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BMSc > Mechanics > Flashcards

Mechanics Flashcards

1
Q

Define ‘kinematics’

A

the study of motion/movement without considering forces

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2
Q

What are the 2 types of movement

A
  1. Linear/Translational
    • Rectilinear
    • Curvilinear
  2. Rotational
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3
Q

SI base units relative to motion

A

Metre (m) - standard unit of length

Kilogram (kg) - standard unit of mass

Second (s) - standard unit of time

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4
Q

Give an example of a supplementary base unit

A

Radian (rad) - standard unit of angle

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5
Q

What are ‘derived units’ and give an example

A

formed by combining base units

e.g. the Newton (N) kg m s-2

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6
Q

What is the COMMON unit of angle

A

the degree (1/360th of a full revolution)

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7
Q

what is one radian equal to approximately in degrees

A

57.296 degrees

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8
Q

definition of a radian

A

one radian encloses an arc which is equal in length to the circle radius

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9
Q

how many radians are equal to a full revolution (360 degrees)

A

2π radians

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10
Q

definition of a scalar and give examples

A

has a magnitude only.

e.g. distance, speed, angle, mass, temperature

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11
Q

definition of a vector and give examples

A

has both a magnitude and a direction.

e.g. displacement, velocity, acceleration, momentum

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12
Q

difference between angular distance and angular displacement

A

angular distance = the total angle turned through (scalar)

angular displacement = the angle turned through and the direction of rotation about an axis (vector)

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13
Q

what are rectangular coordinates

A

three axes at right-angles to each other (x, y and z axes)

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14
Q

rectangular coordinates are an example of orthogonal axes. what does this mean?

A

the axes are all at right angles to each other, so they are independent. a change in position of one axis doesn’t result in a change in position in another axis.

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15
Q

definition of a ‘plane’

A

a flat surface that have zero-thickness and are 2-D.

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16
Q

what are polar coordinates

A

gives an ANGLE of a line and its LENGTH

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17
Q

if an object has 6 degrees of freedom, what does this mean

A

the object is free to move in all directions.

i.e. 3 independent translations, 3 independent rotations

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18
Q

define ‘speed’

A

distance travelled / time taken

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19
Q

define ‘velocity’

A

speed AND direction of travel.

displacement / time (unit: m s-1)

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20
Q

the gradient of a displacement/time graph gives you…

A

the velocity

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21
Q

define ‘acceleration’

A

the rate of change of velocity.

change in velocity/time taken (unit: m s-2)

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22
Q

the gradient of a velocity/time graph gives you…

A

the acceleration

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23
Q

the area under a velocity/time graph gives you…

A

the total displacement

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24
Q

give the equations of linear motion

A

v = u + at

v(squared) = u(squared) + 2as

s = 1/2(u + v) t

s = ut + 1/2at(squared)

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25
Q

define ‘angular velocity’ (ω)

A

the angular displacement of a rotating object travelled per second.

angular displacement / time taken (unit: rad s-1)

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26
Q

the gradient of an angular displacement/ time graph gives you…

A

the velocity

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27
Q

define ‘angular acceleration’ (α)

A

the rate of change of angular velocity

change in angular velocity / time (unit: rad s-2)

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28
Q

the gradient of an angular velocity/time graph gives you…

A

the angular acceleration

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29
Q

define ‘static forces’

A

forces that are acting on a body that is at rest or moving at constant velocity ie. not accelerating

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30
Q

define ‘mass’

A

the quantity of matter that a body is composed of

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31
Q

define ‘weight’ and give the equation

A

the force of gravity acting on a body

W = mg (N)

m = mass of a body, g = acceleration due to gravity (9.81 m/s(squared))

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32
Q

define ‘density’ and give the equation

A

mass per unit volume

density (ρ) = m/v (kg m-3)

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33
Q

Density of a body is constant - T/F? Explain

A

True

If the mass of a body changes, the volume will change proportionately, and vice versa.

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34
Q

define ‘gravity’

A

The acceleration due to the gravitational attraction between two bodies. As the mass of the bodies increase, so does the force of attraction.

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35
Q

define ‘centre of mass’

A

a point where all the mass of a body can be assumed to act

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36
Q

define ‘centre of gravity’

A

a point where the weight of the body can be assumed to act.

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37
Q

do bodies always have a centre of mass?

A

yes, when they have enough mass to not be negligible

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38
Q

do bodies always have a centre of gravity?

