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Homework Help: Help me calculate a calculated bending stress (psi) in cantilever beam?

  1. Nov 17, 2011 #1
    1. The problem statement, all variables and given/known data

    I am using a Vishay strain indicator with it hooked up to a strain gage hooked up 1" away from the wall on the solid beam. The beam is .5" by .5", and it is solid. The beam is 16" long and I am putting weights at the end of the beam. The weights are 1, 2, 5, 10, 15, and 20 lbs. On each weight, I recorded a strain(uin/in). It states to use the modulus of elasticity E_aluminum 10x10^6 to convert to psi. My question is how do I calculate the M part for the bending stress?

    2. Relevant equations

    bending stress = MC/I

    modulus of elasticity E_aluminum 10x10^6 to convert to psi.

    3. The attempt at a solution

    For C, .5/2= .25

    For I, (1/12)*b*h^3 or (1/12)*.5*.5^3 = .0052083

    How do I find my M?

    For 1 lb, would it be M-1lb(16")=0 or M=16 lb.in but I don't get it why it says to use E_aluminum = 10x10^6 psi to convert.

    Here is a picture of the cantilever beam setup I used.

    Last edited: Nov 17, 2011
  2. jcsd
  3. Nov 17, 2011 #2
    The moment is simply the distance the weight is from the point in question. So at the wall, the bending moment is W * 16 = 16W inch pounds.

    The conversion they are talking about is how you convert strain to stress. Stress = strain times modulus
  4. Nov 17, 2011 #3
    Thank you! It makes sense now. The only weird thing is if I just 10^6 instead of 10*10^6, then it will be close to my theoretical value. But if I do 10*10^6, the calculated value is super off from the theoretical value.
  5. Nov 18, 2011 #4
    I have never used strain gauges but does it have some sort of gauge factor associated with it?
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