CQUAD8

Bulk Data Entry

CQUAD8 – Curved Quadrilateral Shell Element Connection

Description

Defines a curved quadrilateral shell element with eight grid points.

Format

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
CQUAD8	EID	PID	G1	G2	G3	G4	G5	G6
	G7	G8	T1	T2	T3	T4	Theta or MCID	ZOFFS

Example

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
CQUAD4	111	203	20	21	50	51	26	94
	95	23	0.125	0.025	0.030	.025	30.	.03

Field	Contents
EID	Unique element identification number. No default (Integer > 0)
PID	Identification number of a PSHELL or PCOMP property entry. Default = EID (Integer > 0)
G1,G2,G3,G4	Grid point identification numbers of connected corner points. Required data for all four grid points. No default (Integers > 0, all unique)
G5,G6,G7,G8	Grid point identification numbers of connected edge points. Optional data for any or for all four grid points. No default (Integer > 0 or blank)
Ti	Thickness of the element at the corner grid points G1 through G4. The thickness of the element with Ti specified will be constant and equal to an average of T1, T2, T3 and T4. Overrides the thickness obtained from the referenced PSHELL or PCOMP entry. Default = blank (Real > 0.0 or blank).
THETA	Material orientation angle in degrees. See Comment 4. Default = 0,0 (Real)
MCID	Material coordinate system identification number. The x-axis of this coordinate system is projected onto the element to define the x-axis of the material coordinate system. If MCID = 0, it specifies the basic coordinate system. If blank, Theta = 0.0 is used, (See Comments 3 and 4). Default is THETA = 0.0 (Integer > 0)
ZOFFS	Offset from the surface of grid points to the element reference plane. See Comment 7. Default = 0.0 (Real)

Comments

Element identification numbers should be unique with respect to all other element IDs.
Grid points G1 through G8 must be numbered as shown here:

$image\cquad8_1.gif$

The element coordinate system is a Cartesian system defined locally for each point ( $image\e.gif$ , $image\cquad8_n.gif$ ).

$image\cquad8_2.gif$

It is based on the following rules:

- The plane containing $image\x_elem.gif$ and $image\y_elem.gif$ is tangent to the surface of the element.

- $image\x_elem.gif$ and $image\y_elem.gif$ are obtained by doubly bisecting the lines of constant $image\e.gif$ and $image\cquad8_n.gif$ .

- $image\x_elem.gif$ increases in the general direction of increasing $image\e.gif$ and $image\y_elem.gif$ of $image\cquad8_n.gif$ .

$image\cquad8_3.gif$

The orientation of the material coordinate system is defined locally at each interior integration point by THETA, which is the angle from the line of constant $image\cquad8_n.gif$ (essentially the same as the $image\e.gif$ -axis) to the material x-direction ( $image\x_mat.gif$ ).

If MCID is used in place of THETA, then the local material x-direction ( $image\x_mat.gif$ ) is obtained at any point in the element by projection of the x-axis of the prescribed MCID coordinate system onto the surface of the element at this point. The local z-direction is aligned with the normal to the surface and the material y-direction ( $image\y_mat.gif$ ) is constructed accordingly to produce right-handed local material system X-Y- $image\z_mat.gif$ .

$image\cquad8_4.gif$

Please note that since $image\x_elem.gif$ changes directions throughout the element based on element shape, the material coordinate system varies similarly. Because of this, an orthotropic or anisotropic material will cause the CQUAD8's stiffness to be biased by both its shape and grid ordering. Use the CQUAD4 element if a constant material coordinate system direction is desired with orthotropic and anisotropic materials.

T1, T2, T3, and T4 are optional. If they are not supplied, then the element thickness will be set equal to the value of T on the PSHELL entry or the sum of the ply thicknesses on the PCOMP entry.
It is required that the midside grid points be located within the middle third of the edge. i.e. the interval (0.25, 0.75) excluding the quarter points 0.25 and 0.75. If the edge point is located at the quarter point, the program may fail with an error or the calculated stresses will be meaningless.
Shell elements can be offset from the plane defined by element nodes by means of ZOFFS. In this case all other information, such as material matrices or fiber locations for calculation of stresses, are given relative to the offset reference plane. Similarly, shell results, such as shell element forces, are output on the offset reference plane.

A positive value of ZOFFS implies that the shell reference plane is offset a distance of ZOFFS along the positive z-axis of the element coordinate system. Note that when ZOFFS is used, both MID1 and MID2 must be specified on the PSHELL entry referenced by this element (otherwise, singular matrices would result).

In OptiStruct, offset is applied to all element matrices (stiffness, mass and geometric stiffness) and to respective element loads (such as gravity). Hence, ZOFFS can be used in all types of analysis and optimization in OptiStruct.

Stresses and strains are output in the local coordinate system identified by $image\x_elem.gif$ and $image\y_elem.gif$ above.
Size optimization of the property referenced by PID is not possible if Ti values are defined here. If the property referenced by PID is selected as a region for free-size optimization, then any Ti values defined here are ignored.

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