There are some Polyurethane compression test specimens (According to ASTM-D575 standard - 12mm thickness) in 3 hardness 55, 65 and 75 ShoreA. The test results are reported in the form of time-force-displacement and now I'm trying to plot true stress-strain curve of compression behavior of these specimens. If the strain will be calculated using "delta l / l" formula (engineering strain), the strain range will be 0-100% that is physically sensible but using the strain integration (integral of "dl/l") (true strain), the strain is from 0 to near 300% (the specimens compressed about 11.8 mm). Now, my question is that, this range for true strain is correct? and what does physically means?

Thanks in advance, best regards.

]]>I recently got a trial version of MCalibration. So far so good, I just had to upgrade my Abaqus from 6.14-1 to ABAQUS2016. I am in the process of getting the academic license.

My ultimate objective is to study the strain rate and temperature-dependent behavior of my material.

It is so clear to me how to capture the temperature dependence using the PolyUmod AB model since there is no temperature input unlike the equation in the book. Does it mean that I have to calibrate for every temperature?

I am trying to relate the material parameters provided by the MCalibration to those required by the equations in your book so that I can check the accuracy of my VUMAT. Kindly let me know if the parameters relate as given in the attachment.

Importantly, how does the parameter Adk relate?

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Thank you very much.

Warm regards,

Melly

]]>Thanks for replying to my previous inquiry, I really appreciate the time taken to help me. For some reason, the reply button did not appear for me so I could not thank you there. I have another question regarding TNM.

When I export my TNM model to my abaqus compression stress relaxation model to verify the TNM model predictions, I apply the same initial stress force (10 MPa) with a general static displacement step with 180 s duration (same as my experiments) then a visco step for 8 hours. The model predicts the relaxation accurately compared to my experiments for (23,40,60,and 80 C). However, things get different when I apply the force (10 MPa) using a general static displacement step with shorter duration than my experiments (30 and 60 s) i.e. higher strain rates. For higher temps like 80 C, when I shorten the displacement duration, the stress relaxation happens faster and the final stress after 8 hours is about 15 to 25% percent less. However, for lower temps such as 23 and 40 C, using a shorter displacement step slightly increases the stress drop or does not even change it. I cannot quite understand why is this occurring, is it the TNM model or Abaqus model or HDPE actually should behave like this in the real world.

Thank you very much for your time and I will be waiting for your reply.

]]>I was wondering if the three network model in polyumod can capture the stress relaxation experimental data of HDPE in both tension and compression at temperatures (20,30,40,50,60,70,80 C). So far it was able to capture the relaxation behavior in compression with good accuracy, However, I just want to be sure that it can capture the relaxation behavior in tension before I go ahead with experimental testing.

Thanks

]]>I have succeeded in executing, with the classical abaqus command line window, a python script which makes use of a subroutine. No problem, calculations went fine.

But when I want to do the same from matlab, this does not work and results into an error.

system();

-> error from matlab: *WindowsError*: The system cannot find the file specified

Does someone know how to solve this problem or is it possible to do this under matlab in the first place ?

thanks

]]>In reference to Abaqus manual, the sum of all the energy components (ETOTAL) should be constant with an error of less than 1%.

This is not the case in my VUMAT subroutine as the ETOTAL increases exponentially in the negative direction with the number of increments.

For comparison purposes, I decided to write a subroutine of one of the Abaqus inbuilt hyperelastic models (Mooney-Rivlin). The stress/strain curves and the contour plots are comparable. The only difference is the ETOTAL with the inbuilt one having a constant value and my subroutine an increasing negative value

Could there be something wrong with my model? what does it mean to have negative ETOTAL value?

Thank you very much

Warm regards,

Melly

]]>

It turns out to be easy to fix this problem.

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]]>I am trying to write a VUMAT to implement the Arruda-Boyce viscoplastic model. After assuming linear elasticity at the beginning of the analysis, the total deformation gradient (**F**) can be easily obtained from defgradNew.

I am a little bit confused about how to decompose **F **into its elastic and plastic components. Should I obtain the plastic components from the state variables and then, of course, find the elastic part via the product of **F** and the inverse of the plastic part?

How about the plastic flow rate, the rate of the plastic deformation, time-derivative of the deformation gradient, and the strain softening. How to incorporate these to capture the material response?

My reference is Chapter 8 of your book (Mechanics of Solid Polymers).

Thank you very much,

Warm regards,

Melly

]]>-Jorgen 8,5,7,6,4

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