Tension Joint Fatigue Analysis 5

[RWilco]

Greetings,

We recently experienced a bolt head failure in a tension loaded installation not unlike the wing root for an F-16.

I'm looking for fatigue analysis methods for tension loaded joints. The NACA reports appear limited to shear/double shear joints.

 

[joekm]

You might try AFGRO [

http://afgrow.wpafb.af.mil/index.php

], they have a formulation for a crack growing into a cylindrical axially-loaded rod.

--
Joseph K. Mooney
Director, Airframe Structures - FAA DER
Delta Engineering Corporation

 

[crackman]

Dear RWilco

You have hit the same wall many many many other fatigue/DTA engineers have hit over the past 30 years. First off, the classic tension fitting analysis used by everyone is the Lockheed bathtub fitting method in their SMM. This method has been copied by everyone including Boeing (its in their own manual verbatum). Any engineer in the industry will tell you that its the only accepted method used by all, for static strength.

Herein lies the problem, the method determines allowable loads for margin calculations but not detailed endpad or sidewall stresses for fatigue analyses. The only attempt at creating a method to develop stresses in tension joints for fatigue purposes that I know of was by a friend of mine at McDonnell Douglas in St. Louis about 15 or 20 years ago. They performed several FEMs with various boundary constraints and I believe they ended up testing as well. You must know that the solution is not a trivial one primarily since the stresses and deflections are heavily dependant on the amount of torque up as well as contact surface between the two fittings.

FEMs can be used but with great caution and in the end the best solution is just to test, test, and test some more.

I have had several experiences with both wing to fuselage tension fittings as well as engine mount tension attach fittings, none were ones I would particularly call good experiences.

Good luck

 

[CoryPad]

You need to review the last references shown here:

faq725-600

There is a lot of analysis of fastener failures, including using AFGRO.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.