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A Question on Tensile Tests

  1. Feb 8, 2009 #1
    For our thesis, my friends and I made a pine needle packaging material made of brown pine needle bits about 1 inch in length and cornstarch paste. We attempted to get its Young's modulus to determine its tensile and compressive strength. This is how we did our experimentation:

    The packaging material became more solid after drying. One half-sheet was then tested for durability by suspending it between two iron rings positioned 5 inches high, while putting 295-gram weights one by one on the sheet. The sheet remained intact until 4 of the weights (which make up a total of 1180 grams) were placed on it, but began to show damage after 4 tries with the weights. The change in length caused by 4 weights in the first try was 1 cm.

    I know how terribly flawed our methodology is T__T. What were the errors we did? How can we properly get the material's compressive/tensile strength? I found somewhere that there is a standardized tensile tests, but we don't have the equipment. Please explain to me also the physics principles I need to consider in this problem. Thank you :).
     
  2. jcsd
  3. Feb 9, 2009 #2
    a couple things:
    -should have continuous loading or application of smaller loads (maybe 5% of ultimate load) at a time, to avoid impact mechanical behavior).
    -Also might want to look into the whole E_t=V_i*E_i+V_r*E_r
    -make sure you have some kind of shoulder so you have a reduced section for the test specimen.
    -the compressive strength may require a cylindar of the substace be made, or a rectangle, you can get dials for more precise measurements. there are also force gages you can get that you can use while pulling on the materials yourself (at a somewhat steady pace).

    you prolly want to use an ASTM standard that uses fiber reinforced polymer matrix tensile test standrad or procedure or something.
     
  4. Feb 10, 2009 #3
    I'm afraid I'm not familiar with the equation you presented E_t=V_i*E_i+V_r*E_r. What is it for?

    Also, I need some help with computing the young's modulus. You see, we conducted 3 tries of putting 1180 grams of weights on the material, and in the 4th try, the material finally gave way. What do these three tries mean in the computation? Will I then triple the force as I compute for the stress?
     
  5. Feb 10, 2009 #4
    Young's modulus describes the stiffness of a material, not its strength, and is defined as:

    [tex]E=\frac{\sigma}{\epsilon}[/tex]

    where [tex]\sigma[/tex] is the applied engineering strain (force applied divided by nominal area across which it is acting) and [tex]\epsilon[/tex] is the engineering strain (change in length divided by original sample length).

    Young's modulus is constant for linear elastic materials up to the yield point of the material, at which the stress-strain relationship is no longer linear. So, for a material tested within this linear elastic regime, the extension of the sample will double as you double the force applied to it.

    Normally this is carried out by manufacturing tensile dumbell or dogbone samples. These comprise a large area that is clamped or held at either end of the sample, and a contraction to a narrow section in between. This is to avoid stress concentrations due to the clamping conditions, and to ensure that the sample fails in the central section. By measuring the change in length of a section of the centre and the applied load, and knowing the initial dimensions of this section, you calculate stress and strain from force-deflection and as a result you can calculate the elastic modulus. By knowing the stress at which the material ceases to behave linearly you can state its yield strength. By continuing to load the material until the material fails, you can calculate its failure strength or ultimate tensile strength.

    As the other poster mentioned, load should be applied gradually to avoid dynamic effects. With your rudimentary resources applying individual masses (gently...don't drop them on!) is perfect for getting an approximate value. If your material is in sheet form at the moment, you can manufacture samples similar to http://web.mit.edu/course/3/3.042/team1_06/solidworks files/3-15 tensile-solidsmall printedunits.jpg.
     
    Last edited by a moderator: Apr 24, 2017
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