Unit 3 - Elastic Deformation Flashcards
Four important material properties associated with elastic deformation
Youngs Modulus
Bulk Modulus
Shear Modulus
Poissons Ratio
Elastic Modulus
Measure of the resistance of a material to elastic deformation
Four types of stress
Pure shear
Biaxial
Hydrostatic pressure
Simple Tension
What is viscosity?
A measure of a fluids resistance to flow
large viscosity = resists motion
low viscosity = flows
Assumptions for Hookes law
Homogenous
Isotropic
Hasn’t exceeded elastic limit
If a force is not normal to the surface it acts up on how can it be resolved?
two components:
Perpendicular to the surface is the tensile stress
Parallel to the surface is the shear stress
4 fundamental elastic constants
Youngs Modulus
Bulk Modulus
Shear Modulus
Poissons Ratio
Standard Poissons ratio for metals, polymers and rubber
Metals = 0.3 Polymer = 0.4 Rubber = 0.5
Difference between engineering and true stress and strain
engineering stress and strain use the original area in the elastic region of the material because strain is less than 0.1%
True stress and strain use the current area of the material
what is the exception to using engineering stress ands strain
Rubber because it undergoes large deformations in the elastic region
What is Youngs Modulus/Modulus of elasticity?
Property of linear elastic material under tensile load.
The measure of resistance to elastic deformation
What is Youngs Modulus a measure of?
Stiffness of a material
What is the yield stress?
The point at which a material plastically deforms.
At this point it will not return to its initial dimensions
Three definitions that describe viscoelasticity
A material which has elastic and viscous properties when undergoing deformation
It loses energy when a load is removed
A molecular rearrangement
How do viscous, elastic and visco-elastic materials typically behave
Viscous - Flow when a shear force is applied
Elastic - Stretch and return to its original shape with load
Viscoelastic - has elements of viscous and elastic and is usually dependant on time
What is creep?
movement or rearrangement of molecules in a solid material.
For example when a stress is applied
to a viscoelastic material such as a polymer, parts of the long polymer chain change positions
describe how a polymer behaves viscoelastically
When a stress is applied to the polymer the materials creeps and accumulates a back stress to counteract the force. When the back stress is equal to the stress the material no longer creeps. As the stress is removed the material the back stresses allow the material to return to its original position.
Describe Bulk Modulus
The inverse of the compressibility of a material
K= 1/C = (F/A0)*(V0/dV)
If K is v big then the material is really incompressible
Shear Modulus definition
the ratio of shear stress to the displacement per unit sample length (shear strain)
It can be experimentally determined using the gradient of a stress strain graph
Conditions of Shear modulus
No normal forces, no compression or tension, no change in volume
Resilience
The ability of a material to absorb energy when deformed elastically and to return it when unloaded
