Stress, strain and the Young modulus

Overview

Recall questions on tensile stress and strain and their units, the Young modulus and its typical size for metals, why it beats stiffness for comparing materials, calculating stress, strain, Young modulus, extension and fracture stress, the stress–strain graph of a ductile metal, and why stress–strain graphs compare wires of different thickness.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Define tensile stress.

   Model answer

   Tensile stress is the tension applied divided by the cross-sectional area over which it acts.

2. 2

   State the unit of stress.

   Model answer

   Stress is measured in pascals (Pa), where 1 Pa is 1 N m⁻².

3. 3

   Define tensile strain.

   Model answer

   Tensile strain is the extension divided by the original length of the specimen.

4. 4

   Why does strain have no unit?

   Model answer

   Strain is an extension divided by a length, so the metres cancel and it is a pure number.

5. 5

   Define the Young modulus.

   Model answer

   The Young modulus is the tensile stress divided by the tensile strain, for a material within its limit of proportionality.

6. 6

   State the unit of the Young modulus and a typical order of magnitude for a metal.

   Model answer

   It is measured in pascals, and for a metal it is of the order of 10¹¹ Pa.

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