DoITPoMS

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Questions

Quick questions

You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!

  1. On a typical tensile stress-strain curve, how is the elastic region represented?

    a By a horizontal straight line
    b By a curved region of increasing gradient
    c By a straight line of positive gradient
    d By a parabolic region

  2. The presence of a small amount of copper in aluminium significantly strengthens the alloy by ...

    a Forming CuAl2 precipitates which hinder dislocation motion
    b Increasing its Young's Modulus
    c Binding atoms together, thus preventing slip
    d Rearrangement of the crystal structure resulting in fewer dislocations

  3. Work hardening strengthens an alloy by ...

    a Removing internal defects in the crystal structure
    b Increasing the dislocation density
    c Decreasing the grain size of the alloy
    d Increasing the lattice resistance to dislocation motion

  4. Annealing ...

    a Softens an alloy by thermally activating dislocation slip
    b Strengthens an alloy by means of thermal stresses
    c Softens an alloy by reducing the dislocation density
    d Does not affect a material's strength

  5. What causes grain growth?

    a Thermal expansion of the grains
    b Larger grains can contain more defects, making them entropically favourable
    c Rearrangement of the grain boundaries into lower energy crystallographic positions
    d Reduction in grain boundary area, since grain boundaries are high energy features

Deeper questions

The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP.

  1. Why is it not possible to obtain the fracture stress of a material from a nominal stress-strain curve?

    a Due to the accuracy of the testing machine, the precise fracture stress is impossible to define on a nominal stress-strain curve.
    b The nominal stress is calculated from the initial sample dimensions and does not take into account the change in cross-sectional area as the tensile test proceeds.
    c Because the nominal strain does not pinpoint the breaking point accurately
    d Because the load on the sample varies throughout this type of test

  2. In a polycrystalline sample, why do the points A (limit of proportionality) and B (yield point) on a graph of true stress against true strain tend not to coincide?
    Graph of stress against strain

    a Because at A the grains are slipping past one another whereas at B they begin to yield internally.
    b At A, slip begins to take place by means of dislocations, whereas at B there is sufficient stress for whole planes of atoms to slide, without need for defects.
    c Because as the stress increases, the grain size changes.
    d At A, slip begins, but only in certain grains which are favourably orientated.B is the point at which yielding occurs across the whole specimen, and is therefore at a slightly higher stress.

  3. Which of the following factors affects yield stress?

    Yes No a Sample geometry
    Yes No b Lattice resistance
    Yes No c Dislocation mobility/interactions
    Yes No d Grain size
    Yes No e Ultimate tensile strength
    Yes No f Presence of precipitate particles


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