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Tensile Testing - Material Failure Prediction

  1. Nov 5, 2011 #1
    Hi all,
    This is not really a homework problem, more of a 'what-if' scenario but I'm not sure which section to put it in.
    For a science project we have been asked to look at material properites and come up with a project (which will have a presentation :eek: but that's another story).
    I'm looking at tensile testing as we have one at our school and I'd like to use that as I think it will be an interesting project.

    1. The problem statement, all variables and given/known data

    What I'd like to know if possible, is there a generic equation that I can use to predict the failure load of a material?
    All the equations I've used require a set force, however tensile testing continually pulls the material (increasing load?) until failure.

    2. Relevant equations
    Not sure. Shear flow?

    3. The attempt at a solution
    As I mentioned it is a theoretical project for the time being so I haven't got anything to show.

    Thanks for any help
     
    Last edited: Nov 5, 2011
  2. jcsd
  3. Nov 6, 2011 #2

    PhanthomJay

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    not really. Steel for example has different failure loads depending on its chemical composition and molecular structure
    correct, increasing load
    You can look up the breaking strength of the material, say for a high strength steel, medium steel, hi strength aluminum rod, etc., and compare with the testing results. You should plot the stress versus strain or load versus extension during the tests for showing the behavior of the material as the load increases.
     
  4. Nov 6, 2011 #3

    Thanks for the response.

    Taking it a level further, what about joints? e.g. a Lap Joint (riveted?). Is it possible to predict the yield force?
     
    Last edited: Nov 6, 2011
  5. Nov 6, 2011 #4

    PhanthomJay

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    No. The rivets or bolts would fail in shear at the ultimate shear stress of the rivet or bolt. This again must be given as the property of the material from which the bolt or rivet is made. The strength of the bolt has nothing to do with the yield strength of the bar/rod.
     
  6. Nov 8, 2011 #5
    Hi,
    So if I wanted to predict the failure load of the joint I would simply compare the shear stress of the rivet and the yield strength of the material?
    Whichever is lower = method of failure and the maximum loading = lowest of above loads?

    Then it's just a case of making the joint and subjecting to load until the joint fails, the failure load should tie in with the above.
    I think that's correct?
    Thanks for your help :cool:
     
  7. Nov 8, 2011 #6

    PhanthomJay

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    I thought you were looking at tensile stressing a rod, but now you've focused on joints which can get a bit difficult to analyze because shear stress in a bolt is just one method of joint failure; the joint could also fail by bearing pressure of the bolt on the steel, or tearout failure at the bolt hole, or failure of the rod itself. And what is meant by failure? Yielding? Breaking? Excess deformation? Exactly what is meant by "we have been asked to look at material properites"? .
     
  8. Nov 10, 2011 #7

    Hi,
    I'll try and put some more meat on the bone.

    I'd like to analyze a single lap joint, riveted together which is then put into a tensile testing machine and subjected (pulled apart) until failure.
    By failure I mean breaking, as in the joint can hold no more force.

    So would I then need to complete several equations?
    From my research I found these failure modes of joints on a British website (http://www.roymech.co.uk/Useful_Tables/Rivets.html).


    • Shearing through one section of the rivet (single shear).
    • Shearing through two sections of the rivet (double shear).
    • Compressive bearing failure of the rivet.
    • Shearing of the plate(s) being joined.
    • Bearing failure of the plate(s) being joined.
    • Tearing of the plates between the rivets.

    Also shown on that website is the equations for the above.
    I believe it's a case of doing all of the calculations and the lowest force is the failure mode?

    However, again I fall back to the issue that the equations require a force. Do I simply rearrange the equation so I can solve for force?
    Thanks
     
  9. Nov 10, 2011 #8

    PhanthomJay

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    Rivets went out of fashion over 50 years ago, having been replaced, for the most part, by high strength bolts.

    Ideally, for economy of design, you'd like the joint to fail in bearing, tear out, and shear at the same time the steel member fails. But since you're looking at joint failure, be sure to oversize the member for tensile load, so that it is is the joint and not the member that fails. Bearing pressure failure by crushing may be difficult to identify, and shear tearout can be eliminated by proper edge distances and bolt spacings. You should calculate what is required for bolt/joint design to eliminate these failur modes, to ensure your failure will be by ultimate shear stress in the bolts. The greater the number of bolts, the greater will be the force required to fail them. Since bolts come in various diameters and various strengths, be sure to know the properties of the particular bolts you will use. Remember that the tensile force applied to the steel member is not the same as the force applied to the bolt. This must be calculated.
     
  10. Nov 10, 2011 #9

    nvn

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    Corsan: Would you be interested in tensile testing a rod? I think I would recommend it. It would be simpler, but still complicated enough, I would think.
     
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