User Guide: FEA Solid Modeling, Meshing and Analysis...User Guide: FEA Solid Modeling, Meshing and...

User Guide: FEA Solid Modeling, Meshing and Analysis A Seminar for Simulation Engineers

George Laird, PhD, PE – Principal Mechanical Engineer Max Breedlove, MSME, FE – Staff Mechanical Engineer

User Guide: FEA Solid Modeling, Meshing and Analysis 2020

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TABLE OF CONTENTS 1. WHY WE ARE HERE: FEA SOLID MODELING, MESHING AND ANALYSIS .......................................................................... 5

1.1 WHAT WE WILL COVER ................................................................................................................................................................................. 5

1.2 WHAT WE DON’T COVER ............................................................................................................................................................................... 5

2. LET’S JUST GET THE DEMO OVER WITH AND WE CAN ALL EXHALE ................................................................................ 6

3. DIFFICULT GEOMETRY IS REAL GEOMETRY ..................................................................................................................... 7

3.1 BUT WAIT – WHAT IS THE REAL TRICK TO A QUALITY MESH? ................................................................................................................................. 8

3.2 DEFEATING MESSY GEOMETRY (WELL, DEFEATURING) ........................................................................................................................................... 9

4. WHY WE STILL NEED HEXING FOR LINEAR STATIC STRESS ANALYIS ............................................................................. 10

5. BEING CLEVER WITH YOUR GEOMETRY BATTLES.......................................................................................................... 11

6. MIXED MESHES CAN BE OUTSTANDING ....................................................................................................................... 12

7. LET’S WRAP IT UP .......................................................................................................................................................... 13



1. WHY WE ARE HERE: FEA SOLID MODELING, MESHING AND ANALYSIS

Pretty much anybody can take a chunk of junk or as one might say a “potato” mesh it, apply some boundary conditions and, with a click-of-a-mouse, generate some colors. But as we know, it can be a long and difficult road to turn colors into a verified simulation that has a chance that it might be accurate. This seminar will focus on showing how to idealize solid geometry into efficient numerical models that are quick to run and can generate accurate results. Why is speed important? Because to get an answer that you can trust often means running the same model dozens of times. So, we will cover how to take a chunk of junk, mesh the impossible, hex what is hexable and tet the rest, discuss element quality and then how to make it run quickly.

1.1 WHAT WE WILL COVER

o Difficult Geometry is Real Geometry: Geometry Cleanup (removing blends), Mesh Preparation

o Why is Hexing so Difficult and Why Should We Care – A Nod to Those that just Tet Everything

o Being Clever with your Geometry Battles – Let’s Step Back and Think about the Analysis Objective

o Why Do So Many Simulation Engineers Think That Mesh Refinement Makes An Accurate Analysis?

o Easy Ways to Check Your Mesh Resolution (Normal Modes Analysis) to Determine If It is Good Enough

o Don’t Fall In Love with 10-Node Tets and If You Do, Use the Iterative Solver

o Lastly, If It Looks Good, Most Likely It Is Good

1.2 WHAT WE DON’T COVER

• Bias Toward Any One FEA Software Program – This Seminar is for Any and All Simulation Engineers

• That FEA is for Everybody, as one Chief Engineer told me “many engineers think that they might enjoy

simulation…” So, we are not going to sugar coat anything. Getting the “right” answer is hard work and no FEA

program created on this wonderful green earth is going to ensure that you will get the right answer – only thru

your hard thinking is the way forward.



2. LET’S JUST GET THE DEMO OVER WITH AND WE CAN ALL EXHALE

Finite element analysis in four steps (and about as many keystrokes)

Import Geometry Assign Material and Mesh

Apply Boundary Conditions (constraint and load) Analyze and Contour Stress Results

What is Verification as compared to Validation?

Why Do So Many Simulation Engineers Think That Mesh Refinement Makes an Accurate Analysis?



3. DIFFICULT GEOMETRY IS REAL GEOMETRY

Every FEA tool has their solid meshing “We Can Mesh EVERYTHING with One Click” option. In FEMAP it is the Geometry Preparation button. The methodology is to keep the underlying geometry while modifying the surface skins.

What We Fear Geometry Preparation Meshed with Some Advisories (Mesh Statistics)

Contour Model Data

Mesh Preparation Slices and Dices and Then Combines Curves and Surfaces

One Can Adjust These Operations in the Mesh Toolbox



3.1 BUT WAIT – WHAT IS THE REAL TRICK TO A QUALITY MESH?

The real gig is to skin the solid with triangular plate elements and then do some cleanup. This may seem a bit old-school

but you arrive with confidence that you have a good mesh.

For tetrahedral meshing, the Jacobian is about as good as it gets for “one-stop-shopping”; however for shells, one should

also check warping, but that is another Seminar.

We Can Do Better Feature Suppression Tri Mesh Jacobian

Mesh Sizing Jacobian < 0.80



3.2 DEFEATING MESSY GEOMETRY (WELL, DEFEATURING)

Just some basic geometry operations that are within the Meshing Toolbox (updated at every revision with more and more stuff).

Meshing Toolbox, Feature Removal

Feature Type, Blends, Limit Size = 0.25

Nothing is perfect and the remaining blends can be removed by Feature Type, Surfaces (no limit size) – one-by-one.



4. WHY WE STILL NEED HEXING FOR LINEAR STATIC STRESS ANALYIS

There is no one size fits all. It is easy to tet mesh but for every 8-node, quasi-parabolic hex, it takes five 10-node tetrahedrals (26 nodes) to fill the same space.

Glued connections between 10-node tet and 8-node bricks is very efficient.

Normal modes analysis is an effective way to determine mesh convergence.

890k nodes, Mixed Mesh 10-node Tet to 8-node Hex

12k nodes, 20.4, 55.2, 71.9, 110 Hz 43k nodes, 20.2, 55, 71.2, 108 Hz 86k nodes. 20.3, 55.4, 71.3, 110 Hz



5. BEING CLEVER WITH YOUR GEOMETRY BATTLES

When Hexing Rules What is the Gig with Hexing (Approach on Surface)

Hexing is about thinking in “blocks” and pushing mesh faces thru the solid. It can handle opposing curved surfaces if they can be matched

Some solids that can be hexed should not be due to poor element quality

One can also blend tet meshes into hex mesh with pyramid elements

Titanium Bone Screw Split Geometry / Hex Sizing Hex Tet



6. MIXED MESHES CAN BE OUTSTANDING

One can mix meshes and get clean transitions

With Pyramid transition, joining 10-node tetrahedrals to 8-node bricks works surprising well

Be careful joining 20-node brick meshes onto 8-node brick meshes (see Modify / Update Elements / Connect Linear/Parabolic Elements)

10-Node Tets, 8-Node and 20-Node Bricks Mixed Mesh with Pyramid Transition Stress Results Under Tension

Something is Not Right Updated Elements Smooth and Mostly Groovy



7. LET’S WRAP IT UP

Don’t Fall In Love with 10-Node Tets and If You Do, Use the Iterative Solver

Lastly, If It Looks Good, Most Likely It Is Good

Good job for hanging in there and working to improve your simulation skill set!

Isoparametric Elements Stress Flows More and More

User Guide: FEA Solid Modeling, Meshing and Analysis...User Guide: FEA Solid Modeling, Meshing and...

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