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2011 – All Rights Reserved

Advanced Modeling and Analysis with Femap and NX Nastran

An advanced tour through the robust modeling and analysis capabili@es of Femap and NX Nastran using a blend of theory and prac@ce that allows the experienced student to build complex and accurate FEA models of structures and systems.


Advanced Modeling and Analysis with Femap and NX NastranPredictiveEngineering.com

Day 1 Course Outline:I. Advanced Femap Modeling Topics

a.) Free Body Diagram Toolbox

i) Technology and Commentary

ii) Workshop I: Free Body Diagrams

b.) Variable Loading

i) Workshop II – Hydrosta@c Loads via Data Surfaces

c.) Connec@ons

i) Theory behind Mul@ Point Constraints

ii) Workshop III: Applica@on and Verifica@on of RBE3s

iii) Workshop IV: Advanced Glued Connec@ons

d.) Loads and Constraints

i) Workshop V: Cyclic Symmetry

e.) Meshing

i) Workshop VI: Advanced Tet Meshing and Geometry Prepara@on

ii) Workshop VII: Advanced Hex Meshing

iii) Workshop VIII: Advanced Surface Meshing



Day 2 Course Outline:II. Advanced Analysis Topics

a.) Linear Contact and Bolted Connec@ons

i) Introduc@on to NX Nastran Linear Contact

ii) Workshop IX: Linear Contact with Solids

iii) Workshop X: Plate Contact with Bolted Connec@ons

iv) Workshop XI: Eigenvalue Analysis with Bolt Preload

b.) Laminate Elemenents

i) Workshop XIIa: Introduc@on to Laminate Modeling

ii) Workshop XIIb: Intoduc@on to Laminate Post Processing

iii) Workshop XIII: Composite Assembly Analysis

c.) Nonlinear Analysis

i) Workshop XIV: Linear and Nonlinear Buckling

ii) Workshop XV: Geometric Nonlinearity

iii) Workshop XVI: Material Nonlinearity



Day 2 Course Outline:III. Troubleshoo@ng and Modeling Guidelines

a.) Understanding Online Help Documenta@on.

b.) Femap and NX Nastran Error Messages

i) Theory/Background on error messages

ii) Workshop – Debug FEA model using error messages with online help

c.) Modeling guidelines and best prac@ces

i) Discussion of element quality indicators

ii) Workshop – contouring element Jacobian over mesh

d.) Open Discussion / Comments



Open Model File: Workshop 1 – Free Body Diagrams / FBD -‐ wingpost -‐ Start.modfem

Movie File: Workshop 1 – Free Body Diagrams / Adv Workshop 1.wmv

Topics Covered:• Free Body Tools.• Free Body Diagrams vs. Interface Loads.• En@ty Selec@on.• Vector Display.• Free Body Contribu@ons.• View Proper@es.

Workshop I: Free Body DiagramsResults Valida,on

With this lash-‐up -‐ you see all the internal forces ac3ng at each node -‐ this is most useful when you are trying to debug interfaces or strange connec3ons.

Introduc)on: FBDs are an invaluable part of any high-‐quality piece of analysis work.



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Introduc)on: Although Rigid Beam Elements (RBE) have element in their name, they are not really elements but represent constraint rela@onships between displacements (u). If one grasps this statement then many of the piballs of their applica@on will be automa@cally avoided.

Connec,ons

Concepts:• The dependent nodes are removed from the displacement formula@on

matrix since their behavior is described by the independent nodes.• Why do we care? (i) One can’t apply displacements to dependent nodes

since they no longer exist within the “u” column; (ii) However, Forces/Moments can s@ll be applied to dependents and independents; (iii) matrix formula@ons are used and inverted. Hence your RBE must not have a mechanism. RBE2’s have 6 DOF for the independent node while RBE3’s are defined by the user (hence mechanisms can occur which result in error messages).

• See Extra Material for addi@onal details on RBE’s.• By using MPC’s, one can open the door to simplicity or to completely

screwing up the analysis. Many a good engineer has wrecked an analysis by not properly checking their applica@on of RBE2 and RBE3’s against reality.

Understanding Nastran Mul@-‐Point Constraints



Workflow:• Run model and inspect F06 file.

Modify and re-‐analyze. SOP with RBE3 is to verify force balance in F06.

• With more advanced model, create RBE3 between beam and plate. Use 6DOF for the Independent nodes. Run and verify the result.

• Use the FBD tool to check the results.• It is recommended that to read the

NX Nastran QRG and the NX Nastran Theory Sec@on on Mul@-‐Point Constraints.

Workshop III: Applica@on and Verifica@on of RBE3

Introduc)on: RBE3 have mechanisms and need a complete DOF set to be constrained. An example model is provided for the student to explore and modify and then check the results using the FBD tool. A more advanced example is also provided for addi@onal prac@ce.

Open Model File: Workshop 3 -‐ Applica@on and Verifica@on of RBE3 / RBE3 Plate and Tube Model -‐ Start.modfem

Movie File: Workshop 3 -‐ Applica@on and Verifica@on of RBE3 / Adv Workshop 2.wmv

Connec,ons



Introduc)on: Hand gluing components together using spring elements or mul@-‐point constraints is a rela@vely mature technology. In the last few years, this step has been automated where the user just defines the components to be glued and the program determines the appropriate parameters. Nevertheless, it doesn’t mean that it always works as the analyst might want it to work.

