Hi All:
I could not resist a few comments on the aluminum fatigue question.
My Reference is "Aluminum Structures" by Kissell & Ferry, which is an
understandable design book based on the Aluminum Association's "Specification
for Aluminum Structures". See www.aluminum.org.
The concept of fatigue is based on cyclical TENSILE stresses (or tensile to
compressive and back cycles). Stress cycles that remain in the compressive
region are not considered to introduce fatigue. Properly designed guyed
towers, which are always in compression, will not experience fatigue stresses
in this concept.
Self-supporting towers and antenna elements will experience fatigue stresses.
To analyze for fatigue, the Aluminum Association provides formulas, and
ratings that apply to categories of the detail material configuration (tube,
extrusion, bolted or welded connection, etc.). The only variable is the
number of cycles to be expected. The upper bound on the number of cycles,
called the Fatigue Limit, is taken as 5 million cycles (This is an arbitrary
value, it does not indicate the material will fail at that limit). There is
also a lower bound, usually taken as 20,000 cycles, below which one need not
consider fatigue.
Thus one may judge that fatigue could be an issue for antenna elements, which
could exceed millions of cycles, and is probably not an issue for SS towers
which likely will not have a large number of full stress cycles during their
life.
For the base metal (Category A), the fatigue stress limit should not exceed
about 50% of the allowable tensile stress value, or about 10,000 psi per the
above reference for unmodified aluminum tube. At this value of stress or
less, more than 5 million cycles should be realized without failure.
What does this mean?
If we assume an antenna element is designed for 80 mph survival in accordance
with structural design specifications (including safety factors), which
prescribes a maximum aluminum tensile stress of 20,000 psi, then the element
has a fatigue stress ratio of 20,000/10,000 = 2. The wind velocity change
required to impose a full fatigue stress cycle is 80/(sqrt 2) = 56 mph. That
is to say, a wind of 56 mph in one direction, followed by a wind of 56 mph in
the opposite direction (both at right angles to the element) would be one
stress cycle. Such a design would withstand more than 5 million such cycles.
This analysis does not consider the vibrations that occur at lower wind
speeds, but one would judge the stresses would be much, much lower, thus the
elements should withstand many millions more cycles than the 5 million
indicated.
For what it is worth.
Bob, W5LT
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