Clarification in stress strain curve input using Multilinear Isotropic Hardening

- October 27, 2021 at 4:50 am
  
  karthikeyan.m
  Subscriber
  
  Im using Ramberg Osgood equation for obtaining the stress strain input data. For this i assume that the yield stress is occuring at 0.2% of plastic strain.
  But i cannot input the 0.2% strain in the first row of strain strain tabular data in ansys. Because we have to enter the first row as 0, Yield Stress.
  So i worried that changing the first point to the 0, yield stress will deviate my ramberg osgood stress strain curve data.
- October 27, 2021 at 11:53 am
  
  Aniket
  Forum Moderator
  
  Plastic strain in the column would be Total strain-Elastic strain, so at yield point you will have zero plastic strain.
  -Aniket
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- October 27, 2021 at 12:39 pm
  
  karthikeyan.m
  Subscriber
  
  Hi Aniket,
  Thanks for your reply.
  I agree your point. We have to input the true Plastic strain data for each true stress values. And first row would be 0(Plastic Strain), Yield stress.
  But for some metals, finding the yield point is not easy. So generally they consider the yield point as the stress level which cause 0.2% plastic strain. It is also called as proof stress.(Refer Image below)
  
  So considering this situation, we have to enter the True Plastic strain and True stress values as mentioned below.
  But ansys will not allow us to enter the stating point other than 0 for the yield point(154MPa). (the yellow field is highlighted by the ansys which indicates the input error).
  So if i want to input the exact stress strain curve by considering the 0.2% Proof stress as a yield stress, what is the correct way to input the Plastic strain-Stress data?
- October 27, 2021 at 1:29 pm
  
  Aniket
  Forum Moderator
  
  simply subtract 0.002 from each row for the first column.
  -Aniket
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- October 27, 2021 at 2:06 pm
  
  karthikeyan.m
  Subscriber
  
  Yes, I tried this also. But subtracting 0.002 from each row will not match the material stress strain curve.
  
  In the above image, ash colour line is the exact material curve. If i reduce 0.002 from all the strain points, the behaviour does not matches with the exact material curve. (Compare green and ash colour line). Here strain values and stress values are not matching with the exact material curve.
  And if i reduce 0.002 from only first point, the behaviour is not matching at the yield point.(compare blue and ash colour line)
  So how to input the plastic strain and Stress data without compromising the exact material stress strain curve?
- November 12, 2021 at 4:56 am
  
  karthikeyan.m
  Subscriber
  
  Can anyone please comment for the above query?
- November 15, 2021 at 5:12 am
  
  karthikeyan.m
  Subscriber
  
  Anyone please help for the above query or suggest any answers for this.
- November 16, 2021 at 1:53 pm
  
  Vigneswaran Sridharan
  Ansys Employee
  
  Hi I suggest you look into this MAPDL reference.
  2.2. Material Model Combinations Using Multilinear Isotropic Hardening (ansys.com)
  Try inserting the MAPDL codes as a snippet in Mechanical once you figure out the appropriate command for your Material model.
  Vigneswaran
- November 16, 2021 at 2:04 pm
  
  Ashish Khemka
  Forum Moderator
  
  Please see if the external tool below is of use. The stress-strain curve will be generated based on 0.2% yield.
  
  P/A RambOs (p-over-a.co.uk)
  
  Regards Ashish Khemka
- May 25, 2022 at 4:29 am
  
  xutaosun
  Subscriber
  
  I think this paper got an answer:
  https://www.researchgate.net/publication/290992225_On_Practical_Implementation_of_the_Ramberg-Osgood_Model_for_FE_Simulation