covers elasticity,plasticity,viscosity states

1. PHYSICAL STATES OF A MATERIAL
By,
K.VINITHA
2018694619
M.TECH (FPE)

2. INTRODUCTION
• Rheology is the study of deformation and flow of
material under the action of applied forces
• Measured using rheometer
• Three important parameters – force, deformation and
time for expressing the mechanical behaviour of the
material.
• Time is the most important parameter when load is
applied to the body
• Three basic properties- elasticity, plasticity and
viscosity.

3. PHYSICAL STATES OF A MATERIAL
• The state of various materials depends on load and deformation to which it is subjected
and also to the environmental factors such as temperature.
• Biological materials-moisture content affects the physical state of the material.
• All materials with linear response behaves corresponding to one or more zones.
Given by,
• Creep compliance function J(t).
• Relaxation modulus function G(t).

4. CREEP COMPLIANCE J(t)
The creep compliance function J(t)- ratio of shear strain to shear stress at any given time t when the load or
stress is kept constant.
Zone 1- corresponds to perfectly elastic material, the state of a material does not depend on the time.
Zone 11- material exhibits both elastic and viscous effect, deformation is proportional to time -the strain
in a material increases with time.
Zone 111- material exhibits rubbery state- shows non-linear elasticity behaviour
Zone 1V- stress is proportional to a perfect fluid or rate of strain

5. • The relaxation modulus function G(t)- ratio of shear stress to shear strain at any given time
when strain is kept at constant.
• Linear responses can be achieved only if critical stress is not exceeded.
• Critical stress is exceeded plastic deformations are observed.
Stress relaxation- gradual decrease of stress when strain is kept constant,
Stress in a viscoelastic material decreases with time
In linear materials – independent of strain level and function of time alone

6. ELASTIC BEHAVIOUR
• Ideal elastic behaviour represented by Hookean body
• The Hooke’s law states that stress is directly proportional to strain
with in elastic limit.
• Materials may hold small strains generally less than 0.1% in some
solids.
• Complete recovery of strain takes place upon removal of stress.
• Biological materials such as fruits and vegetables, cereal grains
etc.- does not exhibit Hookean elasticity even for very small
stress.
Non-linear elasticity

7. PLASTIC BEHAVIOUR
• Ideal plastic behaviour is represented by St.Venant body.
• Represented by a mechanical model consists of a frictional block over a
surface, that exhibits a static friction.
• Pulling force should be greater than the static friction, displacement
gradient is the shearing strain of the plastic material.
• The flow of the material does not start until a limiting value of shearing
stress (yield stress) is reached.
• The material cannot sustain greater strain-flow occurs indefinitely under
the stress.

8. VISCOELASTIC BEHAVIOR
• Viscoelasticity is the property of materials that exhibit
both viscous and elastic characteristics when undergoing
deformation.
• Viscoelastic materials have elements of both of these
properties and, as such, exhibit time-dependent strain.
• Viscoelastic substance loses energy when a load is
applied, then removed.

9. • Depending on the change of strain rate versus stress inside a material the
viscosity can be categorized as having a linear, non-linear, or plastic
response.
• When a material exhibits a linear response it is categorized as a Newtonian
material. In this case the stress is linearly proportional to the strain rate
• If the material exhibits a non-linear response to the strain rate, it is
categorized as Non-Newtonian fluid.

10. PURE VISCOUS BEHAVIOUR
• Newton’s law of viscosity and dashpot- represents pure viscous behaviour.
• Pure viscous flow- slightest force can cause movement of liquid and the
rate of its flow is proportional to the force applied.
• Flow of liquid continues infinitely until the force is removed.
• Like plastic flow – will not regain its original state.
• A material with such a nature has a rheological constant-coefficient of
viscosity.
• Co-efficient of viscosity = shear stress/ shear rate.
• Even if the yield value=0 ,internal friction causes the flow.

11. NON-NEWTONIAN FLUIDS
• Flow properties are not described by a constant value of viscosity.
• Non-Newtonian fluids are ketchup, starch suspensions, paint etc.
• The relation between the shear stress and the shear rate is different, and can even
be time-dependent or purely viscous. Therefore, a constant coefficient of
viscosity cannot be defined.
• Three different types- shear thinning or psuedoplastic, visco-plastic and shear
thickening or dilatant.

12. TIME INDEPENDENT FLUIDS
Pseudo-plastic fluids: Characterized by an apparent viscosity that decreases with
increasing shear rate.
The rate of decrease of the apparent viscosity is not same for each fluid. (display
the opposite properties of dilatant materials)
Visco-plastic fluid:Do not deform when subjected to a shear stress smaller than a
certain value, - yield stress.
If the shear stress in the fluid exceeds the yield stress - fluid deforms as a
(nonlinearly)
viscous fluid -shear thinning, since the fluid structure breaks down progressively
with shear.
A Bingham plastic is a visco-plastic material that behaves as a rigid body at low
stresses but flows as a viscous fluid at high stress.
Dilatant fluids: liquids or solutions whose viscosity increases as stress is applied.
some dilatant fluids have the unique property of being able to turn from liquid to a
solid just by having stress applied.

13. TIME DEPENDENT FLUIDS
• The viscosity of the fluid is dependent on temperature, shear rate and time.
• Non-Newtonian fluids show a time-dependent change in viscosity and a
non-linear stress-strain behavior .
• Thixotropic fluid - time dependent shear thinning property. Takes time to
attain viscosity equilibrium when introduced to a step change in shear rate.
• Rheopectic fluids -exhibit a time-dependent increase in viscosity; The
longer they undergo a shearing force, the higher their viscosity becomes, as
the microstructure of a rheopectic fluid builds under continuous shearing.