materials

what is materials science

how a material’s composition and structure affects its properties and performance

what are the four material families

metals, polymers, ceramics and glasses, composites

reading a materials selection chart

up the chart = higher stiffness

across the chart = higher density

uses logarithmic scales

four mechanical properties

elastic modulus (E), yield strength (σy), fracture toughness (KIC), density (ρ)

metals (examples, used where, properties)

e.g. Aluminium, nickel, steel, copper, titanium

 

Used in reaction vessels, pipes, catalysts etc.

 

PROPERTIES: strong, tough, high stiffness, good conductor, corrosion resistance depends on metal and environment

polymers (examples, used where, properties)

e.g. PVC, PTFE, kevlar, thermoplastics

 

Used in coatings, insulation, making clothes, packaging, sutures

 

PROPERTIES: low density, easy to shape, lower stiffness than metals and ceramics, sensitive to (environmental) conditions

ceramics and glasses (examples, used where, properties)

e.g. Alumina, clay, silica glass, quartz

 

Used in bricks, sanitary ware, glassware, dental ceramics

 

PROPERTIES: high stiffness and resistance to abrasion, many tolerate harsh (corrosive) conditions (e.g. high temp.), vulnerable to cracking/fracture

composites (examples, used where, properties)

e.g. Wood, concrete

 

Used in bike frames, vehicle components, composite decking

 

PROPERTIES: good performance for their weight, expensive, difficult to recycle/repair (sustainability)

what is atomic structure

number of protons and neutrons in the nucleus of an atom

what is crystal structure

arrangement of atoms in the crystalline solid material

what are the three types of metallic crystal structures

FCC, BCC, HCP

FCC (stands for, number of atoms in one unit cell, how to calculate Vc, APF)

face centered cubic

4 atoms (made up of eight 1/8 s and six ½ s)

Vc = 16R³√2

APF = 0.74

BCC (stands for, number of atoms in one unit cell, how to calculate Vc, APF)

body centered cubic

2 atoms (made up of 1 whole atom and eight 1/8 s → more energy required to move the whole atom so moves less freely than FCC)

Vc = ((4 / √3) * R)³

APF = 0.68

what does HCP stand for

hexagonal close packed

atomic packing factor formula and why it’s important

APF = Vs / Vc

Vs = volume of atoms in a unit cell = (4 / 3)𝜋R³ * n → where n is the number of atoms in one unit cell

Vc = total unit cell volume = different for FCC and BCC

APF describes the proportion of a unit cell occupied by atoms and can influence mechanical properties

density formula

ρ = (n A) / (Vc Nₐ)

n = number of atoms in one unit cell

A = atomic weight (g/mol)

Vc = volume of unit cell (cm³)

Nₐ = avagadro’s number (6.022 × 10²³ atoms/mol)

nm (nanometers) → µm (micrometers) → mm (millimeters)

÷1000

what is it called when polymers/ceramics are ordered

crystalline

what is it called when polymers/ceramics have a lack of order

amorphous

what is a cross link

a bond that joins one polymer chain to other polymer chains (makes it stronger)

what happens in extrusion

elongates grain structure at room temperature

structure changes to become longer and thinner (reduces cross sectional area)

movement of atoms are restricted as the grain boundaries are much closer and grain size reduced so becomes stronger

tensile strength equation

σ= F/A

where F is the instantaneous load applied

where A is the original cross sectional area before loading

shear stress equation

𝜏=F/A

where F is the parallel force to the upper and lower surfaces

where A is the original cross sectional area before loading

compressive stress

σ= F/A

where F is the instantaneous load applied

where A is the original cross sectional area before loading

what is engineering stress

force over original area

engineering stress is equal to or smaller than true stress as original area is greater than the instantaneous area

what is true stress

force over instantaneous area

true stress is equal to or larger than engineering stress as instantaneous area is smaller than original area

what is strain

proportional size change

the amount by which a material deforms under stress or force

what is Hooke’s law

stress is proportional to strain

E = σ/ε

where E is young’s modulus (or elastic modulus) in Pa

σ is stress which is force/area

ε is strain which is the change in length over original length

what is stiffness

resistance of a structure to elastic deformation (recoverable unlike plastic deformation which is permanent)

what is yield strength

the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior

σ = E*0.002

where E is elastic modulus (or young’s modulus)

what is necking

thinning in the middle of the material (indication that material will fracture)

how to test for tensile properties in ceramics

bend test

what is toughness

energy to break a unit volume of material (area under a stress-strain curve) - J/m³

what is hardness

resistance to permanently indenting the surface (resistance to plastic deformation)

better wear properties