A

only in a gravitational field!

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39
Q

define ‘friction’ and how does it act?

A

The force arising between two surfaces when they rub against each other. Friction tends to oppose motion and acts at a tangent to the surfaces.

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40
Q

2 forces the maximum magnitude of friction force is dependent on

A
  1. the roughness of the surfaces

2. the size of the force pushing them together

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41
Q

define ‘co-efficient of friction’ and give equation

A

the measure of the maxium friction force between two surfaces
ie. a ratio of the friction force to the force acting normally (perpendicular) to press the two surfaces together
Normally between 0-1.

co-efficient of friction (μ) = F/N

where F = the friction force, N = the force acting normally to the surfaces

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42
Q

3 different types of friction

A
  1. static (most friction)
  2. sliding
  3. rolling (least friction)
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43
Q

is the co-efficient of friction the same for all types of friction?

A

No - each case has its own coefficient!

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44
Q

explain ‘static friction’

A

Present when motion is about to occur betwen two surfaces. The friction force present will be just enough to stop the applied force that is trying to move the two forces over one another.
Once maximum force is exceeded, motion will begin.

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45
Q

explain ‘sliding friction’

A

Only exists when sliding occurs between 2 surfaces

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46
Q

explain ‘rolling friction’

A

Arises between an object and the surface over which it is rolling. It arises because of the deformation of the two surfaces caused by the force acting normally to the two surfaces.

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47
Q

what effect does lubrication have on rolling friction?

A

it doesnt lower it, but it can reduce wear

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48
Q

define ‘pressure’ and give the equation

A

the force exerted per unit area

P = F/A (unit: Pascals/Pa) (N m-2)

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49
Q

define ‘static equilibrium’

A

a body which has no resultant forces acting on it is in static equilibrium. The body is either at rest, or moving at constant velocity ie. it is not accelerating.

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50
Q

what is the first condition of static equilibrium?

A

the sum of all external forces acting on a body is zero

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51
Q

what is Newton’s III Law

A

to every action there is an equal and opposite reaction

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52
Q

what forces are shown in a free body diagram

A

those acting: due to gravity, friction forces, and reaction forces

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53
Q

what is Newton’s I Law

A

The Law of Inertia - a body will remain at rest or at constant velocity, unless acted upon by a resultant force

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54
Q

define ‘Inertia’

A

a body’s reluctance to accelerate. represented by its mass

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55
Q

what is Newton’s II Law and give the equation

A

The Law of Acceleration - the acceleration of a body is proportional to the applied force and inversely proportional to its mass

F = ma (unit: N)

F = force, m = mass, a = acceleration

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56
Q

define ‘dynamic equilibrium’

A

a body which has resultant forces acting on it is in dynamic equilibrium. The body is accelerating or decelerating.

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57
Q

what are the conditions of dynamic equilibrium?

A
  1. the sum off all the external forces = the resultant force

2. from Newton’s II Law, the resultant force is equal to mass x acceleration

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58
Q

Equation for component of weight acting down a slope

A

W = mg x Sinθ (unit: N)

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59
Q

define ‘momentum’ and give equation

A

An expression of a body’s persistence to continue in its present state of motion. Incorporates a body’s resistance to change it motion (it’s inertia properties) and its velocity.

p = mv (unit: kg m s-1)

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60
Q

Change in linear momentum is proportional to applied force. what is the equation for this

A

F = (mv - mu) / t

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61
Q

principle of conservation of momentum

A

A body will continue to move with constant momentum unless an external force acts to change that momentum

total momentum before collision = total momentum after collision

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62
Q

equation for calculating momentum in an elastic collision

A

m1u1 + m2u2 = m1v1 + m2v2

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63
Q

equation for calculating momentum in an inelastic collision

A

m1u1 + m2u2 = (m1 + m2) x v2

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64
Q

what is the ‘moment of a force’

A

the tendency of a force to produce a rotation about an axis (aka torque)

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65
Q

how to calculate the moment of a force

A

the product of the force and length of the line that is perpendicular to the force’s line of action

M = F x d (unit: N m)

M = moment, F = force, d = length of line passing through fulcrum that is perpendicular to force

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66
Q

2nd condition of static equilibrium

A

the sum of all the external moments acting on a body must equal zero - rotational static equilbrium.

(if they are not, the body will angularly accelerate - rotational static equilibrium)

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67
Q

what is a lever system

A

a simple machine that consists of a rigid bar that pivots around a fulcrum (hinge). the lever system is acted on by an effort force and a resistance force.

