Rapid Redesign of Metal Load-Bearing Aircraft Brackets in ......Jun 11, 2014 · FEMAP SYMPOSIUM 2014 Femap Symposium 2014 Discover New Insights May 14-16, Atlanta, GA, USA Rapid

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FEMAP SYMPOSIUM 2014

Discover New InsightsFemap Symposium 2014

May 14-16, Atlanta, GA, USA

Rapid Redesign of Metal Load-Bearing

Aircraft Brackets in Plastic

Tyler Smithson, S.E., P.E., R&D Consultant to C&D Zodiac

2014-05-15


Page 2 Siemens PLM Software

Outline

• Metal Redesign

• Simplified / Estimated Geometry

• Simplified / Initial FEA Analysis

• Refined Geometry

• Refined Analysis

• Finalizing Design, Coordinating

Manufacture & Changes

Tyler Smithson, S.E., P.E., www.smithson1.com

2014-05-15



Metal Redesign

Competition demands lighter and less expensive products

As structural polymers continue to improve in strength and ductility, there are

viable options to trade out metal load-bearing parts and assemblies with lighter,

stronger, less expensive parts

• The days of engineers simply machining

a rough aluminum shape and bolting it

to ‘doublers’ for a quick fix on a

certification test are fading

• A vendor’s qualitative ‘tribal knowledge’

may provide an initial tool with:

• little design flexibility

• expensive raw material

• high part counts

• poor performance

• little innovation

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Metal Redesign

There is always a little time for some good preliminary engineering

• Evaluate strengths and

weaknesses

• Modifications consistent with

manufacturing and service

conditions

• Up/down stream components

for load path and constraint

accuracy

• Part count reduction

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Metal Redesign

Engineering is more than Structural Analysis …

… and Structural Analysis is more than FEA

• What manufacturing processes are cost-efficient for other materials?

• What service and abuse loads and constraints need to be considered in

addition to the main design loading?

• How can inexpensive prototypes be evaluated?

• Could it have been extruded, stamped or cast?

• What alternative materials can be effective?

• Where is inefficient load concentration?

• Where is efficient load distribution?

• Where is inefficient material?AL 6061 Ultem 2300

Shear Load (V) 3000 3300 lb, pin force

Brg width 0.25 0.4 in (2% draft)

Brg depth 0.25 0.3 in

area 0.125 0.24 sq in, (2 locations)

Brg Stress 36000 20625 psi, 3V/2A

Ty 40000 24400 psi

knock down 0.95 0.9

Brg Allow 38000 21960 psi

Area req 0.11842105 0.22540984 sq in

Margin 6% 6%

Weight 0.203 0.143 30% weight savings

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Metal Redesign

Material properties are more than tabular values …

… yet quick linear FEA can still do the trick for many nonlinear materials

• What’s in the Stress-Strain Curve?

• Plasticity & high E of Metal

• Plasticity & low E of Plastic

• Brittleness & moderate E of GRFP

• What are various material ‘tradeoffs’?

• Increase the strength 19%

and keep the same weight

by adding 58% more material

• Reduce the weight 16%

and keep the original strength

by adding 33% more material

Efficient material usage Tu Ty Density B-basis Design

Axially stressed in tension psi psi pci factor Stress

AL 6061 T6 45000 40000 0.1 n/a 40,000

IXEF 1521 (50% glass) 33400 n/a 0.0632 0.9 30,060

Section Weight Load Comparrison

sq in lb lb Volume Weight Strength

AL 6061 Sample 0.250 0.0250 10,000

IXEF 1 (same section) 0.250 0.0158 7,515 100% 63% 75%

IXEF 2 (same weight) 0.396 0.0250 11,891 158% 100% 119%

IXEF 3 (same Load) 0.333 0.0210 10,000 133% 84% 100%

2014-05-15



Metal Redesign

Manufacturing choice can stay a variable to designs

Machining vs Molding …

Machined Molded

Design Flexibility

High design Flexibility after initial prototype. Orthogonal patterns efficient

Low design flexibility after initial prototype. Features can’t be moved

Prototypes, 1st Articles

Low Cost ($100) Short Lead Time (3 days)

High Cost ($5,000+) Long Lead Time (4 weeks)

Repeats High ($5) Low ($.25)

Part Count High (bolts & screws) Low (snap-in clips & adhesives)

Design Issues

stress concentrations @ connections, radii, machining time

draft, sinks, weld lines, gates, texture, coordination with groups/vendors

Typical Designs

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Simplified / Estimated Geometry

Create / import initial geometry

Existing geometry can come from a drafter/designer or can be

regenerated from a picture or drawings

• 15 minutes in AutoCAD:

• 5 min FEA adjustments:

Curves broken at ‘T’

End points coincident

Duplicates removed

Overlaps trimmed

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Create initial FEA geometry

The 2D DXF from the prior slide was imported

into Femap and the following operations took

~20 minutes of a stress engineer’s time

• 4 Boundary Curves & 4 Extrusions

• 3 Copies, 3 Rotations & 2 Translations

• 1 Add & 1 Remove

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Manipulate Geometry

Simple and fast geometry manipulations:

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Simplified / Initial FEA Analysis

Study major behavior patterns - Modes

Is the frequency of the 1ST mode significantly lower

than the 2ND mode?

