Design - Materials

Aesthetic Properties

Properties / aspects of a product that relate to appearance, smell, texture, taste

Alloy

A mixture that contains at least one metal: mixture of metals or mixture of metals and non-metals

Composite

A material comprised of two or more constituent materials that have different properties.

Compressive strength

The ability of a material to withstand being pushed or squashed.

Creep

The slow, permanent deformation of a solid material under the influence of a mechanical stress

Density

The mass per unit volume of a material.

• measured in kg/m³

Ductility

• The ability of a material to be drawn, pulled or extruded into wires / other extended shapes

Elasticity

• The ability of a material to return to its original shape after being stretched, compressed, or deformed, once the force is removed.

Electrical insulator

Reduces transmission of electric charge

Electrical resistivity

The measure of a material's ability to conduct electricity. A material with low resistivity will conduct electricity well.

Electro-rheostatic

This smart property relates to a fluid that can undergo a dramatic change in its viscosity when exposed to an electric field.

Viscosity = how thick or thin a fluid is.

Glass

A hard, brittle and typically transparent amorphous solid made by rapidly cooling a fusion of sand, soda and lime.

Grain size (metals)

• Metals are crystalline structures comprised of individual grains.

• grain size depends on how quickly a metal is cooled.

• Quick cooling = high number of small grains

• slow cooling = low number of large grains.

• as grain size increases, ductility increases, strength and toughness decreases.

Hardness

• a material's ability to resist penetration, scratching, abrasion, indentation or cutting.

• Hardness can be tested in a range of ways

  • Scratch hardness

  • Static & dynamic indentation hardness

Different materials have different hardness scales

• original: Mohs hardness scale

  • Diamond = hardest mineral

  • talc = softest.

• Plastics: Shore Hardness

• Metals: Rockwell/Vickers/Brinell

• Wood: Janka

Magneto-rheostatic

This smart property relates to a fluid that can undergo a dramatic change in its viscosity when exposed to a magnetic field.

Viscosity = how thick or thin a fluid is.

Mass

Relates to the amount of matter that is contained with a specific material. Mass is a constant whereas weight may vary depending upon where it is being measured.

Mechanical properties

The properties exhibited by a material when subjected to different forces, measurable through destructive testing:

• hardness

• stiffness

• toughness

• tensile strength

• compressive strength

• ductility

• malleability

• elasticity

• plasticity

• Young’s modulus

• stress and strain

Oxidization resistance

A property of a metal that means that it does not readily react with oxygen and degrade.

Photochromicity

A material that changes colour in response to an increase in light. When the light source is removed, it returns to its original colour

Physical properties

Physical Properties are properties that can be tested without the damage or destruction of the material.

• Mass

• Weight

• Volume

• Density

• Electrical resistivity

• Thermal conductivity

• Thermal expansion

Piezoelectricity

A property of certain smart materials that allows them to generate a small electrical charge when mechanically deformed, and to change shape (expand or contract) when an electric current is applied.

Plasticity

The ability of a material to be permanently deformed without breaking when a force is applied

Shape memory alloys

A smart material that can be plastically deformed and then return to its original, pre-set shape when exposed to a change in temperature or stress

Smart material

Materials that respond to external stimuli (eg. heat, light, pressure, or electricity) by changing one or more of their properties in a controlled, reversible, and repeatable way.

• human-made

Stiffness

• the ability of the material to resist being bent or deflected

• can be found by calculating the gradient of the linear section of a stress-strain curve

  • steeper gradient = stiffer material

• measured in Gigapascals (GPa)

Strain

• The response of a material due to stress, defined as the change in length divided by the original length.

• no units - is a ratio

Stress

• A force on a material divided by the cross-sectional area of that material.

• Measured in Pascals (Pa)

• 3 types of stress:

  • Tensile Stress: how much force a material can withstand when being stretched or pulled

  • Compressive Stress: how much force a material can withstand when being compressed

  • Shear Stress: a type of stress caused by forces parallel to the material plane

Super alloys

An alloy that exhibits

• excellent mechanical strength,

• resistance to thermal creep deformation,

• good surface stability

• resistance to corrosion.

