I am not a bolt engineer but to the best of my understanding, they are an easier way to achieve consistent clamping. You could use harder bolts but they can't use the same torque spec because they won't stretch.
Also, since this is more of a general tech question, post moved to general section.
Actually, these bolts are called Torque to Yield. Yes Chitty, you are correct. The intent of the design is to achieve a constant clamping force such as on the head gaskets. Uneven clamping force can cause premature gasket failure. Since bolts are elastic, if you were to look at stress strain curves, the curve flattens out in the area of yield. so the stress becomes a constant in that area and the clamping force becomes equal for all bolts. They are a one time use only though.
Just about all bolts "stretch" when tightened... So-called "stretch" (torque-to-yield) bolts are engineered to undergo a further change in their shape (plastic deformation) in order to achieve a prescribed clamping force. Because they actually deform permanently during this "stretch" phase, they should not be re-used, unlike regular bolts that only undergo an elastic deformation and can return to their original shape.
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