[TowerTalk] polyrod

[Roger K8RI on Tower]

<snip>

> I hope everyone takes this the right way.
>
> We tend debate hours on end about cad welding, rebar rust,
> using four direction guys, and all sorts of things and then
> the only thing holding our towers up has an unexplained
> failure problem (that makes no sense) and no one bothers to
> pursue why! What we have here is an engineer at the
> manufacturer saying it can't hurt, sales people saying they
> coil it and store it all the time without problems, and a
> few users who know virtually nothing about the product
> deciding the failure they saw was from being coiled!  Quite
> frankly the idea being coiled ruins it doesn't make any
> logical sense, and the fact the manufacturer responded with

Having spent a lot of time with fiberglass the last few years while building 
an airplane.  This is  an aeroplane capable of aerobatics and I've flown one 
close to 350 MPH while pulling 3 1/2 Gs coming through vertical on the down 
side of a split S.  I've flown 4 G level 360 degree turns and it can do a 
pair of loops with one on top of the other (figure 8) with an entry speed of 
350 MPH at roughly 5  to 5 1/2 Gs.

When I am depending on something I put together to keep my back side safe 
under those conditions I did a lot of studying.

When it comes to strength in fiberglass lay-ups, much, or most depends on 
the construction methods, the amount and kind of resin, and the size of the 
fibers when it comes to flexing, or bending failures.

Fiberglass rod is *relatively* strong lineraly and in shear strength, but it 
does not do well with a bending moment.  The reason being that the outside 
fibers are being stretched with tremendous and relatively localized force 
while the fibers on the inside of the radius are being compressed. 
Individual fibers are relatively weak.   The outer fibers tend to fracture 
and splinter away with the fibers seperating from the resin.  Resin tends to 
flow slightly. Even cured resin.  If you put a bundle of linerar fibers in 
resin and after full cure bend it to just short of the fracturing point. 
Then leave it under that tension. Over time the outside fibers will tend to 
seperate, weakening the rod.

BUT I need to emphasize that how well any particular rod stands up in this 
situation depends on the ratio of resin to fibers, how evenly the mix is 
distributed, the type of resin/epoxy used, the cure rate in some cases, 
whether the fibers have been exposed to moisture, the size of the fibers, 
and how uniformly they are kept parallel, and quite possibly a number of 
other variables.

I would *expect* fiberglass rod to fracture when coiled too tight.  How 
tight is too tight?  I have no idea.  I would expect fiberglass rod that has 
been coiled near its minimum radius to be weakened and *possibly* fracture 
with time.

Another vairable which can be critical is temperature.  The best of resins 
soften with high temperature.  That is why almost all fiberglass airplanes 
are painted white.  I have forgotten the name used for the critical 
temperature when the epoxy softens, but it is relatively low.  Some where on 
the order of 200 F. (give or take)  Vinyl Ester resin has a higher critical 
temperature at which it can be used and there are space age resins used in 
many of our fighterplanes that reach still higher temperatures.    OTOH the 
resins seem to stand up quite well to cold temperatures, but I think I'd 
avoid them at either extreme.

One possiblility for these fractures comes to mind.

IF the rod was coiled when it was warm (room temp or higher), or it was 
allowed to get warm in storage such as an outdoor shed that gets hot in the 
Summer, (I've been in metal storage sheds that would have exceeded the 
critical temperature of even Vinyl Ester Resin and I live in Michigan)  the 
resin could easily have "taken a set" over time.  Then if uncoiled when 
cold, or cool it, straightening it could actually exceed it's bending 
strength, causing it to fracture.

So, at least to me, there are a number of legitimate reasons for fiberglass 
rod to fracture when uncoiled.  Like a garden hose, I'd want to get it good 
and warm before uncoiling if it had been stored for any length of time in 
the coiled condition.  Straightening coiled rod that has "taken a set" can 
cause just as much damage to the rod as coiling it too tight.

I saw one very expensive fiberglass airplane that had been left out in the 
hot sun too long.  You could see where every rib in the wing was located. 
Structurally it was still sound, but it took many hours of work to give that 
wing the proper shape again.

Once the rod is straightened, I would examin the outside surface very 
carefully, looking for "whiskers".  Raised whiskers, or whiskers sticking 
out are a bad sign, not just from a structural standpoint, but they are a 
good way for moisture to get into the rod.  The glass fibers are very 
sensitive to moisture.  In aircraft construction we discard any fiberglass 
material that may have gotten damp.

73

Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com