511-520. , Eds. H. Mihashi and K. Rokugo, Aedificatio Publ., Freiburg, GermanyStang, H., Olesen, J.F., A fracture mechanics based design approach to FRC (2000) Proc. Fifth RILEM Symposium on Fibre-Reinforced Concretes, pp. 315-324. , Eds. P. Rossi and G.Chanvillard, RILEM, Cachan, FranceLuenberger, D.G., (1984) Linear and Nonlinear Programing, p. 491. , Addison-Wesley Publishing CompanyHordijk, D.A., (1991) Local Approach to Fatigue of Concrete, , Doctoral thesis, Delft University of Technology, The Netherlands