Are stretch bolts made of different specification steel to ordinary bolts, or merely tightened beyond a 'yield' point? If so, what was the point of doing it? Are they weakened by the experience, and liable to fail if thus treated again?
There is no such thing as a "stretch bolt". The term is used, erroneously, to indicate a bolt where the manufacturers intend it to be tightened very close to, or past it's yield strength.
All bolts stretch as they are tightened. They behave like springs and stretch according to Hookes Law. Doubling the load doubles the stretch but critically removing the load allows the bolt to contract back to it's original length again. This applies ONLY up to the yield point. If you tighten past yield point the bolt will start to behave non-elastically, and when you remove the load it will have been permenetly lengthened - It won't contract to the original length. If you then tighten it up again to the same load, it will permenetly deform again. And again and again. There is no way of accurately knowing how much it can be permenetly stretched before it will snap - might be 10 tightenings, might be 3.
The load in the bolt is very difficult to predict from the tightening torque because it depends massively on the friction parameters of the bolt thread, nut thread and any lubricant. There can easily be a 50%-200% variation in the load in the bolt just by torque tighteneing. However, angle tightening does produce a known extension in the bolt - turning the bolt 360 degrees stretches it by one thread pitch. If you know the material the bolt is made from and it's dimensions, you can then calculate the load in it required to create that stretch. And vice verca, you can calculate how much angle is required to impart the required amount of pre-load. That's why things like head bolts have angle tightening specs, so the load (on the head gasket) can be accurately controlled.
Metric High Tensile bolts are typically marked 8.8, 10.9 or 12.9. The number before the decimal is the tensile strength of the material in MPa. The number after the decimal is the percentage of this tensile strength that you can use before yield starts to occur. You can down load calculators from "Bolt Science" if you're really interested.