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

Shear Pressure?

  1. Feb 3, 2016 #1
    Hi guys,

    The imagine the picture attached is a cross section of a cylinder. How would I work out the maximum amount of pressure this can be exposed to before the metal shears off at the location labelled "2mm thick"?

    Lets say the following material is used:

    T6 Aluminium
    Ultimate Tensile Strength - 310 MPa
    Tensile Yield Strength - 276 MPa
    Shear Strength - 207 MPa

    Iv googled and googled but can only seem to find information relating to shear force in beams or punching through a material.

    All help greatly appreciated.
    Thanks
     

    Attached Files:

  2. jcsd
  3. Feb 3, 2016 #2

    Nidum

    User Avatar
    Science Advisor
    Gold Member

    Imagine the small diameter section being separate and fitting into the larger diameter part like a piston in a cylinder . The contact area between the two parts represents the area carrying the shear force in the one piece object
     
  4. Feb 3, 2016 #3
    Hi,

    I understand the location, its pretty much where iv marked with a black line on the diagram, I just cant for the life of my figure out how to calculate the max force.

    Regards
     
  5. Feb 3, 2016 #4

    Nidum

    User Avatar
    Science Advisor
    Gold Member

    You may be confused by the blind bore . Pressure acts where you have shown it but also on the area at top of the blind bore . Effectively you can ignore the details of the bore and just treat the upper component as if it was solid for purposes of calculating the effective 'piston area' that pressure acts on .
     
  6. Feb 3, 2016 #5
    Hi,

    Sorry reading my post back I understand I wasn't clear, I dont mean where the arrows are I mean where i've done the black line and labeled it 2mm thick i.e. "the contact area between the 2 parts".
     
  7. Feb 3, 2016 #6

    Nidum

    User Avatar
    Science Advisor
    Gold Member

    It's getting late now . Hopefully someone else will pick up this thread but if not I'll get back to you tomorrow .
     
  8. Feb 3, 2016 #7
    ok thank you
     
  9. Feb 3, 2016 #8

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    You know how to calculate the upward force due to pressure on that upward piece correct? Don't forget to include both the very top and the two horizontal surfaces toward which you have the arrows pointing in your picture.
     
  10. Feb 3, 2016 #9
    Sorry not too sure what you mean. Please could you elaborate a little?
     
  11. Feb 3, 2016 #10

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    Well if you want to know the shear stress where those black lines are, you need two things: a force and an area. So, do you know how to calculate the force you need? What about the area?
     
  12. Feb 3, 2016 #11
    Oh i see what you mean.

    The total area that the pressure is acting on including the upward area is 3.14cm^2.

    For the force cam i simply convert the pressure? For example 100 bar of pressure = 101.972 kgmf/cm^2.
     
  13. Feb 3, 2016 #12

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    So the inner diameter of your larger part of the cylinder is 2 cm?

    You are probably better off doing everything in the MKS units (meter, kilogram, second), so that you do pressure in pascals, force in newtons, and area in m2. What you suggest would work but keeping units straight will be more of a pain.
     
  14. Feb 3, 2016 #13
    Yes 2cm diameter is correct.

    So using MSK....
    Pressure = 100,000 Pa
    Force = 100,000 N/m^2
    Area = 0.0314 m^2

    Where do I go from here?
     
  15. Feb 3, 2016 #14

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    Well first, force is not measured in N/m2. Both of your first two numbers are pressures since Pa = N/m2.

    Second, you have to add a couple extra zeros to your area because 1 cm x 1 cm is not 0.01 m; it is 0.0001 m.

    Then, how do you feel you should proceed to find the force? What does pressure actually mean and how does it relate to force?
     
  16. Feb 3, 2016 #15
    Ahh we are getting there...

    Pressure is force applied to the surface so to speak.

    Pressure = force / area.

    Which gives us...

    Force = pressure x area.
    Force = 100,000Pa x 0.000314m^2
    Force = 31.4N
     
  17. Feb 3, 2016 #16

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    Alright, so now you have a force. Now you have to convert that back into a stress in the region your are interested. Stress is also a force per unit area, but unlike pressure, it can occur in solids and can occur in shear rather than just normal to a surface. Essentially, pressure is the normal component of stress in a fluid.

    Anyway, so you need to find the shear stress. You have a force. You need an area.
     
  18. Feb 3, 2016 #17
    When you say area do you mean the area where the stress will occur?

    If so would it be the circumference x thickness?
    62.83mm x 2mm =125.66mm^2 or 0.00012566m^2

    Or am i completely off the mark?
     
  19. Feb 3, 2016 #18

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    No you're still heading in the right direction (though I didn't check your math).
     
  20. Feb 4, 2016 #19
    Ok so if we used Shear stress = force/ area

    Shear stress = 31.4N / 0.00012566m^2
    Shear stress = =249,880.63 Pa

    Should i factor in the strength of the material? For example wouldnt steel be able to handle more pressure than aluminium?

    (Ps i really really appreciate all your help, thank you)
     
  21. Feb 4, 2016 #20

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    The strength of a material plays into the maximum stress before failure but does not affect the stress in the shape for a given set of forces.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Shear Pressure?
  1. Shear Force (Replies: 5)

  2. Shear modulus of steel (Replies: 6)

Loading...