Tensile strength

The ability of a material to withstand pulling forces.

Thermal conductivity

• A measure of how quickly heat passes through a material when there is a temperature difference across it.

• measured in watts per metre per degree Celsius (W/m·°C).

• Insulation: Materials with low thermal conductivity, like fiberglass, are used as insulators.

  Heat Dissipation: High thermal conductivity materials, such as copper, are used in heat sinks to dissipate heat in electronics.

Metals track their electrical conductivity to match their thermal conductivity, but this does not hold true of other material families

• Copper Pans are considered the best in cooking.

• Copper is chosen for its thermal conductivity for the pan.

• Cast iron is chosen for the handle due to lower thermal conductivity.

• Tin is chosen as a lining for the inside as a sacrificial layer that is more resistant to corrosion

Thermal expansion

A measure of the increase in dimensions when a material is heated.

• length, area or volume.

• measured as the fractional increase in dimension per kelvin increase in temperature

Thermo-electricity

A smart material comprised of two dissimilar conductors that produce an electric current when heated. The temperature difference causes an electric current to form.

Thermoplastic

A type of plastic that can be heated and reshaped repeatedly.

Toughness

• The ability of a material to resist the propagation of a crack under force

• can be found by calculating the area under a stress-strain curve. 

Transparency

Ability to let light be transmitted with minimal scattering allowing a clear view through material.

Young's Modulus

• A measure of a material’s stiffness, defined as stress divided by strain.

• high Young's modulus = material is very stiff (e.g., steel) and doesn't stretch easily

• low Young's modulus = the material is flexible or elastic (e.g., rubber)

• measured in Gigapascals (GPa)

Chemical Properties

Characteristics that describe how a material reacts with other substances, resulting in a change in its chemical composition

• Corrosion Resistance

• Reactivity (Food Safe)

• Hygroscopy (Ability to retain moisture)

• Flammability

Mass

• the measure of the amount of matter

• not affected by gravity - does not change until some matter is added or removed

• measured in kg

Weight

• The force on an object acted upon by gravity, which depends on the gravitational force, and the mass of the object

• Measured in Newtons (N)

• F = ma

Volume

• the amount of space an object occupies

• measured in m³

Electrical conductivity and resistivity

• Electrical conductivity

  • a measure of how easily free electrons flow through a material. 

• Electrical resistivity

• Measures how much a material resists the flow of electricity.

  • They are inversely related; meaning that when a material has high resistive properties, it is a poor conductor of electricity, and vice versa

Factors affecting conductivity: Resistivity is described in terms of the free flow of electrons, which can be caused by...

• Collision with lattice imperfections in the material (could be caused by cold working)

• Collisions with other atoms (e.g. impurities)

• Collisions with thermally induced vibrations (temperature)

RWE: Gold plating is often chosen for sensitive electronic components due to its high conductivity/low resistance, and its low reactivity

Resistivity of Different Materials

• Metals = good conductors

• silicone and water = somewhere in between

• plastics, ceramics and glass = good insulators.

Flammability

How quickly a material can be ignited, and how quickly it burns

• Chemical Composition: Materials with high carbon content are often more flammable.

• Surface Area: Finer particles or thinner materials ignite more easily.

• Environmental Conditions: Oxygen levels and temperature affect flammability

Corrosion Resistance

• The ability of a material to withstand damage caused by oxidization or other chemical reactions

• Both steel and aluminum are used for vehicles, but steel needs a protective layer of paint to avoid corrosion.

• Aluminium does from an oxide layer, but it protects it and doesn’t corrode further.

High humidity and salt increase corrosion rates

Reactivity (food safe)

• The extent to which a material will react with another (including air) to become unsafe for use to come in contact with food.

• Some plastics are food safe, others are not. You should not store water for long periods in a single-use bottle, as over time it will leach chemicals into the water

• Copper is quite reactive, and will react with oxygen/acids/alkalis found in foods and cleaning fluids —> why copper pans are always lined with tin

Hygroscopy

• The ability to absorb moisture from the surrounding environment.