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68
Q

how are lever systems present in the human body?

A

muscles act (the effort force) to move or prevent movement of a limb (the limb is the bar) by overcoming external forces (the resistsance force) such as gravity

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69
Q

what is the mechanical advantage (MA) of a lever system

A

ratio of the force-fulcrum distance / resistance-fulcrum distance

ie force arm/resistance arm

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70
Q

what does a high mechanical advantage tell us

A

the effort force is lower than the resistance force ie. doesn’t take much effort to overcome the resistance, so this is mechanically advantageous

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71
Q

what does a force disadvantage mean and where are force disadvantages commonly found

A

the effort forces are much greater than the forces resisting them.

Found in MSK lever systems, where the forces produced by the muscles are much greater than the forces resisting them. Normally because the muscles insertion points tend to be closer to the fulcrum (force-fulcrum distance shorter).

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72
Q

why do muscles act at a force disadvantage

A

the closer the muscle is inserted to a joint then the smaller the change in muscle length required to produce a correspondingly large limb movement

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73
Q

classes of lever system and state whether they work at mechanical advantage or disadvantage

A

1st class:

  • fulcrum located between effort and resistance
  • either MA or MD

2nd class:

  • resistance located between effort and fulcrum.
  • MA

3rd class:

  • effort located between resistance and fulcrum
  • MD
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74
Q

define ‘tangential linear velocity’ and give equation

A

the velocity measured at any point tangent to a turning wheel

v = r ω,

v = tangential linear velocity, r = circle radius, ω = angular velocity

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75
Q

define ‘tangential linear acceleration’ and give equation

A

the linear acceleration directed at a tangent to the circle formed by the motion

a = r α

a = tangential angular acceleration, r = circle radius, α = angular acceleration

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76
Q

when a body is rotating with constant angular velocity, what will the the tangential linear acceleration be equal to?

A

zero!

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77
Q

define ‘radial acceleration’ and equation

A

Radial acceleration acts to maintain a body on its circular path. It is directed from the body to its centre of rotation

Radial acceleration = r ω(squared)

r = circle radius, ω = angular velocity

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78
Q

define ‘moment of inertia’

A

similar to inertia of a linear body. it is a measure of resistance of a body to angularly accelerate.

79
Q

moment of inertia depends upon…

A
  1. the body’s mass
  2. how the mass is distributed within the body in relation to the axis of rotation (if a mass is distributed further from the axis of rotation it will have a greater moment of inertia, so it will make it harder to accelerate
80
Q

equation for moment of inertia

A

I = mr(squared) (unit: kg m2)

I = moment of inertia, m = mass, r = radius

81
Q

define ‘radius of gyration’ and give equation

A

the radius from the axis of rotation in which all the mass of a body is concentrated in rotary motion

I = mk(squared)

82
Q

define ‘angular momentum’ and give equation

A

incorporates a body’s resistance to change its rotary motion (it’s inertial properites) and its angular velocity

L = I ω (unit: kg m2 rad s-1)

L = angular momentum, I = moment of inertia, ω = angular velocity

83
Q

body segment parameters required in anthropometry

A
  1. length
  2. mass
  3. centre of mass
  4. radius of gyration
84
Q

define ‘work’ and give equation

A

work is done when a force moves its point of application. It is the product of the applied force and the distance through which it moves

W = F x s (unit: joules)

85
Q

define ‘power’ and give equation

A

power is the rate at which energy is expended or work is done

P = W / t (unit: watts)

86
Q

define ‘energy’ and what are the 2 forms

A

energy is a measure of a system’s capactiy to do work

  1. potential energy
  2. kinetic energy
87
Q

what is kinetic energy and give equations for both linear and rotary motion

A

the energy possessed by a body by virtue of its MOTION

KE = 1/2mv(squared) : linear motion

KE = 1/2 Iω(squared) : rotary motion

88
Q

what is potential energy and give equation

A

the energy posssed by virtue of its POSITION

PE = Wh = mgh

89
Q

definition of a structure?

A

an arrangement of materials, designed to sustain a load

90
Q

Definition of a material?

A

Used to construct a structure

91
Q

What is definition of deformation? Do all structures deform when a load is applied?

A

Deformation = the change in shape or size of a structure or any part of it.

Yes, everything deforms to some extent

92
Q

Definition of stress?

