XANSYS Message: 92743 [Go back to message list]
[bookmark on del.icio.us]
No rating yet
Subject: Co-rotation outside of USERMAT
Author: Daniel Aubram
Date: 2008-04-08 16:03:26Dear all,
I have a question for all experts concerning large strain constitutive
equations in USERMAT.
I use a hypoplastic model for soil based on the Jaumann stress rate and
including the Cauchy
stress and the void ratio as state variables. I now know that the
objective integration algorithm
used in ANSYS 11.0 is based on the reference listed below and not on
Hughes-Winget's
scheme anymore. Moreover, ALL quantities in USERMAT (in/out) are
understood to be
co-rotated. The operations for co-rotating and for the "back"-rotation
to the current
configuration is done outside of USERMAT.
However, WHICH rotation matrix is used in ANSYS to rotate the
quantities? This is important
because of consistency of the stress rate used in the constitutive
equation and the rotation matrix.
Hughes (1984) shows that rotation matrix obtained from the polar
decomposition of the total
deformation gradient is consistent with the Green-Naghdi rate, whereas
for the Jaumann rate,
the proper matrix has to be obtained otherwise. In NIKE2D, for example,
the first procedure
is implemented. Thank you!
Regards,
Daniel
Reference:
Hughes, T. J. R., "Numerical Implementation of Constitutive Models:
Rate-Independent
Deviatoric Plasticity", published in Theoretical Foundation for
Large-Scale Computations for
Nonlinear Material Behavior, S. Nemat-Nasser et al. (eds.), Martinus
Nijhoff Publishers,
Dordrecht, The Netherlands (1984).
--
---------------------------------------
Dipl.-Ing. Daniel Aubram
TU Berlin
FG Grundbau und Bodenmechanik - Degebo
Sekr. TIB1-B7
Gustav-Meyer-Allee 25
D-13355 Berlin
www.grundbau.tu-berlin.de
E-MAIL daniel.aubram_at_tu-berlin.de
TEL +49-30-314-72349
FAX +49-30-314-72343
---------------------------------------