Mode I Fracture Behavior in Concrete: Nonlocal Damage Modeling

A nonlocal damage model is used to reproduce typical uniaxial tension tests with reference to a normal strength concrete. A strip and a notched plate subjected to uniaxial tension are first investigated in order to have an insight into the correlations between the specimen geometries and the main constitutive parameters governing the fracture process. It should be noted that the internal length is related to the characteristic length. The former defines the finite volume characterizing the constitutive behavior of the material and the latter identifies the size of an imaginary and uniformly damaged crack band; this relation depends on the geometry used for the calibration, e.g., the strip or the notched plate. The numerical results are shown to be independent on the finite-element type and mesh refinement and to be affected by the law that relates the accumulation of the irreversible strains with the damage growth and by the input constitutive law. A direct tension test on a notched cylinder and a three-point bending test on a notched beam were numerically simulated in order to check the reliability of the model. Both analyses were carried out taking advantage of the tensile strength ft and fracture energy Gf measured by testing the notched cylinder. The cross-examination of the experimental and theoretical results gives the opportunity to reexamine some previous tests concerning mode I fracture. These tests showed the great influence that both the boundary conditions and concrete tensile behavior have on crack propagation. This issue is also investigated by numerically performing a wedge-splitting test, in order to study the evolution of the fracture energy with the opening displacement at the crack tip, and to compare these results with those of the notched cylinder and bent beam.