A

the force exerted per cross-sectional area

93
Q

What factors is stress of a material dependent and independent on?

A

Dependent factors:

  • the material itself
  • the cross-sectional area

Independent factors:
- the structure of the material

94
Q

Definition of strain?

A

the change in length divided by the original length.

95
Q

What factor is strain dependent on ?

A

the length of the bar

96
Q

What is an axial load?

A

A force applied along a geometric axis, producing pure tension (strain) or compression (stress)

97
Q

What is a stress-strain relationship

A

shows how a material deforms (strains) as it is loaded (stressed)

98
Q

What is the Proportional Limit on a stress-strain curve?

A

between the origin and this point, stress and strain have a linear relationship

99
Q

What is the Elastic Limit on a stress-strain curve?

A

the greatest stress that can be applied to a material without causing deformation. after this point the material exhibits plastic behaviour.

100
Q

After the elastic limit on a stress-strain curve, does the material follow he same curve on un-loading?

A

No! The material will follow a new curve because there is a residual or permanent strain.

101
Q

What is the Yield Point on a stress-strain curve?

A

The point on the curve after which there is an increase in strain (elongation) without any increase in stress. The material exhibits plastic behaviour.

102
Q

What is the yield-strength on a stress-strain curve?

A

The stress at the yield point

103
Q

What is the strain-hardening region of a stress-strain curve?

A

Occurs after the large strain during yielding up to the highest point of the curve.
The material changes its atomic structure to resist further deformation.

104
Q

What is the Ultimate Strength on a stress-strain curve?

A

The highest point on the curve

105
Q

What happens after the Ultimate strength point on a stress/strain curve?

A

Stretching (elongation) occurs with an apparent decrease in stress.

This is due to “necking”, where the cross sectional area of the material is reduced (the original cross section is still used to calculate the stress, so it appears reduced).

106
Q

What is the Rupture point on a stress-strain curve?

A

The point where fracture occurs

107
Q

What is the difference between a brittle and a ductile material in terms of their behaviour on a stress-strain curve?

A

Brittle material -
material has an all elastic region, then breaks (no plastic region)

Ductile material -
material has a plastic region (i.e. deforms before breaking)

108
Q

What is Hooke’s Law?

A

describes the behaviour between the origin and the proportional limit on a stress-strain curve, where up to a certain level of stress, the strain produced is proportional to the applied stress.

109
Q

What is Young’s modulus (E)?

A

A proportionality constant, which gives an indication of how difficult the material is to deform under loading i.e. strain under the action of a stress (stiffness).

It is equal to the gradient of the stress-strain curve up to the proportional limit

110
Q

What is the equation for Young’s modulus (E)?

A

Young’s modulus (E) = stress/strain

111
Q

What does a small Young’s modulus indicate?

A

a small amount of stress is required to produce a large amount of strain (i.e. flexible)

112
Q

what does a large Young’s modulus indicate?

A

a large amount of stress is required to produce a small amount of strain (i.e. stiff)

113
Q

What is rigidity?

A

An indication of a bar’s ability to resist axial deformation (either tension or compression).

114
Q

Equation for rigidity?

A

Rigidity = Young’s Modulus (E) x cross-sectional area (A)

Units: N

115
Q

What is stiffness (k)?

A

The force required to produce a unit deflection i.e. shorten or elongate the bar by 1 metre.
It is equal to the rigidity per unit length.

116
Q

Equations for stiffness (k)?

A

Stiffness (k) =
applied force / change in length

Also,

rigidity (EA) / change in length

Units : N m^-1

117
Q

What is flexibility?

A

The deflection of a bar under a unit load, so therefore is the inverse of stiffness.

118
Q

Equations for flexibility?

A

change in length / applied force

1 / rigidity (EA)

1 / stiffness (k)

units : m N^-1

119
Q

What substance exhibits ‘viscous behaviour’?

Describe ‘viscous behaviour’?

A

Exhibited by fluids.

Does not deform instantly on loading - the strain is prolonged. Does not return to original shape and size when load is removed.

120
Q

In viscous materials, stress is dependent on strain - T/F?

A

False - the stresses are dependent on the strain RATE

(since the substance does not deform instantly on loading).

121
Q

Can the behaviour or viscous material be represented by Young’s modulus and Hooke’s Law?

