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Deformation of Solids

Choosing materials: strength, stiffness and cost

  • 3 things to know
  • Unit A2 1

Overview

Breaking stress of a material, difference between a strong material and a stiff material and factor of safety in a design calculation.

What you need to know

Answer each one before you reveal it.

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Key term

Why is the Young modulus of a material important to an engineer designing a bridge?

Key term

What is meant by the breaking stress of a material?

Key term

Why is breaking stress rather than breaking force quoted as a property of a material?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Why is the Young modulus of a material important to an engineer designing a bridge?

    Model answer

    A material with a high Young modulus is stiff, so the deck and cables deflect only slightly under load and the bridge keeps close to its designed shape.

  2. 2

    What is meant by the breaking stress of a material?

    Model answer

    The breaking stress is the stress at which the material fractures, measured in pascals.

  3. 3

    Why is breaking stress rather than breaking force quoted as a property of a material?

    Model answer

    The breaking force depends on how thick the specimen is, whereas the breaking stress is the same for any specimen of that material.

  4. 4

    Explain the difference between a strong material and a stiff material.

    Model answer

    A strong material withstands a large stress before it breaks, while a stiff material has a large Young modulus and strains only slightly under a given stress.

  5. 5

    Why is steel rather than aluminium used for the main cables of a suspension bridge?

    Model answer

    Steel has roughly three times the Young modulus of aluminium and a much greater breaking stress, so for the same cross-section the cables stretch far less and carry a much heavier load.

  6. 6

    Why is concrete reinforced with steel rods?

    Model answer

    Concrete is strong in compression but weak in tension, so the steel rods carry the tensile stresses that the concrete alone could not.

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