Question regarding FEA analysis using abaqus cae

[Kunal Gide]

Hello,

I am trying to simulate a compression test according to ASTM D695 standard. I created a part which is a cylinder (r=6.35mm and h=25.4mm). The material properties I applied are for PEEK polymer. They are as follows:
Elastic Properties
E= 2600 MPa and v=0.378
Plastic Properties
Plastic hardening= Johnson Cook
A=81 Mpa, B=41 MPa, n=0.23, m=0.76, Tm= 343 C, T0=20 C
Rate-dependent type= Johnson Cook
C=0.0141 epsilon0=1

Then I created two RP on the top and bottom surfaces and constrained them using rigid body constraints (Tie). I applied Encastre at the bottom RP and displacement at the top RP as boundary conditions. The step was nlgeom=ON, static general. The mesh size was 0.2mm C3D8R. After I ran the job successfully, I wanted to calculate the elastic modulus by the slope of the stress-strain curve. I selected the whole cylinder and Von Mises and LE Strain (max principle) and took the average value so I got one value per increment for the entire model. Theoretically the resulting modulus should be around 2600 MPa as inputted in material section but I am getting increased modulus around 7000 MPa which is much higher than expected. Can anyone help me figure out the problem in my simulation? Later I want to simulate a similar test with the same material properties for a lattice structure. The material properties were taken from a paper.

Thanks, and regards,
Kunal

 

[SWComposites]

You don’t use VM stress to determine modulus. Look at the equation for VM stress.

To determine modulus from the FEM, pick two points on the gage length of the specimen (where the axial stress should be uniform) determine the displacement between the two points and divide by the distance to get strain. Then use that strain and the gage section stress to calculate a modulus.

 

[Kunal Gide]

Hello,

Thank you for your response. I am still a little confused by how you are asking me to calculate the elastic modulus. Is there a video link or something I could look at to find the modulus that you could share with me?

Thanks,
Kunal

 

[SWComposites]

Maybe, but I don’t use videos.

What is your specific question?

Think about it this way - how is modulus determined from an actual test specimen? (see the ASTM standard). You need to do the same approach with your FEM results in order to compare.

 

[Kunal Gide]

Also which stress should I select from the window option in field output when you say gauge section stress in your previous reply?

In an actual test, we apply load at the top surface at a specific rate and the bottom plate is fixed. we get the stress and strain graph and the slope of the elastic region (linear part) is the modulus.

Thanks,
Kunal

 

[SWComposites]

I don't know what software you are using.
But you should be selecting the axial stress component aligned with the direction of loading.
Re the test, the strain is measured either by a strain gage (with a length) or with an extensometer (with a gage length). The stress is calculated per the equations in the ASTM std (essentially load / (initial cross section area).

 

[FEA way]

Why don't you test this on a single hex element first ? Like examples in the material verification guide in Abaqus documentation. How did you calculate the Young's modulus from the curve obtained in Abaqus ?

 

[Kunal Gide]

I exported it in excel and and calculated the the slope of the linear region @FEA way

 

[FEA way]

Result from a single element test without plasticity:

 

[Kunal Gide]

Hello FEA way,

Coup you share with me the .inp file of the material verification you did?

Thanks,
Kunal

 

[FEA way]

I atached the input file.

 

[Kunal Gide]

Thank you