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Questions and Answers
What defines elastic deformation in materials?
What defines elastic deformation in materials?
- Permanent change in shape under load
- Inability to return to original shape
- Recovery of shape after the removal of load (correct)
- Deformation dependent on the duration of load
How is stress defined in the context of materials?
How is stress defined in the context of materials?
- Force applied to a material per unit length
- Force applied to a material per unit mass
- Force applied to a material per unit area (correct)
- Force applied to a material per unit volume
What does Hooke's law enable us to define for materials?
What does Hooke's law enable us to define for materials?
- Shear modulus
- Young’s modulus (correct)
- Viscosity coefficient
- Strain rate
Which statement is true regarding Newtonian fluids?
Which statement is true regarding Newtonian fluids?
At what conditions do polymers exhibit elastic behavior?
At what conditions do polymers exhibit elastic behavior?
How do polymers behave at high temperatures and low rates of strain?
How do polymers behave at high temperatures and low rates of strain?
What characterizes the elastic properties of polymer fibers?
What characterizes the elastic properties of polymer fibers?
In terms of stress and strain, how is Young's modulus (E) expressed?
In terms of stress and strain, how is Young's modulus (E) expressed?
Flashcards
Stress
Stress
The force applied to a material per unit area. Measured in N/m².
Strain
Strain
The relative deformation of a material when forces are applied. Measured as a ratio of change in length to original length.
Elastic Deformation
Elastic Deformation
A material's ability to stretch or deform under an applied force and return to its original shape once the force is removed.
Hooke's Law
Hooke's Law
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Young's Modulus
Young's Modulus
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Viscoelastic Material
Viscoelastic Material
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Newtonian Fluid
Newtonian Fluid
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Viscosity
Viscosity
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Study Notes
Viscoelasticity
- Elastic deformation is temporary deformation under external force, reverting to original shape upon force removal.
- Stress is force applied per unit area (N/m²).
- Strain is relative deformation in a material (e = dl/l).
- Hooke's Law states stress is proportional to strain (σ = Ee), useful for low strains in polymers. Young's Modulus (E) quantifies this relationship (E = stress/strain).
- Newtonian fluids obey Newton's Law: stress is proportional to strain rate (σ = η(de/dt)), where η is viscosity. This relationship is independent of strain.
Polymer Types
- Elastomers have a Young's Modulus range of ~10⁶-10⁷ Pa.
- Semi-crystalline polymers have a Young's Modulus range of ~10⁸-10⁹ Pa.
- Glassy polymers have a Young's Modulus ~10⁹ Pa.
- Polymer fibers parallel to the fiber axis have a Young's Modulus of ~10¹¹ Pa. Perpendicular direction have a Young's Modulus of ~10⁹ Pa.
Viscoelasticity of Polymers
- Polymer behavior is intermediate between elastic solids and viscous liquids.
- At high strain rates and low temperatures, polymers display elastic properties.
- At low strain rates and high temperatures, polymers exhibit viscous behaviour (flowing like liquids).
- Polymers are thus viscoelastic: exhibiting aspects of both viscous and elastic behavior.
Time-Dependent Behavior
- Creep: Constant stress applied, initial rapid strain increase followed by gradual decrease. Deformation persists.
- Stress relaxation: Constant strain maintained, stress gradually decreases over time.
- Constant stress rates: Stress-strain relationship varies over time while deforming.
- Constant strain rates: Stress-strain relationship changes over time while deforming.
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