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Homework Help: Structures and Materials problem - stress and strain

  1. Sep 24, 2009 #1
    1. The problem statement, all variables and given/known data

    A 4000 kg car traveling at 160 km/h crashes into a concrete post. If all the kinetic energy of the car is used to crush the low alloy steel at the front of the car, how many kg of steel will be crushed before the car stops?

    Properties of low alloy steel from a table of material properties:
    weight per volume = 77 kN/m^3
    stiffness(E) = 200,000 MPa
    yield tensile strength = 420 MPa
    Ultimate tensile strength = 560 MPa
    Compressive strength = 420 MPa
    Resilience = 0.44 MJ/m^3
    Toughness = 135 MJ/m^3

    2. Relevant equations

    E = (1/2)mv^2
    stress (sigma) = F/A

    3. The attempt at a solution

    I am new to this topic and I have no idea which value(s) above I should use. My first attempt at a solution:

    Ek = (1/2)mv^2 = (1/2)(4000 kg)(160 km/h)^2 = 3950617 J
    I divided this by the compressive strength value 420 MPa and calculated that 0.00941 cubic meters of steel was crushed.

    Multiplied this by the weight per cubic meter (77 kN), got a weight of 724 N being crushed.
    Divide it by g, 9.81 m/s^2, to get 73.8 kg of low alloy steel being crushed.

    However my gut feeling tells me this isn't right. Help would be appreciated.
  2. jcsd
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