The marble monument in Figure When a material breaks, it has undergone brittle deformation Figure The stone cylinders in Figure The cylinder on the right looked like the cylinder on the left before it was compressed, with force applied to the top and bottom.
Strain gauges have been glued on to measure the amount of deformation lengthwise and across the cylinders. A material can undergo more than one kind of deformation when stress is applied. The barrel-shaped cylinder of potash in Figure The cylinder was compressed, with stress applied from the top and bottom. Initially, it underwent ductile deformation and thickened in the middle, creating the barrel shape.
But as more stress was applied, the cylinder eventually underwent brittle deformation, resulting in the crack across the middle. A rock is not limited to exclusively brittle deformation, or exclusively ductile deformation. Even the deformed rock in Figure This indicates that a small amount of ductile deformation occurred before brittle failure. For a given rock, deformation will be different depending on the amount of stress applied.
Up to a point, rocks undergo elastic deformation, and will spring back to their original shape after the stress is removed. If more stress is applied, the rock may deform in a ductile manner. If stress increases further, the rock may fracture. The amount of stress required in each case will depend on the type of rock, as well as conditions such as pressure and temperature.
In general, sedimentary rocks will be more likely to undergo ductile deformation than igneous or metamorphic rocks under the same conditions. Rocks within each group will also deform differently. Boudinage structures Figure These structures occur when a stronger rock more prone to brittle deformation is surrounded by weaker rocks prone to ductile deformation.
The stronger rock will fracture into segments, called boudins , and the weaker rock will flow into the spaces between. In Figure The surrounding black layer flowed in to fill the gap where the pinch was happening. Remarkably, the white layer itself contains a dark layer that has fragmented into boudins. Not all boudins break into blocky segments. Some display more ductile deformation Figure At higher temperatures, and under higher confining pressures, rocks are more likely to undergo ductile deformation.
Confining pressure is the stress that a material experiences uniformly from all sides as a result of the weight of material above and around it. The pressure that a diver feels deep in the ocean is confining pressure due to the weight of water above and around the diver. This kind of confining pressure is called hydrostatic pressure.
The aim of this paper was to compare the stress and strain quantities that are related to the failure—deformation process of hard rock. The data used here was obtained from laboratory uniaxial compression tests performed on different types of Fennoscandian hard rocks.
The failure—deformation process quantities were compared at each deformation stage and for each single specimen. Moreover, geological information such as the rock origin process and the rock characteristics of the specimens were studied and linked to the stress and strain quantities.
The purpose was to investigate the influence of the rock origin process and rock characteristics on these quantities. The normalized and volumetric quantities are weakly dependent on the mineral composition. This is a preview of subscription content, access via your institution. Rent this article via DeepDyve. Bieniawski ZT Mechanism of brittle rock fracture. Part I Theory of the fracture process. Article Google Scholar.
Pergamon Press, Oxford. Google Scholar. It involves the study of the different forces such as stress that can be applied to an object which can cause it to change and the resulting deformation that the object goes through which is called the strain. Stress is a force that can cause a change in a physical body. It is the tension that is produced that can cause a body to deform. It is the quantitative measurement of the amount of force that is held within an object.
Stress can be measured and is dependent on the applied force within an area. It is the resistance or internal response of an object to an external pressure. Stress can occur even without strain, but strain cannot exist without stress.
Strain is the change in shape or form of an object when stress is applied. Under applied forces, a physical body is deformed or altered. This is called strain. When a body of rock is compressed in one direction it is typically extended or stretched in another. This is an important concept because some geological structures only form under compressional stress, while others only form under tensional stress.
Most of the rock in Figure The dark rock is chert, which is relatively stronger and remains brittle. As the limestone stretched parallel to the hammer handle the brittle chert was forced to break into fragments to accommodate the change in shape of the body of rock. Figure Like fractures, faults result from brittle breaking of a rock unit.
The key difference is that the bodies of rock on either side of the fault have been displaced relative to each other by the faulting. In geology, deformation refers to folding ductile bending or faulting and fracturing brittle breaking of rocks in response to stress. The deformed rock cannot return to an un-deformed state once the deforming stress is removed.
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