Scallop for Weight Savings

[lakanino]

Hi All,

I remember when i was working for Northrop Grumman, the Chief Engineer warned us of using scallops at the joints for weight savings. Essentially, using 2D edge distance and anything beyond should be scalloped if your after weight savings.

I assume it had to do with Fatigue but I don't remember exactly why scalloping is not recommended.

Any thoughts?

Thanks.

 

[SWComposites]

- harder to fab correctly
- no allowance for MRB or repair
- more complicated for fatigue analysis; available fatigue data might not be applicable
- what type of structure (application)?
- what material?
- what type of fasteners?

 

[GregLocock]

You forgot the biggy

"Bugger all weight advantage for effort expended"

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[rb1957]

you might look up the report on Flight International about the trouble minor weight savings caused Airbus.

several OEMs do what you suggest ... scallop a splice strap. my experience has been attaching secondary structure to primary structure where you want a flexible joint (so that the two sides of the joint can strain at different rates, so that the less stiff side will follow the stiffer side, ...).

my point is whilst i agree with the previous posts (how much are you saving ? how much effort ??), some companies will have long experience with this (and so reluctant not to continue); from an OEM's perspective saving 0.1 lbs (even 0.01 lbs) 1000 times on a plane for the cost of some time on the router (and probably the same time as it'd take to rout a parallel strap) is worth it. BUT be careful ... to do this on a primary splice joint would be to invite trouble.

 

[GregLocock]

When designing lightweight structures I found that it was much better to optimise the thickness of the base structure and then locally reinforce, rather than starting from a thicker structure and removing material where possible/practical/worthwhile.

However I suspect the answer depends very much on details.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[rb1957]

often the thickness of a splice strap (to secondary structure) is dictated by standrd sheet thickness, rather than sized for load.

 

[WKTaylor]

Caution...

Scalloping the edge of a frame, stiffener, longeron, doubler etc is NOT the same as fingering the ends of a doubler or skin.

Scalloped edges allow a thicker part to shed some weight while maintaining thickness for fastener bearing. HOWEVER, the problems with scalloping the edge of a any metal part are multi-fold...

Precisely scalloping an edge to a match a fastener pattern not yet installed can be dicy without precision tooling. Scalloping to match a pre-existing fastener pattern is tedious at best.

Now comes the zingers.

A wavy edge cannot carry significant lengthwise tension or compression without significant increases in peek-to-valley stress concentrations... or compression buckles concentrating at wave valleys.

Transverse bearing-tear-out occurs on a 40 degree angles from hole center [classic Bruhn]. Scalloped edges reduce tearout strength by reducing the hole-edge-to-free-edge distance.

Wavy edges are significantly longer than a straight edge (duhhhh). However cracks tend to form at edges; so the longer the edge, the greater the potential for cracks

Regards, Wil Taylor

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