CAPINC Summer of Simulation Part 3: The 5 Things to do Before Showing Anyone your FEA Results

Shuvom Ghose800-424-2255 [email protected]

The Summer of Simulation #3:

5 things you should do before

showing your FEA results to

anyone

is ’s leading

reseller of SolidWorks CAD and

tools, and

Stratasys 3D printers.

Who we are

Who makes work

with your company’s specific

Who we are

Slides prepared by CAPINC Engineer:

Shuvom Ghose

© CAPINC

Why follow these five steps?

• There will always be people who doubt your FEA results (and FEA use in general).

• Don’t give the doubters any more ammunition.

• Do all you can to solidify your FEA results before showing them to people.

© CAPINC

5 things to do before showing off your FEA results!

1. Ask the right question.

2. Do a sanity check. Any check.

3. Make sure the mesh doesn’t matter.

4. Test a second failure condition.

5. Document your assumptions.

Step 1: Ask the right question

Fixed at 4 corners

Titanium: Ti-6AL-4V

8000 lbs. Total upward

force

If you were given this aircraft bracket to analyze, what would you ask of FEA first?

Many folks might ask: “What is the maximum stress in the bracket? Within, say, +/- 10 percent?”

(Bracket model courtesy of user “optimal_aj” from GrabCAD.com)


Is Fixed a realistic constraint for these bolts?

Is Ti-6AL-4V

the closest alloy?

Is 8000 lbs. really the right force? What if the load is at an

angle?

But if you ask about the absolute stress, now EVERY aspect of your study is called into question:

Is this mesh fine enough for this complex geometry?


Fixed?

Ti-6AL-4V?

8000 lbs.?Worse than that, now you’ll get into the “Percentage Game” with your doubters. You’ll say:

“Given all these questions, the max stress is probably accurate within 15%.”

Mesh fine enough?

“15 percent? HA! You’ll need accuracy better than +/- 5 percent to convince ME!”

© CAPINC


Fixed

Upward force

But instead of maximum stress, what if you asked:

“We’ve got two competing bracket designs. Which one is better?”

© CAPINC


77% Heavier

16% Worse displacement in

the pull direction

Then, with just a 1 minute FEA study, you learn that the lower bracket is:

(Same load, mesh settings, fixtures, material)

© CAPINC


77% Heavier

16% Worse displacement

And the best part is, all those doubts we had with the absolute study fall away, since they are the same for both relative bracket studies!

This means doubters can’t object to the results!

“Um…”

(Same load, mesh settings, fixtures, material)

You can make a LOT of decisions by asking the right questions:

77% heavier16% worse Z disp.

160% heavier50% better Z disp.

216% heavier34% better Z disp.

The RIGHT question

At CAPINC, we call this the “A vs. B comparison”. Use it to make your FEA cycles faster, since EVERY design decision can be boiled down to a choice of: “Do we do this? Or this? Or neither?”

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© CAPINC

Step 2: Do a sanity check

Returning to absolute numbers, with 8000 lbs. upward force on the lugs, the maximum reported stress in the part is:

3.6e5 psi.

Is that way too much? Way too little? In the ballpark? If we’ve never seen a shape like this, how can we tell?

Sanity check: the location of max stress

Is your max stress right next to one of your restrained, rigidly fixed faces?

Then the number is fake.

The fixed face distorts the stress around it, since one end of those mesh nodes can’t move at all, leading to more stress on the element. To get a more accurate number, we’d have to change the fixed restraint, or model the bolts and plate they’re attached to.

© CAPINC

Sanity check: magnitude of stresses

Blue= loaded

Using a Design Insight plot (found in all levels of SolidWorks Simulation), we can see that the back legs carry most of the stress when this bracket is pulled upward.

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Sanity check: magnitude of stresses8000 lbs.

Total upward force

Stresses ~ 4e4 psi?

Using an Iso Clipping plot (found in all levels of SolidWorks Simulation), we can see that the stress in those loaded back legs is around 4e4 psi.

Is that way too much? Not enough? How can we tell?

© CAPINC


And if you look at those back legs and squint a little, that back leg is sort of a cylinder about 0.5” diameter:

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8000 lbs. So what would happen if we took a cylinder of that size and put 8000 lbs. of load on it?

8000 lbs. upward force on test coupon

Stresses = 4e4 psi in 0.5” dia section


8000 lbs. Total upward force on bracket

Stresses ~ 4e4 psi in ~0.5” dia leg

With results in the same magnitude (e4 psi), we can conclude that our initial stress results are… NOT INSANE. That’s all we can tell.

