What’s elastic deformation?
- it’ll return to its original shape once the force is removed
1. when a material is under tension the atoms are pulled apart
2. Atoms can move slightly from their equilibrium positions without changing there position in the material
3. Once the load is removed, the atoms return to their equilibrium position
What’s plastic deformation?
- a material that‘s permanently stretched
1. atoms in the material move their position
2. When the load is removed the atoms don’t return to their original positions
What’s the Hardness of an object?
And an example?
The harder a material the more difficult it is to indent or scratch the surface
-example diamond
What are 2 different ways of measuring the hardness of an object?
- mohs scale of hardness, is a scale of 1 to 10
- Brinell Hardness Number (BHN)
What’s the Stiffness of an object?
- a stiff material exhibits very small deformations
- gradient of a force-extension graph
- high young modulus
- example: steel
How can stiffness be measured?
-modulus of elasticity
What’s the toughness of an object?
- a tough material is able to absorb the energy from impacts and shocks without breaking it
- tough materais undergo plastic deformation to absorb energy
- has a large plastic region
- example: copper & rubber tires
What’s the brittleness of an object?
- a Brittle object will shatter when subject to dynamic shocks
- brittle objects undergo little or no plastic deformation before fracture
- example: glass
What’s the strength of an object?
- a strong object can withstand large forces before it breaks
- the strength of a material depends on its size
- a material with high breaking stress example steel
What’s the malleability of an object?
- a malleable material can be hammered in thin sheets
- example gold
What’s the ductility of an object?
- ductile materials can be readily drawn into wires
- ductile materials are always malleable but malleable materials aren’t always ductile
- example copper
What’s hooke’s law?
It states that up to a given point, the extension/compression of a spring is directly proportional to the force applied to the spring
What does the k represent in F = k multiplied by change in x where F is force & x is extension?
K represents stiffness
It’s called spring constant
What’s the limit of proportionality?
-up until this point it’s called the elastic region where loading & unloading are reversible
What’s the elastic limit?
- beyond this point the wire stops to be elastic, so it won’t return to its original length
- and has undergone permanent deformation
What’s the yield point?
- it won’t contract when the load is removed
- the wire becomes plastic
- in this plastic region the bonds between the atoms are no longer being stretched & layers of atoms slide over each other with no restorative forces
What happens if the load is removed during the plastic phase and then the wire is reloaded?
The wire retains its springiness & has the same stiffness as before
What’s elastic strain energy/elastic potential energy?
- Is the ability of a deformed material to do work as it regains its original dimensions
- sometimes referred to as work done
How do you calculate the work done from a force extension graph?
It’s the area under the line
How do you calculate the stress of a material?
Stress (Pa) = force / cross-sectional area
How do you calculate the strain of a material?
Strain = extension / original length
-strain has no units as it’s a ratio of 2 lengths
How do you calculate the young modulus of a material?
Young modulus (Pa) = stress/ strain
What’s the difference between stress-strain and force-extension graphs?
- stress-strain graphs is always the same for a given material
- where’s force-extension graphs depend on the dimensions of the sample used
How many m2 in mm2?
1 mm2 = 1 x 100-6 m*2
