Materials Flashcards
(30 cards)
Give Hooke’s Law in words.
- The change in length of an object
- Is directly proportional to the force applied,
- Up to the limit of proportionality.
Define density and give the equation (from the data sheet) and units.
- Density is the mass per unit volume.
- Units = kgm-3.
- p = m/v
Use the data sheet to give the Hooke’s law equation.
F = K ΔL
k = spring/stiffness constant
ΔL = change in length
Explain what k represents in Hooke’s law and give units.
- k = spring constant or stiffness constant.
- Specific to a certain spring or wire (not a certain material).
- Larger k = larger force for given extension.
- Units = Nm-1.
Where on a force extension graph should the limit of proportionality (P) be and what does the gradient represent?
- P should be just before it starts to plateau
- Gradient = k
Where on an extension force graph should the limit of proportionality (P) be and what does the gradient represent?
- P should be just before it starts to curve up
- gradient - 1/k
give the equation for calculating the combined spring constant for springs in series
1/k = 1/k + 1/k +…
give the equation for calculating the combined spring constant for springs in parallel
k = k + k +…
Give the equation for energy stored in a deformed object
E = 0.5 x F x ΔL
F = force applied
ΔL = change in length
How can you calculate energy stored from a force-extension graph?
Area under the graph.
Describe elastic deformation.
- Deformation in which the object returns to its original shape and size when the forces are removed.
Describe plastic deformation
- Deformation in which the object does not return to its original shape and size when the forces are removed.
- Object permanently deformed.
Describe the elastic limit.
- Up to this point elastic deformation occurs.
- Beyond this point plastic deformation occurs.
Sketch a loading and unloading curve for a metal wire stretched beyond its elastic limit. Describe key features.
- Loading and unloading lines are parallel.
- Wire is permanently deformed.
- Read permanent extension off the intersection of unloading curve with x axis.
Sketch a loading and unloading curve for rubber (e.g. elastic band).
- Deformation is elastic throughout.
- Rubber returns to original shape and size.
Sketch a loading and unloading curve for a polythene strip (e.g. carrier bag) stretched beyond its elastic limit.
- Undergoes plastic deformation.
- Permanently deformed.
How do you work out the energy stored during loading?
- Area under the loading curve.
How do you work out the energy recovered during unloading?
- Area under the unloading curve.
How do you work out the internal energy stored after unloading?
- Area enclosed between the loading and unloading curve.
Define stress in words and give the equation (data sheet) and units.
- Stress is the force per unit cross sectional area.
- Units = Nm-2 or Pa
sigma = F/A
Define strain in words and give the equation (data sheet) and units
- Strain is the extension per unit length.
- No units.
epsilon = ΔL/L
Give the equation for the Young Modulus from the data sheet. What is Young modulus a measure of? Give the units.
- Young modulus is a measure of stiffness.
- Characteristic of a material.
- Units = Nm-2 or Pa
- Young Modulus = stress/strain
Substitute in expressions for stress and strain into the Young modulus equation.
E = sigma/epsilon = (F/A) / (ΔL/L) = FL/AΔL
On a stress-strain graph, what does the gradient of the straight-line section represent?
- Young modulus.
- Higher gradient = stiffer material.
