3D fastener Pitch for Single Row Joint

[Ryan_aero]

I've typically used 4D fastener pitch for single row CFRP joints. The following reference discusses 3D pitch as optimal to maximize the joint strength.


"Critical Joints in Large Composite Aircraft Structure"

https://ntrs.nasa.gov/api/citations/19870001540/downloads/19870001540.pdf

What is spacing is recommended to baseline for design? 3D feels pretty aggressive so I'm curious how often that low of spacing is used.

If 3D spacing is used - it's conservative to write a pure bearing margin even though net section failure is starting to play a role?

Thanks!

 

[SWComposites]

No, no, no. Don't pay any attention to that old report; it was based on rubbish assumptions and ancient material properties.

Aircraft bolted joints should be designed to be bearing critical NOT bypass (net section) critical. The former allows load redistribution, the latter is a two part fracture that will propagate across the joint leading to separated parts.

Fastener pitch spacing perpendicular to the primary bearing load direction should be 5D to 6D (and these values are on the borderline of between bearing and bypass critical. Fastener spacing between rows (parallel to primary bearing load) should be 4D minimum.

In all cases, both bearing and bypass (net section) margins must be calculated. Bearing is usually critical hot/wet; tension bypass is critical either cold/ambient or hot/wet depending on material, layup, etc. Compression bypass is hot/wet critical.

By the way, I completely rewrote the CMH-17 Vol 3 Chapter 11 Bolted Joints for Rev H (which might get published in about a year).

 

[Ryan_aero]

I'm glad I asked. To confirm - the 5-6D spacing applies to single row joints? I was under the impression tighter spacing could be used since there is no bearing-bypass interaction.

I would love to read through the CMH-17 update once it's available.

 

[SWComposites]

Yes, applies to single row joints.
True, in a single row lap joint, there is no bypass load that goes purely past the joint to another fastener row, BUT, a single row joint loaded in tension reacted bearing can still fail in bypass (net section) if the fastener spacing is small enough. The failure mode cutover spacing is around 5D, plus/minus.
A single row joint loaded in compression reacted bearing will only fail in bearing (there is zero stress at the side of the hole)
A single row joint loaded in shear reacted bearing is similar to a tension reacted joint.

 

[Ryan_aero]

Ok copy, that makes sense. Is tension bypass something you would check for a single row joint? or is bearing forced to be the critical failure mode by using >5D spacing? I imagine a net-section check would have to be rooted in test data?

 

[SWComposites]

If your joint pitch is equal or greater than that used for the bearing strength test specimens, then only a bearing check should be needed.
If the pitch is less than in the bearing test specimens, then a net section check may be needed.
Yes, test data is needed for the net section check.

 

[Ryan_aero]

Makes sense - thanks for the insight!

 

[Ng2020]

SWComp: you mention separate bearing & bypass checks. My recollection is that bearing/bypass interaction is critical in many cases. If you use the bearing test coupon pitch as a design criteria, would that satisfactorily cover interaction failire modes? What if joint loading includes a shear component ?

 

[SWComposites]

NG - yes there are two checks - one for bearing failure mode, using P/dt stress and bearing allowable, and one for bearing/bypass interaction for a net section (bypass) failure mode.

The bearing coupon can be used as a design criteria for a single row joint under tension/compression loads perpendicular to the part edge, since it inherently also covers the bearing-bypass interaction (bypass) failure mode. For a shear loaded single row joint, ideally one has shear reacted bearing test data, though this can be somewhat difficult to achieve (but has been done, though there isn't an ASTM or ISO standard available).