STRESS SUMMARY

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STRESS SUMMARY
Force mass acceleration (units N)
Stress amount of force per unit area (units Pa)
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Lithostatic stress

• r g h
• r density of rock
• g acceleration of gravity
• h height of rock column (or depth in crust)

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Surface Stress
Stress acting on a plane is a VECTOR
quantity (has both magnitude and
direction). This vector can be divided into
normal and shear components.
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Normal and shear components
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Normal
F
Fy F . SinA
Fx F . CosA
Shear
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Principal Stress Components
?1 gt ?2 gt ?3
Principal stresses are NORMAL stresses (NOT
SHEAR) The three principal stresses are always 90
degrees from each other
On the surface of the earth, TYPICALLY one
principal stress component will be vertical.
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Lithostatic Stress

• Similar to hydrostatic pressure
• Magnitude of stress components is the same in all
  directions

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Deviatoric Stress

• This is the amount of stress in each direction
  that is NOT lithostatic

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(No Transcript)
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Magnitude of Normal and Shear Stresses
s1
Normal
s3
Shear
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Strain Rate ?
? e/t
what are the units?
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Strain vs. Time Creep Curve
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Mechanical behavior depends on material properties
Solids strong, rigid Liquids weak, deforms
easily Plastic can flow, but isnt
liquid (play-doh?) Elastic Deforms, but returns
to original shape when stress is released.
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Stress vs. Strain diagrams
Yield Point
Plastic
Elastic
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Increasing Confining Pressure results in greater
strain before failure easier to flowResists
fracture formation
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Rock typeresponsetoconfiningpressure
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Effect of T At low T, failure occurs quickly.
High T, flow is easy
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Effect of Strain Rateslowflow
Fast
Slow
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Effect of Composition
Quartz
Plag.
Olivine
Pyroxene
Calcite
Halite
Gypsum
Slow
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Effect of Composition
30 rule Weakest mineral that makes up gt30 of
the rock will control the deformational style
Examples
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Brittle vs. Ductile Deformation
Brittle deformation Fractures and
Faults Ductile deformation Folds, Foliation,
lineation, shear zones
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COLD
x-axis is differential stress or STRENGTH
HOT