1
Q
Tension definition
A
Produces tensile stress in one direction
2
Q
Compression definition
A
Produces compressive stress in one direction
3
Q
Torsion definition
A
Produces shear stresses
4
Q
Hoop stress definition
A
Produced by pressure differential
between inside or outside of tube
Produces (usually) tensile loads in the
direction around the object
5
Q
Biaxial tension definition
A
Produces tensile stresses in two directions
6
Q
Hydrostatic compression definition
A
Hydrostatic compression, stress same in all directions
7
Q
What is bronze?
A
Bronze is an alloy of copper (Cu) and tin (Sn)
8
Q
What is a material?
A
Substance, matter from which a thing is made.
9
Q
Factors to consider when choosing a material
A
- Stiffness
- Hardness
- Strength
- Wear resistance
- Density
- Damping behaviour
- Corrosion resistance
- Ease of manufacture
- Fatigue resistance
- Thermal conductivity
- Creep behaviour
- Electrical resistance
- Cost
- Magnetic permeability
- Fracture toughness
- Aesthetics
- Thermal expansion
- Recyclability/reusability
10
Q
Classes of Materials
A
- Metals
- Ceramics
- Glasses
- Polymers
- Semiconductors
- Composites
- Natural materials
11
Q
What are metals?
A
An opaque lustrous chemical substance that is a good conductor of heat and electricity and, when polished, is a good reflector of light.
12
Q
Types of metals
A
- Irons and steels
- Aluminium and alloys
- Copper and alloys
- Titanium and alloys
- Precious metals (Gold, silver, platinum)
- Nickel and alloys
13
Q
Electrical properties of Copper (Cu)
A
Highly conductive metal now used on Si chips
Adding impurity atoms increases resistivity
Deforming Cu increases resistivity
14
Q
Mechanical properties of metals
A
- Stiff with a range
- Strong
- Tough
- Fatigue resistant
- Most metals have high density with range
- Melt at high temperature
- Can be shaped and worked
15
Q
What are ceramics?
A
Ceramics are crystalline, mainly metal oxide based, low thermal conductivity, good resistance to chemical attack.
Ceramics have highly defined crystallographic structure.
16
Q
Examples of ceramics
A
- Alumina (Alz03)
- Zirconia (ZrO2)
- Titania (TixOy)
- Clays
- Porcelain
- Silicon Nitride (Si3N4)
17
Q
Properties of Ceramics
A
- Generally stiff and hard, but brittle (Low fracture toughness, low impact strength)
- Medium density
- Low liquid uptake
- Good thermal stability
- Can suffer from slow crack growth (Once a crack is initiated (or even produced during manufacture) grows with small, but repeated loads)
18
Q
What is glass?
A
- Disordered or amorphous material (no long range atomic order)
- Chains of silicon Si (or other metal atoms) and oxygen O atoms with
interspersed atoms e.g. Na) - Trace metals added to control colour
19
Q
Types of glass
A
- Silica (SiOz) and silicate glasses
- Glass fibres for reinforcing
- Bioglasses
- Contain sodium and phosphates
- Bone will bond to them
20
Q
Properties of Glasses
A
- Amorphous
- Medium stiffness
- Very brittle
- Very notch sensitive
- Generally transparent
- Used in optical fibres for data transmission
21
Q
What are polymers?
A
A large molecule composed of many subunits known as monomers these are mostly made from carbon, hydrogen and oxygen - synthetic polymers include plastics and there are natural bioploymers such as DNA.
The polymers are often long chains with cross links between the chains.
Long chain molecules with repeats (mers) of a group of atoms.
Produced by polymerising joining the individual monomers together.
22
Q
Properties of Polymers
A
- Flexible
- Long extension to failure
- Tough
- Low density
- Many soften and the melt at relatively low temperatures (Thermoplastics)
- Some do not melt at any temperature (Thermosets)
- Usually good electrical and heat insulators
- Easily molded
- Can be light and strong
- Their properties can deteriorate when exposed to light - the cross links between chains breakdown
- Difficult to recycle
23
Q
Types of polymers
A
Fibres
Coatings
Adhesives
24
Q
Fibres information
A
Fibres - length/diameter > 100
* Textiles are main use
- Must have high tensile strength
- Usually highly crystalline & highly polar
*Formed by spinning
- extrude polymer through a spinnerette
- Pt plate with 1000s of holes for nylon
*Rayon - dissolved in solvent then pumped through die head
- the fibres are drawn
- leads to highly aligned chains - fibrillar structure
25
Coatings information
Coatings - thin film on surface - i.e. paint, varnish
- To protect item
- Improve appearance
- Electrical insulation
26
Adhesives information
Adhesives - produce bond between two adherents
- Usually bonded by:
1. Secondary bonds
2. Mechanical bonding
* Films - blown film extrusion
* Foams - gas bubbles in plastic
27
Polymers examples
* Polyethylene (PE) - plastic bags
* Polymethylmethacrylate (PMMA or acrylic) - cockpit windows
* Polyamide (PA or nylon) - fibres
* Polystyrene (PS) - foam
* Polyurethanes (PU) - skateboard wheels
* Polyvinylchloride (PVC) - pipes
* Polyethylethylketone (PEEK) - bio
* Polyethylene tetephthalate (PET) -sails
28
Semiconductors information
* They have electronic & optoelectronic properties that can be engineered with tiny amounts of impurities or different alloys.
* Si well known for digital electronics, also Ge, GaAs, AlGaAs, GaN, InP and PbS.
* Not as conductive as metals - Si resistivity depends heavily on the amount of dopants but even the most highly doped Si has about 100000 times larger resistivity than copper.
