Aluminum rim developing new cracks....any thoughts?

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Hello all,I routinely repair cracked or broken aluminum rims.  I did one recently  and the owner has returned for a new crack that developed in a new location.  These are bead cracks the type you get when you hit a curb and the crack leaks air from where the bead seals.Anyway, I repaired the new crack and the guy went on his way with a freshly repaired rim.  He called me today to report that a NEW crack had developed in a location not even remotely near the other two repairs.  I hadn't seen this before as he reports that his son drives the car from home to work, period, and that it is a mystery why it keeps cracking.I suggested two things to him.1.  His son may not be telling him the truth and in fact is banging up the rims when he drives.  or2. The rim has become brittle over time and will continue to develop cracks in the future.  I said this because I know there are grades of aluminum that work harden over time.  I thought it might be possible that the harder the metal, the more tendency to become brittle with age.Is there anything to this?  I am not familiar with the properties of aluminum enough to know how to explain this but it seemed plausible as I could think of no other reason for a rim to crack like this without provocation.Anyone have any thoughts or experience with this happening?I told the customer that I would see if I could find out an explanation and get back to him.  He will be coming by to have me repair the new crack but may alter his plans if it seems that this rim is jinxed and that he is throwing good money away on a dead horse.Thanks,Tony
Reply:Hi Tony,Aluminum does have a "life" in terms of how many "back and forth" (tension and compression) stress cycles it can take before failing, which is known in that event as "fatigue" failure.  Here is a little light reading on the subject:http://en.wikipedia.org/wiki/Fatigue_(material)In a nutshell though, aluminum is generally worse about this (has a shorter lifespan) than steel.  And the higher the material is loaded cyclically, the fewer the number of cycles it can endure before fatigue failure occurs.  Were the wheels relatively lightweight compared to the vehicle they are mounted on?PS - I weld up the cracked wheels for a local wheel repair company.  FWIW, they won't take on any weld repair inboard of the safety bead.  I've done a lot of crack repairs on the lip outboard of the safety bead (where the bead of the tire seats.)  I have often noticed that there is an adhesive type of wheel weights (or evidence of one being there) right in the vicinity of the crack.  It seems like it happens too often to be a coincidence.Last edited by jakeru; 04-20-2012 at 02:47 AM.
Reply:that's good to know... going on an airplane today... 2 hours of thinking how many bend cycles can the fuselage take...--------------------------------------------------------------www.becmotors.nlyup, I quit welding.. joined welder anonymous
Reply:Originally Posted by Donoharmthat's good to know... going on an airplane today... 2 hours of thinking how many bend cycles can the fuselage take...
Reply:Two things you can consider, one you've already nailed. My guess is this kid doesn't know how to drive/drive properly via avoiding potholes and curbs.Second: what size tires are on this car? Lo-pro tires plus potholes can definitely equal cracked aluminum rims. one more thing: what brand is the rim/what quality construction? There are definitely differences, cheap rims show definitely less build quality and potentially use inferior materials. I think those rims are getting to the point of dangerous, if they're cracking this much.Another: Are these straight aluminum or an alloy?Hobart EZ-Tig newbie.
Reply:Is it the same rim every time? Is it possible that the kid has 1 pothole that he drills everyday with the same rim like one on a corner or something?
Reply:x3 on this aluminum rim being old, tired and worn out.  You don't say how old the car or rims are.  Is the owner having the wheels re-balanced after the repairs?  Does he have a bad wheel bearing or shock/strut, or other suspension component that's hammering on the wheel as the car is being driven?Steel, because of it's crystalline structure, has a threshold for fatigue.  If stresses stay below approx 1/2 the yield strength, then no fatigue occurs.Aluminum has no such fatigue limit or threshold.  Every applied stress causes an aluminum part to accumulate fatigue damage.  Fully annealing the rim will erase any accumulated fatigue and reset the clock.  It will also ruin any temper in the alluminum alloy and probably warp the rim severely.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:Toss the rim and replace it.Doesn't really matter now if the driver or car was 'beating' the rim or if the rim was defective or has now reached its End-of-Life, an aluminum rim that is 'sprouting' miltiple random cracks all over the place is toast.re: fatigue of materials.  Steel structures can usually be designed to have an 'infinite' fatigue life, aluminum structures can never be designed to have an infinite fatigue life.  Strong statement I know.It is not whether the metal (or other material) has a crystalline structure or not (almost all metals that are in their solid form are 'crystalline'  or more correctly "polycrystalline" to at least some extent), fatigue life of a material is dependent on micro and macro-level 'defects' in the physical structure, including surface and sub-surface voids, micro-cracks, grain-boundary defects, and so forth.  Fatigue and fatigue life is not really related to 'crystal' structures (barring such things as single-crystal structures such as turbine blades and such, where efforts are made to produce the material with no or as few as possible grain-boundary imperfections as possible in order to maximize physical or other properties).Back to the OP, Tony.Toss the rim(s).  They are shot.  Replace, don't attempt more repairs.  The best laid schemes ... Gang oft agley ...
Reply:most likely not the driver.high milage aluminum rims have been known to crack.miller thunderbolt 250vlincoln square wave tig 175 prolincoln idealarc mig sp250everlast tig 210EXTeverlast power plasma 50chicago electric (hf) 130 tig/90 arcchicago electric 90 amp flux wire3 sets oxy/acet
Reply:Hi fellas (and ladies that might be out there),Thanks for the feedback.  I just got off the phone with my client and he confirmed that his son is running low profile tires.  This then would explain more likely the reason for the mysterious cracks.  In any case I also ran by him the possibility that the rims may have a lot of miles on them (he bought the set used) and they are developing fatigue cracks.He said he is going to go back to the stock rims and not attempt to repair it again.I appreciate the info,Thanks,Tony