Stress Concentration Factor on Fastener Holes 2

[mavechin81]

Hi, Does anybody know what is the stress concentration factor for the countersunk fastener in matellic and composite? Or how do we define or calculate that?

Thanks!

 

[rb1957]

definition is easy (just like any other Kt).

calculation, ah, there's the rub! the CSK creates it's own analysis problems, but there are plenty of papers on that. if there's load transfer, well that's another analysis problem, with fewer open sources references.

Quando Omni Flunkus Moritati

 

[RPstress]

Peterson is the usual stress concentration reference. While neither the original from 1974 nor Pilkey's second edition cover orthotropic materials there is one mention of orthotropy in the contents of the third edition. I hadn’t noticed Ed. 3 come out so I don’t know how well it covers composites. There’s no sign of countersunk holes in Ed. 2 either, but Ed. 3 claims to cover them. Ed. 3 contents is at

http://eu.wiley.com/WileyCDA/WileyTitle/productCd-0470048247.html.

For a composite industry practice has been to use a standard formula for a hole in an infinite orthotropic plate and apply simple compensation for a finite-width strip of material.

Failure of a composite is estimated by a ‘point stress’ criterion or an ‘average stress’ criterion. A small hole in a composite will withstand a higher gross stress than a large one. The point or average stress criterion can be used as a modifier on the basic stress concentration, so that the effective stress concentration is dependent on defect size. It involves calculating a stress a short distance away from the hole. This effect is highly material dependent, so it needs test results to quantify it for each fiber/resin combination and hole size.

For a countersink I look at the big hole (top of csk) and small in a finite width strip and take the worst (it’s always the big one).

For most metals yield ensures that in the presence of holes the net stress at static failure is at least 90% of UTS and often 99%. Fatigue is another matter and needs time-consuming test data.