Bearing Allowable for Graphite/Epoxy Laminate 3

[jargon5]

Anybody have general information on bearing allowables for graphite/ epoxy laminate with 60% uni-directional fibers and 40% +-45 fibers? I generally use half of the compressive strength, approximately 60 ksi, but I would like some good documentation.

 

[SWComposites]

60 ksi is probably in the right ballpark. However, bearing strength is a function of the specific material, layup, temperature, fastener clamp-up, fastener head style, joint configuration, laminate thickness, hole diameter, edge and end distance, etc. Without specific info there is no way to accurately estimate a value. Also, scaling off of the compressive strength is a dangerous assumption and should not be relied on. Having no 90 degree fibers in your laminate will hurt the bearing capability, especially if the edge margins are short.

 

[eap1782]

Remember that bearing strength in composites is also a function of the bypass stress, i.e. you must interact the bearing stress with the bypass stress to calculate the MS of a fastener detail in a composite laminate. The results can be *very* unconservative if you do not, especially if your bypass stress is compressive.

 

[RPstress]

A typical (average) RT dry bearing strength could be as high as 800 MPa (115 ksi) for a good modern unidirectional material with a finger-tight (1.3 N.m, 12 in-lb) fastener. However, by the time you've got reductions for hot wet and B-basis in there it'll usually be below 600 MPa (87 ksi). Bearing is normally good for 1/3 to 2/3 45 plies. One material I used was a 55% Vf five harness satin weave epoxy/old-style fiber (T300), and this had a hot(100 deg C)-wet B-basis allowable of 56 ksi, which was found from plain pin tests (not even finger tight). I've not seen one lower (yet), so I use this as a lower bound for intial sizing. Making it just finger tight raises the allowable significantly, and if you can guarantee a decent torque existing when it's loaded you get quite a bit more.

Finding an allowable outside of an actual qualification programme (which always generate confidential data) is hard. MIL-HDBK-17 has some data on AS4/997. This comes out at high 60's ksi at 180 deg F.

As eap1782 says, any real joint should have a bearing/bypass analysis. MIL-HDBK-17 can help with that as well.

MIL-HDBK-17 is currently about 22 Mb in .pdf format, though volume 2 with the allowables data in it is "only" 3.6 Mb.

 

[WKTaylor]

jargon5.

I heard (a few years ago) that there had been some work to increase hole bearing strength/durability dramatically. The concept is to install bushings in composite structure holes [graphite and Boron... not glass or kevlar] using something like the FTI Force-Mate process... except tailored to the explicit composite structure and CRES bushing employed. The CRES bushings are expanded in-place and "lock" into the exposed ends of the fibers. This allowed the bushing to transfer much higher bearing loads into the structure... approaching the limits of the metal.

Regards, Wil Taylor

 

[SWComposites]

Wil - Work is currently being done to develop expandable bushings and fasteners in composites. However, there are still lots of issues related to hole tolerances and maintenance of interference clamp-up when subjected to environmental and load cycling to be evaluated before this technology can be used.