Here is another good explanation. Just because the spec is tighten to angle does not mean it is TTY. Just because the book says to discard the bolt does not mean it is TTY
GM says to discard any bolt with a torque over 20 lb. ft., way before plasticity. I can't find any bolts on an HHR that are specifically spec'd as TTY.

Not saying it is a bad idea to to discard things like head bolts, where you need a constant clamping force along a large area that is subject to heat distortion.

This guy gets almost to the mind numbing science without going over the line.

 

Thanx Donbrew,
I enjoyed the explanation.

 

I liked this. This would be really entertaining to play in an ME college class. All that Stress and Strain stuff was so dry....

 

Many, and I mean many, Cat engines I have assembled with the "torque to yield bolts" to me was a farce.
If you did the torque turn to the first one and then measured its torque value, you had a number to tighten all the rest.
When you start to install a cylinder head with 24 bolts (or more) that go down to 350 ft/lb I really wanted to make my life a lot easier.
And none of them bit me in the A$$.