Materials Flashcards
Define DENSITY
The mass per unit volume of a material
Equation for DENSITY?
Density (kgm^-30= Mass(kg)/Volume(m^3)
D=M
—–
V
Characteristics of DENSITY
Density does not vary with size or shape, depends on material only
Define HOOKE’S LAW
The extension of a material is directly proportional to the force applied to it until the limit or proportionality is reached
Equation for HOOKE’S LAW?
Force(N)=spring constant(Nm^-1) x extension (m)
F = KX
What is directly proportional in HOOKE’S LAW?
Extension and Force applied
What does a tensile force do to a spring?
Stretch the spring
What does a compressive force do to a spring?
Compress the spring
When is the limit of proportionality broken?
As soon as the graph begins to curve
What is the point called when the graph just begins to curve on a force - extension graph?
The elastic limit
What happens if you continue to apply force beyond the elastic limit?
The material will become permanently stretched and will be longer than before the force was applied.
What is it called when the material is stretched to a point it will no longer return to original length?
Plastic deformation
Think of a practical to investigate extension of a material
- Get a ruler, clamp, clamp stand, material and 1kg of weights
- Set up equipment so weights are hanging off of the material you are testing and the ruler is up against the clamp stand
- Measure original length before adding additional weight.
- When original weight measured increase the weights by 100g and record new length
- Do this until all weights are used.
- Then unload and record lengths per 100g you take back off
- Plot a graph of force against extension
What is elastic deformation?
The material returns to its original state when the force is removed
What happens in terms of the atoms of the material in elastic deformation?
When under tension the atoms are pulled part form one another. They can move small distances from their equilibrium without changing position in the material. When un-loaded the atoms return to their point of equilibrium
Define STRESS
Force applied per unit cross-sectional area
Equation for STRESS
Stress(Pa or Nm^-2) = force(N)/cross sectional area(m^2)
sigma = F
————
A
Define STRAIN
Extension per unit length before extension
Equation for STRAIN
Strain = extension(m)/original length(m)
Large curly E = Delta L
——————
L
When calculating is a tensile force +ve and a compressive force -ve?
YES
What does UTS stand for?
Ultimate Tensile Strength
What is UTS?
The maximum stress a material can withstand
Where is point of UTS on a stress - strain graph?
When the gradient is no longer +ve or zero
What is breaking stress?
When the atoms of a material actually break away from each other causing the material to break
Where on the stress-strain graph is breaking stress?
At the end of the graph
What is elastic strain energy?
The energy required or work done to stretch a material and is stored as potential energy
How would you calculate the strain energy on a graph?
Find the area underneath a force - extension graph
What is the Young’s Modulus a measure of?
The stiffness of a material
Equation for YOUNG’S MODULUS
YM= tensile stress/tensile strain
E(Pa or Nm^-2)= Sigma
————–
Big Curly E
Define YOUNG’S MODULUS
A constant for the linear relationship of a material between tensile stress and strain
Prepare an experiment to determine the Young’s Modulus of a piece of steel wire
- Get a long thin piece of test wire as is possible
- Find the cross-sectional area of the wire using a micrometer in several place along the wire and make an average assume the wire is circular
- Clamp the wire to a bench and put weights on the end of the wire so that it takes the slack then measure the length of the unstretched wire
- Increase the weights and take measurements every increase in weight
- Use results to plot a stress-strain graph and then find the gradient of the straight part to calculate the Young’s Modulus
What is the yield point?
The point when the material stretches without much extra force being required (large amount of plastic deformation - Can be shown on a stress-strain graph by the curves gradient being very close or almost zero
What is the difference between a force - extension and stress - strain graph
Force - Extension shows the force and extension relationship for a specific object that is being tested
Stress- Strain shows the material in generals relationship
If a metal wire is plastically deformed, when unloaded what will happen?
The gradient of the unloading will be the same as the loading gradient when it was elastically deformed (parallel to each other)
Features of a brittle material?
On a stress - strain graph the material will have a very sharp curve at the end or no curve at all when it reaches its breaking stress
There are strong bonds between brittle materials but the atoms are all facing different directions, so when the force is applied atoms can’t move to bend with the material
