INTER9 Elements

INTER9 elements are used in conjunction with variable-node solid elements (e.g. C3D27) and nine-noded shells to model contact between two deformable surfaces.

In FEMGEN the gap is modelled by creating a body between two surfaces on either side of the gap and assigning the INTER9 variant of an HE20 element. The two bodies on either side of the gap are then assigned a variable-node solid element variant. The interface will then create the appropriate mid-face nodes and associate them with the INTER9 elements and associated variable node solids.

The interface will automatically assign a sensible element type (and not create a mid-face node) to those elements which are part of the model but are not adjacent to the gap. The element type is chosen according to which variable-node solid was requested e.g. if C3D27RH elements are assigned to a body the interface will chose C3D20RH variants for the elements not adjacent to the gap.A Jacobian check is performed on all INTER9 elements.

The following rules must be adhered to:

• For the INTER9 elements side 1 of the body must correspond to one side of the gap and side 2 to the other.
• There must be only one layer of INTER9 elements at the gap.
• The bodies either side of the gap must be defined with side 1 at the gap.

The interface is unable to produce any warnings if these rules are not followed and invalid data may be written to the input file. Similar rules must be followed for shell contact problems.

  

Figure 2.2: Example INTER9 Element Definition