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Spar caps: Pultruded carbon profiles against wet roving layup

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Andres

Active Member
Joined
Oct 6, 2018
Messages
29
Location
Stuttgart, Germany
Hi everybody,

since a couple of years we're building a Horten-type of Nurflügel at home in our garage. It's an own design we named Schneewittchen, which stands for Snow-White in English. We're making it completely from scratch and have an extensive build log:
https://hortenmicrolight.wordpress.com/

We also run a channel on Youtube with a couple of videos showing mold and part making, testing, and so forth:
https://www.youtube.com/channel/UClYwEpXe38lMerAH9vf9uCg.

The design aims to meet the requirements of the 120kg microlight/ultralight class (German LTF-L; similar to FAA Part 103). It's almost inevitable to use composites for such a light but still efficient airplane. The main structure elements are carbon/glass composite sandwiches. We've done several structural tests to design the layout. Our latest test regarded the spar caps and I thought the results would be intersting to all homebuilders.

We compared a wet roving layup against a layered layout of laminated pultruded carbon profiles. The key point is that a wet layup is expected to have a higher shear strength than a layered spar cap. However, the profiles are sold by yard, have a quite constant quality, and the caps can be laminated without the use of molds. In our case the geometry of the spar caps is complex and we would need to have four different and huge molds. We bought already the complete material for the spars. This is how it looks like (20 × 0.5 mm carbon with over 60% fiber content; DPP, vDijk Pultrusion Products):

P1070705_m.jpg

We wanted to know how large the difference in shear strength truly is between the two approaches. To have a reproducible and significant estimate, we tested a total of 20 specimens following the standard DIN EN 2563. Half the specimens were a wet layup of Sigrafil C50 rovings and the other half were made of four layers of glued profiles. We used for both specimens the bisphenol-A epoxy resin LG 735 AERO from GRM Systems (similar to Araldite LY 5052 from Huntsman). The resin was thickened with some cotton flakes before joining the profiles. Both specimens were vacuum pressed at room temps, and heat treated afterwards for 15 hours at 60 °C (140 °F):

P1120832_m.jpg


Typical strain-stress curves measured:

profil_vergelich.jpg

Both achieved very high strengths. in contrast to what is expected, the laminated profiles achieved a higher shear strength than the wet layup (64.5 MPa against 59.5 MPa at 0.95 confidence). It seems that the profile solution performed better. I guess it's the constnat quality of the profiels. Hand wet-layups depend stronger on how well they are made and have a higher variance. In my opinion, there is no reason why pultruded profiles should not be used to make layered spar caps ;)

A word of caution:
Surface treatment of the profiles is a must, because they have a release agent that is not soluble in usual solvents. We soda-blasted the profiles and sanded the remaining spots with 80 grit sandpaper. Also, the quality of the resin is critical to achieve such high shear strengths.

Hope that helps you to design your own spars :)

Cheers,
Andrés
 
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