Shear strength of 2117 aluminum rivets 2

[Abbyyyy]

Greetings, everyone!

This is my first time starting a thread, and I'm hoping to receive your help.
I've encountered a problem: commonly used solid rivets, like NAS1097/NASM20426, etc., all have rivets made from 2117 aluminum alloy, otherwise known as AD rivets. According to the technical specification NASM5674, the shear strength of AD rivets is required to be 26KSI. However, after researching and testing actual products, the real shear strength is essentially between 33KSI and 34KSI, exceeding the specification requirement by 23% to 26%. Does anyone know how the shear strength requirement for solid rivets in NASM5674 is established? Why is the specified value in NASM5674 so much lower than the actual ones? Thank you!

 

[WKTaylor]

Bearing-tear-out calculations always have to be performed, for sheet metal alloy/temper + thickness... this is how fasteners are appropriately sized for structures.

Regards, Wil Taylor
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[SWComposites]

Yep we all concur that only checking fastener shear using Fsu for fastener material is not sufficient.

 

[verymadmac]

While we are talking rivet strengths does anyone apply a knock down factors when using less than 7 rivets (I think that's the number) to attach flight critical structure. If thinking behind it is that 7 rivets is the statistical min population to use a B value (I haven't read the memo for years). Its probably an occasional GA issue but I have never seen it done. Its cover in the lockheed stress memo manual and apparently no where else.

 

[LiftDivergence]

While we are talking rivet strengths does anyone apply a knock down factors when using less than 7 rivets (I think that's the number) to attach flight critical structure. If thinking behind it is that 7 rivets is the statistical min population to use a B value (I haven't read the memo for years). Its probably an occasional GA issue but I have never seen it done. Its cover in the lockheed stress memo manual and apparently no where else.

Depends on the data source. For transitional joint strength data such as that reported in a structural repair manual, I would think the reported values would already have statistical knock-downs applied to them to make them B-Basis or A-Basis.

If not, I would think you'd need a lot more than 7 fasteners to get to a B-Basis without a big factor. Remember the one-sided tolerance interval, or k factor (we wouldn't care about overpredicting strength), is based on the the population minus the degrees of freedom. So when you look up the value you're going to be using n-2 = 5 which will give you a pretty bad result to go from mean strength 50/50 to 90/95.

Do you happen to have the Lockheed Memo number??

Keep em' Flying
//Fight Corrosion!