Issue 35

S. Valente et alii, Frattura ed Integrità Strutturale, 35 (2016) 306-312; DOI: 10.3221/IGF-ESIS.35.35 308 This two-stage approach is known in the literature as a Large Time Increment approach [12, 13]. The main consequences of this two-stage approach are: (a) a previous converged load increment is not required. (b) the iterations done during the local stage are characterized by displacement and stress fields which are not real. They are just a way to reach a critical condition at FCT and can be forgot. In this case the node-to-segment friction contact problem is solved by means of the Lagrange multipliers [11]. Once the critical condition has been reached, it has to be saved and plotted as a step of the global stage, which has a clear physical meaning. Since the model outside the process zone behaves linearly and includes a crack, a generic load increment occurring during the local stage will induce a singular stress increment at the FCT. The following two assumptions, related to the FPZ, prevent the onset of a singular stress increment at the FCT: (a) as long as the FPZ is closed, the normal component of the displacement discontinuity vanishes, and therefore the stress intensity factor is K 1 =0. In these conditions, following the Coulomb law, the peak value of the tangential stress is: τ p =c + σ n tan(φ) (b) Since a new step in the global stages starts only when the FCT is in critical conditions, since a rigid-plastic traction- separation law is assumed (Fig.2), the stress intensity factor remains K 2 =0 during the iterations of the local stage. Figure 2 : Traction-separation law applied to the Fracture Process Zone. The effective displacement discontinuity is assumed as: w=( COD 2 + CSD 2 ) 1/2 The value assumed for the joint properties c and φ are shown in Tab. 1. Tab. 2 shows the material properties. Parameters Unit Value Peak cohesion c MPa 0.7 Residual cohesion MPa 0 Tensile strength MPa 0 Friction angle φ deg 30 Softening module H MPa/mm -0.7 Table 1 : Properties of the rock-concrete interface.