Rapid Redesign of Metal Load-Bearing Aircraft Brackets in Plastic

Transcription

1 Tyler Smithson, S.E., P.E., R&D Consultant to C&D Zodiac Rapid Redesign of Metal Load-Bearing Aircraft Brackets in Plastic Femap Symposium 2014 May 14-16, Atlanta, GA, USA FEMAP SYMPOSIUM 2014 Discover New Insights

2 Outline Metal Redesign Simplified / Estimated Geometry Simplified / Initial FEA Analysis Refined Geometry Refined Analysis Finalizing Design, Coordinating Manufacture & Changes Page 2 Tyler Smithson, S.E., P.E.,

3 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 Page 3

4 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 Page 4

5 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? Page 5 AL 6061 Ultem 2300 Shear Load (V) lb, pin force Brg width in (2% draft) Brg depth in area sq in, (2 locations) Brg Stress psi, 3V/2A Ty psi knock down Brg Allow psi Area req sq in Margin 6% 6%

6 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 T n/a 40,000 IXEF 1521 (50% glass) n/a ,060 Page 6 Section Weight Load Comparrison sq in lb lb Volume Weight Strength AL 6061 Sample ,000 IXEF 1 (same section) , % 63% 75% IXEF 2 (same weight) , % 100% 119% IXEF 3 (same Load) , % 84% 100%

7 Metal Redesign Manufacturing choice can stay a variable to designs Machining vs Molding Design Flexibility Machined High design Flexibility after initial prototype. Orthogonal patterns efficient Molded Low design flexibility after initial prototype. Features can t be moved Prototypes, 1 st Articles Low Cost ($100) Short Lead Time (3 days) Repeats High ($5) Low ($.25) High Cost ($5,000+) Long Lead Time (4 weeks) Part Count High (bolts & screws) Low (snap-in clips & adhesives) Design Issues stress connections, radii, machining time draft, sinks, weld lines, gates, texture, coordination with groups/vendors Typical Designs Page 7

8 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 Page 8

9 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 Page 9

10 Simplified / Estimated Geometry Manipulate Geometry Simple and fast geometry manipulations: Page 10

11 Simplified / Initial FEA Analysis Study major behavior patterns - Modes Is the frequency of the 1 ST mode significantly lower than the 2 ND 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 Page 11

12 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. Page 12

13 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 Page 13

14 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 Load envelope must produce a valid stress response. Constraints are often even more critical than loading in the prediction of correct behavior. Page 14 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. Simple constraint option: X bearing on back Y pinching sides Z top edge compressed

15 Refined Analysis Study the results Two constraint systems were used on the adjusted bracket. Page 15

16 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! Page 16 Tyler Smithson, S.E., P.E.,