Crossmember Bars POLL

[amalventano1 at tds.net (Allyn)]

> From my materials engineering textbook: "Fatigue is a form of failur
> that occurs in structures subjected to dynamic fluctuating stresses.

i knew that one was coming. the way this bar works is similar to that bar 
you see the big-ass weight lifters using on tv. the guy lifts the bar near 
the center, with massive weight at either end. these bars get used for years 
and years. it may weaken and bend easier over time (yield strength / 
ultimate strength lowers as it is fatigued), but you never see one just 
snap. the failure mode in this case is to become more bendable, not to 
brittle fracture.

> Do you think the crossmember is breaking due to being
> pushed past it's yeild strength?

yes, this is the exact reason. the metal in the cross member is somewhat 
hardened steel. my guess is that it is hardened enough such that the RTT is 
near ambient temperatures. if you've ever seen the cracks we speak of, they 
are jagged and sharp at the edges. basically brittle fracture. brittle 
fracture is 'sudden, catastrophic failure of a metal under tensile stress 
with little to no plastic deformation (bending)'. it requires a pre-existing 
flaw (small crack, nick in the metal, etc). when a crack propogates in this 
mode, the only way to stop it from propogating is to stop drill it. its 
basically the same as a crack in glass. the stresses are amplified greatly 
at the leading edge of the crack, so it spreads that much easier.

> I don't, because if that was the case
> drilling holes would do no good and the entire crossmember would break
> in half before you even noticed the cracks. The crossmember is
> cracking due to fatigue.

easy stop to that argument is the fact that only the _newer_ (~>85) cross 
members are the ones that are cracking. wouldnt you think that a similar 
designed piece in a 30 year old scirocco would have more fatigue than that 
of a 15 year old one? mileage and horsepower dont seem to have much of an 
impact on it either. from my words above - i think vw hardened this steel a 
bit more or used a different mix that had a lower fracture toughness / RTT. 
i also believe that the cracks don't occur unles some major shock type 
stresses are applied to the member (i.e. accident, bad side mounts, etc).

> Good, now I know how thick the plate is. It didn't look that thick in
> the pictures. But if you look at the geometry of that plate, the

whoa there, dont quote me on it :) its either 1/4 or 3/16. im going on 
eyeball gauge only.

gotta run, party at my place :)
p.s. no flamage here, just opinions, backed by some navy training.
Al