[Antennas] Re: Antenna Vibration (perhaps tower vibration)

[Barry L. Ornitz]

I wrote:

> As long as the vibrations are within the elastic limits of 
> the vibrating components, fatigue does not occur.  

And Wes, N7WS replied:

> Normally it's bad practice to argue with a PhD, but this 
> time I can't help myself. :)
> 
> This just didn't sound right, but it's too early in the 
> morning to call up my metallurgist friend, so a quick Google 
> search finds among others this:
> 
> http://www.foxvalleyspring.com/Glossary.asp
> 
> Fatigue Failure  - When a spring is deflected continually, 
> the metal becomes fatigued and failure may occur at a stress 
> level far below the elastic limit.

Th key here is continual deflection.  On the same website, 
look up a few lines from this definition.  Note the terms 
Endurance Limit, Fatigue Strength (Endurance Strength)  and 
Elastic Limit and remember this website is specifically about 
springs (but the concepts still apply).

"Endurance Limit - Maximum stress which can be repeatedly 
applied without incurring spring failure."

"Fatigue Strength (Endurance Strength) - Stress at which 
failure occurs after a specific number of deflections."

"Elastic Limit - Maximum stress to which a spring can be 
stretched without taking a permanent set."

The Endurance Limit (an alternate term for Fatigue Limit) is 
what actually applies in the tower vibration situation.  It 
has been quite a while since I took my Strength of Materials 
course (and remember I am a ChE and EE and not a ME [*]).  It 
turns out that the endurance limit is less than the elastic 
limit.  It is an asymptotic value estimated from a curve of 
cycles before failure versus stress.

A properly designed spring (or tower) can take vibration for a 
very long time.  Look at the suspension of an automobile and 
figure out how many deflections have taken place over the 
course of a few hundred thousand miles.  And yes these springs 
can and do fail eventually.  Big potholes around here don't 
help!

Thanks for pointing out the difference and for correcting me 
on the proper term to use.

        73,  Barry     WA4VZQ     ornitz@tricon.net

[*]  The course was taught by a Civil Engineer, no less.  And 
he decided to skip the chapter on the buckling of thin-walled 
cylinders - the one of most value to a ChE since it applies to 
pressure vessel and pipe failure!