Quote:
Originally Posted by kaiser
There are a number of experiments where a rod of metal is stretched in a machine and the stress and strain recorded. Quite a number can be found on YouTube.
There is always quite a noticeable deformation taking place, before the rod breaks, and there is a noticeable narrowing as well.
The stretch caused by thermal expansion is most likely only a fraction of the stretch caused by the thread, and nowhere near the yield point of the bolt, in the k4.
I think the confusion arises by some people using the term yield as a simple reference to the bolt "giving" and being designed to do so.
The "yield" point is however quite specific in engineering terms, as the point of no return. I think this could be the cause of some misunderstanding.
I have never seen a k4 bolt stretched. I have however seen a few V6 bolts broken, but they are much shorter. I would not re-use a V6 head bolt, but quite happily a k4.
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Oddly enough when a bolt enters the plastic stage it can produce strain hardening to different degrees, cause by the strain causing dislocations within the metals structure, cold working.
As the strain is increased the strength of the bolt increases to its maximum strength, after this is reached and further strain is applied, bolt strength decreases to eventual possible, but not always necking and then fracture.
This phenomena can cause fastenings taken over their max strength to fracture by being subject to thermal stresses or vibrational stresses.
There was an engine, can't remember which at the moment, in that it was known and expected for one particular bolt to snap in use, even though it was set up correctly by hand or machine, the bolt was very close to an exhaust port.