PSI - Issue 42

Andrea Zanichelli et al. / Procedia Structural Integrity 42 (2022) 118–124 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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In the present numerical model, the matrix is assumed to have a brittle behaviour and the discontinuities due to the fracture process are modelled through a suitable modification of the material properties. In particular, the nucleation of one or more cracks in the finite elements is related to the deterioration of the current matrix stiffness at integration point level. Moreover, in order to describe the fracture process, a cohesive law is introduced to simulate the cracked zone, whereas an elastic law is adopted for the un-cracked region, that is the continuous region. According to the present numerical model, the crack appears when the maximum principal stress inside the finite element reaches the material tensile strength. Such a crack is supposed to pass through the element central point and to have an orientation perpendicular to that of the maximum principal stress. It can be noted that the displacement field across the crack can be written as the sum of a continuous part and a discontinuous one, where this latter one is function of the displacement discontinuity vector, w(x) . Once the components of such a vector across the crack faces are known, the stresses transmitted between the crack can be determined. In particular, the fracture process is modelled by introducing a continuous decreasing exponential law. Such a bridging stress-crack opening displacement relationship is employed in order to quantify the normal stress transferred between the faces of the crack, for a given fracture energy and tensile strength. 3. Experimental campaign on the shot-earth 772 The present section deals with the description of the experimental tests carried out on the shot-earth 772 at the Testing Laboratory of the University of Parma (Vantadori et al. (2022a)). In particular, the present shot-earth consists of 7 parts of excavated soil (stockpiled, left to dry and then sieved and milled), stabilised with 2 parts of Portland cement (cement CEM I 42.5N), and 7 parts of commercial mixed aggregate 0-8 mm (mainly limestone). Moreover, 3% of water by volume is added to the mixture during the spraying phase. The mixture stream is projected at high velocity onto a surface in order to realize panels of compacted shot-earth (Figure 1 (a)). Note that a microstructural, chemical and physical characterization of such a material has been recently performed by Vantadori et al. (2022b) and is available in the literature. Different specimen types have been extracted from shot-earth panels, for both flexural and fracture tests. Four prismatic specimens (Figure 1 (b)), with the nominal geometrical sizes width d 1 =100 mm, depth d 2 =100 mm, length L=350 mm, have been subjected to three-point bending tests in order to determine the shot-earth flexural strength. The tests have been performed according to the UNI EN 12390-5 (2019), under load control at a rate of 130 N/s and with a loading span of 300 mm. Moreover, six prismatic specimens (Figure 1 (c)), with the nominal geometrical sizes width B=40 mm, depth W=80 mm, length L=375 mm, and with a notch a 0 =27 mm in the lower part of the middle-cross section, have been subjected to three-point bending tests in order to determine both the elastic modulus and the fracture toughness by means of the Modified Two-Parameter Model (Vandatori et al. (2018), Vantadori et al. (2021), Zanichelli et al. (2018)). The loading span was equal to 320 mm.

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Fig. 1. (a) Mixture stream projected at high velocity onto a surface; (b) prismatic specimen extracted from shot-earth panel for flexural test; (c) prismatic specimen extracted from shot-earth panel for fracture test.