© CAPINC

8000 lbs. upward force on test coupon

4e4 psi

Objection!

“You’re estimating the stresses in your part with a cylinder under tension?

I can do what with a simple hand calculation!

Why do we need some fancy, expensive FEA package to accomplish that!?!

Back in my day…”

Answer: the cylinder gives a sense of the magnitude, but can’t tell you anything about stress distribution, or A vs. B comparison of choices

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© CAPINC

Other sanity checks: hoop stress for pressurized vessels

On-line Hoop Stress Calculator:http://www.engineersedge.com/calculators/hoop-stress.htm

http://www.engineersedge.com/calculators/hoop-stress.htm

http://www.engineersedge.com/calculators/hoop-stress.htm

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Other sanity checks: cantilevered beam calculator inside of SolidWorks

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Other sanity checks

• What’s the MOST this number could be?

• What’s the LEAST?

• What is the range of your plot’s legend?

• Are deformations still “Elastic”, i.e. are they under 0.2% strain? (Linear solvers assume all deformations are elastic.)

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© CAPINC

Step 3: Make sure the mesh doesn’t matter

31,000 mesh

elements

126,000 mesh

elements

How many mesh elements are

‘enough’?

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What’s happening as you add more mesh elements

One big mesh element is like one big spring, pretty stiff…

Many mesh elements are like many small springs, added all together, they bend more while staying in

the elastic zone…

© CAPINC

But meshes are tetrahedrons! It still works! (Try this at home!)

Tip displace:5.14e-8 in

Tip displace:5.27e-8 in

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But the affect of adding more mesh elements eventually decreases

Increasing mesh density

Result

‘Real’ answer

What ‘should’ happen as you increase mesh density

© CAPINC

And this is what happens as you increase mesh density

Mesh elements

Z displacement, (in)

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But the time to solve (red) also increases

Mesh elements

Z displacement, (in)

Mesh elements

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So how can we keep our solve times reasonable?

Ask the right question!

Mesh accuracy doesn’t matter if the second A vs. B design is heavier AND worse in displacement!

77% Heavier

16% Worse Z Displacement

1 minute FEA study

© CAPINC

What does it mean if you get this graph as you increase your mesh density?

Mesh elements

Your result number

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© CAPINC

Step 4: Test a second failure condition

8500 lbs.?

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5000 in-lbs.

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But those tests were all part of the original specification

Who remembers what we checked for in Summer of Simulation #1?

Things Breaking Things Bending too much Things Overheating Things Shaking at Nat. Freqs. Long, Slender Things Buckling Fatigue Failure

© CAPINC

BreakBendBurnBuzzBuckleFat

BBBBBFat!

We checked for:

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Fatigue

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Natural Frequency

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SolidWorks Simulation Professional:

Break and Bend Buzz

Fatigue Optimize

Burn

Buckle

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BBBBBFat!

© CAPINC

What do you think is your BIGGEST FEA assumption?

Restraints

Material Properties

Loads

Mesh Geometry Simplification

Contact Conditions

© CAPINC

What do you think is your BIGGEST FEA assumption?

Restraints

Material Properties

Loads

Mesh Geometry Simplification

Contact Conditions

© CAPINC

Most likely sources of error(in general)

• Restraints

• Material Properties

• Loads

• Mesh • Geometry Simplification

• Contact Conditions

BIGGER assumptions

Smaller assumptions

© CAPINC

How to document restraints?(How it’s usually done- BAD)

• Bracket variant 142A was restrained with fixed faces at each of its bolt corners.

Appendix F: Assumptions

© CAPINC

How to document restraints?(Better)

• Bracket variant 142A was restrained with fixed faces (green) at each of its bolt corners:

Section 2: Assumptions (BEFORE results!)

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How to document restraints?(Best!) (For this example, changed away from fixed restraints to make a more interesting picture)

• Bracket variant 142A was restrained with bolt faces only allowed rotational motion (green cylinders) and bottom faces only allowed sliding motion (orange planes):

Section 2: Assumptions

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BBBBBFat!

© CAPINC

The Summer of Sim is over, but you can still watch our other recorded FEA webinars:

• 3 rules to do material selection right

• 10 designs by hand, 100 by FEA

• 5 things to do before anyone sees your FEA results

http://www.capinc.com/events/webinars/recorded-webinars

http://www.capinc.com/events/webinars/recorded-webinars/

If you want cutting edge or , contact

For more information

If you want more info on the FEA concepts presented, contact:

([email protected])

([email protected])

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CAPINC Summer of Simulation Part 3: The 5 Things to do Before Showing Anyone your FEA Results