fracture_mechanics - I. Experimental Fracturing in Rocks A....

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103 I. Experimental Fracturing in Rocks A. Methodology 1. Experimental Work in Rock Mechanics a. Rock core samples placed in triaxial press b. Pressure applied until strength exceeded c. Fractures examined to provide insight into Mohr relations. B. Basic Concepts and Variables 1. Rock failure a. critical stress relations at which sample is unable to support further stress increase without permanent deformation 2. Strength a. critical stress conditions at which failure of rock sample occurs 3. Brittle Failure: brittle cracking of sample a. Brittle Fracture: surface zone across which rock sample loses cohesion (1) atomic bonds broken at subatomic level 4. Ductile Failure: rock material becomes permanently deformed without losing cohesion 5. Confining Pressure: pressure applied to and surrounding the exterior of the sample 6. Pore Fluid Pressure: pressure of fluids contained in pore spaces of rock 7. Temperature: may be controlled in experimental aparatus 8. Axial Stress: stress applied parallel to core cylinder axis 9. Radial Stress: stress applied perpendicular to core cylinder axis a. i.e. confining pressure C. Common experimental conditions 1. Axial compression experiments (positive stress) a. axial stress = sigma1 b. radial stress = sigma2=sigma3 2. Axial tension experiments (negative stress) a. axial stress = negative = pull = sigma3
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104 D. Concepts from Rock Fracture Experiments 1. Mode I, II and III Fractures Commonly Produced in Experiments a. Review of Terminology (1) Mode I: extension fractures, separation perpendicular to fracture surface (2) Mode II: strike-slip shear fracture (3) Mode III: dip-slip shear fracture b. Extension Fractures (Mode I) (1) From under positive, compressive stress (2) fracture plane perpendicular to minimum principal stress sigma3 (3) fracture plane parallel to maximum principal stress sigma1 (4) displacement normal to fracture surface c. Tension Fractures (Mode I) (1) Form under negative, tensile stress (2) fracture plane perpendicular to minimum principle stress sigma3 d. Shear Fractures (Mode III) (1) Form under conditions of confined compression (2) Commonly form at angles < 45 degrees to maximum compressive stress, sigma1 (3) Displacement, by shear, parallel to fracture surface (4) If under triaxial conditions: sigma1 > sigma2 > sigma3. .. (a) shear fractures form parallel to intermediate principal stress, sigma2 2. Conditions of Tension a. Tensile strength of rock (1) critical tensile stress (negative sigma3) at which rock undergoes brittle failure to form tension fractures b. Fracture plane angle (1) angle between maximum principal stress (sigma1) and the fracture plane c. Fracture angle (1) angle between maximum principal stress (sigma1) and a
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105 normal to the fracture plane d. For a tension fracture: (1) tension fracture perpendicular to sigma3 (2) tension fracture parallel to sigma1 (3)
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This note was uploaded on 10/02/2011 for the course GEOLOGY 406 taught by Professor Taylor during the Spring '11 term at Western Oregon University.

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fracture_mechanics - I. Experimental Fracturing in Rocks A....

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