Random analysis - stress plot
Random analysis - stress plot
(OP)
Hi everybody, I hope someone could help me solve a problem I'm having in anlyzing a component.
The component is basically a short mast, with circular section, crimpled to a support, at the other end there are some instruments.
I am performing random analysis through MSC.Random. Then eveluating the PSD response and the stress using the built-in tool of MSC.Random.
I'm having very strange results though: basically in every test I have performed (many test, with different constraints and different meshing) I realized that the elements where I attach the input acceleration are the most stressed. The problem increases when the mesh becomes smaller and more detailed.
Looking at the acceleration plot response I realized that there is a strong damping effect in the first element, as it absorb a big quantity of energy.
The stresses at the input nodes are incredibly high and unrealistic. (I'm talking of X*10^3 Mpa!! on a component that weight less than 200 grams and it not bigger than 250mm with moderate PSD loading)
Another strange effect is that plotting the CRMS of the Stress It apperars evident how the biggest part of the stresses is accumulated on the lowest frequency (10-80 Hz), while the hightest PSD inputs are between 50-300 Hz and the first frequency is around 160 Hz.
Basically seems some sorf of effect that tends to concentrate the deformation and so the stresses on the input nodes, but I cannot realy explain why.
I'm attaching some pictures:
Stresses:

Details:

CRMS Stress Plot:

Another example of a different meshes with even higher results, it seems that the smaller the mesh become the worst are the results!

I thank everybody who will be able to help me!
Ned
The component is basically a short mast, with circular section, crimpled to a support, at the other end there are some instruments.
I am performing random analysis through MSC.Random. Then eveluating the PSD response and the stress using the built-in tool of MSC.Random.
I'm having very strange results though: basically in every test I have performed (many test, with different constraints and different meshing) I realized that the elements where I attach the input acceleration are the most stressed. The problem increases when the mesh becomes smaller and more detailed.
Looking at the acceleration plot response I realized that there is a strong damping effect in the first element, as it absorb a big quantity of energy.
The stresses at the input nodes are incredibly high and unrealistic. (I'm talking of X*10^3 Mpa!! on a component that weight less than 200 grams and it not bigger than 250mm with moderate PSD loading)
Another strange effect is that plotting the CRMS of the Stress It apperars evident how the biggest part of the stresses is accumulated on the lowest frequency (10-80 Hz), while the hightest PSD inputs are between 50-300 Hz and the first frequency is around 160 Hz.
Basically seems some sorf of effect that tends to concentrate the deformation and so the stresses on the input nodes, but I cannot realy explain why.
I'm attaching some pictures:
Stresses:

Details:

CRMS Stress Plot:

Another example of a different meshes with even higher results, it seems that the smaller the mesh become the worst are the results!

I thank everybody who will be able to help me!
Ned





RE: Random analysis - stress plot
I create a test case, with a cilinder + flange and I create the .bdf using MSC.Random of Patran 2010.1.2 and then I run the .bdf on 4 different solver (mine Nastran 2012.1, the one of Nastran 2007 on the pc room of my University, Nastran 2010 and MD Nastran 2.1 of two other professionist I know).
Results: -> 2 sets of different results. Nastran 2010 and 2012.1 provide wrong results but these results are equal. Nastran 2007 and MD Nastran 2.1 provide another set of results, equal, and apparently correct.
Now, I can't really explain this, but I think it may be due to a set of instructions I provide to the solver in the wrong way. Maybe a change in providing the instructions to the solver that happened between the version 2007 and 2010.
I don't really know... any idea??
RE: Random analysis - stress plot
If you upload your .bdf file(s), I can take a quick look at it.
RE: Random analysis - stress plot
CODE -->
$ NASTRAN input file created by the Patran 2010.1.2 64-Bit (MD Enabled) $ input file translator on January 07, 2013 at 14:45:21. $ Direct Text Input for Nastran System Cell Section $ Direct Text Input for File Management Section $ Delete xdb file before re-running job ASSIGN dbc='Random_X_standard.xdb', unit=40 unformated delete init master(s) $ do not save database $ Direct Text Input for Executive Control $ $ Frequency Response Analysis, Modal Formulation, Database SOL 111 CEND $ Direct Text Input for Global Case Control Data $ TITLE = Job Created by Patran Random ECHO = NONE RESVEC = NO LOADSET = 102 SDAMPING = 1 SUBCASE 1 TITLE=Created by Patran Random SUBTITLE=msc_random_base_X METHOD = 1 FREQUENCY = 1 SPC = 2 DLOAD = 103 ACCELERATION(PLOT)=ALL SPCFORCES(PLOT)=ALL MPCFORCES(PLOT)=ALL STRESS(PLOT,BILIN)=ALL FORCE(PLOT,BILIN)=ALL BEGIN BULK $ Direct Text Input for Bulk Data $ $ ====================================================================== $ =============== Bulk data cards created by Patran Random ================ $ ============================= Enforced Motion ============================= Param,Enfmotn,ABS $ ====================================================================== $ $ $ ====================== End of Patran Random Created Cards =============== $ PARAM POST 0 PARAM WTMASS 1. PARAM GRDPNT 0 PARAM,NOCOMPS,-1 PARAM PRTMAXIM YES FREQ2 1 19.9 2000.1 50 FREQ4 1 19.9 2000.1 .03 5 FREQ 1 20. 50. 300. 2000. TABDMP1 1 CRIT 10. .05 50. .05 300. .05 2100. .05 ENDT EIGRL 1 -.1 2000.1 0 $ HERE ARE THE MODEL DESCIRPTION - ELEMENTS, MATERIALS, GRID $ Loads for Load Case : msc_random_base_X SPCADD 2 101 102 RLOAD1 105 106 1 ACCE LSEQ 102 106 107 SPCD 107 6567 1 1. RLOAD1 108 109 1 ACCE LSEQ 102 109 110 SPCD 110 6568 1 1. RLOAD1 111 112 1 ACCE LSEQ 102 112 113 SPCD 113 6569 1 1. RLOAD1 114 115 1 ACCE LSEQ 102 115 116 SPCD 116 6570 1 1. DLOAD 103 1. 1. 105 1. 108 1. 111 1. 114 $ Displacement Constraints of Load Set : msc_random_X_unit_accel.101 SPC1 101 1 6567 6568 6569 6570 $ Displacement Constraints of Load Set : crimpled_RBEs SPC1 102 123 6567 6568 6569 6570 $ Referenced Dynamic Load Tables $ Dynamic Load Table : msc_random_Flat_Field TABLED1 1 10. 1. 2100. 1. ENDT $ Referenced Coordinate Frames $ $ ENDDATARE: Random analysis - stress plot
Note: I cancelled the nodes, points (GRID) and elements list from the bdf to be able to post the bdf on this page.
RE: Random analysis - stress plot
What it looks strange to me is SPC1 card. I think you should remove X in ID 102, like this one;
$ Displacement Constraints of Load Set : crimpled_RBEs
SPC1 102 23 6567 6568 6569 6570
RE: Random analysis - stress plot