The structural behavior of quasi-brittle materials and brittle-matrix composites ranges from stable to unstable depending on material properties, structure geometry, loading condition and external constrains. In this paper the fracture behavior of a composite characterized by a bilinear cohesive law is analyzed by means of a cohesive-crack model and a bridged-crack model. It is shown that the complex changes in the shape of the load-deflection curve for a three-point bending beam are controlled by two dimensionless parameters. The first, s_E=G_F/σ_uh, depends on the beam depth, h, and on the composite fracture toughness, G_F, and tensile strength, σ_u. The second, G_b/G_(IC)~m, is the ratio of the energy necessary to develop the bridging mechanism of the secondary phases, G_b, to the intrinsic fracture energy of the matrix, G_(IC)~m. It fundamentally depends on the shape of the cohesive law.
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