Scalloping effect on fatigue critical parts

[banag]

From fatigue aspect, is it possible that scalloping the components at free edges would be more critical than non-scalloped component?
Assume, minimum edge distances of holes are maintained, it is well designed that no stress concentration interactions between fastener holes and scalloped portion (would be like a notch), and scalloped portion stress concentration also under control. In that case can we go for scalloping for all the fatigue critical components without any hesitation (which would reduce more weight)? Or is there any other hidden risk?

 

[rb1957]

sure I think you could radius the straight edge between fasteners. I think you'd add something to the cost and save a very small amount (of weight). and you might need to think about verifying the result ... have you increased the stresses around the fasteners ? panels often have a significant amount of shear along the edge.

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

 

[tbuelna]

The operative phrase in your OP is "fatigue critical components". The FAA requires special procedures with any Fatigue Critical Structure (FCS). So before you "....go for scalloping for all the fatigue critical components without any hesitation....." you may need to consider regulatory requirements such as FAR part 25.

Here is a good reference on the subject:

http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC 25.571-1D .pdf

Hope that helps.
Terry

 

[rb1957]

2nd thought ... should you be trying to save ounces/grams on "fatigue critical" parts ?

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

 

[tbuelna]

That's a great point. The full cost/benefit of implementing the change must be considered. If it saves say 10 ounces per airframe, and the total non-recurring and recurring costs to implement the change work out to $200 per aircraft ($320/lb), that might be considered cost effective for many commercial aircraft programs.

 

[banag]

Thanks for the answers...

The increment in cost for having scalloped edges might vary for types of manufacturing?. For example, having a scalloped edges in a machined doubler comparing to a sheet doubler. Honestly, I have no clue about how much it will cost involves for making a component. And, My question is mainly focussing on initial design of a component rather than MOD.

As I don't have much experience, I am not able to understand the consequences happens, if I go for scallop. I mean, the analyses says OK to go for scallop for FCS, but, little scared if it goes for test.
For example, cabin window surround stiffener (forging) analyses are OK to scallop the free edges which would save a lot of weight (across all the windows). But, this is one of the most fatigue critical location, so the strong argument is "do not experiment, can't afford, instead stick with conventional design i.e. no-scalloping".

Deviation from original question: how would it be, if we rank the FCS based on fatigue loading for jet aircraft? Off course, FAA advisory circular gives the list of FCS, not the order of most criticality.
For fuselage, 1) emergency exit door cut-out surround above wing 2) cabin window cut-out surround 3) wind-shield stiffeners ...

 

[rb1957]

you're at an OEM ? The problem I have with this is that we carve out weight from critical primary structure 'cause we're doing a bunch of analysis on these parts; but possibly much more weight could be saved from secondary structures ?

sure, you can save a little weight on a window frame scalloping the edges; expect people to marvel at your intensive design or to mock you mercilessly ... it's the way it goes ! but you could probably save more by thinning the thickness slightly.

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

 

[rb1957]

here's an example of what you're suggesting ... this is an overhead bin brkt.

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

 

[tbuelna]

The increment in cost for having scalloped edges might vary for types of manufacturing?. For example, having a scalloped edges in a machined doubler comparing to a sheet doubler. Honestly, I have no clue about how much it will cost involves for making a component. And, My question is mainly focussing on initial design of a component rather than MOD.

From my last post: "...The full cost/benefit of implementing the change must be considered........the total non-recurring and recurring costs to implement the change....".

Non-recurring costs are typically one time expenses like engineering/analysis, tooling/fixtures, etc. Recurring costs are those associated with producing each component like raw material, processing, QA, etc. For example, more expensive machining operations for a scalloped flange would mean higher recurring costs. While for an FCS the added expense for more extensive fracture and fatigue analysis work required by the scalloped flange would mean higher non-recurring costs. For an FCS there would also be added recurring costs for the extra QA required, such as documentation/traceability of materials and processing, and special inspection procedures during manufacturing.

On one hand, I agree with rb1957 that when it comes to weight reduction efforts it is always best to go after the "low hanging fruit" first. And on every production aircraft design I have ever seen there were plenty of opportunities for reducing weight in non-critical areas. I have worked on a couple commercial aircraft programs and there was usually some cost/lb metric for implementing a change. It was lower on short haul aircraft than on long haul aircraft, but on a short haul commercial jet it might be something like $300/lb.