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Mechanical Properties of Materials

Measuring the Young modulus and fracture stress of a metal

  • 3 things to know
  • Components 01, 02 and 03

Overview

Recall questions on the wire arrangement and reference wire, measuring diameter at several points, what is plotted, safety and clamping, readings taken while unloading, and finding fracture stress.

What you need to know

Answer each one before you reveal it.

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Key term

Describe an arrangement for measuring the Young modulus of a metal in wire form.

Key term

Why is a long, thin wire chosen when measuring the Young modulus?

Key term

Which quantity is measured with a micrometer, and why does it matter most?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Describe an arrangement for measuring the Young modulus of a metal in wire form.

    Model answer

    Clamp a long thin wire between flat plates at one end, run it over a pulley or hang it vertically, load it with known masses, and measure the extension against a fixed scale alongside a reference wire.

  2. 2

    Why is a long, thin wire chosen for this measurement?

    Model answer

    A long thin wire gives the largest extension for a given load, which reduces the percentage uncertainty in measuring the extension.

  3. 3

    Which quantity is measured with a micrometer, and why does it matter most?

    Model answer

    The diameter of the wire, because it is very small and it is squared in finding the area, so any error in it has a doubled effect.

  4. 4

    Why is the diameter measured at several points along the wire?

    Model answer

    The wire may not be perfectly uniform, so a mean of several readings gives a better estimate of the cross-sectional area.

  5. 5

    Why is a second, unloaded reference wire hung alongside the test wire?

    Model answer

    Both wires expand equally with any change in room temperature, so comparing them removes thermal expansion from the measured extension.

  6. 6

    State what is plotted and how the Young modulus is obtained from the graph.

    Model answer

    Plot stress against strain and take the gradient of the straight portion, or plot force against extension and multiply the gradient by original length divided by cross-sectional area.

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