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

[Wes (N7WS) and Linda]

At 11:47 PM 1/22/2003 -0500, Barry L. Ornitz wrote:

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

To which Barry replied:
>
>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.

Actually, after writing the above I did call my metallurgist friend and
discussed this. He said he did not like the term "elastic limit" as it is
too imprecise, and would never use the term in any report he authored.

73,

Wes  N7WS