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enviromental effect in stress rock mechanics

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    محمد عادل الاحول
    محمد عادل الاحول غير متواجد حالياً

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    تاريخ التسجيل: Dec 2007
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    enviromental effect in stress rock mechanics

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    من مواضيع محمد عادل الاحول :


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    التعديل الأخير تم بواسطة محمد عادل الاحول ; 2007-12-13 الساعة 06:08 AM

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    alshangiti
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    الصورة الرمزية alshangiti


    تاريخ التسجيل: Mar 2007
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    The behaviour and mechanical properties of rocks depend on a number of environmental conditions. (1) Confining pressure increases the elasticity, strength (e.g., yield point and ultimate fracture stress), and ductility. (2) Internal pore-fluid pressure reduces the effective stress acting on the sample, thus reducing the strength and ductility. The effective, or net, confining pressure is the external hydrostatic pressure minus the internal pore-fluid pressure. (3) Temperature lowers the strength, enhances ductility, and may enhance recrystallization. (4) Fluid solutions can enhance deformation, creep, and recrystallization. (5) Time is an influential factor as well. (6) The rate of loading (i.e., the rate at which stress is applied) influences mechanical properties. (7) Compaction, as would occur with burial to depth, reduces the volume of pore space for sedimentary rocks and the crack porosity for crystalline rocks.
    Rocks, which are typically brittle at the Earth's surface, can undergo ductile deformation when buried and subjected to increased confining pressure and temperature for long periods of time. If stress exceeds their strength or if they are not sufficiently ductile, they will fail by fracture—as a crystal, within a bed or rock, on an earthquake fault zone, and so on—whereas with ductility they can flow and fold.



    Some strengths for various rock types under different temperatures and confining pressures are listed in the Table. The plastic yield strength here is the stress at a 2 percent strain; the ultimate strength, as stated above, is the highest point (stress) on the stress-strain curve.
    An increase in confining pressure causes brittle fracture to become shear slippage and eventually causes flow (ductile) behaviour. This transition is also aided by higher temperature, decreased internal pore-fluid pressure, and slower strain rate.



    The Table gives the values of some elastic constants—bulk modulus (k), Young's modulus (E), shear modulus (m), and Poisson's ratio (sp)—at room pressure (1 bar) and high confining pressure (3,000 bars). The values for clastic sedimentary rocks would be particularly variable.

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