13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- [ ] [ ] 1 , 0, n n t t T + ⊂ . In particular the nonlinear solution at time 1 nt + has been calculated by means of an elastic prediction – plastic and/or damaging correction procedure. The interphase element has four nodes and zero-thickness. The integration of the stiffness matrix has been solved by applying the Reduced Selective Integration method, that is two Gauss are used for the integration in the direction normal to the interphase plane while one Gauss point is used in the tangential one. The numerical applications presented in this work regard uniaxial compression and diagonal compression tests on masonry specimens. All numerical examples have been carried out under the hypothesis of plane stress state. 4.1. Uniaxial compression tests on masonry. Uniaxial compression tests have been carried out to assess the performance of the interphase element. A brick-mortar-brick system uniformly compressed (Fig. 3) has been considered with the aim to show the formation of damage in the bulk material when the stiffness of the bricks is lower than that one of mortar. In this case, in fact, the bricks tend to expand more than the mortar in the horizontal direction because of the Poisson effect, but the higher stiffness of the mortar opposes to this displacement. The bricks are therefore subjected to compressive stresses while the mortar to tensile ones along the x-axis. Figure 3. Uniaxial compression test on a masonry block. The parameters used for tests are reported in Table 1, while the results are shown in Fig. 4. The numerical test has been performed under displacement control. In this paper the results obtained with a load multiplier equal to 3 and for a FE model with 80 interphase elements are reported in terms of stresses along the mortar layer. Table 1. Parameters used for compression test. Ebrick 500 MPa vbrick 0.3 Emortar 15000 MPa vmortar 0.2 c0 4.5 MPa φ 30° σ0 1.0 MPa hp 1 ζ0 0.001 MPa hd 0.0025 MPa
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