is it possible tp over stress spring steal without it fracturing? Like a tortion bar. Do they stretch?

Answer 1

I'm not sure what your question is, but I'll try to explain torsion bars. A torsion bar isn't stretched, it's twisted. There is a point where the force on the bar exceeds it's torsional strength. At this point, the bar will snap.

Torsional strength can only be measured by destructive testing. This can be done by twisting the bar until it fails. Or it can done by stretching the bar until it fails: this is gives maximum strength, and torsional strength can be approximated from that. Or it can be measured by applying a shear load to the bar until it snaps. For a VW bar, torsion strength is probably in the neighborhood of 80-100 KSI, assuming a yield strength of around 230KSI.

To understand how deformation occurs, think about a circlular cross section through the bar. As the bar twists along it's longitudinal axis, a point on the surface of the metal has to move through more distance than a point closer to the center. In fact, a point exactly at the center of the bar wouldn't move at all. So the stress on a torsion bar concentrates near the surface. If the bar is subject to unusual stress, or if it's simply exposed to normal stresses for long enough, the surface will reach it's yield point, and microcracks will begin to form at the surface. These cracks cause the bar to weaken, and the car will sag. In time, the cracks will progress deeper into the bar and it will fracture. For that reason, I prefer to replace sagging bars than adjust them.

In normal use, there will be a certain amount of plastic deformation, or set, which occurs as a result of stresses concentrating at the surface of the bar. For this reason, you should never swap the left and right torsion bars, even if they are mechanically identical: reversing this set will rapidly cause the bar to fail. You may find that new bars sag immediately after installation. This is the result of the bars acquiring set, and it's one case where the bars can safely be adjusted.

Answer 2

You haven't been paying attention to those overloaded pickup trucks doing landscaping or moving an entire household. Their leaf springs are bent backwards. Once that happens the strength of the spring is gone and it does not hold it original shape. Yes it is possible.