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RHEOLOGY AND LITHOSPHERIC STRENGTH |
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| Rheology is the study of flow, the response of material to applied stresses. We describe first the relationships between stress and strain then stress and strain rate that characterise the macroscopic behavior of rocks. These relationships are called constitutive equations as they depend on the constitution of the material. They derive from experimental deformation in which rock mechanics is investigated as function of stress, temperature, pressure, grain size, composition and chemical environment. |
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| Continuum Models of Material Behavior |
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The graph on the right shows the flow curves for the three elementary stress-strain relationships:
• Linear Elastic flow (mechanical analogue a spring) occurs as soon as a stress is applied. When stress is removed the material recover its initial shape: strain is reversible. The elastic flow curve (in blue) is linear its slope is 1/E with E the Young modulus.
• Ideal Plastic (analogue: a block on a plane), after an amount of elastic strain the stress reaches a treshold called the yield stress (in A) and strain accumulates at constant stress (green line). When stress stops (in B) the elastic component of strain is recovered but the plastic component remains. |
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• Linear Viscous (analogue: a dashpot), flow occurs as soon as a stress is applied. Strain accumulates at varying and also constant stress (dashed purple line from D) and when the stress is removed the flow stops but the material does not return to its undeformed state. |
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