Compressive Strength:-
Compressive strength is the capacity of a material to withstand axially directed
compressive forces. The most common measure of compressive strength is the uniaxial compressive strength or unconfined compressive strength. Usually compressive strength of rock is defined by the ultimate stress. It is one of the most important mechanical properties of rock material, used in design, analysis and modelling.
Young's Modulus and Poisson’s Ratio:-
oung's Modulus is modulus of elasticity measuring of the stiffness of a rock material. It is defined as the ratio, for small strains, of the rate of change of stress with strain. This can be experimentally determined from the slope of a stress-strain curve obtained during compressional or tensile tests conducted on a rock sample.
Similar to strength, Young’s Modulus of rock materials varies widely with rock type. For extremely hard and strong rocks, Young’s Modulus can be as high as 100 GPa. There is some correlation between compressive strength and Young’s Modulus, and discussion is given in a later section.Poisson’s ratio measures the ratio of lateral strain to axial strain, at linearly-elastic region.For most rocks, the Poisson’s ratio is between 0.15 and 0.4.
Stress-Strain at and after Peak:-
With well controlled compression test, a complete stress-strain curve for a rock specimen can be obtained.
Strain at failure is the strain measured at ultimate stress. Rocks generally fail at a small strain, typically around 0.2 to 0.4% under uniaxial compression. Brittle rocks, typically crystalline rocks, have low strain at failure, while soft rock, such as shale and mudstone, could have relatively high strain at failure. Strain at failure sometimes is used as a measure of brittleness of the rock. Strain at failure increases with increasing confining pressure under triaxial compression conditions.
Rocks can have brittle or ductile behaviour after peak. Most rocks, including all
crystalline igneous, metamorphic and sedimentary rocks, behave brittle under uniaxial compression. A few soft rocks, mainly of sedimentary origin, behave ductile.
Tensile Strength:-
Tensile strength of rock material is normally defined by the ultimate strength in tension, i.e., maximum tensile stress the rock material can withstand.
Rock material generally has a low tensile strength. The low tensile strength is due to the existence of microcracks in the rock. The existence of microcracks may also be the cause of rock failing suddenly in tension with a small strain.
Tensile strength of rock materials can be obtained from several types of tensile tests: direct tensile test, Brazilian test and flexure test. Direct test is not commonly performed due to the difficulty in sample preparation. The most common tensile strength determination is by the Brazilian tests.
Shear Strength:-
Shear strength is used to describe the strength of rock materials, to resist deformation due to shear stress. Rock resists shear stress by two internal mechanisms, cohesion and internal friction. Cohesion is a measure of internal bonding of the rock material.
Internal friction is caused by contact between particles, and is defined by the internal friction angle, φ. Different rocks have different cohesions and different friction angles. Shear strength of rock material ca be determined by direct shear test and by triaxial compression tests. In practice, the later methods is widely used and accepted.
Compressive strength is the capacity of a material to withstand axially directed
compressive forces. The most common measure of compressive strength is the uniaxial compressive strength or unconfined compressive strength. Usually compressive strength of rock is defined by the ultimate stress. It is one of the most important mechanical properties of rock material, used in design, analysis and modelling.
Young's Modulus and Poisson’s Ratio:-
oung's Modulus is modulus of elasticity measuring of the stiffness of a rock material. It is defined as the ratio, for small strains, of the rate of change of stress with strain. This can be experimentally determined from the slope of a stress-strain curve obtained during compressional or tensile tests conducted on a rock sample.
Similar to strength, Young’s Modulus of rock materials varies widely with rock type. For extremely hard and strong rocks, Young’s Modulus can be as high as 100 GPa. There is some correlation between compressive strength and Young’s Modulus, and discussion is given in a later section.Poisson’s ratio measures the ratio of lateral strain to axial strain, at linearly-elastic region.For most rocks, the Poisson’s ratio is between 0.15 and 0.4.
Stress-Strain at and after Peak:-
With well controlled compression test, a complete stress-strain curve for a rock specimen can be obtained.
Strain at failure is the strain measured at ultimate stress. Rocks generally fail at a small strain, typically around 0.2 to 0.4% under uniaxial compression. Brittle rocks, typically crystalline rocks, have low strain at failure, while soft rock, such as shale and mudstone, could have relatively high strain at failure. Strain at failure sometimes is used as a measure of brittleness of the rock. Strain at failure increases with increasing confining pressure under triaxial compression conditions.
Rocks can have brittle or ductile behaviour after peak. Most rocks, including all
crystalline igneous, metamorphic and sedimentary rocks, behave brittle under uniaxial compression. A few soft rocks, mainly of sedimentary origin, behave ductile.
Tensile Strength:-
Tensile strength of rock material is normally defined by the ultimate strength in tension, i.e., maximum tensile stress the rock material can withstand.
Rock material generally has a low tensile strength. The low tensile strength is due to the existence of microcracks in the rock. The existence of microcracks may also be the cause of rock failing suddenly in tension with a small strain.
Tensile strength of rock materials can be obtained from several types of tensile tests: direct tensile test, Brazilian test and flexure test. Direct test is not commonly performed due to the difficulty in sample preparation. The most common tensile strength determination is by the Brazilian tests.
Shear Strength:-
Shear strength is used to describe the strength of rock materials, to resist deformation due to shear stress. Rock resists shear stress by two internal mechanisms, cohesion and internal friction. Cohesion is a measure of internal bonding of the rock material.
Internal friction is caused by contact between particles, and is defined by the internal friction angle, φ. Different rocks have different cohesions and different friction angles. Shear strength of rock material ca be determined by direct shear test and by triaxial compression tests. In practice, the later methods is widely used and accepted.
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