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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Thank you. I understand the theory, the experiments and deformation (necking) that occurs when rods are placed under tensile stress - I did the experiments regularly when I was a slip-of-a-lad studying my City & Guilds General Course in Engineering and Shipbuilding. I now teach this stuff to engineering students as a hobby!
What I am trying to get to the bottom of is whether the bolts when torqued-up in accordance with MGR's specification are below or above the yield point i.e. in the elastic or the plastic region. My guess is that they are in the elastic region but not far from the yield point and can enter the plastic region under certain circumstances - perhaps temperature cycling, reuse etc.Therefore the term torque-to-yield is used loosely (although not unreasonably) but does not mean that the bolts are torqued up to the yield point on the stress-strain curve i.e. they do not permanently stretch under normal circumstances.
Alternatively, that the bolts are in the plastic region when torqued-up to spec and the above term is being used precisely. But they go out of spec due to further stretching as a result of the above types of circumstances.
If the above were not the case, why would MGR's repair manual for the k4 specifically document that the length of the bolts should be measured and the bolts discarded if above a certain length? The possibility of a change in length implies that at least some bolts can be found to have entered the plastic region.
PS marinabrian attached the relevant page of the k4 repair manual specifying the need for length measurement in a previous thread on the topic during 2016.