* In metals the charge carriers are electrons. In semiconductors the charge carriers can be electrons, or a missing electron (called a hole).
* In some semiconductors when the electrons and holes meet they emit light.
29
Superconductors information
* About half the elements in the periodic table exhibit superconductivity at low enough temperature (approaching absolute zero).
* Superconductors can support current with zero resistance, and produce high magnetic fields.
* Applications include MRI machines, levitating trains, power transmission, CERN, detectors for astronomy, quantum computers.
30
What are composites?
* Two (or more) phase material
* Phases can be seen as separate on a microscope
* Generally used to optimise the properties of the phases
31
Composites examples
* Fibreglass (glass in resin)
* Carbon fibre reinforced polymers (CFRP)
* Dental composites (glass in thermoset polymers)
32
Properties of Composites
?????
* Depends on:
- Component materials
- Method of production of the composites
- Amounts and shapes of components
- etc
33
Natural Materials examples
* Wood
* Cork
* Leather
* Bone
34
Properties of Natural Materials
* Lowish modulus (<70GPa)
* Good strength to weight ratio
* Smart materials
- React to their (mechanical) environment
35
What is stress?
Force per unit area
Applying a forces produces deformation.
36
What is strain?
Change in length divided by length
* Applying force/stress to an object causes it to deform
* Normalised deformation is called strain
* As with stress can be normal to surface under consideration or perpendicular to this direction
- Tensile/compressive strain
- Shear strain
37
What is tensile strain?
Under tension object extends and contracts in perpendicular direction.
38
What is Poisson’s Ratio?
* A tensile stress produces longer and thinner specimen
* Compression produces shorter and fatter specimen
* The relative amounts of lenthening/shortening is called
Poisson's ratio v (nu)
39
Effect of Poisson's ratio
* Virtually always positive
- Few rare foams made with negative Poisson's ratio
* Value is between 0 and 0.5
* For v = 0.5
- no change in volume with load
- rubber approximately 0.49
* For v < 0.5
- Volume increases in tension
- Decreases in compression
- Cork approximately 0.0 (very little lateral expansion when compressed)
40
What is Hooke’s Law?
* Stated that for small strains the strain is nearly proportional to stress
* Under these conditions materials are called "linearly elastic"
41
What is moduli?
Modulus describes the stress-strain relationship
42
Types of moduli
Young’s modulus
Shear modulus
Bulk modulus
43
What is Young's modulus (E)?
Ratio of linear stress to linear strain.
44
What is Shear modulus?
Shear modulus or modulus of rigidity (G) is the ratio of shear stress to shear strain.
45
What is bulk modulus?
Bulk modulus (K) - resistance to uniform compression.
46
What is atomic spectra?
When elements emit and absorb light at specific wavelengths.
47
Bohr Model of Atom 2
* Nucleus (neutrons and protons) provides mass
* Electrons can jump between orbits gaining or losing energy
* Energy is absorbed or emitted as an optical photon.
* Electrons have negative charge
* Protons have positive charge
48
What are isotopes?
Isotopes are variants of a particular chemical element.
* Number of neutrons differentiates between isotopes and changes the atomic mass accordingly.
49
Properties of electrons:
* According to quantum mechanics, electrons have wave-like and particle-like properties.
- This means that electrons around the nucleus are in atomic orbitals defined by a spatial probability distribution.
- Each orbital at discrete energy level defined by quantum numbers.
50
Electron energy states:
Electrons…
* Have discrete energy states
* Tend to occupy lowest energy state
* Electrons (Fermions) are anti-social only 2 (spin up, spin down) per energy level
51
Atomic Orbitals
* The p subshells have 3 possible orbitals that are orientated mutually perpendicular to one another.
* The shape of the orbital is related to the probability of finding an electron in relation to the nucleus which is at the center.
* These atom orbitals are combined to make bonds between atoms and give the directionality of covalent bonds
52
Types of Primary (strong) bonds:
- lonic
- Covalent
- Metallic
53
Types of Secondary bonds (weaker):
- van der Waals
- Hydrogen bonds
54
What is an ion?
An atom that has acquired charge by losing or gaining electrons.
55
Ionic Bonds
* Function with positive and negative ions
* Eg. NaCl (...table salt)
* Thus one electron transfers from Na to Cl leading to Na+ and Cl-
56
Covalent Bonding
* Sharing of electrons
* Two atoms combine and share electrons
*The covalent bond can be regarded as a linear combination (a summation) of atomic orbitals.
* Can be very strong
* Leads to stiff materials with a high melting point e.g. diamond
* Bonds are generally sp3 hybrids thus directional
57
Metallic Bonds
* Occurs in metals, metal alloys and to some extent in semiconductors
* Outer electrons are weekly bound to metal atoms and they leave the nuclei and wander around the solid. In this case the electrons are delocalised.
* The positive nuclei are in a sea of negative electrons
* No directionality
* High electrical and heat conductivity
58
van der Waals - Secondary bond
* Results from dipole-dipole attraction between non charged atoms
* All atoms experience constant vibrational motion
* Average electron position is symmetrical, out TIME VARIES thus nonsymmetrical
59
Hydrogen bonds - Secondary bond
*In certain hydrogen-containing molecules, the hydrogen electron is shared
* Leaves a bare hydrogen nucleus (+ve) which therefore attracts negative end of next molecule
* Hydrogen bonds determine the crystal structure of ice
* Water expands below 4°C and ice is less dense than water so ice floats on top of water
* Hydrogen bonds cause water to have a relatively high boiling point 100°C (373K)
60
First Year Aeronautical Engineering
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