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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
    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 ?


    • #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 Bergstrom, Ph.D. PolymerFEM Administrator