A boundary element method based numerical model is presented to simulate the nonlinear fracture process zone in cementitious materials. A cohesive type stress-separation constitutive relationship (σ-wcurve) is incorporated in the model to represent the process zone. Numerical algorithms for both force-controlled (prescribed loading history) and crack-tip-control-led (prescribed crack tip position) are implemented to allow whole range simulations, including strain-softening and snap-back behavior. The numerical program includes special features to permit re-adjustment of nodal points such that accurate determination of the crack-tip position is achieved. A series of numerical simulations on both 3-point beams and double cantilever beams (DCB) are conducted to investigate the development of the inelastic process zone with respect to load level, loading configuration, specimen size, and the stress-separation relationship in the process zone. Size effect on fracture resistance is clearly demonstrated. Conclusions are drawn regarding the importance of determining the details ofσ-wcurve (i.e., the values offtandwc)and the need for re-evaluating theR-curves approach in cementitious material
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