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Liner Static vs NL Static analysis thin plate

Solution Partner Creator Solution Partner Creator
Solution Partner Creator

Hello!

I am confused by the results of the analysis of a thin plate under its own weight.

E=21000 kN/cm

u=0.33

Plate 300х29cm with thickness 0.1cm

In liner static i have inadequate resalts, total translation 464.5 cm

2017-09-10_13-11-01.png

In nonliner static its ok, total translation 1.118 cm

2017-09-10_13-11-48.png

I understand that non-linear analysis typically reduces displacements, but not as much)

Please explain why I can not consider this task in a linear formulation and why this happens?

10 REPLIES

Re: Liner Static vs NL Static analysis thin plate

Solution Partner Phenom Solution Partner Phenom
Solution Partner Phenom

Hello!,

You are suffering stiffening effect, in linear static is totally ignored, but in nonlinear analysis this is detected as a geometric nonlinearity, please post your model and we will take a look to it, OK?.

Best regards,
Blas.

Blas Molero Hidalgo, Ingeniero Industrial, Director
IBERISA • 48004 BILBAO (SPAIN)
WEB: http://www.iberisa.com
Blog Femap-NX Nastran: http://iberisa.wordpress.com/

Re: Liner Static vs NL Static analysis thin plate

Solution Partner Creator Solution Partner Creator
Solution Partner Creator

Test model.

Re: Liner Static vs NL Static analysis thin plate

Solution Partner Phenom Solution Partner Phenom
Solution Partner Phenom

Post the model again, not available!.

Best regards,
Blas.

Blas Molero Hidalgo, Ingeniero Industrial, Director
IBERISA • 48004 BILBAO (SPAIN)
WEB: http://www.iberisa.com
Blog Femap-NX Nastran: http://iberisa.wordpress.com/

Re: Liner Static vs NL Static analysis thin plate

Solution Partner Creator Solution Partner Creator
Solution Partner Creator

Sorry! Sent again)

Re: Liner Static vs NL Static analysis thin plate

Solution Partner Phenom Solution Partner Phenom
Solution Partner Phenom

Not success, I don't see the file, make sure to ZIP the FEMAP file, also check that is set public, not private, for any reason your file do not appear visible to me ready to download.

Best regards,
Blas.

Blas Molero Hidalgo, Ingeniero Industrial, Director
IBERISA • 48004 BILBAO (SPAIN)
WEB: http://www.iberisa.com
Blog Femap-NX Nastran: http://iberisa.wordpress.com/

Re: Liner Static vs NL Static analysis thin plate

Solution Partner Creator Solution Partner Creator
Solution Partner Creator

I have a bad day realy)

https://drive.google.com/open?id=0B4DAnh4O_N-zZUFKZi1lclJuOEE

Thank you for your patience!

Re: Liner Static vs NL Static analysis thin plate

Solution Partner Phenom Solution Partner Phenom
Solution Partner Phenom

Dear Kolotilov,

You are considering an extreme case, then the big differences found:

  • The plate thickness t=0.1 is very low, compared with element size = 5.0, the ratio is 5/0.1=50, then the behaviour is extremely nonlinear by geometry, what you get here with a linear static analysis is useless. Please note you get a maximum resultant displacement of 465, when the thickness is 0.1 units, then NX NASTRAN is telling you that the problem is nonlinear, forgot linear static results, OK?.
  • Also the plate is pinned at both ends, not fully constrained. If you run a linear static analysis and plot reactions forces using a FREEBODY diagram you will see only vertical reactions. Not lateral forces exist, than the assumption is reasonable, from the point of view of simplistic linear static analysis. 

nonlinear-vs-linear-vertical-reactions-only.png

  • But if you run a nonlinear analysis activating the geometric nonlinearities (large displacements effect + stress stiffening effects) and plot reactions forces you will be surprised to see the same vertical reactions forces of the linear static analysis, plus also HORIZONTAL reactions forces. But please note, the value of horizontal reaction forces is 2.2473/0.03294 = 68.2 times bigger than the vertical ones, how could it be if I only have a vertical load of gravity AZ=-1g??.

nonlinear-vs-linear-vertical-reactions-plus-horizontal.png

This nonlinear analysis consider the membrane forces of the shell elements, when the linear static analysis simply ignore at all. The plate is under dead gravity load, then is working in tension, so here we are the stress stiffening effect.

The term "stress stiffening effect" refers to a coupling between membrane stress and lateral displacements associated with bending. The bending stiffness of a beam, arc, plate or shell is increased by tensile membrane stress, and decreased by compressive membrane stress, ie, stress softening effect, ie, buckling.

Stress stiffening effect is negligible for massive bodies, but the example here is a extremely thin-walled plate, then the problem is extremely non-linear by geometry, understood?.

In summary, double-check your results always running both linear & nonlinear analysis, OK?.

Best regards,
Blas.

 

Blas Molero Hidalgo, Ingeniero Industrial, Director
IBERISA • 48004 BILBAO (SPAIN)
WEB: http://www.iberisa.com
Blog Femap-NX Nastran: http://iberisa.wordpress.com/

Re: Liner Static vs NL Static analysis thin plate

Solution Partner Creator Solution Partner Creator
Solution Partner Creator
Thak you very match, Blas!

Re: Liner Static vs NL Static analysis thin plate

Solution Partner Creator Solution Partner Creator
Solution Partner Creator
Is there a general rule (i.e ratio max displament/plate thickness) to undertand when Non Linear is essential ? Thank you franco