Quote:
Originally Posted by Spurshot
I do engineering for a living. If you yield a bolt twice by permanently torque stretching, as in Torque To Yield (read that as permanent deformation), it will be that much closer to full failure.
A piece of metal will only stretch so far before it fails. When a given piece of metal stretches within a certain limit, it can return to its original length. That's called "within it's elastic limit". If it stretches and doesn't return to original length, it has stretched into its "plastic region" or "beyond its elastic limit". A little more stretch and it will fail.
How much a piece of engineering steel stretches either within its elastic limit or plastic limit depends greatly on alloy and hardness. As the steel gets harder, the range of stretch in either of these regions drops down dramatically.
Your correlation of miles verses stretch of those bolts is mistaken. First these bolts are not primarily in tension in their function. They are mostly in shear. If they were to permanently stretch, due to their use/mileage, they would become loose. This is not the case. They permanently stretch (yield is the engineering term) due to the extreme torque applied to them. If you stretch them too far, they will fail. Repeated (once, twice, ?) yielding (permanent stretching) will fail the bolt. That's why they are one time use.
A TTY bolt saves going to the next size bolt that would be normal torque to stretch within elastic limits. This can save weight, cost and space.
I recommend that if you re-use these bolts, they be torqued to normal (stretch within elastic limits) torque values for that size if re-using rather than torque to yield TTY of 30lbs + 90*, which permanently stretches and puts the bolt at greater risk of failing from over stretching. The mechanism that keeps a bolt tight is the elastic stretch, which is why all bolts are torqued.
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You are spot on
I have been in more GM service departments than I can count doing suspension training. GM has used TTY bolts in calipers for several decades. In all my interactions I met one, ONE, GM World Class Tech that actually replaced the caliper bolts. I have lost count of the SCCA, NASA and HPDE pits and paddocks I have been in and cannot recall seeing a single racer replacing caliper bolts.
I am not advocating not replacing them. Clearly, the best practice is replacing them and we frequently do. I have had issues getting replacement nuts and bolts. When we can't get new TTY caliper or suspension bolts and nuts we reuse them with LocTite.
So the question for the metallurgy and engineering experts is -- Based on what I have seen, the vast majority of repairs made reuse TTY bolts on driveways or service departments reuse TTY bolts and nuts and it has been this way for quite some time. Why don't we see more issues and failures?