4: Testing Materials Flashcards
What is Hooke’s law?
Extension is proportional to force
Describe what happens to a wire when it is supported at the top and has a weight attached to the bottom. Talk about forces
It stretches
The weight pulls down with force F, producing an equal and opposite force at the support
What is k, in the equation F=kx?
Force or stiffness constant. Also for springs called the spring constant
Describe tension in a material (When a material is stretched)
Stretching a material creates tension across it. Forces of tension act along the same line as the forces stretching the material but in the opposite direction at each end of the material - they ‘pull’ on the object at either end of the material
What is need for a spring to change length?
The application of a pair of opposite forces
Describe the extension in the spring. When is it negative?
The extension of a spring is proportional to the force applied. If the forces are compressive, the spring is squashed and the extension is negative
In springs, is k the same if the forces are compressive or tensile?
The same value
Do all materials obey Hooke’s law?
No, but they all obey it up to a point
When does Hooke’s law stop working?
When the load is great enough
What does a material obey Hooke’s law look like on a force against extension graph?
Straight line through (0,0)
When does the graph of force against extension start to curve?
When the force becomes great enough. Starts to curve after the limit of proportionality
What happens if a material exceeds the elastic limit?
The material will be permanently stretched
When all force is removed, the material will be longer than at the start
What happens to the relationship between force and extension after the elastic limit?
The material will stretch further for a given force
A stretch can be elastic or ….
Plastic
When does a material show elastic and plastic deformation?
Elastic up to the elastic limit then plastic after the limit
What does elastic deformation mean?
The material returns to its original shape once the forces are removed
Describe elastic deformation microscopically
When a material is out under tension, the atoms of the material are pulled apart from one another
Atoms can move slightly relative to their equilibrium positions, without changing position in the material
Once the load is removed, the atoms return to their equilibrium, distance apart
If a deformation is plastic, the material is ….
Permanently stretched
Describe plastic deformation microscopically
Some atoms in the material move position relative to one another
When the load is removed, the atoms don’t return to their original positions
Describe the investigation of extension by stretching an object. How can you find k?
Support the object being tested at the top and measure its original length
Add masses one at a time to the bottom of the object
After each weight is added, measure the new length of the object, then calculate extension
Plot a graph of force (weight), against extension. Where the line of best fit is straight, the object obeys Hooke’s law, so gradient = k. If you’ve loaded the object beyond its limit of proportionality, the graph will start to curve
Describe the set up for the experiment investigating extension
Object, with weights attached, attached to a clamp, which is attached to a clamp stand. A ruler is put next to the clamp stand to measure the length of the object
If the force on the material stretches the material is it compressive or tensile?
Tensile
If a force squashes the material is it compressive or tensile?
Compressive
What causes a strain?
A stress
Define stress
The tension (the force applied) divided by the cross-sectional area
What are the units of stress?
Pa or N/m²
Define strain
What are the units?
The extension, i.e. the change in length, divided by the original length of the material
There are no units, it’s given as a number or percentage
What is the fracture stress?
A stress big enough to break a material
Describe the fracture stress microscopically
The effect of the stress is to start to pull the atoms apart from one another
Eventually the stress becomes so great that atoms separate completely, and the material fractures (breaks). Distress at which this occurs is called the fracture stress
What does the fracture stress look like on a graph of stress against strain?
It is the point where the line or graph stops
What is the ultimate tensile strength?
This is the maximum stress that the material can withstand before breaking
What is elastic strain energy?
The energy stored in a stretched material
When is work done in deforming a material?
When a material is stretched or compressed
On a graph of force against extension what represents the work done?
The area under the graph
Before the elastic limit where is all the work done, by stretching or compressing the material, stored?
Before the elastic limit, all the work done in stretching or compressing the material is stored as energy in the material
What is the work done on an elastic material equal to?
The work done on an elastic material, in stretching it, is equal to the energy stored in the material as elastic strain energy
Explain why the work done = Fx/2
Work done equals force * displacement
However, the force on the material isn’t constant. It rises from zero up to force F. To calculate the work done, use the average force between zero and F, F/2
Describe what happens with work done once a material is stretched be on the elastic limit
If the material is stretched beyond the elastic limit, some work is done separating atoms. This energy will not be stored as elastic strain energy, and so is it released when the force is removed
Up to the limit of proportionality which two values are proportional to each other?
Stress and strain
Explain, when testing Young’s modulus, why the wire should be as thin and as long as possible
The longer and thinner the wire, the more extends for the same force. This reduces the uncertainty in your measurements
Describe the experiment for the investigation of Young’s modulus of a very long wire
Calculate the cross-sectional area and the unstrechted length of the wire.
If you increase the weight, the wire stretches and the marker moves.
Increase the weight in steps, recording the marker reading each time. Use a digital scales to accurately find the weight added at each step
Use your results to calculate the stress and strain on the wire and plot a stress-strain graph
From a stress strain graph how can you find the Young’s modulus?
The gradient of the graph gives you the Young’s modulus.
What does the area under a stress strain graph give you?
The elastic strain energy per unit volume
Explain the term brittle
Brittle materials break suddenly without deforming plastically.
If you apply a force to brittle material, it won’t deform plastically, but will suddenly snap when the force gets to a certain size.
Brittle materials can also be quite weak if they have cracks in them.
Explain the term ductile
Ductile materials can be drawn into wires without losing their strength.
You can change the shape of ductile materials by drawing them into wires or other shapes. Important thing is that they keep their strength when they’re deformed like this
Why is copper perfect for making electric wires?
Copper is ductile, and with its high electrical conductivity this means that it’s at the ideal for electric wires
Describe the term strong
Strong materials can withstand high stresses without the forming or breaking.
Strength is a measure of how much a material can resist being deformed (bent, stretched, fractured etc.) by a force without breaking. This can be resisting a pulling force (tensile strength) or a squeezing force (compressive strength)
Describe the term stiff
Changing the shape of stiff materials is really difficult as they are resistant to both bending and stretching. Stiffness is measured by the Young’s modulus – the higher the value of the stiffer the material
Describe the term tough
Toughness is a measure of the energy and material can absorb before it breaks. Really tough materials can absorb a lot of energy so are very difficult to break.
Stress strain graphs for ductile materials…
Curve
On the stress strain graft for a ductile material, what is the elastic limit?
At this point the material starts to behave plastically.
From this point onward, the material would no longer return to its original shape once the stress was removed
On a stress strain graph for a ductile material, what is the limit of proportionality?
After this, the graph is no longer a straight line but starts to bend.
At this point, the material stops obeying Hooke’s law, but would still return to its original shape if the stress was removed
On a graph of stress against strain for a tactile material, what does the graph look like before the limit of proportionality?
The graph is a straight line through the origin. This shows that the material is obeying Hooke’s law
On a stress strain graph for ductile material, what is the yield stress?
Here the material suddenly starts to stretch without any extra load.
The yield stress is the stress at which a large amount of plastic deformation takes place with a constant or reduced load
Describe briefly the stress and strain graph for a brittle material
Brittle materials don’t tend to behave plastically. They fracture before they reach the elastic limit, therefore most of the graph is a straight line
Investigation of Young’s modulus:
Describe how to accurately find the length of the unstretched wire
Clamp the wire to the bench. Start with the smallest weight necessary to straighten the wire (don’t include this weight in your final calculations).
Measure the distance between the fixed end of the wire and the marker – this is your unstretched length
Is silicon a conductor, semi-conductor or an insulator?
Semi-conductor
