Hank,
I am curious if you have any insight as to where this
old wives tale about aluminum and work hardening
originated. I remember hearing this kind of thing
repeated 15 years ago when I was going to school.
The version I heard suggested that aluminum had
a propensity to crystalize when it work hardened.
The advice I was given was that if I were on an
aluminum tower and it stopped swaying in the
breeze, that meant that it was about to fail. I sort
of imagined the tower just breaking off and falling
over :). Granted I have seen aluminum yagi element
just break off cleanly mid-section. I had assumed
this was from fatigue failure due to wind induced
vibrations. Of course, this would have been due
to literally millions of fatigue cycles given the
relatively high frequency of vibration. I wonder
if you could comment on this sort of fatigue failure.
Thanks,
Mike, W4EF............
----- Original Message -----
From: <kr7x@attbi.com>
To: <towertalk@contesting.com>
Sent: Thursday, February 14, 2002 2:19 PM
Subject: [TowerTalk] Universial Freestanding Tower
>
> Talkians:
>
> Sometimes I just can't believe the amout of
> misinformation and absolutes that get pawned off as
> verifiable truth on this website. Ordinarily it does no
> harm or potential harm. However this discussion of the
> aluminum vs steel towers and reduction of strength due
> to "work hardening" is in need of some scientific and
> engineering basis.
>
> The first order of business is the posting of an
> individual who relates that he installed a 75 foot
> cantilever aluminum tower without a concrete base in a
> hole that is 12" in diameter by 6 feet deep. Sir, if you
> installed that tower in the dirt then you are at risk of
> a failure of the soil at some time in the future due to
> the soil getting saturated with ground water or by the
> interaction of the soil and the aluminum. At 12 inches
> in diameter it must be a round section and not a
> latticed type tower.
>
> The next item is the term "work hardening." This maybe a
> term used in the welding and fabrication shops but the
> accepted terminology is Fatigue and Fatigue stress.
>
> "Fatigue, as used in this Specification, is defined as
> the damage that may result in fracture after a
> sufficient number of fluctuations of stress. Stress
> range is defined as the magnitude of these fluctuations.
> In the case of a stress reversal, the stress range shall
> be computed as the numerical sum of the maximum repeated
> tensileand compressive stresses or the sum of the
> maximum shearing stresses of opposite direction at a
> given point, resulting from differing arrangement of
> live load.
>
> 1. Loading Conditions; Type and Location of Material
>
> In the design of members and connections subject to
> repeated variation of live load, consideration shall be
> given to the number of stress cycles, the expected range
> of stress and the type and location of member or
> detail. ........"
>
> (Manual of Steel Construction, Allowable Stress Design -
> 9th Edition, AISC)
>
> Two things are important; the concept of cycles of
> loading and a stress limit value to eliminate fatigue.
>
> ".... Aluminum, like other materials, may fail in
> fatigue after large numbers of applications of load.
> Fatigue failure is almost always traceable to a stress
> raiser such as a notch, hole, or sharp reentrant corner,
> or to local bending resulting from joint eccentricity.
> Concentrations of stress and local bending can
> frequently be alleviated by proper design, fabrication
> and maintenance, resulting in greatly improved fatigue
> life.
> It is not possible to design precisely for a specified
> fatigue life, even in cases where a constant, know load
> cycle is applied to the structure, ..........
> Nevertheless, approximate rules for design of structures
> subjected to repeated loads have been developed on the
> basis of fatigue tests of structural joints."
>
> (Task Committee on Lightweight Alloys: Suggested
> Specifications for Structures of Aluminum Alloys 6061-T6
> and 6062-T6 and Suggested Specifications for Structures
> of Aluminum Alloy 6063-T5 and T6, ASCE)
>
> For both materials and all structural metals the design
> for the elimination of fatigue failure involves the
> concept of number of cycles of stress reversal and the
> magnitude of the stress range.
>
> Example time:
>
> Cantilever Tower for Amateur Radio: Design Life 25
> years, Daily cyclical stress reversals say 5, that is 5
> maximum wind load occurances every day. Total cyclical
> loading = 5 x 365 x 25 = 45,625 cycles for the design
> life.
>
> Both the AISC for steel and ASCE for aluminum give max
> stress range based on cycles of loading below which
> fatigue is not a factor.
>
> for aluminum : 100,000 cycles Stress range if max
> stress is larger than 1/2 max allowable, = F, the max
> allowable for static loads.... i.e., no reduction in max
> allowable stress due to fatigue. ( 6061 T-6 )
>
> For steel: 100,000 cycles, 63,000 psi stress range.
> i.e., max stress = 63,000 / 2 = 31,500 psi > .6 Fy
> =21,600 psi... no reduction in max allowable due to
> fatigue. (A 36)
>
> Neither material has a greater propensity for fatigue
> than the other.. properly designed.
>
> Other comments:
>
> No such animal as aging of steel or aluminum due to
> flexing... if properly designed.
>
> I would like to hire any individual who can; by eyeball
> and gut feel tell me the stress values in a structure
> without calculating them from the loading conditions and
> structural geometry.
>
> Most catastrophic tower failures occur due to
> compression buckling of either a tower leg or brace.
>
> There is no progressive derating of any structure.
>
> There is, in my experience, no such animal as:
> Coefficient of Flexation. Maybe the Modulus of
> Elasticity is what the person meant.
>
> Again the whole concept of one material ("work
> hardening") fatiguing faster than another is
> unfounded, given the load cycles that towers see in
> their lifetimes. Properly designed.
>
> Any metal that is loaded below its threshold fatigue
> stress level and unloaded can,theoretically be loaded
> and unloaded forever...
>
> Rule of thumb is to use one half the yield stress of the
> material, this keeps the stress well within the elastic
> range for the material.
>
> The question of using a steel or aluminum tower of
> comparable load carring capacity is not one of which
> will fatigue before the other but one of enviromental
> corrosion resistance, weight, transportatability,
> absolute strength, size of structure required and cost.
> All of these vary for each material, intended usage and
> total load carrying requirements.
>
> I have at various time designed aluminum, steel, fiber
> reinforced composite, and timber structures to support
> elevated loads of, freestanding and guyed, antennas and
> other equipment. The decisions were never based on the
> fatigue preformance of the materials but rather on cost,
> availability, transportation, and future expandability
> of the proposed structure.
>
> As we contribute to this reflector, we as individuals
> need to consider what we are saying and what factual
> basis we are using to support our statements.
>
> Hank Lonberg, P.E./ S.E., KR7X
>
> ________________________________________________________________________
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> Order online at http://store.eham.net.
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