doubts QUAD4 formulation

Hi all,
I have a doubt about QUAD4 Nastran Element. I'm looking into its formulation and I'm not clear.
QUAD4 documentation says the normal to the midsurface remains normal to the midsurface during bending which is just Kirchhoff theory. But this other document states QUAD4 are Mindlin shell elements. Other point is the stresses outputs in QUAD4 which are Sx, Sy and Sxy, but if it's a Mindlin element, what about the rest of the non zero components?
Thanks and regards

 

[BlasMolero]

Hello!,
The formulation of the CQUAD4 and CTRIA3 elements in NX NASTRAN are based on the Mindlin-Reissner shell theory. These elements do not provide direct elastic stiffness for the rotational degrees-of-freedom which are normal to the surface of the element.

CQUAD4 is NX Nastran’s most commonly used element for modeling plates, shells, and membranes. The CQUAD4 is a quadrilateral flat plate connecting four grid points. It can represent in-plane, bending, and transverse shear behavior, depending upon data provided on the PSHELL property entry.

You should use the CQUAD4 element when the surfaces you are meshing are reasonably flat and the geometry is nearly rectangular. For these conditions, the quadrilateral elements eliminate the modeling bias associated with the use of triangular elements, and the quadrilaterals give more accurate results for the same mesh size. If the surfaces are highly warped, curved, or swept, you should use triangular elements.

Under extreme conditions, quadrilateral elements will give results that are considerably less accurate than triangular elements for the same mesh size. Quadrilateral elements should be kept as nearly square as possible, because their accuracy tends to deteriorate as their aspect ratio increases.

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/

 

Sorry, but what about the questions?

 

[BlasMolero]

Hello!,
If I request the stress output on CQUAD4 elements in NX Nastran using the "STRESS(PRINT,CORNER) = ALL" Case Control command I will get the following:
• σx, σy – Normal stresses in the x- and y-directions.
• τxy – Shear stress on the x face in the y-direction.
• Major and minor principal stresses.
• Angle between the x-axis and the major principal direction. This angle is derived from σx, σy, and τxy.
• von Mises equivalent stress if you request STRESS(VONM) or maximum shear stress if you request STRESS(MAXS). These stresses are derived from σx, σy, and τxy.

NX NASTRAN plate elements use the Mindlin theory which assumes that the transverse shear deflections in the element are second order effects compared to the bending deflection.

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/