Quote:
Originally Posted by HerrK
Hey no problem disagreement is ok with me. I'm a test engineer, sometines I get to try and actually break things. Ceratinly won't claim to be a stress engineer. What I envisioning is the rim being an equally loaded beam (for conservatisim fixed at the ends) because I believe the casting is brittle - not ductile. Now the tire gets a high point load at impact and even though the load is spread locally (say 3 -5") across a circumferential portion of the rim, high localized stress over 1-2" is higher that the tensile strength of the rim. Crack.
Anyway it would be nice to see a cross section of the crack under a few SEM photos. We'd be able to tell if the metal was rapidly torn or stretched. A mass spec of the rim material would help differentiate the alloy and properties. Unfortunately I no longer have easy or "free" access to these methods as we now subcontract out that work.
An old 30 series RFT could be relatively easily sectioned, supported and deflection tested. It would at least give a crude visual load path.
It just seems reminicent of a test when we split a tank of LH2 wide open. Stress engineers said the flight adapter was softer and spread the load into the tank better than our hard stiff test fixture leading to stess fracture of 180ksi CRES.
BMW should be exploring all the potential failure paths or bones of the fish. I love a good fishbone!
I noted what seems to be a month year cast designation on the back of one of the spokes 2 digits for year followed by 5 lines as mine are (10 l : l l l l) I'm guessing that number and location of the dimples in between the lines is relative to yearly quarter and the month. Front rims 1 dot (Jan of 2010) & rear rims have 2 dots (Feb of 2010) It would be a starting point to gather that information for material lot issues.
Hoping that these 2 instances are out of family occurances and not tending to be the norm. Its a big puzzle and reverse engineering this thing could be done but it would require some time & $. I hope I don't have to go there with my 296's but I will if it happens to me and I am pushed/ignored by BMW. It's too late tonight and I don't even want to start thinking about F=ma, free body diagrams, and dynamic factors. Its really much easier for someone with FEA experience and it would be a complex model to say the least and that is not my bag.
Thanks for your thoughts.
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Are you a destructive testing engineer?
I see where your going with your analysis and agree the wheel is brittle. We have no empirical data - it's all guesswork - so your theory is just as valid as mine.
It would be nice to do so some real test and analysis of the tire and the wheel, which is the only way to know for sure how/why it happened. I suspect that enough of these turn up cracked you may have your wish.
Cheers