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Experiments with rock: remarks on strength and stability issues
No full textIn the classical work of Fairhurst and co-workers on machine stiffness, it was shown that the failure response of a rock specimen can be a mixture of material and structural behavior. This is due to the development, around peak stress, of a localized region of microcracking called the intrinsic zone, which may be a property of the rock. The inhomogeneity formed by the intrinsic zone has a
fundamental importance for defining the system behavior in terms of material strength and post-peak instability such that size effects appear. In addition, the testing environment, that is, the load frame, with its finite stiffness and load arrangement, has an impact on the overall response. Observations of the intrinsic zone are presented, and the topics of strength and stability are discussed in relation to tensile and compressive testing of rock
Simulating fracture in rock using a micropolar peridynamic formulation
No full textMode I opening and mixed-mode I-II fracture experiments were performed with a homogeneous, fine grained sandstone using the three-point bending test. Specimens were notched at various lengths and positions of the beam edge to produce the desired loading condition. A micropolar peridynamic model was used to simulate the fracture initiation and propagation process. The analytical implicit formulation was derived by defining a specific macroelastic energy density function for micropolar non-local lattices, which depends on three deformation parameters: bond stretch, bond shear deformation accounting for the rotational degrees of freedom, and the particle's relative rotation. The micropolar non-local lattice model is capable of handling a variable Poisson's ratio, and is suitable for modelling the mechanical behavior of Cauchy isotropic solids subjected to non-homogeneous deformation fields and fracture. A preliminary analysis on a smooth boundary specimen was performed in order to validate the results obtained with the conceived peridynamic model adopting irregular discretizations. The failure process in notched sandstone specimens was simulated numerically in quasi-static conditions. Numerical results were compared with experimental data obtained from electronic speckle pattern interferometry (ESPI) tests, which were used to quantify and detect the fracture phenomena. Due to the intrinsic features of peridynamic theory, realistic crack patterns and crack initiation angles were obtained from the numerical simulations
Opening and Mixed-mode Fracture Initiation in aQuasi-brittle Material
No full textresolution displacement data from fringes that are formed by the subtraction of laser
speckle patterns, was constructed to study fracture initiation in a quasi-brittle material.
Mode I opening and mixed-mode I and II fracture experiments were performed with a
homogeneous, fine grained (0.1 - 0.8 mm grain size) sandstone using the three-point
bending test. Specimens were notched at various lengths and positions of the beam
edge to produce the desired loading condition, with KII/KI = 0 - 13%. The experimental
results indicate that the length of the localized damage zone at peak load for mode I
fracture is 6 - 7 mm, about ten times the (largest) grain size. From the mixed-mode
loading tests, the zone length at peak load increased to 10 - 12 mm, and the length
was more or less constant for KII/KI = 5 - 13%. ESPI also provided detailed
information on the horizontal displacement profiles along the damage zone. For center
notch specimens at peak load, the horizontal (opening) displacement at the notch tip
was 40 μm, which can be interpreted as the critical opening displacement if the
damage zone is fully formed at peak. For mixed-mode specimens, the critical
horizontal displacement at peak load is 60 - 80 μm, but the vertical displacement is
needed to resolve the critical opening and sliding components
Simulating fracture in rock using a micropolar peridynamic formulation
No full textMode I opening and mixed-mode I-II fracture experiments were performed with a homogeneous, fine grained sandstone using the three-point bending test. Specimens were notched at various lengths and positions of the beam edge to produce the desired loading condition. A micropolar peridynamic model was used to simulate the fracture initiation and propagation process. The analytical implicit formulation was derived by defining a specific macroelastic energy density function for micropolar non-local lattices, which depends on three deformation parameters: bond stretch, bond shear deformation accounting for the rotational degrees of freedom, and the particle's relative rotation. The micropolar non-local lattice model is capable of handling a variable Poisson's ratio, and is suitable for modelling the mechanical behavior of Cauchy isotropic solids subjected to non-homogeneous deformation fields and fracture. A preliminary analysis on a smooth boundary specimen was performed in order to validate the results obtained with the conceived peridynamic model adopting irregular discretizations. The failure process in notched sandstone specimens was simulated numerically in quasi-static conditions. Numerical results were compared with experimental data obtained from electronic speckle pattern interferometry (ESPI) tests, which were used to quantify and detect the fracture phenomena. Due to the intrinsic features of peridynamic theory, realistic crack patterns and crack initiation angles were obtained from the numerical simulations
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