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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 because they depend on the constitution of the material. They derive from experimental deformation in which rock mechanical behaviors are 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 three elementary stress-strain relationships:
Linear Elastic (mechanical analogue a spring), flow occurs as soon as a stress is applied. When the stress is removed the material recover its initial shape: the deformation is reversible. The elastic law is a straight line (in blue) of slope 1/E with E the Young modulus.
Ideal Plastic (block on a plane), after an amount of elastic deformation the stress reaches a limiting value called the yield stress (in A) and deformation accumulates at constant stress (green line). When stress stops (in B) the elastic component of deformation is recovered but the plastic component remains permanent. |
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| Linear Viscous (dashpot), flow occurs as soon as a stress is applied. Deformation 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 stage. |
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