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Calculating change in length under max load

  1. Feb 2, 2012 #1
    Please help, i cant answer part B, but think i am using the right equation! Thanks

    A student is designing a suspension pull rod for a car. The rod consists of a central tube
    section that has a wall thickness of 1.0 mm. The following tube diameters are available: 10mm,
    8 mm and 6 mm and the maxmimum allowed normal stress in the tube is 100 MPa. If the pull rod
    is designed to withstand a maximum load of 2500N what is the minimum tube diameter that
    can be used?

    2b) Calculate the change in length for the chosen tube and length 400 mm under max load
    conditions, E = 210 GPa.

    2c) Each end of the pull rod is to be mounted on a wishbone rocker by using a steel bolt in double
    shear. Using the design load of 2500 N and assuming a factor of safety of 1.5, what is the
    minimum bolt size that can be used if the shear stress allowed in the bolt is 150 MPa.
    Assume bolt sizes available are 4 mm, 5 mm, 6 mm and 8 mm.
  2. jcsd
  3. Feb 2, 2012 #2
    If you know the max stress is 100Mpa (100x10^6 N /m^2) and you know the load is
    2500N then you can work out the min cross sectional area of material required.
    With the given diameters and wall thicknesses it should be possible to select what will do the job. I have not calculated anything yet but I will have a go.
    The change in length should be a straight forward calculation because you are given Young's modulus = 210 GPa
    Youngs modulus = stress/strain and strain = extension/original length
    Hope this helps
  4. Feb 2, 2012 #3
    Are you using these equations for part b

    E=σ/ε where σ is your stress and ε is your strain. You would use this equation to calculate your strain.

    For change in length you would use this equation

    ε=ΔL/L where ΔL is the change in length and L is the original length.

    These equations should solve your problem.
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