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Essential utility tools developed using HyperMesh-Tcl.
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Create Linear Gaps
Use the Linear Gap tool to create linear gap analysis cards for Nastran and OptiStruct.
Linear Gap
Linear gap is the simplest contact form used in Nastran simulations.

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Aerospace
Essential utility tools developed using HyperMesh-Tcl.
- Assembly Tools
  Create an assembly structure from a .csv file definition.
- Load/BCs Tools
- Connection Tools
  - Convert CFAST Elements
    Use the cfast converter tool to convert CFAST elements into CBAR/RBE3 (spiders). For thermoelastic analysis, the CFAST has no thermal expansion coefficient. CBAR is thermoelastic compatible.
  - Stuck Node Creation
    Use the Stuck Node Creation tool to create (double) RBE3 connection to "stick" nodes on QUAD or TRIA elements to make the node displacements following the element ones.
  - Create Linear Gaps
    Use the Linear Gap tool to create linear gap analysis cards for Nastran and OptiStruct.
    - Linear Gap
      Linear gap is the simplest contact form used in Nastran simulations.
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Linear Gap

Linear gap is the simplest contact form used in Nastran simulations.

Linear gap can solve a large class of problems, such as hole bearing, heel-toe interaction and fitting contacts. Linear does not include friction, large displacements and material non-linearity. CGAP contact elements (node-to-node defined in the FEM environment) run in both linear static (SOL101) and non-linear analysis (SOL106).

The linear gaps provide for simple contact (compressive) forces only. MPC’s are used to define the gaps. The displacements and MPC-forces are used to determine if each gap is "welded" or "free".

Displacement criteria: no penetration (MPC displacement > =0)
MPCFORCE criteria: compressive force only (MPC force < =0)

The solution converges when there is no penetration of MPC's (gap deflection > = 0.0) and there are no tensile forces. An iterative procedure (CDITER-constrained displacement iterative technique) is used to solve the model based on displacements/MPC forces. The image below is an example where the CGAP is modeled.

Figure 1.

The equation for the gap displacement can be written as:

Uygap = Uyopening – Uyclosing + Uyinitial

Rearranging in MPC format and setting Uyclosing as the dependent (first) term:

Uyclosing – Uyopening + Uygap – Uyinitial = 0

The Nastran commands to use for linear gaps are the following:

SPOINT (scalar point definition) is used to define the U_gap and U_init :

SPOINT 55 155

The SUPORT entry specifies reference degrees-of-freedom for rigid body motion, and it puts SPOINT 55 (U_gap) in the R-set for iteration:

SUPORT 55 0

SPC selects a single-point constraint set to be applied and defines the initial gap opening = .05 (U_init):

SPC 101106 155 0 .05

MPC entry: (equations of motion: Uy3 – Uy2 + U_gap – U_init = 0)

$ SID G1 C1 A1 G2 C2 A2

MPC 101106 3 2 1.0 2 2 -1.0

$ G3 C3 A3 G4 C4 A4

55 0 1.0 155 0 -1.0

Related MSC/Nastran Bulk Data Entries are:

PARAM, CDITER (integer) maximum number of iterations to determine open/closed
PARAM, CDPRT (yes/no) print iteration history of constraint violations
PARAM, CDPCH (yes/no) punches DMIG, CDSHUT entries for final solutions
DMIG, CDSHUT: input vector of initial open/closed state of gaps (default = all closed)

Figure 2.