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Help with Tensile testing

  1. Aug 5, 2016 #1
    • Homework posted in wrong forum, so no template
    I'm asked to design a rod, it is aluminium and it is to withstand force of 200kN.
    maximum allowable stress is 170MPa with a strain of 0.0025mm.mm^-1
    rod must be at least 3.8m long but must deform elastically no more than 6mm when the force is applied.

    UTS = 170 MPa
    e = 0.0025mm.mm^-1
    F = 200kN
    Lf = 6mm + 3.8m (?)
    Lo = 3796.5087mm (?)
    Gauge length = Lo ??
    I'm unsure about the gauge length, Lo and Lf
    do i need to also consider the force 200kN
     
  2. jcsd
  3. Aug 6, 2016 #2

    haruspex

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    Applied how? I'll assume this is tension.
    I'm not quite sure what that is saying. Is it that at a tension of 200kN the stress must be no more than 170MPa? What does that tell you about the cross-section?
    I'm unclear how those two relate. The first seems to say that at 200kN tension the rod must not extend more than 0.25%, while the second says it must not extend more than 6/38 %, or about .15%.
    Since it must withstand that force, that seems a rather crucial consideration.
     
  4. Aug 6, 2016 #3
    I've come up with Lo = 3796.51mm and Ao = 0.0012msquared
    using the formulas:
    e = Δgaugelength/initial length (taking that Δgaugelength = 3.8m + 6mm)
    σ = P/Ao (P=0.2MN , σ = 170MN/msquared)

    is this right????
     
  5. Aug 6, 2016 #4

    haruspex

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    That satisfies the max allowable stress, but what will the extension be if at that stress it extends 2.5 mm/m?
     
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