Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Average normal stress

  1. Feb 5, 2004 #1
    Two solid cylinderical rods AB & BC are welded together at B & loaded as shown. Knowing that d(1)=30mm & d(2)=50mm, find the average normal stress in the midsection of (a)rod AB, (b) rod BC.
    125kN--->| |
    A______________| |
    60kN<---| | |
    | | |
    |______________| |
    |<- 0.9m------>| |

    stress = P/A

    (a) A= pie r ^2=pie (30mm/2)^2=.000707 m^2
    stress=P/A= (60x10^3 N)/(.000707)=84.9MPa

    (b) A= pie r^2)= pie(50mm/2)^2=.001963
    stress= P/A= (250x10^3)/.001963=106.1 MPa

    the stress for (b) is wrong; it's suppose to be 96.8MPa Can someone show me what I have done wrong and explain why it should be done differently? Thanks
  2. jcsd
  3. Feb 5, 2004 #2
    figure for post

    ..............125kN--->| ..........|
    ........A______________|.......... |
    60kN<---|............. | ..........|
    ........| .............| ..........|
    ........|______________|.......... |
    ........|<- 0.9m------>|.......... |
  4. Feb 5, 2004 #3
    figure for post

    I can't seem to get the figure to turn out right sorry about this, I will attempt to explain it in words.

    the first rod AB is 0.9m long and has a diameter of d1=(30mm); 60kN<---- from the A end of this rod the second rod BC attaches to the B end of the first rod it is 1.2m long and has a diameter is d2=(50mm); 125 kN is applied both above and below where it is attached to the first rod on the B end 125kN---->
  5. Feb 6, 2004 #4


    User Avatar
    Science Advisor
    Gold Member

    Maybe I just have a short attention span, but I don't see the situation. I've got rod AB welded to BC (connected at B, naturally). A is on the lefthand side and C is on the righthand side of my scrap paper. Tell me the following and I'll try to help you:

    - Force applied at point A and the direction the force is pointing.

    - Force applied at point B (and the direction).

    - Force applied at point C (and the direction).

    - Whether or not the rods are attached to anything (e.g. mechanical ground) and how.
  6. Feb 7, 2004 #5

    force at A is pointed to the left and is in the center on the end
    force at B is pointed to the right and is at the top and bottom of where C attaches. Can I send you a pic of the thing? It keeps telling me here that the attachment is bigger then 400X 400 and i made sure it wasn't
  7. Feb 7, 2004 #6


    User Avatar
    Science Advisor
    Gold Member

    OK, I think I get it. The way you're describing it is a little difficult to follow and may be the reason you're having trouble solving it.

    In cheesy ASCII art, here's my sketch (sans dimensions):

    Code (Text):

            125kN    __________
                --->|          |/
              ______|          |/
             |      |          |/
    60kN  <--|      |          |/
             |______|          |/
                    |          |/

             A      B          C
    So you've got A B and C going from left to right. At surface C, the bar is up against a wall (for all intents and purposes, connected to mechanical ground). At surface A, there is a force of 60 kN to the left. At surface B, there are two points of loading, each with 125 kN, as sketched.

    Member AB is in tension. Since the loading is 60 kN and the area is as you have computed, the normal stress is ~84.9 MPa, as you calculated.

    Member BC is in compression. The compressive force is (125+125-60)kN = 190 kN. (There is a reaction force normal to the wall that balances this force). The normal stress is again given by &sigma; = P/A. Use the proper value for P with the area you computed and you will find the answer.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook