Fastener Stiffness at Bolted Joint
Fastener Stiffness at Bolted Joint
(OP)
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
Any suggestion is appreciated.
lenosrep





RE: Fastener Stiffness at Bolted Joint
RE: Fastener Stiffness at Bolted Joint
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
RE: Fastener Stiffness at Bolted Joint
http://pe
RE: Fastener Stiffness at Bolted Joint
RE: Fastener Stiffness at Bolted 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.
RE: Fastener Stiffness at Bolted Joint
RE: Fastener Stiffness at Bolted Joint
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.)
RE: Fastener Stiffness at Bolted Joint
RE: Fastener Stiffness at Bolted Joint
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.
RE: Fastener Stiffness at Bolted Joint
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.
RE: Fastener Stiffness at Bolted Joint
RE: Fastener Stiffness at Bolted Joint
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