A

No - it is represented by the Coefficient of Viscosity,

stress/strain rate,
(unit Nm^-2 . s)

where strain rate is the change in strain / time

122
Q

What substances exhibit ‘viscoelastic behaviour’?

Describe ‘viscoelastic behaviour’?

A

Exhibited by articular cartilage and cortical bone.

A material exhibits both viscous (responds to rate of loading) and elastic (returns to original shape and size after load removed) behaviour.

123
Q

What is creep?

A

describes the behaviour of a material over time when it is subjected to a CONSTANT LOAD.
the material undergoes an initial elastic deformation, then slowly (creeps) continues to elongate (strain) over time

124
Q

What are the stages of creep on a strain-time graph?

A
  1. elastic strain
  2. constant creep rate equal to the gradient
  3. material necks, and deformation accelerates until fracture
125
Q

What is stress relaxation?

A

describes the behaviour of a material when it is subjected to a CONSTANT DEFORMATION.
the stress in the material will gradually decrease over time when at a constant strain.

126
Q

List the different types of loading that a solid bar can be subjected to?

A
  1. Tension (axial loading)
  2. Compression (axial loading)
  3. Bending
  4. Shear
  5. Torsion
127
Q

What is shear stress?

A

forces are acting in opposite directions to slip/shear surfaces within a material i.e. slippage of adjacent surfaces

128
Q

Equation for shear stress?

A

Shear stress (tau) = sheared force/sheared area

Units : N m^-2 (Pa)

129
Q

What is shear strain?

A

a quantification of the angular deformation a material undergoing shear stress has been subjected to.

130
Q

Equation for shear strain?

A

shear strain = angle sheared

distance sheared (x) / distance between two shearing forces (d)

Units : radians

131
Q

What is shear strength?

A

The max shear stress a material can withstand before fracturing

132
Q

Equation for shear strength?

A

shear force required to fracture the material / sheared area

133
Q

What is Modulus of Rigidity (G)?

A

a shear modulus.
Combines the measure of shear stress and shear strain into one.
Ratio of shear stress to shear strain.
Equal to the gradient of a shear stress/shear strain curve up to a limiting stress.

134
Q

What is the equation for Modulus of Rigidity (G)?

A

G = shear stress/shear strain

Unit = Pa

135
Q

When a bar is subjected to a tensile or compressive force, what type of force also always arises?

When is this force largest?

A

There is always a shear stress, as well as the axial one.

The largest shear stress occurs at 45 degrees to the axial loading, and is equal to half the axial stress.

shear stress (max) = 
axial stress / 2
136
Q

at what angle does the maximum shear stress occur in a bar under an axial load?

A

45 degrees

137
Q

What is bending stress?

A

Arises when a material is acted upon by forces or moments which tend to bend or curve it, causing one side to be elongated and the other side to be compressed.

138
Q

What is cantilever bending?

A

the bar is fixed to a wall at one end, and force is applied to the other end.

139
Q

What is three-point bending?

A

Compressive and tensile forces act on either side of the bar.

140
Q

What is the neutral axis when a bar is subjected to a bending load?

A

occurs between the elongated and compressed side of the bar, where there are no stresses and the bar remains the same length

141
Q

What is the stress in any layer in a bent beam dependent on?

A

the displacement of the layer from the neutral axis.
The further the layer is from the axis, the greater the stress.
The maximum stress will occur at the surfaces of the beam , so the material will fail at the surfaces rather than within the material.

142
Q

What is a bending moment?

A

The internal moment that balances the externally applied moments when a bar is subjected to a bending load

143
Q

Where does the maximum bending moment occur in a bar?

A

when the displacement, x, from the applied bending force, F, is equal to the length of the bar, L. i.e.

M = FL

144
Q

In bending moment diagrams, when is the bending moment + ve and when is it - ve?

A

Bending moment is +ve, when the force causes sagging of the beam.

Bending moment is -ve when the force causes hogging of the beam.

145
Q

What is the ‘bending strength’ of a beam?

What factors is it dependent on?

A

The maximum magnitude of a bending moment that a beam can resist. (unit: Nm)

  1. the strength of a material
  2. the cross-sectional area
  3. the cross-sectional shape
146
Q

What equation combines the factors in bending stress - the differing stresses in each layer, and the bending strength?

A

MISYER equation

M/I = stress/y = E/R

where
M = max bending moment
I = second moment of area
stress = max bending stress
y = displacement of the layer from the neutral axis
E = young’s modulus
R = radius of the circle containing the neutral axis

147
Q

What is the ‘second moment of area (I)’ in bending stress?

