Section 5 - Materials Flashcards
What is density?
Mass per unit volume
Does density vary with size or shape?
Neither
What is the density of water?
1gcm cubed
What does Hooke’s law say?
Extension is proportional to force
What do tensile forces do?
Stretch the spring
What do compressive forces do?
Squash the spring
When does Hooke’s law stop working?
When the force exceeds the limit of proportionality
What is the equation for Hooke’s law?
F = k delta L
What is elastic deformation?
The material returns to its original shape and size
What is plastic deformation?
The material is permanently stretched
What happens to atoms in a material when it is put under tension in elastic deformation ?
The atoms are pulled apart from each other
What happens to atoms after the load is removed in elastic deformation?
They return to their equilibrium distance part
Is energy conserved while stretching?
Yes
Is elastic deformation, what is the work done stored as?
Elastic strain energy
What happens in elastic deformation after the stretching force is removed in terms of energy?
The elastic strain energy is transferred into other forms
In plastic deformation, what is the work done stored as?
Energy is not stored as strain energy. Work is done to separate atoms
In terms of energy is transferred, why are crumple zones good?
Some energy goes into changing the shape of the vehicle’s body as it deforms plastically, so less energy is transferred to the people inside
What does a stress cause?
A strain
What is the material called when a force can stretch it?
Tensile
What is the material called when a force cannot stretch it?
Compressive
Define tensile strain
The change in length divided by the original length of the material
Define tensile stress
Force applied divided by the cross-sectional area
What is the unit of strain?
It has no unit
What is the unit of stress?
N/m^2
What is breaking stress?
A stress big enough to break the material
What is the UTS on a graph?
The ultimate tensile stress, the max stress that a material can withstand
Describe the stress-strain graph for a material where the force is constantly increasing
Straight line through the origin, then starting to curve as it exceeds limit of proportionality, then UTS where the curve starts going down
What does the area under a force-extension graph represent?
Elastic strain energy
What is the Young’s Modulus?
Stress/strain
What is the unit of Young’s Modulus?
N/m^2
What does Young’s Modulus measure?
The stiffness of a material
Describe the experiment for Young’s Modulus
- Get a long and thin wire
- Measure cross-sectional area multiple times and average
- Clamp wire to bench and add weights
- Measure distance between fixed end of wire and marker
- As weight increases the marker moves
- Increase weight in steps
- Calculate stressStrain using FL/A delta L
What does the gradient of a stress-strain graph give?
Young’s Modulus
What are the 3 important points on a stress-strain graph?
- P is the limit of proportionality (when the graph is no longer a straight line)
- Below P is the straight line through the origin which shows that the material is obeying Hooke’s Law
- E is the elastic limit, where the material starts to behave plastically
- Y is the yield point where the material stretches without any extra load
What type of line do you get for a brittle material on a stress-strain graph?
Straight line (no curves)
Describe the stress-strain graph for a BRITTLE material where the force is constantly increasing
Straight line through the origin to show the material is obeying Hooke’s law, then a sudden break in the material as it doesn’t deform plastically
What is the difference between a force-extension graph and a stress-strain graph?
F-E - specific for the tested object and depend on its dimensions
S-S - Describe general behaviour of material, as stress and strain are independent of dimensions
Describe the stages for a force-extension graph for a material where the force is constantly increasing
- Straight line through origin
- Starts to curve when material exceeds elastic limit, and curves even more when the material starts to plastically deform
- When load is removed, unloading line is parallel to loading line
- Unloading line finishes at 0 force, but a longer extension as the material has been permanently stretched
What does the area between a loading and unloading line on a force-extension graph represent?
Work done to permanently deform a wire
