COMPRESSION FAILURE OF QUASIBRITTLE MATERIAL - NONLOCAL MICROPLANE MODEL

The previously presented constitutive model of microplane type for nonlinear triaxial behavior and fracture of concrete is used in nonlocal finite element analysis of compression failure in plane strain rectangular specimens. For specimens with sliding rigid platens there is a bifurcation of the loading path at the beginning of postpeak softening; a symmetric (primary) path exists but the actual (stable) path is the nonsymmetric (secondary) path, involving an inclined shear-expansion band that consists of axial splitting cracks and is characterized by transverse expansion. The secondary path is indicated by the first eigenvalue of the tangent stiffness matrix but can be more easily obtained if a slight nonsymmetry is introduced into the finite element model. In specimens with bonded rigid platens there is no bifurcation; they fail symmetrically, by two inclined shear-expansion bands that consist of axial splitting cracks. The transverse expansion produces transverse tension in the adjacent material, which serves as the driving force of propagation of the axial splitting cracks. Numerical calculations indicate no significant size effect on the nominal stress at maximum load.