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  #11  
Old April 11th 07, 02:27 PM posted to rec.aviation.rotorcraft
Don W
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Posts: 52
Default control failure



Stuart & Kathryn Fields wrote:
Don: Be sure to share what you find. I think that I'm only scratching the
surface of some stuff. One thing I found and had never seen before was an
equation relating the increase in stress due to a crack. It scares the hell
out of me. I'm afraid to use a metal fork in my salad. The equation
basically says that the max stress is 2 times the load divided by the area
times the square root of the crack length divided by the radius of curvature
of the end of the crack!!! If the radius of curvature was equal to the
crack length, the max stress is already twice what you would calculate using
the applied load and the element cross section. Now put a reasonably sharp
crack and see what happens....as the radius approaches 0.001 times the
length of the crack......????


That is because the load is not equally spread
across the part, but is concentrated at the end of
the crack.

Think about a piece of metal bar in tension with a
crack halfway across it. The cross sectional area
that has already seperated cannot bear any load at
all as it has already failed. The end of the
crack is taking a lot of load because the crack
pulls apart when it is under tension.

Solution: Don't use cracked parts!! (Well duh)

Don W.

  #12  
Old April 25th 07, 04:19 PM posted to rec.aviation.rotorcraft
Stuart & Kathryn Fields
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Posts: 328
Default control failure

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.

--
Stuart Fields
Experimental Helo magazine
P. O. Box 1585
Inyokern, CA 93527
(760) 377-4478 ph
(760) 408-9747 publication cell
"Don W" wrote in message
...


Stuart & Kathryn Fields wrote:
Don: Be sure to share what you find. I think that I'm only scratching
the surface of some stuff. One thing I found and had never seen before
was an equation relating the increase in stress due to a crack. It
scares the hell out of me. I'm afraid to use a metal fork in my salad.
The equation basically says that the max stress is 2 times the load
divided by the area times the square root of the crack length divided by
the radius of curvature of the end of the crack!!! If the radius of
curvature was equal to the crack length, the max stress is already twice
what you would calculate using the applied load and the element cross
section. Now put a reasonably sharp crack and see what happens....as the
radius approaches 0.001 times the length of the crack......????


That is because the load is not equally spread across the part, but is
concentrated at the end of the crack.

Think about a piece of metal bar in tension with a crack halfway across
it. The cross sectional area that has already seperated cannot bear any
load at all as it has already failed. The end of the crack is taking a
lot of load because the crack pulls apart when it is under tension.

Solution: Don't use cracked parts!! (Well duh)

Don W.



  #13  
Old April 25th 07, 07:42 PM posted to rec.aviation.rotorcraft
Don W[_2_]
external usenet poster
 
Posts: 1
Default control failure

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.

 




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