How would you pick a material for a task?

By looking at what properies are required for the task and seeing which materials have those properies.

What are the 3 classes of materials?

1. Ceramics: These materials are usually hard and brittle. Examples include china and pottery.

2. Metals: These materials are usually strong and tough, as well as malleable and good electrical conductors.

3. Polymers: These materials are usually very tough.

What are composite materials?

Materials that contain a mixture of different classes of materials in order to combine their properties. An example is bone, which consists of calcium phosphate; a ceramic which is strong under compression and collagen, a polymer which is strong under tension.

What is stress?

Force applied over the surface area it's applied across and is mathematically defined as σ = F/A.

There are several types of stress; including tension (stretching), compression, torsion (twisting)...

What is Strong/strength?

Strong: a large stress is needed to cause failure.

Strength: The maximum stress a material can withstand before failure (yielding/breaking).

What is weak?

Requires a small stress to fail.

What is toughness?

Toughness refers to materials that undergo considerable plastic deformation before breaking and absorbs lots of energy. Toughness is defined as:

1. The energy required to create new surface area. Jm^{-2}

or

2. The energy absorbed per unit volume. Jm^{-3}

What are the properies of a tough material?

1. Needs a large amount energy to break.

2. Undergos a large amount of plastic deformation before breaking.

3. Resists crack propagation.

What is brittle?

Opposite of tough. Brittle materials do not undergo a lot of crack propagation before breaking and break as soon as their elastic limit is reached. Cracks propagate easily through it.

What is hard?

Resistant to dents and scratches.

What is elastic?

Changes shape when stress is applied but returns to its original shape when stress is removed.

What is plastic?

Changes shape when stress is applied but doesn't return to its original shape when stress is removed.

What is ductile?

Can be drawn into wires.

What is malleable?

Can be easily pressed/hammered into shape.

What is Hooke's law?

Extension is proportional to force applied if object obeys Hooke's law, or F=ke; where F is force applied, k is a constant and e is extension.

What is the relationship between Hooke's law and springs?

Extension of springs is proportional to force applied, so springs obey Hooke's law. k is called the spring constant, where the bigger the value of k, the stiffer the spring.

What is the shape of a force-extension graph for metal wires?

1. The first part of the graph is a straight line, meaning that the wire obeys Hooke's law and force is proportional to extension.

2. The line then begins to curve beyond the elastic limit. Beyond which point the wire undegos plastic deformation and no longer returns to original shape.

What is the equation for strain?

Strain = extension/original length (ε = x/l).

What is the Young modulus?

A measure of how stiff a material is. Measures how much stress is required to cause a certain strain and is mathematically defined as E = σ/ε = (F/A)/(x/l) = Fl/Ax.

What is conductivity?

A measure of how good a material is at letting an electric current flow through and is mathematically defined as σ = Gl/A where l is lengh, A is cross-section area and G is conductance.

What is resistivity?

How bad a material is at letting an electric current flow through and is mathematically defined as ρ = RA/l where l is length, A is cross-section area and R is resistance.