4. Tests on steels
page 18

Interactive graphic on forces
Interactive graphic on forces
Interactive graphic on forces
Picture 4.1 Diagram of three ways of stressing a sample of material.
Mechanical properties of steel
The way in which a material behaves is described as its mechanical properties. When the forces tend to stretch a material it is said to be in tension. When the forces squash the material it is said to be under compression. When forces tend to twist the material or to make one part of it move relative to another part it is described as being in shear.
    The main properties that we can test are:
    Stress and strain are the quantities we use to compare fairly the effects of a force on a material. Instead of the applied force we use stress; and instead of extension (or compression), we use strain.

    Stress is the force per unit area.

    What does stress mean?
    Interactive graphic on stress
    Interactive graphic on stress
    Picture 4.2 Illustrations of stress and strain.

    Stress has units of newtons per square metre, N m-2, or pascals, Pa. It is often expressed in: megapascals [1 MPa = 106 Pa] or gigapascals [1 GPa = 109 Pa].

    Stress allows us to get a fair comparison of the effects of a force on different samples of a material. A tensile force will stretch and, possibly, break the sample. However, the force needed to break a sample will vary - depending on the cross sectional area of the sample. If the cross sectional area is bigger, the breaking force will be bigger. However, the breaking stress will always be the same because the stress is the force per unit area.

    In picture 4.2, sample A breaks with a force of 60 kN. Sample B breaks with a force of 120 kN because it has twice the area (you can think of it as being two pieces of sample A next to each other, with each one needing a force of 60 KN to break it). Although the force is bigger for sample B, the stress is the same for both samples – it is 60 kN cm2.

    So breaking stress is a more useful measurement than breaking force because it is constant for a given material. It allows us to fairly compare the strengths of different materials.

    For example:
    The force needed to break a piece of steel wire with a cross sectional area of 2 x 10–6 m2 is 2 400 N.
    a) What is its breaking stress?
    b) What force would be needed to break a steel bar with a cross section of 5 x 10–4 m2?

    a) The breaking stress = breaking force ÷ area
    = 2400 ÷ 2 x 10–6
    = 1, 200, 000, 000 N m–2
    = 1.2 GPa.
    b) To break the steel bar, the force needed = breaking stress x area
    = 1.2 x 109  x  5 x 10–4
    = 600, 000 N

    Photo of strength test
    Picture 4.3 A special heavy duty machine puts a huge force on test samples of steel bars to break them.
    Use the links below to see a video clip of strength testing.

    higher quality (528k)
    lower quality (176k)

    Tensile strength
    A tensile test is used to find out what happens when a material such as steel is stretched. A steel bar is placed in a device that pulls one end away from the other fixed end. The tensile strength is the maximum stress that the bar can withstand before breaking.
    So far, we have looked at only the breaking stress of a material. But what about the stretching effect of a force. We would expect that different samples of the same material would stretch in the same way when we apply the same stress to them. However, in order to compare the amount they stretch, we need to think of more than simply their extension. This is strain.

    On the next page we will look at strain and stiffness.

    Question 4-1.
    a) How many megapascals make up one gigapascal?

    b) Why is stress a more useful measurement than force for comparing different materials?

    c) The breaking stress of a particular steel is 1.2 GPa. What force will be needed to break a piece of wire with a diameter of 0.5 mm?

    Summary                   Close
    • the main tensile measurements we can make on a material are stress, strain and strength
    • stress is the force per unit area
    • strength is the force needed to break it