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III. Strain and Stress. Strain Stress Rheology. Reading Suppe , Chapter 3 Twiss&Moores , chapter 15. http://www.alexstrekeisen.it/english/meta/deformedoolite.php.

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## III. Strain and Stress

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**III. Strain and Stress**• Strain • Stress • Rheology Reading Suppe, Chapter 3 Twiss&Moores, chapter 15**http://www.alexstrekeisen.it/english/meta/deformedoolite.php****Photomicrograph of ooid limestone. Grainsare 0.5-1 mm in**size. Large grain in center shows well developed concentric calcite layers.**Thin section (transmitted light) of deformed oolitic**limestone, Swiss Alps. The approximate principal extension (red) and principal shortening directions (yellow) are indicated. The ooids are not perfectly elliptical because the original ooids were not perfectly spherical and because compositional differences led to heterogeneous strain. The elongated ooids define a crude foliation subperpendicular to the principal shortening direction. Photo credit: John Ramsay. http://www.rci.rutgers.edu/~schlisch/structureslides/oolite.html**Deformed Ordovian trilobite. Deformed samples such as this**provide valuable strain markers.Since we know the shape of an undeformed trilobite of this species, we can compare that to this deformed specimen and quantify the amount and style of strain in the rock.**III. Strain and Stress**• Strain • Basics of Continuum Mechanics • Geological examples Additional References : Jean Salençon, Handbook of continuum mechanics: general concepts, thermoelasticity, Springer, 2001 Chandrasekharaiah D.S., Debnath L. (1994) Continuum Mechanics Publisher: Academic press, Inc.**Deformation of a deformable body can be discontinuous**(localized on faults) or continuous. • Strain: change of size and shape of a body**Basics of continuum mechanics, Strain**• A. Displacements, trajectories, streamlines, emission lines • 1- Lagrangianparametrisation • 2-Eulerian parametrisation • B Homogeneous and tangent Homogeneous transformation • 1- Definition of an homogeneous transformation • 2- Convective transport equation during homogeneous transformation • 3-Tangent homogeneous transformation • C Strain during homogeneous transformation • 1-The Green strain tensor and the Green deformation tensor • Infinitesimal vs finite deformation • 2- Polar factorisation • D Properties of homogeneous transformations • 1- Deformation of line • 2- Deformation of spheres • 3- The strain ellipse • 4- The Mohr circle • E Infinitesimal deformation • 1- Definition • 2- The infinitesimal strain tensor • 3-Polar factorisation • 4- The Mohr circle • F Progressive, finite, and infinitesimal deformation : • 1- Rotational/non rotational deformation • 2- Coaxial Deformation • G Case examples • 1- Uniaxial strain • 2- Pure shear, • 3- Simple shear • 4- Uniform dilatational strain**A. Describing the transformation of a body**• Reference frame (coordinate system): R • Reference state (initial configuration): k0 • State of the medium at time t: kt • Displacement (from t0 to t), • Velocity (at time t) • Position (at t), particle path (=trajectory) • Strain (changes in length of lines, angles between lines, volume)**A.1 Lagrangian parametrisation**• Displacements • Trajectories • Streamlines**A.1 Lagrangianparametrisation**• Volume Change**A.2 Eulerian parametrisation**• Trajectories: • Streamlines: (at time t)**A.3 Stationary Velocity Field**• Velocity is independent of time NB: If the motion is stationary in the chosen reference frame then trajectories=streamlines**B. Homogeneous Tansformation**• definition**Homogeneous Transformation**• Changing reference frame**Homogeneous Transformation**• Convective transport of a vector Implication: Straight lines remain straight during deformation**Homogeneous transformation**• Convective transport of a volume**Homogeneous transformation**• Convective transport of a surface**Tangent Homogeneous Deformation**• Any transformation can be approximated locally by its tangent homogeneous transformation**Tangent Homogeneous Deformation**• Any transformation can be approximated locally by its tangent homogeneous transformation**Tangent Homogeneous Deformation**• Displacement field**D. Strain during homogeneous Deformation**• The Cauchy strain tensor (or expansion tensor)**Strain during homogeneous Deformation**• Stretch (or elongation) in the direction of a vector**Strain during homogeneous Deformation**• Stretch (or elongation) in the direction of a vector • Extension (or extension ratio), relative length change**Strain during homogeneous Deformation**• Change of angle between 2 initially orthogonal vectors • Shear angle**Strain during homogeneous Deformation**• Signification of the strain tensor components**Strain during homogeneous Deformation**An orthometric reference frame can be found in which the strain tensor is diagonal. This define the 3 principal axes of the strain tensor.**Strain during homogeneous Deformation**• The Green-Lagrange strain tensor (strain tensor)**Strain during homogeneous Deformation**• The Green-Lagrange strain tensor**Strain during homogeneous Deformation**• Rigid Body Transformation**Strain during homogeneous Deformation**• Rigid Body Transformation**Strain during homogeneous Deformation**• Polar factorisation**Strain during homogeneous Deformation**• Polar factorisation**Strain during homogeneous Deformation**• Pure deformation: The principal strain axes remain parallel to themselves during deformation**Some properties of homogeneous Deformation**• The strain tensor is uniquely characterized by the strain ellipsoid (a sphere with unit radius in the initial configuration)**Some properties of homogeneous Deformation**• The strain tensor is uniquely characterized by the • strain ellipsoid (a sphere with unit radius in the initial configuration)**Some properties of homogeneous Deformation**• Note that knowing the strain tensor associated to an homogeneous transformation does not define the uniquely the transformation (the translation and the rotation terms remain undetermined)**Homogeneous Transformation**x= RSX + c c x1=SX x2=Rx1 x= x2+c**Classification of strain**The Flinn diagram characterizes the ellipticity of strain (for constant volume deformation: with**E. Infinitesimal transformation**Infinitesimal strain tensor**Relation between the infinitesimal strain tensor and**displacement gradient**The strain ellipse**NB: The representation of principal extensions on this diagram is correct only for infinitesimal strain only**Rk: For an infinitesimal deformation the principal**extensions are small (typically less than 1%). The strain ellipse are close to a circle. For visualisation the strain ellipse is represented with some exaggeration**Relation between the infinitesimal strain tensor and**displacement gradient**F. Finite, infinitesimal and progressive deformation**• Finite deformation is said to be non-rotational if the principle strain axis in the initial and final configurations are parallel. This characterizes only how the final state relates to the initial state • Finite deformation of a body is the result of a deformation path (progressive deformation). • There is an infinity of possible deformation paths to reach a particular finite strain. • Generally, infinitesimal strain (or equivalently the strain rate tensor) is used to describe incremental deformation of a body that has experienced some finite strain • A progressive deformation is said to be coaxial if the principal axis of the infinitesimal strain tensor remain parallel to the principal axis of the finite strain tensor. This characterizes the deformation path.**Non-rotational transformation**A a B b x= SX + c

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