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Huth Fastener Flexibility

Huth Fastener Flexibility

Huth Fastener Flexibility

Regarding the Huth Fastener Flexibility equation (1) in the ASTM STP 927 paper, has anyone been able to reproduce the results shown in Fig. 15? Specifically, the top plot labelled 'A' which  applies to the double shear specimen. I'm using Eqn. 1 for doubler shear (n = 2) with the specimen geometry shown in Fig. 12, and I'm calculating a first fastener pin-fraction 0.306, while Fig. 15 shows about 0.230. I have good correlation to the Fig. 15 results for the single shear specimen, but I'm way off for the double shear. I'm aware of the typo in Eqn. 1 in the STP paper, and am using the corrected third term (ie. 1/(2t1Ef) instead of 1/(nt1Ef)). Any suggestions would be appreciated.

RE: Huth Fastener Flexibility

I got about 0.26 for the first fastener for E = 10Msi, t1 = 0.05", t2 = 0.05". I could not immediately find what they used for the modulus and thickness, which will affect the result.

You can check it against this free software, which has the Huth equation built in:


For the double shear case, it is moot between the original German and ASTM version. The single shear is where is it mixed up....(or do I have that backwards, haha).


RE: Huth Fastener Flexibility

Forgot to mention something about double shear fastener flexibility, that can be easily be done incorrectly.

Make sure that you are consistent with how you define the fastener flexibility. That is, the Huth paper sets it up to be the total deformation and should represent -1- spring in the compatibility matrix. However, some people like to set it up as -2- springs in the compatibility matrix. As such, you need to adjust the way you interpret Huth's equation (by a factor of 2).

There is a large aircraft company that has been doing it incorrectly. If you are using their software, then this is one likely reason. Though I had used it for years, I only recently discovered this problem. You can check with my software or modify theirs manually with the flexibility adjusted by 2.0 to see. I passed it on to them and hopefully they have, or will soon, correct the problem.



RE: Huth Fastener Flexibility

That was the problem. The flexibility of the fastener for double shear is, as you wrote, based on a single-spring assumption (per Fig. 4 of Huth paper). In my spreadsheet, I model double shear with two springs and the flexibility I was using per Eqn. 1 of the Huth paper requires a factor of two as a result. I've corrected this and can now match the Huth results.

I've been staring at this problem for weeks, and you just saved me from staring at it any longer. Thanks very much.

And thanks for the link to your spreadsheet. Very nice.


RE: Huth Fastener Flexibility

Don't feel too bad. Like I said, a major aircraft company has done this incorrectly as well (and they have tremendous resources and people using/checking). It is really hard to spot unless you go over everything with a fine tooth comb.

I ran into a lot of issues like this while developing the software. It was good timing since I am writing a book on composites. All of this is addressed in the book.


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