Import NASTRAN File: Elements

Description

Elastic Line Elements

NASTRAN uses CBAR to define a simple beam element. Straus7 creates a beam element with end releases and offsets. Offsets are adjusted to account for the difference in the default location of line elements between Straus7 and NASTRAN.

NASTRAN uses CBEAM to define a beam element. Straus7 creates a beam element with end releases and offsets. The warping values SA and SB are ignored. Offsets are adjusted to account for the difference in the default location of line elements between Straus7 and NASTRAN.

NASTRAN uses CBEND to define a curved beam element. Straus7 creates a straight beam element or curved pipe element with radius determined by the GEOM option.

NASTRAN uses CDAMP1 to define a scalar damper element. Straus7 creates a connection element.

NASTRAN uses CDAMP2 to define a scalar damper element with no property entry. Straus7 creates a connection element.

NASTRAN uses CELAS1 to define a scalar spring element. If one node is defined, Straus7 creates a translational or rotational node stiffness. If two nodes are defined, Straus7 creates a connection element with connection UCSs.

NASTRAN uses CELAS2 to define a scalar spring element with no property entry. If one node is defined, Straus7 creates a translational or rotational node stiffness. If two nodes are defined, Straus7 creates a connection element with connection UCSs.

NASTRAN uses CGAP to define a gap or frictional element. Straus7 creates a normal point contact element.

NASTRAN uses CROD to define a tension-compression-torsion element. Straus7 creates a truss element.

NASTRAN uses CONROD to define a rod element with no property entry. Straus7 creates a truss element.

NASTRAN uses CTUBE to define a tension-compression-torsion tube element. Straus7 creates a truss element with torsion.

NASTRAN uses CFTUBE to define a fluid tube element. Straus7 creates a beam element.

NASTRAN uses CVISC to define a viscous damper element. Straus7 creates a spring-damper element.

Reference Nodes

The beam types CBAR, CBEAM, CBEND and CGAP include either a reference node or orientation vector in order to define the local beam axis system.

Where a reference node is provided, Straus7 posts this directly as the beam reference node.
Where an orientation vector is provided, Straus7 creates a reference node at the position specified by the magnitude and direction of the orientation vector with respect to the position and local coordinate system of the first end of the beam.

For more information on NASTRAN local axis system orientation, see NASTRAN File: Conventions.

Offsets

The beam types CBAR and CBEAM may include offset vectors at each end of the beam. Each offset has three components defined in the local coordinate system of the node at which it acts. Straus7 imports these offsets in the following way:

If either end node has a local coordinate system, the offsets on that node are converted into components in the global XYZ system.
Each offset is converted into components acting along the beam's principal axes.
A single offset vector is created using the average of the two offset vectors (since Straus7 only has a single beam offset vector, not one at each end).
The offset components perpendicular to the beam axis are applied (since Straus7 does not support offsets acting along beam axes).

Because the default node location of beams in NASTRAN is the shear centre, whereas in Straus7 it is the centroid, offsets are automatically added to account for these different conventions.

End Releases

The beam types CBAR and CBEAM may include end releases for each end of the beam. Each end can be fully released in up to five of the six degrees of freedom. Straus7 imports end releases for each end and fully releases the appropriate degrees of freedom.

Pipe Radius

The beam type CBEND may include a GEOM flag to define the bend element. Straus7 assigns a pipe radius to the appropriate beam elements with a pipe property.

Connection UCS

The beam types CELAS1 and CELAS2 may include local coordinate systems at each end of the beam different to that of the principal axis system of the beam. Additionally, different stiffness components (translational and rotational) may be defined at each end of the beam, although the reference system may be different. Straus7 assigns a connection UCS to each end of the beam if appropriate. If the required coordinate system is not available, Straus7 generates additional user-defined coordinate systems with the default title "NASTRAN Node UCS N".

The Straus7 connection element does not allow different stiffness components to be defined at each end of the beam. If the components at each end are translational or rotational, Straus7 can account for the different components via the connection UCS assignment. Straus7 does not support interaction between translational and rotational components at each end of the connection element.

Beam Numbering

New Straus7 beams are numbered sequentially (starting at 1), whereas beam elements in the NASTRAN file can be arbitrarily numbered (as long as all IDs are unique). The NASTRAN ID number will be assigned as a beam ID attribute in Straus7.

Elastic Surface Elements

NASTRAN uses CQUAD4 to define a quadrilateral plate element. Straus7 creates a Quad4 plate element, with a local angle and offset.

NASTRAN uses CQUAD8 to define a curved quadrilateral shell element. Straus7 creates a Quad8 plate element, with a local angle and offset.

