Rheology

Rheology

1. Science of the flow and deformation of matter under the effect of an applied force.

   • Stress.- Measure of internal forces acting within a deformable body.
     • Shear stress- Deformation from the action of a force per unit of area.
       • Newton's Law
   • Deformation
     • Change of the shape and the size of a body due to applied forces (external and internal)
       • Force exerted is larger than the internal force. - Solids and liquids
     • Static & Dynamic

2. Principal characteristics of materials

   • Elastic
     • Matter is reverted to the original form after stress is removed
       • All energy added is stored in the material
   • Viscous
     • Viscosity depends of temperature and shear rate
       • All energy added is dissipated into heat
   • Plastic
     • Permanent deformation without breaking down
       • Viscoelastic= Viscoelestic + elastic behavior
       • Real plastic= Viscous + plastic behaviour

3. Important concepts

   • Yield stress
     • The amount of stress that an object needs to experience for it to be permanently deformed.
   • Yield strenght
     • The stress at which a material begins to plastically deform.
   • Yield Value
     • A certain shear strees must be applied before the flow occurs.

4. Elastic Solids

   • Mechanical energy of a solid depends on shape as well as volume. Resist forces act to change its shape.
     • Responds ELLASTICALLY to forces that are less tnan its yield strength
       • When the force exceeds the yield strength, it deforms PLASTICALLY and permanently changes its shape.
         • Hooke's Law

5. Fluids

   • Newtonians
     • Shear stress is directly proportional to the rate of shear strain or velocity gradient
   • Non Newtonians
     • Fluids that NO depends of time
       • Bingham plastic
         • Pseudoplastic fluids
           • Dilants fluids
     • Fluids that depends of time
       • Thixotropic
       • Rheopetic
     • Do not follow the Newton's Law

6. Viscoelastic fluid

   • Show both viscuous and elastic properties
   • Exhibit an elastic recovery of the deformations that occur during flow

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