• Wood = absorbent material + needs protecting from wet rot, dry rot, and fungal/lice attack.

• Not all timber products absorb moisture at the same rate.

• MDF for example, is very absorbent and needs sealing and coating or it expands and becomes ineffective

• Natural textiles eg. cotton readily absorb moisture

• Synthetic fibres like nylon = much less absorbent.

This important distinction helps designers choose appropriate fabrics based on their intended use.

Thermochromism

Thermochromic materials undergo a reversible chemical reaction in response to a temperature increase.

Strength

• a material's resistance to stress or force without bending or breaking

• 3 types of strength:

  • Tensile strength: the ability of a material to withstand pulling forces without breaking or deforming

  • Compressive strength: the ability of a material to withstand forces that try to squash or compress it without breaking or deforming

  • Shear strength: the ability of a material to withstand forces parallel to the material plane

Young’s Modulus Graph

Stiffness

• calculated by finding the gradient of the straight line at the start of the curve

• stiffer material = steeper line

Elastic Limit

• Any stress beyond this value will result in the material turning from elastic to plastic and will not return to its original shape

Ultimate tensile strength

• the maximum force a material can endure

Necking

• the localized thinning of a material before it breaks. Less force is required to continue to stretch the material

Failure point

• the strain at which the material snaps

Stress

• the force divided by the cross-sectional area. (Pascals)

Strain

• a measure of how much the material stretches. It’s calculated by dividing the change in length by the original length. (no units)

Malleability

• the ability of a material to be be plastically deformed under compressive forces

• beaten, rolled or pressed into different shapes.

• the material must display good amounts of plasticity to be malleable

General Properties of Metals

• typically hard and shiny

• high electrical and thermal conductivity

• Metals in pure element form are usually either too soft, brittle, or chemically reactive for practical use within design. Understanding how to manipulate the properties of these metals is key to their success

Categories of Metals

• Ferrous Metals: Ones which contain iron

  • Low carbon steel, wrought iron, mild steel, cast iron

• Non-ferrous metals: Ones which do not contain iron

  • Aluminium, Titanium, Copper, Brass

• Alloys: A mixture of two or more metals

  • Stainless steel, Iron-nickel

Smelting: Extracting Metal From Ore

The process of refining metal ores into usable metals

• Metal is rarely found in its pure element form - it’s often chemically bonded with impurities of other elements, like oxygen.

• To remove these other elements and extract the pure metal, a process like smelting is required

Smelting:

• Iron ore is heated in a furnace with a reducing agent (often carbon).

• The carbon reacts with the oxygen and you are left with a mix of iron and carbon.

• The impurities bind with the carbon to produce waste called slag.

Iron Oxide (+impurities) + Carbon (reducing agent) → Steel (Iron + carbon) + Slag

Copper

• excellent thermal and electrical conductivity.

• Highly ductile and malleable

• Used in:

  • electrical applications, cookware, pipes for plumbing

Aluminium

Aluminium & others (rarely used pure)

• Highly formable

• recyclable,

• good corrosion resistance.

• High strength to weight ratio + thermal conductivity

• Used in:

  • electrical applications, cookware, pipes for plumbing

Stainless Steel

Iron, carbon, chromium & others

• excellent corrosion resistance

• similar mechanical properties to a range of low to high carbon steel

• Used in:

  • general purpose metal work, medical utensils, cutlery

Wrought Iron

<0.08% Carbon

• highly malleable,

• soft,

• lower strength and stiffness compared to other steels

• Used in:

  • ironmongery, decorative metal work

Low-Carbon Steel (mild steel)

0.05 - 0.3% Carbon

• highly malleable, ductile,

• tough, stiff and strong

• Used in:

  • general purpose metal work

High-Carbon Steel (tool steel)

>0.5% Carbon

• very high strength, stiffness and hardness.

• Lower ductility and toughness than mild steel

• Used in:

  • cutting tools, vehicle chassis

Cast Iron

>2.5% Carbon

• very stiffness and hardness.

