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Old 01-29-2011, 09:56 PM   #341
EnderTheX
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Joined: Mar 2009
Location: DFW Area, TX
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Quote:
Originally Posted by Bayner View Post
Thanks Ender, that's good info.

Anytime

A quick comment on the theory behind corner design in mechanical systems...

Outward pointy corners (example: the corners of a cube) are no problem for stress.

Inward pointy corners (like on that bar) are problems for fatigue, anything that experiences cyclic loading may eventually fail due to a crack originated at the point.

How do we avoid this?... simple add a radius to the corner.

While parts with a radius or a sharp corner have minimal overall strength differences the resistance to fatigue stress is significant for a smooth corner with a generous radius.






On a complete side note and mainly for my own amusement, I have to say none of this holds true for composites and parts made of carbon fiber or kevlar layers may exhibit the exact opposite characteristics. One of my favorite stories is about some engineers at Bell laboratories working on a composite helicopter rotor assembly part originally made of aluminum. When they mimicked the shape of the aluminum part (radii in the corners) it would tear itself to pieces. No one knew what to do until one engineer had a revelation and eliminated the radii from the composite part. The forces experienced by the part were exactly 90 deg apart and isolated from each other. This means the radii in the composite part were transmitting forces from one direction to the other... the composite was designed for strength only in the required direction and the fibers pulled themselves apart because they did not have any support in the opposite direction. Once the radii were eliminated the part functioned perfectly. (It's been a while since I've told this so I could be missing one part of the story but you should get the point )
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