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Creep Constants

Question asked by Andrew Paino on Dec 30, 2013
Latest reply on Dec 31, 2013 by Jared Conway

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I am trying to determine the constants described in the help file to model creep life with simulation. Does anyone have any experience doing this? So far, I haven't been able to find a valid solution using a system of four equations. I found the link showing an example problem where C2 is assumed to be 1 (http://www.motovated.co.nz/newsletter/aug13/FAQID__x394.pdf); the only problem with this is they say it is usually safe to assume C2 = 1, but don't say when it is not. Below is an image of the creep life data we're using to determine the constants. We will be using Heat No. WWS.

Any help would be greatly appreciated.

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Shaun Densberger Dec 30, 2013 8:10 PM
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Any value less than 1 for C2 results in a smaller amount of creep strain, so it's a conservative assumptions to set C2 = 1. To calculate the values of C0 and C1 for Heat No. WWS, you'll need to use either the 1600 deg F or 1800 deg F data sample. This will also assume that you're analysis is done at a constant elevated temperature.

For 1600 deg F you have two equations:
1. 0.005 =[C0*(55 ksi)^C1]*(42 hr)
2. 0.01 =[C0*(55 ksi)^C1]*(77 hr)
Here you have two equations and two unknows, so a unique solution exists.

For 1800 deg F you have two equations:
1. 0.005 =[C0*(22 ksi)^C1]*(139 hr)
2. 0.01 =[C0*(22 ksi)^C1]*(178 hr)
Here you have two equations and two unknows, so a unique solution exists.

Make sure to use consistant units when calculating your constants.
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Jared Conway Dec 30, 2013 8:08 PM
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C2=1 should be eliminated from the KB in the near term
we recently did a creep project for a customer and interfaced pretty heavily with the developers and determined that was only valid for the case that it was written for.
if you do want to keep C2=1 the method that shaun describes will work
the other option is a guess and check
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- Shaun Densberger @ Jared Conway on Dec 30, 2013 8:26 PM
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  Jared, is there any additional information that came about from the developers out of this project?
  
  If we want to leave C2 as an unknow constant, then we can still solve for C0, C1, and C2.
  
  For 1600 def F at Heat No. WWS:
  
  0.005 =[C0*(55 ksi)^C1]*[(42 hr)^C2]
  0.01 =[C0*(55 ksi)^C1]*[(77 hr)^C2]
  0.02 =[C0*(55 ksi)^C1]*[(124 hr)^C2]
  
  Here you have three equations and three unknows, so a unique solution exists.
  
  For 1800 def F at Heat No. WWS:
  
  0.005 =[C0*(22 ksi)^C1]*[(139 hr)^C2]
  0.01 =[C0*(22 ksi)^C1]*[(178 hr)^C2]
  0.02 =[C0*(22 ksi)^C1]*[(214 hr)^C2]
  
  Here you have three equations and three unknows, so a unique solution exists.
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  - Jared Conway @ Shaun Densberger on Dec 30, 2013 8:31 PM
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    shaun, you are totally correct
    in the case of this project we actually had creep curves that needed to be converted so your method could have worked but the curve would be skewed based on what points were chosen
    we used the guess and check method to determine constants that matched the curve as much as possible
    we also need to address that the length of the creep time far exceeded that of the curves and had to come up with a reasonable extrapolation method and also make sure our constants made a curve that matched reasonably
    
    i should also second your comment about making sure the units are consistent and also making sure they are in the unit system expected by the software. this is well documented in the solidworks kb.
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    - Andrew Paino @ Jared Conway on Dec 31, 2013 8:48 AM
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      Jared and Shaun,
      
      Thank you this is very good information. It looks like the calculation is fairly straight forward if the part is at a constant temperature, unfortunately the part I'm analyzing is not so I will need a CT value. Judging by what you both have said it doesn't sound like I have enough data, is there any way of determining all four constants with the data shown above?
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      - Jared Conway @ Andrew Paino on Dec 31, 2013 11:13 AM
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        same as above
        your constants are going ot characterize 2 curves
        from there you should be able to build a more general curve that defines any temperature to a certain extent
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        Andrew Paino @ Jared Conway on Dec 31, 2013 11:38 AM
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        Jared, I think I understand what you're saying to do. You mentioned in an above comment that you had to find a reasonable extrapolation method to make sure the constants matched, would you mind elaborating on how exactly you ended up doing that? The part I'm analyzing must have a creep life of 9,600 hours so I will definitely need to look into that.
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        Jared Conway @ Andrew Paino on Dec 31, 2013 11:41 AM
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        BN is a power curve so we used that for extrapolation
        but we also had to use a linear interpolation in one case because we were at the very end of the curve and were fitting the curve to multiple other curves
        
        my suggestion would be to start digging for more material properties for that material, i don't think you have enough information to characterize the situation you're running into
        
        we're going to have a blog article up in the next month or so about what we learnt but it still needs some polishing before we can post it. in the meantime if you're looking for more one-on-one guidance, we can certainly help you based on your experience.
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