The frictional nature of fiber pullout is a major reinforcing mechanism in brittle matrix composites. The objective of present work is to develop an interface failure model for use as a closing pressure formulation. A fracture mechanics model for the debonding of a fiber from a cementitious matrix is used to calculate the stiffness degradation due to propagation of a stable debonding zone. A formulation based on the growth of a mode II crack along an elastic-perfectly plastic 1-D interface is used. The crack growth criterion is defined using the strain energy release rate of a partially debonded interface. It is assumed that the fracture toughness of the interface increases as a function of the debonded length in the form an an R-Curve approach. In addition, a frictional shear stress of constant magnitude acts over the debonded length. The R-Curve parameters are obtained by numerical solution of the resulting differential equations. The load-slip response is obtained from the R-Curves, and a parametric study of fiber type, length, and interface properties are conducted. Results are compared with a finite element model based on Coulomb frictional models.
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