MSC APEX WORKSHOP - Patran vs MSC Apex LBCs & MPCs

ABSTRACT

The aim with this training workshop is to assist Patran users to understand how MSC Apex defines Loads, Boundary Conditions (LBCs) and Multiple Point Constraints (MPCs).
1 - First things First
If all else fails, check the exported BDF from MSC Apex to see exactly what was included and how it was defined since what is in the BDF, is what MSC Nastran will solve.
Time to study the MSC Nastran Quick Reference Guide (QRG)!

Unsafe and safe way to export a BDF

Unsafe BDF: An exported BDF from the file menu is probably not in run-ready state.

Reasons why it’s unsafe:

  • The BDF includes all the defined LBCs in the model.
  • No events (Patran: Load cases) are defined.
  • The model is not checked for any errors.

Safe BDF: An exported BDF from a scenario (Patran: Job) is ready to be sent to MSC Nastran for analysis.

Reason why it’s safe:

  • A green “thumbs up” icon shows that the scenario passed all the model checks and is ready to be solved with MSC Apex or MSC Nastran.
  • All the LBCs or only those selected in an event (Patran: Load case) will be exported.
  • Events defined in the scenario will be exported as MSC Nastran load cases.

Patran vs MSC Apex Analysis setup terminology

With the development of MSC Apex, common terms used by FE analysts were changed to be more engineer friendly by adopting more common terms. The table below lists the most important ones:
Patran/MSC Nastran
MSC Apex
Select Entire Model or Group for analysis
Select Model, subassembly or part for analysis
Solution Type
Environment Type
Available Jobs
Scenarios
Full Run
Run (Solve with MSC Apex)
Analysis Deck
Export BDF (from Scenario, not the file menu!)
Or
Load case
Event
Define Load case
Define Event
Output Requests
Output Requests
This is a block of text. Double-click this text to edit it.
NB: When ever a new load or constraint has been added to a model in MSC Apex, they are NOT added to any existing study. 
2 - Constraints
With the intent to use more commonly used terminology and rely less on the analyst’s correct use of general constraints, MSC Apex provide pre-set constraints for commonly used boundary conditions while also offering general constraints for the user to define.
The following image shows how to create a general constraint in Patran vs MSC Apex:
Apart from the general constraint which can be used to setup any type of boundary condition, MSC Apex provides a few pre-set constraints:
For and combination of Tx, Ty,Tz, Rx, Ry & Rz.
General constraint
Clamped (fixed) constraint
For Translation X, Y or Z
Axial (sliding) constraint
For planes Xy, Yz or Zx
Symmetry constraint
Spherical constraint
(Single point!)
Why are any of the “new” constraint and connection options in MSC Apex useful?
Motivation: If you understand exactly what they define, you can do more in less time and demand a higher salary!
Or you can leave it to your colleagues to outperform you…
For constraints, Patran provides three constraint type options:
For the Nodal type, only Nodes are selected and for the Element Types, only 2D or 3D elements are selected.
In MSC Apex, the user has two application method options: Direct or at a Remote Location.
For the Direct option, the constraint is applied to all the applicable nodes, directly, while the remote location option connects all the applicable nodes to a specified location where the constraint is applied (i.e. the nodes are not directly constrained).
For each type of constraint option, the selection filter guides the user as to what can be selected to apply the constraint to and what not:
The selection filter on the left changes dynamically according to the options available to the user.
Note: Orange selection filter items
are geometry entities and purple (magenta) selection filter items
are nodes, elements or ties (MPCs), while red selection filter items
are constraints or loads.
The remote location can be defined anywhere in space, relative to anything selectable or relative to the volume centre of the selected entities:
By selecting or grabbing any of the locator axes, the point (of rotation in this case) can be specified interactively by dragging the axis or typing in a distance from the current location.
Tip: In MSC Apex, the remote location does not need to be predefined with a point or node. The node, for analysis purposes, will be created automatically for you.
For the remote location option, an additional option becomes available namely the distribution type:
The distribution types (rigid and compliant) will be explained in the next section.
3 - Connectors, Joints and Ties (MPCs)
MSC Apex does not use the term MPC in the UI, however it does use RBE2s and RBE3s internally when defining connectors, joints, and ties. This is evident when a BDF is exported and is the recommended method to confirm your understanding of what you defined:
The MSC Nastran Quick Reference Guide can be referenced to understand what the above means. For example, the 1st term shows that an RBE2 is defined, and the 4th term (123456) means that all 6 DOF of the connected nodes are linked.
Although MSC Apex uses RBE2s and RBE3s internally, the only descriptions used in the UI is Rigid (RBE2) and Compliant (RBE3).
When defining the spherical constraint below, notice the effect of the two distribution type options on the behaviour of the plate where the constraint is applied:
Note that the MPC was automatically created based on the chosen distribution type and remote location.
The Rigid option keeps the application region rigid (no local deformation occurs) while the Compliant option allows local deformation while keeping the structure constrained to the location of the constraint.
To replicate the above constraint in Patran, the following steps are required:
Patran: Create node at rotation location, create RBE2 between nodes defining 6 linked DOFs, create constraint on created node defining locked translations and free rotations.
Amount of work: Minimum number of 28 mouse clicks in ±90 seconds.
In MSC Apex, the above steps are covered on this single form with only 9 mouse clicks in ±30 seconds.
±70% less work!

Connectors (Not joints)

Connectors can be defined through the Interactions Tools group in the right-side toolbar:
A connecter consists of two MPCs and one 1D element between the two MPCs. The connector above is defined as follows:

The first step is to chose what type of element should be between the two MPCs. The options are:

  1. Spring element
  2. Damper element
  3. Spring-Damper element
  4. Bushing element
  5. Rigid (RBAR) element
  6. Beam element
  7. GAP element
Following the chosen 1D element, the type of connections on either side are defined:

Joints

Joints are also pre-defined combinations of MPCs and constraints to ensure that the required DOFs between two parts are as desired, without the headache of ensuring that you define the connections correctly.

In the exact same manner as with connectors, creating a joint is a 3 step process:

  • Choose the type of joint (Revolute, Cylindrical, Prismatic (Translational), Planar & Spherical).
  • Define the 1st side’s entities to connect and the type of connection (Rigid or compliant)
  • Define the 2nd side’s entities to connect and the type of connection (Rigid or compliant)

Ties

MSC Apex has 3 types of ties that can be defined: Mesh independent, Mesh dependent and Discrete ties.

Mesh independent ties

A mesh independent tie is simply glued contact between multiple different meshes. It creates a permanent bond between meshes without any shared nodes or other connectors.

Mesh independent ties can be used between any types and sizes of meshes (i.e. they are independent of one another) and is ideal to eliminate hours of work to create matching meshes.

Mesh dependent ties (2D elements only)

A mesh dependent tie can connect edges to faces and edges to edges. Mesh dependent ties create coinciding nodes on the tied meshes which looks and act identical to stitched meshes, however it holds the following advantages over stitched meshes:
  • Tied meshes can belong to different parts (i.e. the assembly structure stays in tact) while stitched surfaces need to belong to the same part.
  • Tied meshes can be moved relative to one another (during modelling) while stitched meshes need to be unstitched before they can be moved relative to one another.

Mesh independent ties

Discrete ties are MPCs which can be individually created, just like in Patran if none of the previous options suites your requirements. Use for care! Test your model using a Modal analysis to ensure you set up the model correctly to display the correct degrees of freedoms.
TIP: When ever you are in doubt, run a modal analysis and carefully watch all the mode shape animations. If the part moves and flexes correctly, it will do so as well in other analysis types.
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