Week 2 - Material Properties
09 October 2022 17:11
Week 2:
Material choice is a compromise. Ei silver is a better conductor than copper but more expensive.
In theory interatomic bond strength multiplied by the number of bonds would get u the value of material strength,
however in practice materials are not perfect and include voids, misaligned crystals and foreign matter. Some inclusions
can create improvements in a material. Very difficult to get a pure material due to the massive number of atoms,
99.999% pure still have millions of impurities
Static mechanics : Things that don’t move.
Dynamics and vibration: Excitation in the same frequency of the natural frequency produces resonance. Resonance =
bad: can cause failure of components.
Assessments:
Coursework - 3 online quizzes - 25%
Closed book exam - 75%
Course text - Materials science & engineering an introduction, Callister, William - Version 7/8
- Describe a wide range of materials
- Understand corrosion
- Appreciate standards and regulatory requirements
- Understand reasons for and best practice in professional laboratory working practices
Materials: Broad characteristics (there are exceptions)
- Metals: Strong, good conductors of heat and electricity, high melting point, shiny, smooth
- Ceramics: Brittle, hard, good insulator, matte
- Polymers: plasticy, can be literally anything, fabrics ect
- Semiconductors: Semi conductive, natural insulator but can easily be turned into a conductor
- Composites: made up of more than one thing, Tungsten carbide is a compound not a composite. Properties of 2
materials not combing materials to make something new
- Other
Defining properties:
- Hardness: Resistance to plastic deformation. No scratches or indentations
- Brittleness: Falls apart under shock load
- Stiffness: Resistance to bending
- Malleability: How easily it can be worked. Under impact the material spreads out, not brittle
- Ductility: Can be drawn into wires
- Plasticity: Movement of atoms, stays that shape. Ei Bending metal rod
- Elasticity: When deformed goes back to its original shape
Valence Electrons:
In chemistry and physics, a valence electron is an electron in the outer shell associated with an atom, and that can
participate in the formation of a chemical bond if the outer shell is not closed.
Electrons and protons don’t collapse together
Aufbau principle: --->
Filling Rules:
- Entropy: Seeking the lowest energy. Thermal energy of coffee degrades, give up heat to the room. Electrons want
to fill the lowest energy shell first.
- Rule 1: Lowest energy orbitals fill first
- Rule 2: Pauli Exclusion Principle - Only two electrons are permitted per orbital and they must be of opposite spin.
Two electrons with opposite spins found in the same orbital are referred to as being paired
- Rule 3: Hund's Rule - The most stable arrangement of electrons in a subshell occurs when electrons are added to
orbitals in a subshell singly and with parallel spins before orbitals are occupied in pairs.
What are solids:
Nuclei are unimaginably dense but tiny, lots of space between nuclei. The space between nuclei is filled with
electromagnetic fields. Solids are mostly electromagnetic bonds by volume.
Links:
https://www.youtube.com/watch?v=LhveTGblGHY - Easy entry into atomic structure
Lecture 2:
Intro and Structures:
- Ionic bonding - one molecule gives up an electron to another so they can both have full outer shells. Ionically
bonded molecules are stable and non-reactive. They are conduct poorly.
- Covalent bonding - Molecules share electrons to have full outer shells. Ei H2O.
- Metallic bonding - delocalised electrons flowing around many metal +ions. Highly conductive.
Di-pole, two poles/polarised. These are very useful molecules, an example is water. The oxygen gains a slight - charge
with the hydrogen being slightly +.
Bravais Lattices:
14 different lattices. Represent the crystalline structure of materials.
Crystal and grain formation.
Strength:
Strength is important to designing structures, but how is strength defined.
- Tensile strength (pulling things apart)
- Compressive strength
- Sheer strength (forces acting in opposite directions, ei scissors)
Materials and Mechanics (30 Credits) Page 1
, - Sheer strength (forces acting in opposite directions, ei scissors)
Resonance - a vibration of large amplitude in a mechanical or electrical system caused by a relatively small periodic
stimulus of the same or nearly the same period as the natural vibration period of the system.
