Materials Flashcards
What is Hookes Law?
Extension (Change in L) is directly proportional to force applied (F) given that the environmental conditions are kept constant and the limit of proportionality hasn’t been surpassed.
F= k * Delta L
(k =spring constant)
(Delta L = change in length)
What is Meant by Tensile Stress?
The Force Applied per Cross sectional Area
F/Area
Nm^-2
What is meant by Tensile Strain?
What proportion of a materials original length it has extended by
A Measure of how the material stretches
Delta L / L
What is the Difference Between Plastic and Elastic Deformation
Elastic: When a force is removed the object will return to its original shape
PlasticL After the force is removed the object will not return to its original shape
What is Breaking Stress
The Maximum Stress a material can endure before fracturing
What is meant when a material is described as brittle
A Material deforms elastically, but breaks immediately when the stress reaches a certain value (elastic limit)
What is Elastic Limit
The point which stress causes a material to exhibit plastic behaviour
If surpassed the material will begin to undergo plastic deformation
What does the area be near a force-extension graph represent?
The work done to deform the material
WD = 0.5F Delta L
How do you calculate Elastic Strain Energy
0.5k(Delta L)^2
What is Young’s modulus
Tensile Stress/Tensile Strain
Describes a materials elasticity
How do you find Young’s modulus from a stress strain graph
The Gradient
On a force Extension graph what is the significance of the unloading line?
Unloading line tells you if the material has undergone plastic deformation
If Unloading line doesn’t go through the origin then the material is permanently stretched
How does a force extension graph show if a material is obeying hookes law?
Constant Gradient Line through the origin
F and Delta L are directly proportional
What is the limit of proportionality
The point after which Hookes Law is no longer obeyed
Shown by a curve forming after a constant gradient on a force-extension graph
Describe & explain the relationship between loading and unloading lines on a plastically deformed material
Parallel but not identical
The Spring constant k hasn’t changed
The forces between the atoms are the same when loading and unloading
But the material is permanently deformed so when no force is applied it is not at its original length
Under Plastic deformation why isn’t all the work done stored as elastic strain energy
Work is done to move the atoms apart.
Energy as a result is dissipated as heat energy
Explain one uses for the knowledge of plastic and elastic deformation
. Crumple Zones deform plastically to minimise kinetic energy transfer to the passengers
. seat belts stretch to convert the passengers kinetic energy to elastic strain energy
Outline the Energy Changes when a spring with a fixed top is pulled down and released
Elastic Strain (WD pulling down) —> Kinetic (As Spring is released) —> Gravitational (as spring rises)
How do you find the ultimate tensile stress
Highest point on a stress strain graph
Maximum stress a material can withstand before fracture
What is meant when a material is described as ductile
A Ductile material can withstand a larger amounts of stress when deforming plastically, before fracturing after the elastic limit has been reached
How do you calculate the spring constant for 2 spring connected in series?
1) Calculate extension of all springs and equate it to f/K
2) Total Extension = Extension in first + Extension in second
3) Sub in f/K
4) as the f applied will be the same for both you can divide through it
1/K = 1/(first spring K) + 1/(second spring K)
How do you calculate the spring constant for 2 spring connected in parallel?
1) Force total = Force(first Spring) + Force(Second Spring)
2)Force total = (K(first) + K(second))* Delta L
3) K Total = (K(first) + K(second)) as F/Delta L = K
Describe an Experiment to test young’s modulus (6 Points)
1.) Find the cross-sectional area of the wire. Use a micrometer and measure the diameter of the wire in several places. Then use formula for area of a circle for finding cross sectional area.
2.) Clamp the wire to the bench so you can hang weights off one end, add a marker at suitable distance from the clamp point
3.) Measure the distance between the fixed end of the wire and the marker - this is the un-stretched length
4.) Then if you increase the weight, the wire stretches and the marker moves
5.) Increase the weight hung off of the end of the wire in steps, recording the marker reading each time - the extension is the difference between this reading and the un-stretched length.
6.) You can use your results from this experiment to calculate the stress and strain of the wire and plot a stress-strain curve
Why do Armco Barriers damage crash test dummies less then concrete walls.
Armco barriers deform plastically, this means the contact time between :) the car and the barrier has increased while the barrier deforms
Impulse = Change in momentum = force * time
Dummies as a result receive a smaller impulse so they incur less damage