What factors is the second moment of area dependent on?

A

Gives an indication of a structure’s resistance to bending.

It is dependent on the cross-sectional shape of a beam - the further the material of a beam is concentrated away from its neutral axis the larger its second moment of area.

148
Q

What is the equation for the second moment of area (I) and y(max) of a RECTANGULAR cross-sectional shape?

A

I = bd^3/12 (units m^4)

y(max) = 1/2d

149
Q

What is the equation for the second moment of area (I) and y(max) of a CIRCULAR cross-sectional shape?

A

I = (pi)(d^4) / 64

y(max) = 1/2d

150
Q

What is the equation for the second moment of area (I) and y(max) of a HOLLOW CIRCULAR cross-sectional shape?

A

I= (pi)/64 x (d^4 outer - d^4 inner)

y(max) = 1/2d (outer)

151
Q

What is torsional stress?

A

Torsion stress is caused by twisting due to the application of a moment. A bar under the action of a twisting moment is said to be in torsion.

152
Q

When is a bar said to be in pure torsion?

A

When the cross-section of the bar retains its shape (remains circular and radius is unchanged)

153
Q

In torsion, is deformation uniform throughout the bar?

A

No - it is zero at the central axis and max at the outer surface.

Therefore angle of twist, shear strain and shear stress are max at the outer surface

154
Q

What equation is used in torsion questions?

A

M/J = tau/R = G(angle) / L

M = applied twisting moment
J = polar second moment of area
tau = shear stress 
R = radius of the cross-section
G = modulus of rigidity 
angle = angle of twist in radians 
L = length of bar
155
Q

What is the ‘polar second moment of area’ (J)?

A

Gives an indication of how resistant a material is to torsion

156
Q

What is the equation for the polar second moment of area (J) of a SOLID circular cross-section bar?

A

J = (pi)(d^4) / 32

Units: m^4

157
Q

What is the equation for the polar second moment of area (J) of a HOLLOW circular cross-section bar?

A

J = (pi) (d^4 outer - d^4 inner) / 32

units: m^4

158
Q

What computerised methods are available for stress analysis?

A
  1. Strain gauges
  2. Photoelasticity
  3. The finite element method
159
Q

How are stress-strain curves produced in an experimental setting?

A

Performing a tensile test.

Applying a tensile load to a specimen and gradually increasing until it fractures.

Strain and Stress can be measured and plotted as a graph.

160
Q

In ductile materials, why is the rupture strength less than the ultimate strength?

A

Due to necking - the cross-sectional area is reduced, so the specimen appears to carry less stress when it is calculated using the original cross-sectional area rather than the reduced cross-sectional area.

161
Q

Explain the process of development of a ductile fracture?

A
  1. Application of a tensile load
  2. Formation of microscopic voids at centre of bar - due to high stress causing separation of grain boundaries
  3. Voids grow and connect producing larger cavities
  4. Metal to metal contact area reduced so much that metal is unable to sustain load and fracture occurs.
  5. Deformation by shearing also occurs (max at 45 degrees to axial tensile load)
162
Q

What are the characteristic features of a ductile fracture?

A

necking

flat granulated central portion

small shear lip

cup and cone fracture surface

163
Q

In what situations will a normally ductile material respond like a brittle material?

A
  • When it has been exposed to fatigue loading (repeated loading and unloading)
  • If the material contains a notch or crack at the tip
  • Decreasing temperature and increasing strain rate by rapidly loading the material (impact loading)
164
Q

What are the characteristic features of a brittle fracture?

A

flat fracture surface, perpendicular to the load applied

granular appearance

chevron pattern (separate crack fronts fan out from origin of the crack).

165
Q

What are ‘stress concentrations’ within a structure, and how do they arise?

A

points in a structure where the stress level is greater than the average stress with the material.

Caused by sudden changes in shape in the structure (termed ‘stress risers’).

166
Q

How are the locations of stress concentrations within a structure found?

A

Using stress trajectories - the points of high stress are where the trajectories are tightly packed.

167
Q

What is ‘fracture propagation’?

A

development of a fracture from the tip of a crack or notch (where stress concentration is highest - notch sensitivity)

168
Q

Since all materials will contain some pores and cracks, how is fracture propagation halted within structures?

What is the advantage/disadvantage of doing this?

