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Nice write up. My only comment is that I'm not sure what sort of load would allow for a dead straight (and REALLY long) Stage I and Stage II area that runs parallel to the longitudinal axis. Whether sitting or driving it should be torsionally loaded. What stress state would allow the fatigue crack to initiate and propagate in that fashion?

Yeah, we need a closer look, but he's over the pond...


We don't know how long Stage I is and these multiple origins may be small stress risers such as rock dings, mechanical tool marks, or some sort of material imperfection from its orignal bar stock; however I do agree and am puzzled by the long flat Stage II region. I suspect it may be related to a shear component that preceeds a torsional fracture on a solid bar.

Considering the car is "lowered" means the torsion bar may be under zero torque; however, the reverse loading from muliple bumps on the road may be the driving force for a fatigue fracture (essentially a reverse loading scenario).

I agree we would benefit by seeing it after it is cleaned and using oblique lighting to have a better idea of what may be going on.