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Andreas Burger's blog

Abaqus: equally space amplitude


I've been trying to apply a displacement to a body in an implicit model using equally spaced amplitudes.

Abaqus implicit, rigid body-deformable body contact, negative dot product


I am running a simulation where I have a rigid body (shell elements) pushing into a deformable part.  Unfortunately, the simulation aborts with the following error:

The dot product -0.19758 cannot be negative for element 30050 instance xzy with record ID 844 at step 2 inc 1 kitgen 5.

This element (C3D4H) belongs to the deformable part and is within the body, so no not close to the contact at all.  At the point where the simulation aborts, this element is not deformed and the stress is basically 0 (E-6).

Bergstroem Boyce Model - inelastic part


 I was wondering if anyone has experience in implementing the Bergstroem-Boyce Model (Constitutive modeling of the time-dependent and cyclic loading of elastimers and application to soft biological tissue, 2001)? I am struggling with the implementation of the viscoplastic part (Network B) of the model:

NTVDM CPU has encoutnered an illegal instruction


I'v been using the Abaqus command window to run my jobs without any problems.

Today, however, I suddenly got the following message:

writing data to dat file and plotting values during job execution


I am in the process of debugging a UMAT.  In my UMAT code I am currently using write statements to output certain variables to the dat file.  I was wondering if there are any other ways to output data during the execution of a job.  Is it possible to print/plot variables, for example using Matlab (maybe even realtime)?

Also, has anyone experience in accessing the fill files (e.g converting them to ASCII and then use Matlab to look at data?).



Pressure over closure function for Polyurethane or (nearly) incompressible materials in general


Is anyone aware of a pressure-overcosure function for (nearly) incompressible materials.

 I am aware of functions for PE and articular cartilage but they do not work with v=0.5.   I have been trying to find similar functions for elastomers, but can't seem to find anything.

 I am grateful for any input.


UMAT: Calling a another subroutine in UMAT, Errors during job execution


I am in process of implementing the Bergstroem-Boyce Model. In a first
step I am just considering the hyperlastic part of it (Network A:
8-chain). I have implemented this part in a UMAT using a numerical
approach for the elasticity tensor (Forward Difference Approximation
with perturbation of F).

I have two problems:

In order to keep my UMAT as easy to read as possible I have tried to keep the FDA in a separate file using

call KFDA_Jaumann(dfgrd1,......)

Perturbation technique based on forward difference approximation


 I am trying to implement (in Abaqus) the elasticity tensor using a numerical approach.  I have come across a paper by C.Miehe (Numerical computation of algorithmic (consistent) tangent moduli in large-strain computational inelasticity, 1996).

There are a few things I don't quite understand.  

1 PK, 2PK vs true, nominal stress


 if I have a material model defined in cauchy stress then I use test data with true stress (calculated from nominal stress: sigma_true=sigma_nom(1+epsilon_nom).

If I wanted to use nominal stress data, which stress tensor would I have to use to define my material model? First or second Piola Kirchhoff?


Problem with nominal vs true values


 I have cyclic uniaxial compression data of a PU.  When I look at nominal stress vs nominal strain then the relation is clearly nonlinear (increasing stiffnes with increasing  strain).  When I convert to true stress vs true strain I still have a nonlinear relation, however, not as strong and the stiffness decreases with increasing strain.
When I look at true stress vs stretch, however, the relation seems to be almost (I'm only talking about the first loading cycle) linear. Nominal stress vs stretch is still nonlinear. 

Viscoelasticity in Abaqus

Hi All,

 I would like to use Abaqus to model the viscoelastic material  behaviour of a polymer.

I have material data from a simple uniaxial creep experiment (nominal strain vs time).  I tried to use the viscoelastic material model in Abaqus.  I am a bit confused as to how I need to enter my data (what format). 

The manual says I have to specify the normalized bulk and normalized shear compliance

Modeling polyurethane

I've started to look into the modeling of polyurethane.  So far I'v found one paper that specifically looks into how to model the mechanical behavior of PU. It describes a possible constitutive model for PU:

Stress-strain behavior of thermoplastic polyurethanes, Qi,H.J.; Boyce, M.C., Mech. Mater. 2005, 37, 8, 817-839

Does someone know of any other resources on the modeling of PU?  Thanks, Andreas

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