A

building in features like smooth holes to blunt cracks.

although it reduces chance of propagation, the loss of material weakens the structure.

169
Q

What is impact loading?

A

A sudden intense blow to a structure

170
Q

How is the resistance of structures to impact loading tested?

A

Charpy impact test

Pendulum released from a known height and breaks specimen when it reaches the bottom. Pendulum continues to swing until it reaches its peak.

Peak swing will be less than the height the pendulum was released from. Change in potential energy is equal to energy absorbed by the specimen - impact energy

PE = W x (initial height - finish height)

171
Q

In what way does temperature affect a material’s ability to absorb energy?

A

Able to absorb more energy with increasing temperature, because the material is changing from brittle to ductile

172
Q

What is fatigue fracture?

A

Fracture due to repeated loading, often lower than that required to fracture the material.

173
Q

What are the two distinct regions of a fatigue fractured surface of a metallic component?

A
  1. concentric clamshell markings
    - show where crack has stopped at various positions as it propagates intermittently
    - allows origin of fracture to be located
  2. granular or fibrous region
    - granular region if material undergoes rapid brittle fracture
    - fibrous appearance if material undergoes ductile fracture
174
Q

What are fatigue fractures of bone also known as?

A

Stress fractures

March fractures

175
Q

What are the main contributing factors to fatigue fracture in bone?

A

Frequency of repetition of load - more likely when frequency is too fast for remodelling process

Magnitude of load

Muscle fatigue

176
Q

What is corrosion?

A

A result of chemical reaction of a metal with its environment

177
Q

Why are orthopaedic implants resistant to corrosion, and how are they made resistant?

A

The fluids of the body are very corrosive to most metals.

An inert film (passivation layer) covers the surface of the material

178
Q

How does corrosion affect the fatigue behaviour of metals?

A

Reduces the fatigue resistance, resulting in lower fatigue life and no endurance limit

179
Q

2 groups which metals can be divided into?

A
  1. Ferrous metals (contain iron)

2. Non-ferrous metals (no iron)

180
Q

What are metal alloys and why are they formed?

A

An alloy is formed by addition of various elements to a basic metal.

Improves the mechanical and corrosive properties.

181
Q

What are ferrous alloys?

Give the most common example of a ferrous alloy?

A

Iron-based alloys.

Steel - an iron-based alloy of iron and CARBON

182
Q

What is stainless steel?

A

An iron-based steel alloy that contains CHROMIUM to give it its corrosion-resistant property.

183
Q

What are non-ferrous alloys?

Give the most common example of a non-ferrous alloy?

A

Metal alloy that doesn’t contain iron.

Titanium based alloy

184
Q

What are the main advantages of titanium-based alloys?

A
  1. Lower density compared to steels
  2. Higher strength-to-weight ratio compared to aluminium
  3. Excellent corrosion resistance
185
Q

What are the main disadvantages of titanium-based alloys?

A
  1. High material cost compared to steel and aluminium
  2. High fabrication cost compared to steel and aluminium
  3. Lower Young’s modulus than stainless steels
186
Q

What are polymers and give the two most common examples of polymers?

A

Lightweight, corrosion-resistant electrical insulators.

e.g. plastics and elastomers

187
Q

What are the small units which polymers are made from?

A

Monomers

188
Q

Describe the stress-strain behaviour of polymers?

A

Non-linear and time dependent.

189
Q

What are the two types of plastic polymers and describe them?

A

Thermoplastic - can be reformed by heating

Thermoset - cannot be reformed by heating

190
Q

What is the main characteristic of elastomers?

A

Can undergo massive strain (700%)

191
Q

What is the structural shape of ceramics?

A

Crystaline

192
Q

3 examples of ceramics?

A

Diamond
Brick
Glass

193
Q

Explain why ceramics are suitable for the heads of hip prostheses, but not for the stems?

A

They have a very large Young’s Modulus, so are very hard. This means they are wear-resistant and have low friction.

However they are brittle and not tough, so they cannot be used for stems as they would snap easily.

194
Q

What are composite materials and explain the 3 types?

A

Two or more materials are joined to give a combo of properties.

  1. Particle-reinforced composites
    - hard brittle material dispersed in a soft ductile material
  2. Fibre-reinforced composites
    - fibres of a hard brittle material are incorporated into a soft ductile material
  3. Laminar-reinforced composites
    - layers (either thick or thin) are arranged in such a way to increase strength and corrosion-resistance.

BMSc (6 decks)

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