Materials Flashcards

1
Q

Define density, giving units

A

mass of the object per volume it takes up
ρ = m/v
kgm^-3

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Explain density qualitatively

A

Measure of how compact a substance is. Therefore, doesn’t vary with size or shape, just depends on what the object is made of
(kind of like resistivity)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What determines whether an object sinks or floats

A

The average density of the object
A solid object will float on a fluid if it has a lower density than the fluid

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is the density of water

A

ρ = 1 gcm^-3

so 1 cm^3 of water has a mass of 1g

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Why do objects deform ie stretch, twist, bend etc

A

Opposite forces acting on the object

e.g. force down on a spring and reaction force up from the support

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

State Hooke’s law qualitatively

A

The extension of a stretched object is proportional to the load or force, given that environmental factor i.e. temperature remain constant

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is the equation for Hooke’s law

A

F = k∆L
where k is a constant, being the stiffness, which depends on the object being stretched

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is the difference between tensile and compressive forces

A

Tensile forces stretch the object

Compressive forces squash the object

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Why are springs a special case of Hooke’s law

A

The value of k, known as the spring constant for springs, is the same for both tensile and compressive forces

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

How is Hooke’s law shown on a graph

A

Plot a graph of force against extension and will be a straight line

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Why does Hooke’s law stop applying

A

Hooke’s law assumes reversibility from the deformation so will not apply if the deformation begins to be permanent due to too much force being applied

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is the point on the force/extension graph where Hooke’s law no longer applies

A

Up to the limit of proportionality, where the graph starts becoming curved instead of a straight line

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is the elastic limit

A

The point on the graph (after the limit of proportionality) where any more force would cause permanent deformation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Define elastic deformation

A

Deformation in which the material returns back to its original shape and size once all forces are removed

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Explain why elastic deformation occurs

A

Tension on a material pulls atoms apart. Atoms have an equilibrium point so they can move small distances and then return back to the equilibrium point after the force is removed

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Define plastic deformation

A

Deformation in which the material is permanently stretched

17
Q

Explain why plastic deformation occurs

A

When a material is stretched past its elastic limit, the atoms move too far from their equilibrium points so they do not return once the force is removed

18
Q

Explain the energy transfer during elastic deformation, giving example

A

Work is done when stretching the material, which is stored as elastic strain energy
When the force is removed, this energy is transferred to other forms

e.g. elastic band flies across room when released from stretch

19
Q

Explain the energy transfer during plastic deformation

A

Work is done to separate atoms and this energy is not stored as strain energy, it is mainly just dissipated as heat

20
Q

Explain how the energy transfers during plastic deformation are helpful in the real world

A

Crumple zones in cars are designed to deform plastically in a crash
This means energy goes in to changing the shape of the car and then dissipated as heat instead of being stored and transferred to passengers.

21
Q

How can you calculate the elastic strain energy that a material stores

A

Area under a force/extension graph = 1/2FΔL

22
Q

Derive the formula for elastic strain energy

A

Elastic strain energy stored = work done on material = force x extension

As the force is not constant and rises from 0 to F, the average force is 1/2F

Therefore ESE = 1/2FΔL

Since ESE is only stored when obeying Hooke’s law, F = kΔL

Therefore, ESE = 1/2kΔL^2

23
Q

What does unloading look like on a force extension graph

A

The unloading graph is a straight line parallel to the loading one (because value of k is the same)
However, it is shifted to the right as when the force is 0, there is still extension due to plastic deformation

24
Q

What is the area between the loading and unloading curves

A

Work done in permanently deforming the metal

25
What is the difference between a stress strain and force extension graph
Force extension graphs describe the behaviour of a specific object Stress strain graphs describe the behaviour of the material
26
Explain why stress and strain may be used instead of force and extension
Stress and strain takes into account the area and length of a material, which would affect how much force it can withstand. This makes it easier to compare how different materials behave under similar loading conditions
27
Define tensile stress, giving units
Force applied/ cross-sectional area Units are N/m^2 or Pa
28
Define tensile strain, giving units
The extension/ original length of the material Strain has no units, can be written as number or percentage
29
How do the equations for stress and strain compare for tensile and compressive forces
Same equation but force and extension is negative for compressive
30
Define the breaking stress of a material
The stress at which the atoms in the material separate completely and the material breaks, depends on conditions e.g. temperature
31
Define the ultimate tensile stress of a material
The maximum stress that the material can withstand, depends on conditions e.g. temperature
32
Define the yield point
Point on a stress strain graph after the elastic limit, where plastic deformation actually becomes noticeable. Mainly seen in ductile materials.
33
Define the Young's Modulus of a material
A measure of the stiffness of a material Stress/Strain when Hooke's law is being obeyed = FL/ΔLA
34
What are the units of young's modulus
N/m^2 (same as stress as strain doesn't have units)
35
How can you find Young's modulus from a stress/strain graph
The gradient
36
What does the area under a Stress/Strain graph give
The strain energy = energy stored per unit volume Therefore, 1/2 x stress x strain = energy stored per unit volume
37
Explain the stress strain graph for a brittle material
The material doesn't behave plastically at all, it first behaves elastically and then just breaks when the stress or force reaches a certain point
38
Explain the stress strain graph for a ductile material
It has limit of proportionality, then elastic limit, then yield point, then ultimate tensile strength and then breaking point
39
Explain the stress strain graph for a plastic material
limit of proportionality,then elastic limit, then breaking point, no yield point