Workshop IV: Advanced Glued Connec@on NX Nastran Users Guide Chapter 20: An op3on to glue elements together during a solu3on is available in NX Nastran. Glue is a simple and effec3ve method to join meshes which are dissimilar. It correctly transfers displacement and loads resul3ng in an accurate strain and stress condi3on at the interface. The grid points on glued edges and surfaces do not need to be coincident. Glue creates s3ff springs or a weld like connec3on to prevent rela3ve mo3on in all direc3ons. For discussion purposes, the internal elements created by the solver from glue defini3ons will be referred to as “glue elements” in the documenta3on.

Assembly Modeling

Topics Covered:• Glued connec@on on edges

Workflow:• It is basically straighborward.• The secret of a good glued

connec@on is to mimic reality with your idealiza@on.

• Keep in mind that the connec@on is made with 6 DOF spring elements.

Open Model File: Workshop 4 -‐ Advanced Glued Connec@ons / Edge Gluing -‐ Start.modfem

Movie File: Workshop 4 -‐ Advanced Glued Connec@ons/ Adv Workshop 4.wmv



NX Nastran Linear Contact for Sol 101:• It is almost necessary to read the manual…. (NX Nastran User’s Guide,

Chapter 19).• The NX Nastran linear contact has a legacy directly from SDRC-‐Ideas. It

is a well-‐proven technology and they are s@ll improving it.• Be careful of the defaults for Min and Max Contact Search Dist. Actual

contact elements are created within the solver based on the regions that you create in Femap and then the distances specified in these entries. One can create virtual contact elements where one least suspects it….

• Num Allow Contact Changes default of 0.0 is open too rigorous. One sugges@on from an experienced user is that it should be 1% of the number of contact elements formed within the solver (see your F06 file – it’ll be listed in the first contact itera@on dialog).

• Serng up contact regions can be done automa@cally but it may be more effec@ve in the long run to carefully select your contact regions via the Connec@on Region dialog boxes.

• The last dialog box to make contact. Think of the total process as 1.) Connec@on Property; 2.) Region and 3.) Connector.

• Extra Material – complete set of linear contact notes.

Linear Contact



Introduc)on: For bolt preload to work properly you need to capture the deforma@on under the head and nut of the bolt; i.e., this means 3-‐D elements with contact. In the workshop below, contact will be enforced between the two parts and a bolt preload of 5,000 lbf applied. The analysis will be debugged and verified (bolt axial load checked). A normal modes analysis will be run with the bolt preload ac@ve and without. The pre-‐s@ffened modal analysis provides an interes@ng comparison to that where only the bolt s@ffness is used.

Bolt Preload

Workflow:• Set Contact card to Defaults• Create Regions using Property• Bolt preload is 5,000 lbf• Run analysis | Debug• Fix via Contact Property Card• Re-‐Run• Normal modes with SUBCASE (see

Femap Commands, 8.7.1.9 Modal Analysis)

• Run with standard normal modes.• Update model by decreasing the bolt

diameter to 1/8”. Re-‐analyze model in stress-‐s@ffened mode.

Workshop XI: Eigenvalue Analysis with Bolt Preload

Open Model File: Workshop 11 -‐ Eigenvalue Analysis with Bolt Preload / Bolt Preload and Eigenanalysis -‐ Start.modfem

Movie File: Workshop 11 -‐ Eigenvalue Analysis with Bolt Preload / Adv Workshop 11.wmv



Introduc)on: Laminate modeling is simply an extension of plate modeling. All of the plate modeling techniques, like building non-‐manifold geometry, using the meshing toolbox, crea@on connec@ons, etc. translate seamlessly to laminate modeling. However, the analyst must also model the increased complexity of the material proper@es and the layup plies.

Composite Modeling and Analysis

Workflow:• Create a Layup• Set the material angle. • Analyze the model.• Adjust the material angle• Reanalyze the model and compare

with previous results.• Update the model with a new, more

complex layup and corresponding property.

• Interrogate the results and determine if they match the predicted outcome based on material proper@es, layup, material angle and loading condi@ons.

Workshop XII: Part 1 -‐ Introduc@on to Composite Modeling

Open Model File: Workshop 12 -‐ Introduc@on to Composite Modeling / Laminate Intro -‐ Start.modfem

Movie File: Workshop 12 -‐ Introduc@on to Composite Modeling / Adv Workshop 12A.wmv



Introduc)on: Now that we have had a basic introduc@on to the modeling of laminate elements, we must explore the post processing in greater depth. In this workshop, the example model is kept simple to allow the analyst to compare hand calculated failure indices to those recovered from the FE model.

Composite Modeling and Analysis

Workflow:• Based on the model dimensions, layup

thickness and loading condi@ons, hand calc the stresses in the laminate

• Choose a failure method and include a Bond Shear Allowable in the property card.

• Given material data, hand calc the failure index and compare to FEA results.

• Reverse the direc@on of the load and repeat the process.

• Create a 4 ply layup.• Reanalyze• Extract inter laminar shear stresses and

calculate bonding failure.• Compare the results with the model.

Workshop XII: Part 2 -‐ Introduc@on to Composite Modeling

Open Model File: Workshop 12 -‐ Introduc@on to Composite Modeling / Laminate Failure -‐ Start.modfem

Movie File: Workshop 12 -‐ Introduc@on to Composite Modeling / Adv Workshop 12B.wmv

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