Maximum Doubler Thickness Ratio to Web 1

[sdiguana]

I am doing repair doublers for a flight test vehicle, and the rule of thumb as I understand it is that a single sided doubler should be greater than 2x the thickness of the web. As far as I understand it, this is to prevent excessive eccentricity of the built up section.

My question is how does this rule change if the doubler is double-sided? (e.g. sandwiching the web) 4x thickness? Eccentricity is removed, but if it gets too thick its going to be a hard-point. I'm inclined to say that the limit is just the sizing of the perimeter fasteners since they'll take most of the load.

Lastly, is there any reference I can go to for the thickness ratio, I've never seen it in my texts, but I may not be looking in the right spot/books.

Thanks!

 

[Sparweb]

"...greater than 2x the thickness of the web. "

Who told you that?

I suggest you look through the Structural Repair Manual for this aircraft, or one that is similar, to get a feel for the proportions of materials to use in repairs. There is a lot of "tribal knowledge" in this subject, sorry, so even the SRM isn't really enough. But it will have numerous repair examples illustrated and materials detailed for many structural shapes, skins, channels, angles, etc.

Repairs to cracked structure are different from repairs to crushed or buckled structure. Repairs to aluminum structure look different from repairs to steel or titanium. Composites, and repairs to them, are a whole subject unto themselves. Any one of these cases could use a "doubler". Please understand I'm not being picky, I'm just afraid it's too easy to assume you're doing one thing when maybe you mean another.


STF

 

[sdiguana]

This is for a flight test vehicle, we are still in production so there is no SRM. I am working spar and rib webs since I left it out on the previous post. The doublers are due to adding penetrations for flight test sensors and conduits.

The customer wants to turn one or all of the FTV into revenue items. So we are doing for full static fatigue and DT.

 

[sdiguana]

Oh and another side question... I am finding (some) webs are being allowed to buckle at ultimate, if pristine. However since flight test is making holes of them is it enough to recover the webs original capability or does it really need to be supported such that the web won't buckle at ultimate. I am told the reason the do not let production webs with cut outs buckle is concern of leaking fuel.

 

[rb1957]

i think the rules of thumb about dblr and web thicknesses are based in practicality ...

the load into the dblr is coming through the web. why make the dblr X times the web thickness ? i'd be surprised at any design that wanted a dblr much thicker than the web it sits on.

i really don't understand "the reason they do not let production webs with cut outs buckle is concern of leaking fuel" ?? what the relationship between webs with cut-outs and fuel leakage ?

yes, certainly webs are allowed to elastically buckle at ultimate, or to develop elastic diagonal tension buckles.

Quando Omni Flunkus Moritati

 

[LiftDivergence]

I think the answer is that the doubler needs to be thick enough to recover whatever strength you have lost. Make sure the load path is no weaker than the original. Rules of thumb can be useful, but you should just do some analysis to size the part. Thicknesses are dependent on a number of factors including the material you choose.

Maybe I'm missing something, but I also don't understand the initial statement that doublers are generally greater than 2X the original thickness to prevent excessive eccentricity. Assuming the doubler is mechanically fastened, for a single shear joint, the eccentricity would increase with part thickness.

Keep em' Flying

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