NASTRAN uses CQUADR to define a quadrilateral plate element (no membrane-bending coupling). Straus7 creates a Quad4 plate element, with a local angle.

NASTRAN uses CSHEAR to define a shear panel element. Straus7 creates a Quad4 plate element.

NASTRAN uses CTRIA3 to define a triangular plate element. Straus7 creates a Tri3 plate element, with a local angle and offset.

NASTRAN uses CTRIA6 to define a curved triangular shell element. Straus7 creates a Tri6 plate element, with a local angle and offset.

NASTRAN uses CTRIAR to define a triangular plate element (no membrane-bending coupling). Straus7 creates a Tri3 plate element, with a local angle.

NASTRAN uses CTRIAX6 to define an axisymmetric triangular cross section ring element with no property entry. Straus7 creates a Tri6 plate element, with a local angle. Note that the CTRIAX6 element has a different numbering sequence to the other triangular elements.

Local Angles

The plate types CQUAD4, CQUAD8, CQUADR, CTRIA3, CTRIA6, CTRIAR and CTRIAX6 may include a local axis angle. This angle adjusts the orientation of the local plate axis system.

However, the local axis systems in Straus7 and NASTRAN are not defined the same way. Usually, there is a non-zero angle between the local x-directions as defined by Straus7 and NASTRAN. This angle difference is added to the local axis angle before it is posted to Straus7, so that the local axes are in alignment.

For more information on NASTRAN local axis system orientation, see NASTRAN File: Conventions.

Offsets

The plate types CQUAD4, CQUAD8, CTRIA3 and CTRIA6 may include an offset in the local plate z-direction. Straus7 stores this value as a plate offset attribute.

Plate Numbering

New Straus7 plates are numbered sequentially (starting at 1), whereas plate elements in the NASTRAN file can be arbitrarily numbered (as long as all IDs are unique). The NASTRAN ID number will be assigned as a plate ID attribute in Straus7.

Elastic Solid Elements

NASTRAN uses CHACAB to define a six-faced acoustic absorber element with 8-20 nodes. Straus7 creates a Hexa8, Hexa16 or Hexa20 brick element.

NASTRAN uses CHACBR to define a six-faced acoustic barrier element with 8-20 nodes. Straus7 creates a Hexa8, Hexa16 or Hexa20 brick element.

NASTRAN uses CTETRA to define a four-faced brick element with 4-10 nodes. Straus7 creates a Tetra4 or Tetra10 brick element.

NASTRAN uses CPENTA to define a five-faced brick element with 6-15 nodes. Straus7 creates a Wedge6 or Wedge15 brick element.

NASTRAN uses CHEXA to define a six-faced brick element with 8-20 nodes. Straus7 creates a Hexa8, Hexa16 or Hexa20 brick element.

NASTRAN uses CHEX20 to define a six-faced brick element with 8-20 nodes. Straus7 creates a Hexa8, Hexa16 or Hexa20 brick element. Note that CHEXA and CHEX20 have a different connection sequence.

Brick Element Types

Straus7 brick elements are either linear (no mid-side nodes), or quadratic (every edge has a mid-side node). In contrast, NASTRAN brick elements may or may not have a mid-side node on any edge. When importing a NASTRAN brick, Straus7 adopts the following convention:

If every edge has a mid-side node, Straus7 posts a quadratic brick element (Hexa20, Wedge15 or Tetra10).
If at least one edge has no mid-side node, Straus7 ignores all the mid-side nodes and posts a linear brick element (Hexa8, Wedge6 or Tetra4).
If the NASTRAN brick is a CHEXA, CHEX20, CHACAB or CHACBR element with 16-19 nodes, check to see if the sides without mid-side nodes are parallel. If this is the case, Straus7 posts a HEXA16 brick element.

Brick Numbering

New Straus7 bricks are numbered sequentially (starting at 1), whereas brick elements in the NASTRAN file can be arbitrarily numbered (as long as all IDs are unique). The NASTRAN ID number will be assigned as a brick ID attribute in Straus7.

Mass Elements

NASTRAN uses CMASS1 and CMASS2 to define a scalar mass element. Straus7 creates a translational node mass for each of the two end nodes. Each mass is half the mass of the scalar element. The component numbers C1 and C2 are ignored. Straus7 gives each mass the same value in each direction (no beam element is created).

NASTRAN uses CONM2 to define a concentrated mass at a grid point. Straus7 creates a translational node mass for the specified node. The rotational part of the inertia matrix is transformed to a principal axis system so that it is diagonal. The resulting rotational mass and UCS are created.