Micromechanical model for the rate dependence of the fracture toughness anisotropy of Barre granite

• Published in: International journal of rock mechanics and mining sciences (Oxford, England : 1997) Vol. 63; pp. 113 - 121
• Main Authors: Dai, F., Xia, K., Nasseri, M.H.B.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01-10-2013; Elsevier
• Subjects: Anisotropy; Applied sciences; Barre granite; Buildings. Public works; Crack–microcrack interaction; Exact sciences and technology; Fracture mechanics; Fracture toughness; Fracture toughness anisotropy; Geotechnics; Granite; Loading rate; Mathematical models; Microcracks; Micromechanics model; Notched semi-circular bend (NSCB); Rock; Soil mechanics. Rocks mechanics; Strength of materials (elasticity, plasticity, buckling, etc.); Structural analysis. Stresses; Fracture toughness anisotropy; Barre granite; Micromechanics model; Notched semi-circular bend (NSCB); Crack–microcrack interaction; Microcrack; Rock mechanics; Fracture mechanics; Mechanical model; Interaction; Crack―microcrack interaction; Modeling; Dynamic test; Granite; Fracture toughness; Anisotropy; Numerical simulation
• ISSN: 1365-1609; 1873-4545
• DOI: 10.1016/j.ijrmms.2013.08.011

Laboratory measurements of mode-I fracture toughness of Barre granite under a wide range of loading rates were carried out with an MTS machine and a split Hopkinson pressure bar (SHPB) system using the notched semi-circular bend (NSCB) specimen. The fracture toughness anisotropy was found to decrease with the increase of the loading rate. A micromechanics model is utilized in this work to understand this experimental observation, invoking crack–microcrack interactions. Two micromechanics models are constructed based on the microstructural investigation of Barre granite samples using the thin-section method. In both models, the rock material is assumed to be homogenous and isotropic. The main crack (i.e., the pre-crack in the NSCB specimen) and the closest microcracks are included in the numerical analysis. Numerical results show that stress shielding occurs in the model where the two microcracks form an acute angel with the main crack and the nominal fracture toughness is bigger than the intrinsic one, while stress amplification occurs in the model where the microcrack is collinear to the main crack and the nominal fracture toughness is smaller than the intrinsic one. Assuming that the intrinsic fracture toughness of the rock material has the usual loading rate dependency, we are able to reproduce the decreasing trend of the fracture toughness anisotropy as observed from experiments.
●Investigated microstructures of recovered NSCB samples using the thin-section method.●Developed two micromechanics models to understand experimental observations.●Showed the stress shielding/amplification effects of the main crack due to microcracks.●Explained the measured apparent fracture toughness anisotropy of Barre granite due to microcracks.●Reproduced the decreasing trend of the fracture toughness anisotropy against the loading rate.