Stuart & Kathryn Fields wrote:

Don: Reading in a text book on Fatigue design for aluminum structures,
there is at least one design technique that assumes that there are cracks
inherent in the material to start with that are not readily detectable by a
visual inspection. The design philosophy progresses from there. This whole
area of study is explaining why the time life specs on parts can be very
meaningful. It also can allude to the excessive safety factors that can be
used in determining time life specs.
It looks like I've got a lot more studying to do in this area.

Hi Stuart,

If you've ever looked at aluminum under a high
power microscope you have seen that it is not a
smooth material at all, but consists of "grains"
of material more or less mashed together. If it
is alloyed with copper, you can see the copper
grains around the edges of the aluminum grains.

Fatigue of material is an interesting subject. Do
you remember the problems introduced at the drive
shaft on the Rotorway 162F when people started
changing out the chain drives for belt drives?

The chain drives needed an oil bath which required
oil seals, and a housing all the way around the
chain. Since the only way to change the bottom
seal when it started leaking is to remove the
housing, and that requires removing the chain and
drive sprockets it seemed that replacing the chain
sprockets and chain with a gilmer belt and
tensioner was a good idea--at least until the
drive shaft from the motor started failing right
on the other side of the support bearing.

The problem was/is that the drive shaft is
supported by only one bearing below the drive
pulley, and the gilmer belt must be kept in much
more tension than a drive belt. Consequently the
end of the drive shaft is bent ever so slightly by
the belt tension. This would be okay except that
when it rotates, the direction of the bend keeps
changing. The effect is like bending a piece of
metal back and forth 50 times per second. Even
the fairly thick drive shaft failed in a hundred
hours or so given that treatment. The failures
puzzled everyone at first, and they called it
"fretting fatigue"--I guess because the designers
were fretting over it ;-)

Don W.