Materials and Mechanics (30 Credits) Page 2
, Week 3 - Crystal Structures
10 October 2022 08:59
Two key parts to the structure
- The crystalline structure *
Tensile Testing - For a perfect crystal with the load shared equally across all of the bonds the tensile strength would ne
the sum of all bond strengths but this is rarely the case
- Hard to produce perfect crystals
- Difficult to apply a perfect axial load
- Edge effects come into play
A material has a maximum load it can take - ei 238, we could use 119 (half its maximum load) to have a safely factor of 2.
People often misuse items which means we must design things with unpredictable factors in mind. Corrosion also occurs
and makes things less strong or functional. This leads to a lot of overdesigning to make things safe.
Edge effects - Atoms in the middle of a structure are bonded on all sides. Atoms on the edge have less bonds putting Friction can cause the peaks to break, there may also be physical
them in a state of imperfection. This makes edge electrons and their valent electrons likely to bond with other atoms, ei interlocking and bonding from the edge atoms with their valent
oxygen to cause oxidisation(corrosion), or boundary layer adhesion. Less bonds = less strength therefore materials are electrons.
likely to break at the edge first. Double bonding may also occur causing molecular distortion - evident in liquids where
this is seen as surface tension.
We use fluid/lubrication to reduce friction. (To fill the gaps).
Slip Planes in Crystals:
If a shear force is applied whole plane of bonds could be forced to move to the adjacent nuclei.
Plastic Deformation - Permanent distortion
Elastic Deformation - Non-permanent distortion that snaps back to the original shape after the force stops being applied.
Defects in Crystals:
Substitutional - Foreign atom in crystal structure.
Vacancy - Atom missing from lattice (frequently occur when 2 grains come together)
Dislocation - Slight mismatch where 1 or more bonds missing. Atoms out of place.
Interstitial - Smaller atoms sitting in between other atoms in the lattice.
Defects can sometimes resolve themselves and form stronger materials. Dislocations can move outside of the crystal
when a load is applied.
Edge Dislocation *
Screw dislocation * ^Slip Planes^
Grain Boundaries:
When copper cools the edge cools before the middle. The heat in the middle must travel through the solidified copper
on the outside. Larger crystals form in the middle and smaller on the outside as the middle as cooled slower.
When things cool, materials often contract. When the middle of the copper is liquid and the edge has solidified it causes
gaps in the grain boundaries as the liquid contracts when cooling. *insert steel. Carbon steel, pearlite, layers of iron and iron carbide
Foreign species trapped in between grain boundaries.
Crystals form at all different angles.
^all these reasons show why the theoretical strength of materials cannot be relied on^
Single Crystal or Granular:
In tightly controlled conditions pure materials solidify as a single crystal. Usually when a liquid is poured it will form fr om
all surfaces. Crystallisation occurs at the many points. As the crystals grow they come together with discontinuous
boundaries. Each crystal is known as a grain.
Stress and Strain:
Stress - Force applied over a component (pressure) Stress = Load/Area
Strain - extension of materials due to a load
Poissons Rations
Hookes law
Youngs modulus of elasticity*
Hardness testing:
Hardness is a materials resistance to surface deformation. It is tested but pressing a known shape under know load
conditions and measuring the deformation. Several different methods are used each with different ranges, Brinell and
Vickers hardness scales are common.
Work hardening - doesn’t snap when pulled through a die
Malleability - Capacity to deform under impact loads. Gold can be flattened to a thin sheet, glass
wouldn’t cope with this as well.
Toughness - Amount of energy required to cause fracture under an impact load. A tough material can
absorb more energy.
Fatigue - Fatigue occurs when the load is cyclic and the maximum load may initially be much less than
the tensile yield strength. Microcracks occur and get bigger over time, eventually leading to catastrophic
failure.
How to test for fatigue resistance - put under a small cyclical load and see if it breaks.
Creep:
Occurs at average loads considerably less than yield strength. Failure mechanism is strain and
Cracking occurred in the top left and after many more cycles the crack developed
Materials and Mechanics (30 Credits) Page 3