• Brittle, slightly higher thermal resistivity, and corrosion resistance

• Used in:

  • cookware, manhole covers

Gold

• aesthetic appeal,

• high malleability,

• excellent electrical conductivity,

• high value

• Used in:

  • electrical connections, jewellery, monetary purposes

Brass

copper, zinc & others

• Highly formable, ductile, malleable.

• Good corrosion and microbial resistance.

• Low coefficient of friction

• Used in:

  • musical instruments, couplings, hinges, gears, bearings, ammunition

Bronze

copper, tin & others

• excellent corrosion resistance,

• highly formable,

• ductile and malleable

• Used in:

  • ship propellers, currency, bearings and bushings, 

Titanium

• very high stiffness, strength, toughness, strength to weight ratio.

• High operating temperatures.

• Difficult to form, expensive

• Used in:

  • Turbines, prosthetics, sports equipment

Lead

• soft, high density, toxicity to life.

• Most uses of lead have now been illegal due to its toxicity and danger to living things

• Used in:

  • radiation shielding, bullets, car batteries

Tin

• low melting point,

• high formability,

• high thermal and electrical conductivity,

• good corrosion resistance

• Used in:

  • soldering and plumbing joints, tinning of copper cookware and steel cans

Effect of Rate of Cooling on Metals

• When a metal solidifies, it forms into grains (crystals). The crystalline structure formed will impact the properties of the material.

• Cooling Quickly:

  • material will form a higher number of smaller crystals.

  • results in a material that is harder, stiffer and stronger, but brittle

• Cooling slowly:

  • material will form a lower number of larger crystals.

  • results in a material that is softer, more ductile, more capable of plasticity

Metal Grain Size - Relationships of Properties

• As metal grain size increases

  • ductility increases

  • strength + toughness decreases

Metal: Normalising

• Heating up to a critical temperature and then left to cool in air.

  • reduces the grain size,

  • produces a more uniform structure,

  • relieves some internal stress within the material.

  • reduces hardness within the material. 

Metal: Annealing

• Heating up to a critical temperature and then being allowed to cool very slowly.

  • makes the metal soft and ductile,

  • reducing hardness.

  • often done to allow further cold working without risk of material fracture

Metal: Hardening

• Heating up to a critical temperature of 800°C and then being quenched rapidly in water or oil.

• This converts the crystalline structure and then cools it quickly, leading to the storage of internal stresses within the material.

• Hardened steel is very difficult to abrade or cut

  • great for the edges of knives, or the shackles on padlocks

Tempering

• Heating up to a low temperature (sub 500°C) for several hours, and being left to cool in air.

• This process is done after hardening, and it is performed to bring back some of the ductility of a material and increase its toughness.

• A padlock or knife or sword = requires both hardness and toughness, so they are nearly always tempered.

Advantages of Alloying of Metals

Base metals are rarely used in their pure form, because there are great benefits of alloying them with another metallic element

• Advantages of Alloying Metals

  • An increase in yield or tensile strength

  • Increased resistance to thermal shock

  • Increased strength at higher temperatures

  • Increased corrosion resistance

  • Increased creep resistance

Superalloys

This group of metallic alloys have unique properties:

• Retain high stiffness and strength at up to 70% of their melting point

• Extremely resistant to corrosion from heat, moisture and chemicals

• Creep and fatigue resistance

• Excellent resistance to thermal shock

Superalloys all exhibit an Face-centered cubic (FCC) structure and fall into three main groups:

• Iron-nickel based alloys

• Nickel based alloys

• Cobalt based alloys

Applications:

• aircraft jet engines

• gas turbines for power plants

• used in specialized medical prosthetics, dental equipment

Timber

• Timber is wood that has been harvested from trees and processed for use in manufacturing, construction, and carpentry.

• renewable building material that uses the sun’s energy to replenish itself in a continuous cycle

  • Softwoods (from evergreen/coniferous trees)

  • Hardwoods (from deciduous trees)

  • Man-made

Whilst most of its production emits significantly less CO2 than steel or concrete, consideration needs to be given to the consequences of mass-deforestation.

Wood VS Timber

• Wood = raw form (tree trunk)

• Timber = wood turned into useable formats that designers and builders can use effectively.

