Geometry

FEMGEN will automatically generate a finite element mesh of nodes and geometric element types. These geometric element types are related to the analytical element names of a particular analysis package by an element variant number. The element variant mappings may be displayed by using the FEMGEN command 'UTILITY TABULATE FE BERSAFE', and the mappings for BERSAFE are shown below:

 

ELTYPE BERSAFE ELEMENT VARIANTS

BE2 1=EP4 2=EX4 3=EZ6 4=CSB12 5=PEB12 6=GBE12 7=HBE12

BE3 1=GBE17

TR3 1=EP6 2=EP6F 3=EX6 4=EX6F 5=EX9N 6=EX3T 7=EP3T

QU4 1=EP8 2=EX8 3=EX12N 4=EX4T 5=EP4T

TR6 1=EP12 2=EP12F 3=EX12 4=EX12F 5=EX18N 6=EX6T 7=EP6T

8=FS12 9=CS24

QU8 1=EP16 2=EX16 3=EX24N 4=EX8T 5=EP8T 6=CS32

QU9 1=EP18 2=EX18

PE6 1=EZ18

HE8 1=EZ24

PE15 1=EZ45 2=EZ45F

HE16 1=CS40

HE20 1=EZ60

The thick shell element CS40 has been mapped to an HE16 brick element because the CS40 requires 8 pseudo nodes in addition to the 8 normal nodes. This element should be used with caution because of this unusual mapping. The interface uses the nodes on face 1 of the brick element as the CS40 nodes, and the nodes on face 2 of the brick element as the CS40 pseudo nodes. Care must be taken to ensure that face 1 of the element is aligned with the surface to be modelled and NOT face 2.

Element variants are assigned using the command 'MESHING TYPES part ELTYPE variantno'. The variant number is optional and the default is 1.

The model geometry is translated into card types 11 (element topology), 25 (nodal coordinates), 31 (element attributes), and where appropriate types 32 and 33 (additional beam element data). All of these cards are written in fixed format.

For 16 and 20 noded brick elements the number of nodes field on card type 11 is set to number of nodes + 1 and a continuation card used according to the BERSAFE convention.

A Jacobian check is performed on all elements. Where a negative Jacobian is found, the node order of the element is changed to produce a positive value. For TR3, TR6, QU4, QU8, and QU9 elements the nodes are re-ordered in an anticlockwise direction. For brick elements, the nodes on the first face are swapped with those on the opposite face. For CS40 elements, the node order is never changed. For models where one or more nodes lie outside the X-Y plane, the interface will not re-order nodes on TR3, TR6, QU4, QU8, and QU9 elements.

In beam element models it is often necessary to specify beam orientation vectors using the command 'PROPERTY PHYSICAL ORIENTATE ...'. The interface uses these vectors to create pseudo nodes. The pseudo nodes are added to the end of the list of type 25 cards, and refered to later on in the deck on type 32 cards. One pseudo node is created for each FEMGEN line, and all beam elements generated on that line will have the same pseudo node. The pseudo node coordinates are generated by adding the beam orientation vector to a point mid way between the ends of the line. When using beam orientation vectors on curved geometries, the vector must be scaled such that the resulting pseudo node is created on the correct side of the line of beam elements.