Non linear spring in NX Nastran 8.0 1

[spikefer]

Hello Everybody,

Does anyone can tell me how can be a non-linear (Force vs displ.) modeled in Nx Nastran 8.0? I need to include non linear spring in a 106 (or 601) simulation, and I know that Nastran allow to do that using the PBUSHT card. Unfortunately I didn't find any reference about that.

any other suggestion is well accepted!!

Thanks in advance!

 

[BlasMolero]

Dear Spikefer,
With FEMAP V10.3.1B and NX NASTRAN V8.1 for basic nonlinear analysis (SOL106 & 129) I strongly suggest to use the CBUSH element that feature nonlinear force-deflection properties defined on the PBUSH and PBUSHT property entries. The CBUSH element is geometric linear; the orientation is not updated in case of large deformation. The CBUSH1D element is both geometric and material nonlinear.

The CBUSH element is a generalized spring and damper. Stiffness and damping can be specified for 6 degrees-of-freedom, 3 translations and 3 rotations. In SOL 106 and 129, you can specify 6 nonlinear force-deflection curves instead of combined stiffness values. The properties of the CBUSH element are defined on the PBUSH and PBUSHi entries. The PBUSHT entry refers to TABLEDi entries for force-deflection curves. The CBUSH element is geometric linear only. The circulation is not updated.

In the following pictures you have the way to define the nonlinear properties for the CBUSH element:

The CBUSH1D element is a rod-type spring and damper, it is a 1D version of the CBUSH element. The CBUSH1D element can be geometric and material nonlinear. General nonlinear functions of the axial forces versus axial displacement and for velocity are defined with the PBUSH1D entry. The CBUSH1D element is the only element which can model rotation damping.

The CBUSH and CBUSH1D elements are recommended over the CELASi, CVISC, and CDAMP elements. The bushing elements always insure rigid body invariance and they offer nonlinear capabilities.

The use of CBUSH element has many advantages over CELASi elements. For example, if you use CELASi elements and the geometry isn’t aligned properly, internal constraints may be induced. The CBUSH element contains all the features of the CELASi elements plus it avoids the internal constraint problem.

In nonlinear solution sequences, the linear stiffness and damping is used for the initial tangent stiffness and damping. When nonlinear force functions are defined and the stiffness needs to be updated, the tangents of the force-displacement and force-velocity curves are used for stiffness and damping. The BUSH1D element is considered to be nonlinear if a nonlinear force function is defined or if large deformation is turned on (PARAM,LGDISP,1). For a nonlinear BUSH1D element, the element force output is the sum of the elastic forces and the damping forces. The element is considered to be a linear element if only a linear stiffness and a linear damping are defined and large deformation is turned off.

Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48011 BILBAO (SPAIN)
WEB:

http://www.iberisa.com

Blog de FEMAP & NX Nastran:

http://iberisa.wordpress.com/

 

[spikefer]

Dear BlasMolero,

thank you so much for your comprehensive reply!
since I notice your experience in Femap/Nx nastran, just another question: what is your suggestion to preload tha above-mentioned non-linear spring? It is better to manage a restart-analysis or is there some kind of initial offset of the Force_VS_Displ curve of the bush (in order to have non-zero force at zero deflection)?

Thanks in advance another time!