5: Materials Flashcards
What is the equation for density?
mass/volume
What is Hooke’s law?
the extension of a stretched wire is proportional to the load or force
what is the equation for Hooke’s law?
F = kΔL
What do tensile forces do to a string?
Stretch the spring
What do compressive forces do to a spring?
Squash the spring
What is represented by the gradient of a force-extension graph?
Spring constant
What is meant by the elastic limit?
The point where the force applied to an object becomes so great it no longer obeys Hooke’s law, and the graph starts to curve
What happens if you increase the force past the elastic limit?
- the material will be permanently stretched
- When all the force is removed, the material will be longer than at the start
What is meant by the limit of proportionality?
- The point at which a material no longer obeys Hooke’s law
How could you investigate how extension varies with force?
- The object should be supported at the top, e.g. using a clamp, and a measurement of its original length should be taken using a ruler
- Weights should be added one at a time to the other end of the object
- The weights used will depend on the object being tested - a trial investigation should be completed to work out the range and size of the weights needed
- After each weight is added, the extension should be calculated
- extension = new length - original length
- A graph should then be plotted
What does it mean if a deformation is elastic?
The material returns to its original shape once the forces are removed - so it has no permanent extension
How does elastic deformation work in terms of atoms?
- when a material is put under tension, the atoms of a material are pulled apart from one another
- atoms can move small distances relative to their equilibrium positions without actually changing position in the material
- Once the load is removed, the atoms return to their equilibrium distance apart
What does it mean if a deformation is plastic?
- the material is permanently stretched after the force is removed
- Some of the atoms in the material move position relative to each other
- When the load is removed, the atoms don’t return to their original positions
What is meant by tensile stress?
the force applied, F, divided by the cross-sectional area
- stress = F/A
What is meant by tensile strain?
- the change in length (extension) divided by the original length
- ΔL/L
How does stress work in terms of atoms?
- It starts to pull atoms apart from one another
- The stress eventually becomes so great that atoms separate completely, and the material breaks
What is meant by the breaking stress?
The stress great enough to break a material
what is UTS?
- ultimate tensile stress
- the maximum stress a material can withstand
What is represented by the area under a force-extension graph?
Elastic strain energy
the energy stored by the stretched material is equal to…
The work done on the material in stretching it
What is the equation for elastic strain energy?
E = 1/2FΔL
Energy changes in an oscillating spring can be summed up as:
change in kinetic energy = change in potential energy
What is the purpose of crumple zones in cars?
- They deform plastically in a crash
- This means some of the car’s kinetic energy goes into changing the shape of the vehicle’s body, so less is transferred to the people inside
What is the Young’s modulus?
The constant produced by stress/strain when a material has not reached its limit of proportionality
What is the equation of the Young modulus?
E = FL/AΔL
How could you investigate the Young’s modulus of a material?
- Use a long, thin wire to reduce uncertainty
- Find the CSA of the wire (micrometer)
- Clamp the wire to the bench so you can hang weights off one end of it
- Start with the smallest weight necessary to straighten up the wire
- Measure the distance between the fixed end of the wire and the marker
- if you increase the weight, the marker moves
- Increase the weight in steps, recording the marker reading each time - the extension is the difference between this and the original length
- Use results to work out stress and strain then plot a graph
- The Young modulus is the gradient of the graph
What is meant by the yield point of a material?
- the point at which a material starts to stretch without any extra load
- the stress at which a large amount of plastic deformation takes place with a constant or reduced load
Why is the loading line parallel to the unloading line on a F-ΔL graph?
the spring constant remains the same
How do you know a wire has been stretched past its elastic limit and deformed plastically on a F-ΔL graph?
- The unloading line doesn’t go through the origin
What would the stress-strain graph look like for a brittle material?
- A straight line that does not curve
- Always goes through the origin, always obeys Hooke’s law
- Same for F-ΔL graphs
Do brittle materials deform plastically?
no
- When stress reaches a certain point, the material snaps
What is meant by brittle fracture?
- when a stress is applied to a brittle material cracks at the material’s surface get bigger and bigger until the material breaks completely
- These cracks are able to grow due to the rigid structure of the material
