On Sep 29, 10:40*pm, jim beam wrote:
On 09/29/2009 03:15 PM, Jobst Brandt wrote:

Jay Drew wrote:

What is feathering?

A fading out of one cross section into another where they meet:

*http://www.gtgtandems.com/tech/brazing.html

wrong. *feathering is simply filing the edges of the lug.



The sudden step in cross section causes a stress concentration that
leads to cracking.

which is why it's not a straight cross section, it's got curved features
that mitigate - effectively radius reduction, a standard fatigue
mitigation technique.

Not surprisingly, jim beam has things exactly backwards. When
blending a transition from a larger sized piece (like the lug) to a
smaller sized piece (like the fork blade) the idea is to increase, not
decrease the radius. The ultimate in radius _reduction_ would be a
sharp corner, which generates a high stress concentration. Large
radii are the opposite and generate lower stress concentrations.

You can play around with different radius values at
http://www.fatiguecalculator.com/cgi-bin/findkt.pl
to investigate this.

For example, for D=25 mm, d = 23 mm and r=1 mm, the stress
concentration factor is 1.84 - that is, peak stress in the area of the
shoulder will be 1.84 times the stress further away. If you reduce
the radius (as jim seems to recommend) to 0.1 mm, the stress
concentration factor jumps to 3.52

- Frank Krygowski