Unrestricted © Siemens AG 2014
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
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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
Unrestricted © Siemens AG 2014
Page 3 Siemens PLM Software
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%
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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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Simplified / Estimated Geometry
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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Simplified / Estimated Geometry
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
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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.
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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