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Re: Trouble with "flexure of beams" apparatus



At 14:18 12/8/00 -0600, Fred Lemmerhirt wrote:
Does anyone have any experience with the old Cenco "beam flexing apparatus"
used to determine Young's modulus? ///
He is both varying the load while maintaining a constant distance between
the supports, and varying the distance while maintaining a constant load.
Then he plots the appropriate graphs (deflection vs load, and deflection vs.
cube of distance), getting nice straight lines, and using the slopes to find
Young's modulus. But one method is giving results in the 3.3 to 3.8 (times
some power of 10) range while the other gives values in the 1.8 to 2.2
range. I've examined his work and haven't been able to discover any
mistakes. Do you know of any typical trouble spots in this procedure or
have any idea what might be wrong?

______________________________________
Fred Lemmerhirt

As a modest reality check, I used a commercial beam code (Archon Beam)
to verify the relations:

a simple beam with pinned supports and a central point load
gave a linear increase in maximum deflection with load, thus:
beam 100 units long, central point load 1000 units, deflection 0.718 units
500 0.359
250 0.18

A simple beam with pinned supports and a fixed central point load of 1000
units
100 units long, deflection 0.718 as before
126 units long 1.413
144 2.145
159 2.887

This confirms the basic premise.
I conclude that the analytical derivation of modulus from slope is in error.
I am unfamiliar with the procedure given. What is it?


brian whatcott <inet@intellisys.net> Altus OK
Eureka!