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Materials

Stress-strain graphs, ductile and brittle materials

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  • Papers 2 and 3

Overview

Recall questions on the shape of a ductile metal's graph, breaking stress, what the gradient gives, and how a brittle material differs.

What you need to know

Answer each one before you reveal it.

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What does the gradient of a stress-strain graph give, what is breaking stress, and what is the equation for elastic strain energy?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What does the area enclosed between the loading and unloading curves represent?

    Model answer

    It represents the energy per cycle that is not recovered but is dissipated within the material, mainly as thermal energy.

  2. 2

    How does a force-compression graph for a spring compare with its force-extension graph?

    Model answer

    It shows the same straight-line proportional behaviour and the same stiffness at small compressions, but the line ends abruptly once the coils touch and the spring can be compressed no further.

  3. 3

    What is the shape of the stress-strain graph for a ductile metal such as copper?

    Model answer

    It is a straight line up to the limit of proportionality, then curves over at the yield point, extends by a large plastic strain with little extra stress, and finally fractures.

  4. 4

    What is meant by breaking stress?

    Model answer

    It is the stress at which the material actually fractures.

  5. 5

    What does the gradient of the straight-line region of a stress-strain graph give?

    Model answer

    It gives the Young modulus of the material.

  6. 6

    Why is a stress-strain graph more useful than a force-extension graph when comparing two materials?

    Model answer

    Stress and strain do not depend on the dimensions of the sample tested, so the graph describes the material itself rather than one particular specimen.

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