4.1 Properties of Materials

Density

The mass per unit volume of a material.

Hardness

The resistance a material offers to penetration or scratching.

Thermal Expansion

A measure of the degree of increase in dimensions when an object is heated. This can be measured by an increase in length, area or volume. The expansivity can be measured as the fractional increase in dimension per kelvin increase in temperature.

Thermal Conductivity

A measure of how fast heat is conducted through a slab of material with a given temperature difference across the slab.

Electrical Resistivity

This is a measure of a material’s ability to conduct electricity. (Ω⋅m).

Mechanical Properties

tensile and compressive strength, stiffness, toughness, ductility, elasticity, plasticity, Young’s modulus, stress and strain

Tensile Strength

The ability of a material to withstand pulling forces.

Stiffness

The resistance of an elastic body to deflection by an applied force.

Toughness

The ability of a material to resist the propagation of cracks.

Ductility

The ability of a material to be drawn or extruded into a wire or other extended shape. (NOT malleability, ability to shape material).

Stress

Stress = force / area (pascal units, Pa).

Strain

The ratio of a change in dimension to the original value of that dimension.

Elasticity

The extent to which a material will return to its original shape after being deformed.

Plasticity

The ability of a material to be changed in shape permanently.

Young’s Modulus

Measure of the stiffness of a material.
E = Stress (F) / Strain (S).
Stiffness: The resistance of an elastic body to deflection by an applied force.
Stress (F) = experience of a force (N/m²).
Strain (S) = change of length (ΔL) / original length (L₀).

Yield Stress

This is the stress at the yield point on the stress/strain graph. Beyond the yield point, the material undergoes plastic deformation.
Beyond yield stress the material undergoes plastic deformation → the material deforms past this point.
This shows us when a material will buckle or fail in operation (safety in design, danger!).
Once the material is past this it cannot return.
Knowing all this can save time, money and effort in a design situation.

Aesthetic Properties

taste, smell, appearance, and texture

Smell

The ability to perceive odours such as sweet, acrid or fragrant.

Texture

It is how something feels or looks, it can be rough or smooth.

Color

Can be warm (e.g. browns) or cool (e.g. blues). Can have psychological effects (e.g. greens are calming).

Appearance

The design of appearance in a product must be aesthetically pleasing to attract a customer, unless it is for a certain market.

Smart Materials

piezoelectricity 

shape memory alloys

photochromicity

magneto-rheostatic

electro-rheostatic

thermoelectricity

Electro-Rheostatic (ER) and Magneto-Rheostatic (MR) Materials

Fluids that can undergo dramatic changes in their viscosity.
They can change from a thick fluid to a solid in a fraction of a second when exposed to a magnetic (for MR materials) or electric (for ER materials) field, and the effect is reversed when the field is removed.

Applications of er and mr

MR fluids are being developed for use in car shock absorbers, damping washing machine vibration, prosthetic limbs, exercise equipment, and surface polishing of machine parts.
ER fluids have mainly been developed for use in clutches and valves, as well as engine mounts designed to reduce noise and vibration in vehicles.

Shape Memory Alloys (SMAs)

SMAs are metals that exhibit pseudo-elasticity and shape memory effect due to rearrangement of the molecules in the material.
Pseudo-elasticity occurs without a change in temperature.
The load on the SMA causes molecular rearrangement, which reverses when the load is decreased and the material springs back to its original shape.
The shape memory effect allows severe deformation of a material, which can then be returned to its original shape by heating it.

Piezoelectric Materials

When a piezoelectric material is deformed, it gives off a small electrical discharge.
When an electric current is passed through it, it increases in size (up to a 4% change in volume).
They are widely used as sensors in different environments.
Specific details of crystalline structure are not required.

Applications of Piezoelectric Materials

Can be used to measure the force of an impact, for example, in the airbag sensor on a car.
The material senses the force of an impact on the car and sends an electric charge to activate the airbag.
Also used for guitar pickups (convert vibrations into electrical signals).

Thermoelectricity

Electricity produced from heat using movement and energy from hot and cold temperatures.
Thermo-electric effect using semi-conductors of alloys (such as Bismuth telluride).

Photochromicity (Photochromism)

Reversible colour changes with light intensity.
A chemical reaction to UV light.
Photocromatic lenses chemicals (chlorine and silver reaction).

phyical properties

density, electrical resistivity, thermal conductivity, thermal expansion, hardness, mass, and weight

mass -

amount of matter contained in a speciifc material ( kg

weight

vector force that the measure of mass in relation to gravity. depends of gravity rate (newtons)