• If so, the first mode will generate much easier than

the higher modes and constrain acccordingly.

Is the shape of low modes consistent with the

expected deformation under design load?

• If so, the geometry may already be optimal, so skip

further FEA and go directly to design refinement.

(Do not pass Go, do not spend another $200.)

The main vertical rib will curve in the back as the

base plate bends.

The thin base plate will have high stresses next to

the thick vertical rib

2014-05-15



Simplified / Initial FEA Analysis

Validating questions:

Validating Questions:

• Is the base stiffer than the wall (are rigid constraints valid)?

• Is the load on a loose or tight bolt (slip critical for full hole

application or contact style reduced rbe3 application)?

• Is vibration or fatigue a potential issue and do these results

provide guidance?

Recommendations:

• Hole patterns appear excessive. Perhaps it should be a plastic part with a few

lateral ribs and a chemical bonding adhesive for load transfer.

• No significant deformation should be allowed, so try a carbon or GFRP.

• This part may be able to be produced inexpensively in a compression mold

with low prototype costs and the load point hole a secondary process.

• A thicker bearing area to the load point will need to be sized.

• The width of the base plate may be excessive and the

square corners can be rounded.

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Refined Geometry

From preliminary engineering and initial analyses

This geometry is again developed from a

2D AutoCAD DXF and Femap (in 40 min)

• 30% lighter than the metal bracket

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Refined Analysis

Now it’s time for refined FEA

Now that materials & basic structural parameters are understood and

manufacturing methods are defined, we can refine the analysis

Simple constraint option:

X – bearing on back

Y – pinching sides

Z – top edge compressed

• Load envelope must produce a valid stress response.

• Constraints are often even more critical than loading in

the prediction of correct behavior.

“Spider” or rigid elements can pull and alter stiffness

so consider RBE3 and reduced RBE2 locations.

Pinning the entire inside surface is common, but it

incorrectly chemically bonds an infinitely rigid bolt.

Flexible contact modeling with bolts can be accurate,

but is often impractical due to complexity and time

constraints.

The option shown must align with the load direction.

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Refined Analysis

Study the results

Two constraint systems were used on the adjusted bracket.

2014-05-15



Finalizing Design, Coordinating Manufacture & Changes

Iterate on recommendations

Read the paper for further recommendations.• The stress contours near the load point show that bearing is significant for this loading. Mesh refinement may show a clearer distribution of

stresses at the load point, but since the metal part certified without significant deformation (yielding) at this load point, perhaps the sizing

calculations in combination with this simple analysis provide sufficient confidence in this area.

• The forces from the insert constraints are perhaps too high for standard insert designs, so the base could be

lengthened and additional inserts added to the middle of the part.

• The common approach of placing a metal ‘doubler’ with adhesive to increase the potted insert strength could

be avoided by using the adhesive on the part directly. It would then allow for the higher insert forces – but

would provide sufficient resistance on its own as well (red Tu is encapsulated, green Ty is well distributed and

the average shear is << Ty).

• The front area could have some rib adjustments to better distribute the compression load (more evenly).

• The middle/side rib could be thinned.

• Modeling of adhesive is possible but an exhaustive analysis is involved. The image shows such an analysis for another part.

The system is now ready for manufacturing discussions, including mold-flow

iteration with automated fiber orientation in FEA. Tooling commitments will set

most of the design as final and testing will prove the design out. Follow-up

programs with variations will also be linked to this documentation and

certification by analysis options are many.

Unfortunately for the conference attendee, some of the final details of the

assembly remain private, but rest assured that while the new bin above

you on the flight home could be lighter & less expensive – it will hold your

heavier carry-on for the certification load!

Tyler Smithson, S.E., P.E., www.smithson1.com

Documents

Rapid Redesign of Metal Load-Bearing Aircraft Brackets in ......Jun 11, 2014 · FEMAP SYMPOSIUM 2014 Femap Symposium 2014 Discover New Insights May 14-16, Atlanta, GA, USA Rapid