Testing materials

Question 1

How would you pick a material for a task?

Answer 1

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

Question 2

What are the 3 classes of materials?

Answer 2

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.

Question 3

What are composite materials?

Answer 3

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.

Question 4

What is stress?

Answer 4

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)…

Question 5

What is Strong/strength?

Answer 5

Strong: a large stress is needed to cause failure.

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

Question 6

What is weak?

Answer 6

Requires a small stress to fail.

Question 7

What is toughness?

Answer 7

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

Question 8

What are the properies of a tough material?

Answer 8

1. Needs a large amount energy to break.
2. Undergos a large amount of plastic deformation before breaking.
3. Resists crack propagation.

Question 9

What is brittle?

Answer 9

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.

Question 10

What is hard?

Answer 10

Resistant to dents and scratches.

Question 11

What is elastic?

Answer 11

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

Question 12

What is plastic?

Answer 12

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

Question 13

What is ductile?

Answer 13

Can be drawn into wires.

Question 14

What is malleable?

Answer 14

Can be easily pressed/hammered into shape.

Question 15

What is Hooke’s law?

Answer 15

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.

Question 16

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

Answer 16

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.

Question 17

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

Answer 17

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.

Question 18

What is the equation for strain?

Answer 18

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

Question 19

What is the Young modulus?

Answer 19

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.

Question 20

What is conductivity?

Answer 20

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.

Question 21

What is resistivity?

Answer 21

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.