Non-linear Buckling analysis with ANSYS
Non-linear Buckling analysis with ANSYS
(OP)
Hi all,
I'm currently having some troubles analising a thin walled conical conduct subjected to the atmospheric pressure on the outside (vacuum inside). The problem is that no matter what I do I can't find the buckling load (or mode) with a non-linear analysis: the structure simply deforms like there was no imperfections and never buckles.
If anyone could help me I'd appreciated.
Thanks in advance.
Here is the code that I used to perform the analysis:
/SOLU ! Enter the solution mode
ANTYPE,STATIC
PSTRES,ON
!boundary conditions
SFA,ALL,1,PRES,-0.1, !Apply atmospheric pressure
LSEL,S,LOC,z,0
DL,ALL,,UX,0
DL,ALL,,UY,0
DL,ALL,,UZ,0
LSEL,S,LOC,z,900
DL,ALL,,UX,0
DL,ALL,,UY,0
lsel,all
asel,all
nsel,all
SOLVE
FINISH
!************************Eigenvalue buckling analysis ***************************
/SOLU ! Enter the solution mode again to solve buckling
ANTYPE,BUCKLE ! Buckling analysis
BUCOPT,LANB,6
SOLVE
FINISH
/SOLU ! Re-enter solution mode to expand info - necessary
EXPASS,ON ! An expantion pass will be performed
MXPAND,6 ! Specifies the number of modes to expand
SOLVE
FINISH
/POST1 ! Enter post-processor
SET,LIST
SET,FIRST ! Read in data for the desired mode
PLDISP ! Plots the deflected shape
!****************************Nonlinear buckling analysis *************************
*GET,MYLOADF,ACTIVE,,SET,FREQ
/PREP7
/INQUIRE,MYJOBNAME,JOBNAME
UPGEOM,0.005,1,1,%MYJOBNAME(1)%,RST
!F,1254,FX,1
/SOLU
ANTYPE,STATIC
NLGEOM,ON
TIME,MYLOADF*1.1
NSUBST,20,1E5,20
OUTRES,ALL,ALL
LNSRCH,ON
SFSCALE,ALL,1.1*MYLOADF
/SOLU
SOLVE
I'm currently having some troubles analising a thin walled conical conduct subjected to the atmospheric pressure on the outside (vacuum inside). The problem is that no matter what I do I can't find the buckling load (or mode) with a non-linear analysis: the structure simply deforms like there was no imperfections and never buckles.
If anyone could help me I'd appreciated.
Thanks in advance.
Here is the code that I used to perform the analysis:
/SOLU ! Enter the solution mode
ANTYPE,STATIC
PSTRES,ON
!boundary conditions
SFA,ALL,1,PRES,-0.1, !Apply atmospheric pressure
LSEL,S,LOC,z,0
DL,ALL,,UX,0
DL,ALL,,UY,0
DL,ALL,,UZ,0
LSEL,S,LOC,z,900
DL,ALL,,UX,0
DL,ALL,,UY,0
lsel,all
asel,all
nsel,all
SOLVE
FINISH
!************************Eigenvalue buckling analysis ***************************
/SOLU ! Enter the solution mode again to solve buckling
ANTYPE,BUCKLE ! Buckling analysis
BUCOPT,LANB,6
SOLVE
FINISH
/SOLU ! Re-enter solution mode to expand info - necessary
EXPASS,ON ! An expantion pass will be performed
MXPAND,6 ! Specifies the number of modes to expand
SOLVE
FINISH
/POST1 ! Enter post-processor
SET,LIST
SET,FIRST ! Read in data for the desired mode
PLDISP ! Plots the deflected shape
!****************************Nonlinear buckling analysis *************************
*GET,MYLOADF,ACTIVE,,SET,FREQ
/PREP7
/INQUIRE,MYJOBNAME,JOBNAME
UPGEOM,0.005,1,1,%MYJOBNAME(1)%,RST
!F,1254,FX,1
/SOLU
ANTYPE,STATIC
NLGEOM,ON
TIME,MYLOADF*1.1
NSUBST,20,1E5,20
OUTRES,ALL,ALL
LNSRCH,ON
SFSCALE,ALL,1.1*MYLOADF
/SOLU
SOLVE





RE: Non-linear Buckling analysis with ANSYS
RE: Non-linear Buckling analysis with ANSYS
Theoretically there is no imperfection in the structure. But to analyse buckling with the non-linear analysis, I´ve indroduced a small geometric perturbation using the command: UPGEOM,0.005,1,1,%MYJOBNAME(1)%,RST
This updates the geometry by introducing a perturbation that is 0.5% (0.005) of the buckling mode obtained by the eigenvalue buckling analysis. I've tested this approach in other test cases and it always worked.
RE: Non-linear Buckling analysis with ANSYS
Any help is appreciated.
Thanks
RE: Non-linear Buckling analysis with ANSYS
RE: Non-linear Buckling analysis with ANSYS
RE: Non-linear Buckling analysis with ANSYS
I am getting out of my depth here but presumably eigenvalue buckling is based on elastic theory and will not reflect nonlinear behaviour. Maybe the nonlinear behaviour is dominant and not secondary, as in some sort of membrane effect.
I remember seeing some kids science program on tv that tried to explain why soap bubbles always end up spherical even when they start off random shapes, and was wondering if there might be some sort of analogy!
RE: Non-linear Buckling analysis with ANSYS
Yes, I've tried different loads and none seems to be enough to cause buckling with a non-linear analysis.
RE: Non-linear Buckling analysis with ANSYS
To all of you that have been following this thread, I just wanted to tell you that I have found what was wrong with my analysis. Apparently, to perform this kind of analysis, the boundary conditions need to be aplied on the nodes rather than on lines and areas (go figure :)).
Thanks to all