Topic: nlgeo in nonLinearStatic
Hello all,
how to include nlgeo from each element in NonLinearStatic analysis?
Just retrieve initial stiffness matrix and assemble with stiffness matrix?
Pages 1
You are not logged in. Please login or register.
OOFEM forum → General Help → nlgeo in nonLinearStatic
Hello all,
how to include nlgeo from each element in NonLinearStatic analysis?
Just retrieve initial stiffness matrix and assemble with stiffness matrix?
Hi,
Could you be more specific?
Do you want to use geometrically nonlinear analyisis it or to implement it?
What kind of element and constitutive law are you going to use?
Hello,
I just want to include geometrical non-linearity in non linear analysis (material non-linearity can be present as well). In many other solver I just have an option to include nl geo (eg. Abaqus).
So, you need to specify nlgeo 1 at each element.
Thanks - simple and effective, this mught be specified in the manual (afaik, it is specified only in the Element manual).
One more consideration: for a simple Linearstatic analysis, activating nlgeo for each element will include second order effects in results? I suppose the answer is no, because we need iterations to reach equilibrium once geometric stiffness has been applied.
IMHO, this is just why the nlgeo feature has to be set from the analysis. What do you think about it?
You are right, when you use it with linearstatic you don’t get the correct results. However, I don’t think the nlgeo should be specified at the analysis level. You can, for example, have combination of linear and nonlinear elements, such as linear beams with nonlinear cables, etc.
Mixing lin. and nl elements represents a rare modelling choice - anyway, I need further clarifications on that.
I noticed that only elements that inherit the NLstructuralElement class have the "nlgeo" options. In mitc4shell for example, computeBHmatrixAt is not implemented, and it seems that inheriting NLstructuralElement is futile.
In particular, for beams I see that LIBeam supports nlgeo. For beam3d, I'm just wondering if the geometrical stiffness matrix computed in a linear buckling analysis has anything to do with nonlinear geometry support (spatial vs. material configuration). Can you help understanding this?
The default implementation of nlgeo in oofem is based on the deformation gradient and the first Piola Kirchhoff stress, i.e., the standard B matrix is replaced by Bh matrix, and stress is replaced by the 1PK stress. Otherwise, calculation of the stiffness matrix and internal forces stays the same. However, usually, for beams or shells, formulation based on the Green-Lagrangian strain and 2PK stress is more suitable. In this formulation, the stiffness matrix, in comparison with the small strain theory, is enhanced by the geometrical part. Moreover, B matrix for calculation of the nonlinear part of the GL strain needs to be implemented. You can see this approach in truss3dnl, see also the attached file. Note that it shouldn’t be that difficult to implement this approach for MITC. There are also several approximate approaches as co-rotational formulations etc.
OOFEM forum → General Help → nlgeo in nonLinearStatic
Powered by PunBB, supported by Informer Technologies, Inc.
Currently installed 4 official extensions. Copyright © 2003–2009 PunBB.