Unit 3 - Elastic Deformation

Question 1

Four important material properties associated with elastic deformation

Answer 1

Youngs Modulus
Bulk Modulus
Shear Modulus
Poissons Ratio

Question 2

Elastic Modulus

Answer 2

Measure of the resistance of a material to elastic deformation

Question 3

Four types of stress

Answer 3

Pure shear
Biaxial
Hydrostatic pressure
Simple Tension

Question 4

What is viscosity?

Answer 4

A measure of a fluids resistance to flow
large viscosity = resists motion
low viscosity = flows

Question 5

Assumptions for Hookes law

Answer 5

Homogenous
Isotropic
Hasn’t exceeded elastic limit

Question 6

If a force is not normal to the surface it acts up on how can it be resolved?

Answer 6

two components:
Perpendicular to the surface is the tensile stress
Parallel to the surface is the shear stress

Question 7

4 fundamental elastic constants

Answer 7

Youngs Modulus
Bulk Modulus
Shear Modulus
Poissons Ratio

Question 8

Standard Poissons ratio for metals, polymers and rubber

Answer 8

Metals = 0.3
Polymer = 0.4
Rubber = 0.5

Question 9

Difference between engineering and true stress and strain

Answer 9

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

Question 10

what is the exception to using engineering stress ands strain

Answer 10

Rubber because it undergoes large deformations in the elastic region

Question 11

What is Youngs Modulus/Modulus of elasticity?

Answer 11

Property of linear elastic material under tensile load.

The measure of resistance to elastic deformation

Question 12

What is Youngs Modulus a measure of?

Answer 12

Stiffness of a material

Question 13

What is the yield stress?

Answer 13

The point at which a material plastically deforms.

At this point it will not return to its initial dimensions

Question 14

Three definitions that describe viscoelasticity

Answer 14

A material which has elastic and viscous properties when undergoing deformation
It loses energy when a load is removed
A molecular rearrangement

Question 15

How do viscous, elastic and visco-elastic materials typically behave

Answer 15

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

Question 16

What is creep?

Answer 16

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

Question 17

describe how a polymer behaves viscoelastically

Answer 17

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.

Question 18

Describe Bulk Modulus

Answer 18

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

Question 19

Shear Modulus definition

Answer 19

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

Question 20

Conditions of Shear modulus

Answer 20

No normal forces, no compression or tension, no change in volume

Question 21

Resilience

Answer 21

The ability of a material to absorb energy when deformed elastically and to return it when unloaded