13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- The diagrams depicted in Fig. 4 clearly show the points were the damage nucleation takes place. The first fracture forms in the middle of the layer and grows during the following steps. As a consequence, a redistribution of stresses takes place in the two parts on sides of the fracture. When the thermodynamic force in another Gauss point or group of Gauss points overcomes the threshold value a new fracture appears symmetrically with respect to the middle of the mortar joint. On the other hand, while σx stresses suddenly fall down and tend to zero, the σz values are little influenced by fracture. This is due to the fact that damage affects only internal and mixed terms of the free energy. 0 4 8 12 16 20 d [cm] 0 10 20 30 40 50 60 σx [MPa] x 10-1 0 4 8 12 16 20 d [cm] -20 -40 -60 -80 -100 -120 σz [MPa] 0 4 8 12 16 20 d [cm] -0.6 -0.4 -0.2 0 0.2 0.4 0.6 τ [MPa] x 10-1 x 10-1 Figure 4. Uniaxial compression test on a masonry block: results at a load multiplier equal to 3. (a) normal stresses along x-axis; (b) vertical stresses; (c) tangential stresses. 4.2. Diagonal compression tests on masonry. Diagonal compression numerical tests have been carried out on a masonry panel and compared with the experimental results obtained in laboratory. With reference to Fig. 5, the specimen is made up of four courses of sandstone blocks with calcium-cement mortar. It has a squared shape with a length of 67 cm for each side. A single block is 33 cm long and 16 cm high. The mortar layer has a thickness of 1 cm. A total number of 256 plane stress 2D solid elements and 72 interphase elements has been used to create the finite element model. hinge hydraulic jack Load cell Figure 5. Diagonal compression test on a masonry panel. Experimental setup and finite element model. The mechanical variables for blocks and mortar and the parameters used for the finite element model are reported in Table 2. (a) (b) (c)
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