Fasteners in Composite Materials

[daxman1]

It looks like I'm getting back into major composite builds again and I have a few questions I'm hoping someone on this site can answer. The last time I worked composites most of the time we used what we called "chicken" rivets to hold something into place along with the adhesive bond. As I recall all we used in these cases were cherry max or the like but never solid rivets and never interference fit fasteners because they had to be driven.

The reason I'm writing this is I talked to an engineer on the 787 who told me they are now using solid and interference fasteners in their MRB repairs. Have composites gotten to the point that fasteners can be driven into their holes without needing NDI to check for installation issues?

 

[WKTaylor]

Daxman... I take a swing...

NOTE. From my F-16 and F-15 composites experiences You are/were generally correct. However, the 787 [as I recall], has incorporated a large number of fasteners... so highly stressed composites, machining and fastening practices, have to be very mature.

SAE recently published AIR5367 Machining of Composite Materials, Components and Structures which has heavy emphasis on quality of holes.

NOTE. I believe the fasteners type-design/alloy/finishes/hole-fits/etc for have to be very-carefully matched to the composite structure matrix and fibers they are installed within... and whether the structure is assembled 'dry', 'wet-with adhesives', 'wet-with sealant' [pressure or fuel], etc...



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[SWComposites]

Have composites gotten to the point that fasteners can be driven into their holes without needing NDI to check for installation issues? > No, No, No. Not unless you use very specific types of interference fit fasteners used for fuel tank installations, and have worked out very precise installation processes. I suspect you either got misinformation from the Boeing person, or they were referring to the specific wing fuel tank fasteners that they developed for 787. But those are not rivets or blind fasteners.

 

[daxman1]

I would like to expand on the use of fasteners in composite material. While working with metallic parts a knife edge was considered a countersink which penetrates more than 67% of material thickness at Boeing and slightly higher at other companies I have worked for. With that in mind is there a general range that would be considered a knife edge for composite material?

 

[Kwan]

I've worked for an OEM that had very good test data on composite joint allowables. It's good to have these joint tests done but quite expensive and is usually proprietary.

I found a couple references that may help:

https://www.niar.wichita.edu/coe/NC...d_fastener_clearance_effects_Jeff_Hendrix.pdf

http://www.dtic.mil/dtic/tr/fulltext/u2/a199171.pdf

 

[ESPcomposites]

daxman1,

Here is an excerpt from Chapter 11 of the book "Practical Analysis of Aircraft Composites"

The countersink depth should not be more than 70% of the laminate thickness (Ref 40, 11). This prevents knife-edging, excessive reductions of the bearing strength, detrimental effects to the fastener, and reduced fatigue performance. A slightly more conservative approach is to allow the countersink depth to be no greater than 2/3 of the laminate’s thickness (Ref 42, 43).

11. CMH-17-3G, Composite Materials Handbook, Volume 3, Chapter 7, 2012.
40. Military Handbook MIL-HDBK-17-3F, Composite Materials Handbook, Volume 3,Chapter 12, June, 2002.
42. Walker, T.H., Minguet, P.J., Flynn, B.W., Carbery, D.J., Swanson, G.D., and Ilcewicz, L.B., “Advanced Technology Composite Fuselage-Structural Performance,” NASA Contractor Report 4732, April, 1997.
43. Kassapoglou, C., Design and Analysis of Composite Structures, Chapter 11, John Wiley and Sons, Chichester, West Sussex, 2010.

Brian

http://www.espcomposites.com

 

[WKTaylor]

Composites have driven use of 120-Deg and even 130-Deg flush-dome-head [solid, blind] fasteners that minimize Csk depth... hence maximize shank bearing and spread tension load over thinner/flatter fastener heads. Also includes special formed-tails and collars that prevent crushing the fibers around holes during install.

Controlled fastener-shank swelling within the holes also allows improved engagement to fiber ends within the hole bores.

Clean/solid holes are mandatory for good fastener performance.

I still prefer metallic-composites construction where composites skins are bonded to [titanium] 'picture frames' that are then conventionally fastened/adhesive-bonded together [with well-established practices] into major segments/assys.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[daxman1]

If I may I would like to continue on with metallic vs composite fastener issues. While I worked on the C-17 we preferred 2.5D for ED but would buy down to 1.5D if it was not in a high stress area. Other companies and aircraft I worked on were looking for 2.0D but again would buy down to 1.5D if not in a high stress area.

The last A/C I worked on was all composite and they required 3.0D but would buy down to 2.0D for their structures. In general, is that the norm for composite AC mainly in the fuselage?

 

[rb1957]

that's my experience too. but "buy" requires some supporting data; 3D is typical for composites (and I assume there's plenty of data for this), allowing 2D as a repair or manufacturing NCR/MRB would require some data (or some hand waving, eg low loads in this specific area, ...)

another day in paradise, or is paradise one day closer ?

 

[ESPcomposites]

Regarding the original design, from "Practical Analysis of Aircraft Composites"

----
The following recommendations may be considered for the initial design, but must be validated by testing. To prevent shearout and cleavage-tension failures, there are various recommendations for the edge and end distance ratios. One suggestion is to use an edge distance ratio of 2.5 and an end distance ratio of 3.0 (Ref 1) Another recommendation is for both the edge and end distance ratios to be at least 3.0, which prevents shearout and cleavage-tension failure modes (Ref 17, 30). For the ATCAS program, an edge and end distance of 2.5D plus a manufacturing tolerance of 0.050 inch (1.27 mm) was used (Ref 42); the same approach is suggested in another publication (Ref 43). Other recommendations are for the edge distance ratio to be at least 3.3 (Ref 11) at least 2.5 (Ref 41) or 2.5–3.0 (Ref 39) Note that for metals, it is standard practice to use an e/D of 1.5–2.0 (Ref 39) but 2.0 is commonly used for the original design since this allows for repairability, manufacturing defects/tolerances, etc.
----

1. Nelson, W.D., Bunin, B.L. and Hart-Smith, L.J., “Critical Joints in Large Composite Aircraft Structure,” NASA Contractor Report 3710, Contract NAS1-16857, August 1983.
17. Garbo, S.P. and Ogonowski, J.M., “Effects of Variances and Manufacturing Tolerances on the Design Strength and Life of Mechanically Fastened Composite Joints,” Volumes 1, 2 and 3, AFWAL-TR-81-3041, April 1981.
30. Tan, S.C., “Analysis of Bolted and Bonded Composite Joints,” WR-TR-92-4084, Interim Report, September, 1992.
39. CMH-17-3G, Composite Materials Handbook, Volume 3, Chapter 2, 2012.
41. CMH-17-3G, Composite Materials Handbook, Volume 3, Chapter 14, 2012.
42. Walker, T.H., Minguet, P.J., Flynn, B.W., Carbery, D.J., Swanson, G.D., and Ilcewicz, L.B., “Advanced Technology Composite Fuselage-Structural Performance,” NASA Contractor Report 4732, April, 1997.
43. Kassapoglou, C., Design and Analysis of Composite Structures, Chapter 11, John Wiley and Sons, Chichester, West Sussex, 2010.


Brian

http://www.espcomposites.com

 

[WKTaylor]

Also...

AGARD-CP-427 Behaviour and Analysis of Mechanically Fastened Joints in Composite Structures



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]