  • Subtractive and lamination processes, rather than moulding are used to create parts.

Advantages of Timber

1. Good strength-to-weight ratio.

2. Can be machined, sanded, and finished easily.

3. Naturally insulating (thermal and acoustic).

Disadvantages of Timber

• cannot be easily moulded; you need a piece of timber that has been cut from a piece of wood big enough for the part you are making.

• Can be affected by moisture, insects, or rot if untreated

Natural timber ranks significantly higher in strength over manufactured timber, so why is manufactured board used so commonly?

• Manufactured board can be processed into much larger sheets, which makes it ideal for making things out of. \

• It can also be made very cheaply out of recycled scraps

Strength of Steel VS Timber VS Plastic

The strength of most materials are dwarfed by that of steel, but due to its weight, expensive and energy intensive production process, we don’t see steel being used for everything.

Softwood

• comes from coniferous trees

• evergreen, needle-leaved and cone-bearing.

*the fibres of these trees are larger and allow for nutrient transport which allows for faster growth

Properties of Softwood

• Rapid growth - reaching maturity in 30 years

• grows in colder climates

• Strength

  • Softwoods are not soft, and many have comparable tensile strength to hardwoods

• Cheaper than hardwood:

  • Quicker to replenish, and can grow all year round in harsher climates means wood can be harvested and replaced quickly

• Sustainability

  • The quicker growth rate allows for a more sustainable scale of production

• Aesthetics

  • Softwoods have an attractive appearance and are often left bare to show off the natural grain of the wood

• Not Moisture Resistant

  • All types of timber need to be protected from moisture, with softwood being no exception

• Requires preparation

  • All natural wood needs to be seasoned; where the moisture content is reduced to below 20%. This provides many benefits, such as resistance to rot and fungi, increased strength, and better appearance.

Examples of Softwood

• Douglas Fir

• Siberian Larch

• Scots Pine

• Red Cedar

Hardwood

• come from mainly deciduous trees or angiosperm trees which are seed or fruit bearing with broad leaves

*the presence of larger vessels that transport nutrients, with smaller fibres that do not

Properties of Hardwood

• Hardwood trees take much longer to grow (up to 150 years to mature).

• They can grow in the tropics, and in temperate climates, but not in very cold conditions

• Strength

  • Hardwoods in general are stronger  and denser than softwood, however exceptions like Balsa wood do exist

• Cost

  • slow growth rate - significantly more expensive than softwood

• Sustainability

  • Slow growth rate means that demand can easily outstrip the supply, making hardwood less renewable

• Aesthetics

  • Hardwoods are a premium cost also due to their aesthetic appeal

• Moisture Resistance

  • Hardwood can be more moisture resistant than softwood, but still need to be protected 

• Preparation

  • All natural wood needs to be seasoned; where the moisture content is reduced to below 20%. This provides many benefits, such as resistance to rot and fungi, increased strength, and better appearance.

Examples of Hardwood

• Oak

• Walnut

• Cherry

• Balsa

Man-Made Timber

manufactured by a series of industrial processes, where fibres, chips or layers are glued together

General Properties

• cheaper than natural timber

• larger sheet sizes than natural timbers

• Susceptible to water damage unless protected

• Easily machined from CAD

• Very strong in both directions due to the lamination being perpendicular

Man-made timber: Plywood

• Construction:

  • Layers called veneers laminated with the grain perpendicular to one another

• Strength:

  • Very strong in both directions due to the lamination being perpendicular

• Moisture resistance

  • Good- especially if using marine ply, but still needs to be treated and protected

• Cost

  • Most expensive

• Appearance

  • Comes in different grades, but generally looks appealing

• Uses

  • Boats, furniture, flooring.

