Stress, Strain, and Young Modulus

Question 1

What is stress? What’s the formula?

Answer 1

Stress is the force per unit area.

Stress (sigma)= Force(N)/surface area (m^2)

Question 2

Why is stress useful as a measurement of comparison?

Answer 2

It takes into account the force per unit surface area so materials of varying sizes can be compared for fair comparison.

Question 3

What is the yield stress? What happens to materials as they experience plastic deformation?(i.e. in bridges)

Answer 3

This is the maximum stress needed before plastic deformation happens, which is important when building bridges, as the material will start to buckle. In metals they will neck as the material thins.

Question 4

What’s the fracture stress?

Answer 4

This is the maximum stress before the material breaks.

Question 5

What’s strain? What’s the formula?

Answer 5

This is the % of extension to the original length.

Strain (E) = x/L

Question 6

Why is strain a useful measurement?

Answer 6

It’s a ratio so allows for fair comparison.

Question 7

If length doubles, the extension….

Answer 7

Doubles.

Question 8

What does a stress(y axis), strain, (x axis), graph look like for brittle materials such as glass?

Answer 8

There’s a straight line, linear, as glass shatters very quickly, experiencing little plastic deformation.

Question 9

What does a stress-strain graph look like for materials like copper and mild steel?

Answer 9

Mild steel is very tough, so it experiences lots of plastic deformation before shattering. Therefore, as the limit of proportion is reached, the graph slopes as the material deforms, gaining round edges. A rod of mild steel may neck under tension.

Question 10

What is the young modulus? (E)

Answer 10

This is the stiffness of a material rather than specimen in Nm^-2.

Question 11

What’s the formula for Young Modulus?

Answer 11

E = Stress / strain

When stress and strain are directly proportional in a linear graph this is constant.