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Determining stress strain curve and elastic modulus for POM

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  • Determining stress strain curve and elastic modulus for POM

    Hi all,

    I have tensile test data for POM (injected test specimens) in different strain rates.
    I will try to explain my question with an example.

    When I use the 50 mm/min strain rate test data I get yield point in stress of 62 Mpa, which corresponds to a strain of 0.12 this should give me a Youngs modulus of ~63/0.12=525 [MPa], in the data sheet it states that the Young modulus for the grade I am using is in the range of ~2800 [MPa].
    I guess they are using another strain rate to get this number.

    My questions are:

    1. If I am using different strain rates for different analysis (I dont have a strain rate dependent material in my FE pacakge - FEMAP) I guess I should use the young's modulus that I got from the test data for this strain rate - am I right?
    2. How can I know what is the strain rate used by the manufacturer to get this ~2800 [MPa] youngs modulus?
    3. How can I know If my test data is OK? what should I look for in the manufacturer data sheet to "validate" my test data at different strain rates?
    4. Is there a way to "average" this? using a simple non-linear plastic material (with a single stress-strain curve) to capture a range of strain rates in one material property setup? (I know its probably not possible, just some fantasy I have for making my life easier... )

    Thanks for your replies.

  • #2
    Hello,
    I am a question along similar lines. I have tested a polycarbonate material that is non-linear in the elastic region.
    1. I have been using the 0.2% offset to determine the yield point but i wonder if this is too conservative for such non-linear materials? Is there a more accurate approach?
    2. At higher rates, I see that the yield strength based on 0.2% offset starts to drop. So I really doubt if this is the right approach?
    3. I am seeing marginal increase in Young's modulus with higher rates. Could I just use the modulus determined from quasi-static loading rate to build the material model in FEA ?

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    • #3
      1. You can use any definition of the yield stress that you like. In my opinion, it is more important to select a suitable material model than to focus too much on the yield point. I often use the TN model for POM.
      2. The yield stress almost always increases with strain rate.
      3. The Young's modulus is usually very weakly dependent on the strain rate.
      -Jorgen
      Jorgen Bergstrom, Ph.D. PolymerFEM Administrator

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