  • Its high strength + ability to be produced in large sheets = a good choice for many applications

Man-made timber: Particle Board (MDF/HDF)

• Construction:

  • Fibres of recycled or scrap wood, pressed and glued together under immense pressure and heat

• Strength:

  • Low stiffness and strength compared to plywood

• Moisture resistance

  • Poor resistance to moisture, needs to be treated as it acts like a sponge

• Cost

  • Fairly inexpensive

• Appearance

  • Poor; never used in its natural state. Often veneered with PVC or hardwood

• Uses

  • Used primarily for cheap mass produced furniture due to its ability to be machined easily and low cost

Man-made timber: Chipboard

• Construction:

  • Fibres of recycled or scrap wood, pressed and glued together under immense pressure and heat

• Strength:

  • High stiffness, low weight but not as strong as plywood

• Moisture resistance

  • Poor resistance to moisture, can be protected but typically never used outside

• Cost

  • Fairly inexpensive

• Appearance

  • Poor, never used in its natural state. Often veneered with PVC or melamine

• Uses

  • Similar to MDF, and used in kitchen counters, IKEA furniture and packaging blocks

Timber Treatment: Seasoning

reducing the moisture content of timber to make usable lumber

• a drying process where the moisture content is lowered to below 20%.

• prevents wood from rotting, shrinking, twisting or warpingover time.

• can take place in an oven, or in a naturally dry environment.

Timber Treatments: General

• Timber treatments are designed to protect the wood from insects or fungi which would attack and decompose the wood.

• They penetrate the wood fibres and usually stain the wood a darker colour.

Fungal Decay - Wood Rot

Timber Finishes

• applied to the surface of a wood and mostly sit on or near the surface rather than penetrate the wood fibres.

• form a barrier which protects from moisture + are often used to improve the aesthetic quality of the wood

Types of Timber Finishes

• Varnish

  • Forms a tough, protective barrier which often gives the wood a shiny and smooth appearance.

  • Several layers, with sanding in between are required for full effect.

  • Often used on boats to protect timber from the harsh environment of salt water

• Wax

  • Often beeswax is used, dissolved into a solvent, and wiped on with a cloth.

  • Offers only low levels of protection as a barrier to moisture as there is no thickness compared to varnish.

  • Brings out the aesthetic grain of natural timber

• Oil

  • Applied with a cloth, and forms a thin protective barrier to moisture.

  • The range and scope of different oils used for different purposes is wide

    • Mineral Oil (derived from petroleum)

    • Linseed Oil

    • Sunflower Oil

    • Tung Oil

    • Walnut Oil

Glass

• A hard, brittle and typically transparent amorphous solid made by rapidly

  cooling a fusion of sand, soda and lime

Glass General Properties

• Hard

• Brittle

• High compressive strength

• Low tensile strength

• Transparent

• Low thermal and electrical conductivity

Advantages of Glass

• Non-porous and low reactivity

  • Glass doesn’t absorb liquids or react with most chemicals

  • can be wiped clean easily and, when manufactured under circumstances is resistant to changes in heat

  • chemical + culinary: used in chemical-resistant lab glassware (like beakers) and airtight jars for storing food.

• Insulating

  • often has a layer of air between multiple layers which prevents the heat from escaping via conduction

• Structural

  • widely used building material.

  • great aesthetic qualities, bringing light and warmth to a space,

  • its insulating properties keep heat trapped inside,

  • can support large forces and can be molded into precise and complex forms

• Recyclable

  • Because glass isn’t easily fused to other materials + it melts, it can be easily separated and recycled.

  • It won’t decompose if dumped in a landfill, but also won’t leech chemicals.

  • Up to 95% of recycled glass can be used when making new glass, if no specialist properties are required

• Flexible

  • With the right composition + specialist tooling, glass can be drawn out into tiny long fibres that are flexible.

  • used either for textiles to create fibreglass, or in a hollow form called optical fibre, which transmits the majority of digital data across nations in the form of light

Types of Glass

• Annealed glass

• Toughened glass

• Borosilicate glass - copes well with thermal expansion and temperature change

• Laminated glass

Annealed Glass

• The most common and untreated type of glass.

• Made by

  • heating silica (the primary component of sand) to a high temperature where it melts,

  • mixing with various compounds to alter its properties

  • slowly cooling the glass to relieve internal stress.

• Weak —> breaks into large, sharp shards, which can be dangerous.

• Used in:

  • Picture frames, regular drinking glasses, standard windows (where safety isn’t a big concern).