[Harp-L] Fatigue and reed life

["Aongus MacCana" <amaccana@xxxxxxxxxx>]

As I remember from second year engineering school, susceptibility of a
material to fatigue failure  could be predicted by a "notch sensitivity"
test. A test where an ingot of the material to be tested has a standard
notch machined out of it and is given a whack by a standard pendulum to see
what it takes to break it off at the notch.
A fatigue crack is most likely to start at a surface imperfection or a
sudden change in section of a component e.g. injudicious scraping with the
tuning file or chisel (especially across the reed!)or at the junction of the
base of the reed where it's section is reduced after the rivetting flange.
This suggests that 'tuning passes' would be best made along the reed rather
than across it and a rotary grinding tool might give less surface
discontinuity than the regular file or scraper does.
Hot rodders have a fetish for highly polishing highly stressed components
like engine conrods to try and ensure that there are no surface
imperfections or stress raisers to breed fatigue cracks. They also compress
the outer surface of such components by shot peening them (bombarding them
with steel shot)but this does not look like a feasible strategy for
harmonica reeds.
Aongus Mac Cana