Materials

Question 1

What is the difference between the limit of proportionality and the elastic limit?

Answer 1

• The limit of proportionality is the point at which Hooke’s Law stops being obeyed i.e. when the graph starts to curve. However, when the force is removed, the material will still return to its natural length.
• The elastic limit is the point at which the material will be permanently deformed (stretched or compressed)

Question 2

What is plastic behaviour of a material under load?

Answer 2

1. Some atoms in the material move position relative to each other.
2. When the load is removed the atoms don’t return to their original positions.
3. The material is permanently stretched (plastic deformation)

Question 3

What are tensile stress and strain?

Answer 3

• Stress is the tensile force applied divided by the cross sectional area
• Strain is the ratio between the extension and the natural length of the material

Question 4

Describe the effect of increasing the tensile stress on an object.

Answer 4

• Stress does work by pulling atoms apart from each other
• When the stress is great enough, the atoms separate completely and the material breaks ( breaking stress)
• Before breaking stress, the UTS or ultimate tensile stress is reached, which is the maximum stress that the material can withstand, after which the force required to extend it further decreases.

Question 5

What does the area under a force-extension grpah represent?

Answer 5

Elastic Strain Energy

Question 6

Draw a typical stress-strain graph and label the key points.

Answer 6

Question 7

What does the stress-strain graph for brittle materials look like?

Answer 7

• Straight line at first, showing they obey Hooke’s Law.
• Once the stress is high enough, the material suddenly breaks, without deforming plastically, so the line stops (without curving).

Question 8

What is the difference between stress-strain graphs and force-extension graphs?

Answer 8

Stress-strain graphs will always look that same for a particular material, whereas force-extension graphs are also affected by the dimensions of the sample.

Question 9

Draw and label the key points of a force exention graph for a sample stretched beyond its elastic limit.

Answer 9

Question 10

How can you find the Young modulus from a stress-strain graph?

Answer 10

The gradient of the straight line part

Question 11

What is the formula for elastic strain energy per unit volume?

Answer 11

EPE/V = 1/2 * stress * strain