Fastener Stiffness at Bolted Joint 3

[lenosrep]

In order to model a major aircraft joint to correctly represent its stiffness in a FE model. I have been trying to find a suitable method for calculating fasteners stiffness. After evaluating methods used by major airframers and nasa pubs. It is not an easy choice to pick as they all come up with very different answers. The Boeing effective thinkness method seem to give a conservative number but I am not able to find any publication that it is based upon.

Any suggestion is appreciated.

lenosrep

 

[rb1957]

if you're talking about shear stiffness, Michael Niu's book describes an approach. I prefer the approach of Huth, who's published some papers on the topic ...

 

[MNLiaison]

See Heimo Huth's paper, "Influence of Fastener Flexibility on the Prediction of Load Transfer and Fatigue Life for Multiple-Row Joints" which can be found in Fatigue in Mechanically Fastened Composite and Metallic Joints, ASTM STP 927 dated 1985.

I purchased my copy through the ASTM website. The fastener flexibility equation that was derived was compared against Tate and Rosenfeld, Douglas, and the Boeing equations for flexibility. It was found that the Huth equation was the most accurate for predicting individual fastener loads in a joint.

Regards

 

[rb1957]

here's a link a found (yahooing "huth +fastener") ...

http://perso.orange.fr/Jas-n-Geoff/Geoff/thesis/thesis.html

 

[lenosrep]

There are two methods quoted by the Boeing manual. Huth's method was one of them. When I compared Huth's method with the effective thinkness method (which is in the same Boeing manual). The later one predicts greater stiffness and hence more conservative for the joint that I was looking at. But I don't seem to find the literature for this method.

 

[rb1957]

in the greater scheme of things, i doubt that it matters which formula you use, in part 'cause there's so much variation between formulas, in part 'cause no formula will accurately predict your specific design, in part 'cause there should be a reasonable amount of redundancy in the joint.

i'd be a little careful with "conservative". assumptions that are usually conservative on one part of a design are usually unconservative on another.

 

[lenosrep]

Good point! Thanks for the comments!

 

[SWComposites]

The effective thickness method is based on Boeing Proprietary documents and data, so you are unlikely to find anything in the open literature.

Higher fastener stiffnesses are not necessarily conservative. None of the published fastener flexibility methods are accurate; however, large differences in flexibilty values usually produce only relatively small differences in joint load distributions. But, if you are sizing a joint to zero MS using a single set fastener flexibility values, you are just kidding yourself that the MS prediction is accurate. Critical joints should be designed using a range of flexibilities giving a range of load distributions ("overlapping assumptions").

Further, if you are using an FE model of the joint, you need to calibrate the FE model idealization and stiffnesses using a single joint configuration so that it matches the closed form fastener flexibility equations. You cannot directly calculate stiffness values for the FE model from the fastener flexibility formula. The calbration must account for the type of elements in the straps (shells, solids), and elements used to model the fasteners (rigid elements + springs, beams, bars + springs, etc.)

 

[rb1957]

i've used zero length spring elements to represent fasteners, using the stiffness from an equation ...

 

[40818]

Test Test Test.

The only way you will ever get accurate fasteners stiffnesses is to carry out testing on an accurate structural representation.
One point to note though, if you are working on a boeing aircaft, use a boeing method, and likewise for other manufacturers (lockheed, airbus etc)applying the methods they will have used will be closer to the overall aircraft strutcural response than mixing and matching.

 

[tbuelna]

lenosrep,

If you're working on a commercial passenger aircraft, review the appropriate FAR.

You should also consult your DER. He/she will give you guidance on how they will want to see your analysis presented.

Finally, most design specs require a higher MS for validation by analysis than they do for validation by test.

Here's a link to NASA's fastener design manual:

http://gltrs.grc.nasa.gov/reports/1990/RP-1228.pdf

Good luck with your project.

 

[737eng]

In the Huth formula for fastener flexibility (Huth on Influence of Fastener Flexibility). If you have a single shear joint with two different sheet thicknesses (t1 and t2), which sheet thickness do you utilize for t1 and t2(i.e. the thin sheet for t1 and the thicker sheet for t2 or vice versa). Due to the 1/2nt2E3 factor in the right hand side bracket, this multiply by two in the denominator can make a difference depending on which thickness you utilize. Also, since this equation is also good for double shear with n=2 in lieu of 1, I am wondering if this 2 is a typo. My example: I have a 0.056" thk skin doubler on a 0.050" thk skin, I am obtaining my fatigue stress from the original skin which I am making t1, the doubler, which is thicker, is then t2.

 

[crackman]

I assume that all of this is in support of load transfer determination for the purposes of fatigue analysis. I have spent the better part of 20 years using various methods and correlating them. I have tried them all starting with Tate and Rosenfeld, Boeing, Lockheed, Grumman, etc. All are empirically derived. The Huth equation is by far the best that is commercially available. However, I would caviat this with the fact that it is not very accurate for very thick stackups with large steel or titanium joints (note limitations based on hardware used in test). Basically, its good for sheet metal joints in transport type construction but not so good for very robust joints such as those used in fighter type structure. Also, make sure to account for multiple layers or gaps by using parallel springs as well. For example, if there are 3 layers, there is a stiffness between each layer but also between the combination of two with the third layer. Grumman had a very good method for doing this and was included in all their development work and I believe some was even published in some public domain USN documents.
Finally, if you are analyzing joints for fatigue, the stiffnesses you use can have a large impact on the analysis. This is particularly important when analyzing large repairs with multiple layers as you could end up analyzing the wrong part.
Good luck