Materials Flashcards
What is the equation for density?
Density = mass/ volume P = m/v
Define Density
Mass per unit volume
In words what does hooks law state?
That the extension of a wire, change in l is proportional to the force applied.
As an equation what does hooks law state?
F= k x change in length .
What is an objects stiffness constant ?
The force needed to extend it by 1m , it depends on the objects size an shape.
Does hooks law work for compression?
Yes
How do you calculate extension
New length - old length
On a graph how do you know hooks law is being obeyed?
A straight line through the origin.
What is the limit of proportionality?
The point beyond which the force is no longer proportional to the extension. The hooks law limit.
What is the elastic limit?
The point at which the material will be permanently stretched and not return to the same size.
If a material is stretched past its elastic limit it deforms …..
Plasticly
Something that has undergone plastic deformation has …? What does this mean?
Gone past the elastic limit and so will not return to its original shape.
Elastic deformation means that …
The material will return to its original shape.
Why can materials stretch?
The atoms in the material can move small distances relative to their equilibrium positions without changing the structure of the material.
Tensile stress is calculated by
Stress = force/ area
What is strains units?
It has no units
What is the equation for strain?
Tensile strain =
Change in length/ starting length
What are the units for stress?
Nm^-2 or pascals, Pa
What is breaking stress?
Stress that I large enough to break apart the material
What is the maximum strength a material can withstand called?
Ultimate tensile stress (UTS)
When an item is stretched, before the elastic limit where is the energy stored?
As elastic potential energy
When a material is stretched what is the elastic potential energy called?
Elastic strain energy
What is elastic strain energy?
the name for when a material is stretched and energy is stored as the elastic potential energy.
What is the area under a stress strain graph?
Energy per unit volume
How would you find the energy stored (work done) using a force/ extension graph
Find the area under the line,
1/2 x force x change in length
What is the equation. For energy stored in a stretched/compressed object? What is necessary for this to be true? (Graph)
Energy= 1/2 x force x change in length.
The material must be obeying hooks law.
How can the energy stored equation also be written?
E=1/2k x L^2
* F is replaced by Kl (hooks law)
Points on how 1/2xFx delta L is derived. (Energy stored)
Energy stored = work done Work done = Fx delta L Graph is F by change in L area under graph is Energy Line is proportional and through 0 Triangle equation for area 0.5BH So energy stored is 1/2 x F x delta L
What is the point where the material stretches without extra load being added called?
The yield point
What is the youngs modulus of a material?
It is a measure of how stiff a material is, it makes it useful for comparing materials and calculating extension.
What is the symbol for youngs modulus?
E
How is Young’s modulus calculated ( 2 equations)!
Stress / strain
FL/ A x change in length
Describe an experiment to find the Young’s modulus of a wire.
Set up a wire, fixed to a bench at one end and a pulley at the other.
Make sure the wire is thin and long, so it extend more.
Place a marker in the wire. And a mm rule underneath this.
Start with the smallest possible weight to make the make the wire straight with no slack.
This is the un-stretched length.
Increase the weight in steps,recording the change in length from the marker to the start each time.
Use a micrometer to measure the diameter if the wire. Take repeats and take an average to calculate the CS area.
Use this data to calculate Young’s modulus, E
What could you use to calculate the cross sectional area?
Use a micrometer to find the diameter and then use pieR^2
Energy per unit volume =
1/2x stress x strain
Area under a stress strain graph.
How can you recognise a graph of a brittle material?
It follows hooks law with a straight line through the Pravin and then stops, it does not curve of deform plasticly, it snaps.
What is 1mm^2 in metres
1 x 10 ^-6 M^2
Describe the structure of a brittle material.
The atoms are often in a crystalline structure. In a brittle material the strong bonds and arrangement means that the atoms can not move much from their equilibrium position.
Brittle fracture is when?
When stress is applied to a brittle material and tiny cracks at the materials surface get bigger and bigger until the material breaks.
When stress is applied to a brittle material and the cracks grow and grow as the atoms can not move much, it is called?
A brittle fracture
