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Fastener edge margin on composite parts

Fastener edge margin on composite parts

Fastener edge margin on composite parts


For composite part design; as far as I know, there's a requirement of 2.5D edge margin. For metallic part, the edge margin is defined as the distance of fastener hole center to the nearest edge of part, but for composite part, how do we define this E/D? In addition, for the composite sandwich panels, there's core and fiberglass ply lay-up, how do we know what's the min. edge distance of the fastener to the the edge of core ramp, and staggered ply drop since I could not find any information about this requirement in design handbook or standard.

Thanks for your inputs.


RE: Fastener edge margin on composite parts

Edge distance is defined in the same way as for metal parts.

There should not be ply drops between the hole and edge of the panel. Edge distance into the panel is (conservatively) defined as the distance to the first ply drop.

The minimum edge distance for composites depends on your test data. Typically the minimum edge distance is 2.5D to 3.0D. Anything less than 2.5D runs the risk of undesirable failure modes and hole damage/wear under fatigue loads.

RE: Fastener edge margin on composite parts

Embedding a metallic [typ Titanium] edge member in the laminate, or installing FTI GromEx Hole [CX'ed hole reinforcement sleeve], adds tremendous bearing/tear-out resistance to fastener holes in composite structures.

Regards, Wil Taylor

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RE: Fastener edge margin on composite parts

2.5D? Doesn't sound like enough, and what about the laminate's geometry? Shouldn't fiber orientation play a role?

Bearing strength data for laminates is rare, and all I have handy right now is for the most popular carbon and glass fabrics. This data doesn't give an allowable for 45 or 90 degrees, which I'd expect to be different from the 0 degree allowable for bearing. Any laminate panel will likely be a mixture of orientations of ply, so I would expect a minimum edge distance to be a conservative value, to be safe at any orientation. Or do the calculation for every single rivet/laminate combination...

Is any test data available?


RE: Fastener edge margin on composite parts

Bearing Strength is a function of:

- Single shear versus double shear joints
- Stabilized versus unstabilized joints
- Installation torque
- Fastener pin and collar/nut
- Environment
- Hole fit
- Countersink or not
- Other

There are various recommendations for edge and end distances to prevent shearout and cleavage-tension failure modes. [R1] Suggests an edge distance of 2.5D and end distance of 3.0D. [R2] and [R3] recommend the edge and end distances be at least 3.0D to prevent shearout and cleavage-tension failure modes. [R4] used an edge and end distance of 2.5D plus a manufacturing tolerance of 0.05 inch. The edge and end distances are defined in figure X. The variable e/D is often used to describe both edge and end distances.

Though bearing strength is a function of fiber percentages, it can often be approximated as a constant value for bolted joints that are approximately quasi-isotropic. Per Ref. [R5], the bearing strength is relatively constant if the 0 degree plies are 20-40%, ±45 degree plies are 40-60%, and 90 degree plies are a minimum of 10%. For the room temperature strengths, table X (not in this post) shows that the dramatically different layups have approximately the same bearing strength (5% different). However, for the ETW condition, the strengths are relatively different. The three tested laminates are the extremes or practical fiber percentages.

NCAMP also has some bearing strength data for popular material systems.


[R1] 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.

[R2] Garbo, S.P. and Ogonowski, J.M., “Effects of Variances and Manufacturing Tolerances on the Design Strength and Life of Mechanically Fastened Composite Joints,” Vol 1, 2 and 3, AFWAL-TR-81-3041, April 1981.

[R3] Tan, S.C., “Analysis of Bolted and Bonded Composite Joints,” WR-TR-92-4084, Interim Report, September 1992.

[R4] 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.

[R5] Military Handbook MIL-HDBK-17-3F, Composite Materials Handbook, Vol. 3, Chapter 6, June, 2002.


RE: Fastener edge margin on composite parts

Thank you ESP! You did a lot of legwork for that reply. Mil-Hdbk-17 was right under my nose, of course.


RE: Fastener edge margin on composite parts

Typically it is 2.5D to 3.0D, but to be conservative, we are using 3.0D.
However, it is recommend to do a test and confirm your data.

RE: Fastener edge margin on composite parts

man, how can we top ESP's post ?

"how do we know what's the min. edge distance of the fastener to the the edge of core ramp" ... i don't think the edge of the ramp is an edge.

"and staggered ply drop" ... i don't think ply drop counts as an edge either.

the "requirement" is "only" a design rule of thumb ... if you want to develop the maximum strength use 3D. you pretty much need to test each design, or develop an extensive database of tests to extrapolate.

another question ... how wide to make edge reinforcement plies ? 6D ?

Quando Omni Flunkus Moritati

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