I like how the pitch changes. I have seen that before.
I had a project where weld studs were snapping off in the field and the question was, were they over torqued and being twisted off. I had some sample studs welded and made an impromptu test - enough to see the quality of the failure rather than to quantify it.
That result you have is what I was seeing. At the the highest torque the studs weren't twisting off but the pitch was changing. This meant that the assemblers were seeing a false torque because of interference in the threads and so the clamp load was not being developed. Being loose on a vehicle that saw moderate vibration and where the studs then became the shear path led to rapid fatigue at the weld stud base. Somehow it had something to do with dis-assembly and re-assembly. They were fine when initially installed, but failed after servicing.
I don't recall what the action after that was. It has slipped my memory after 25 or so years. But I do recall the satisfaction at seeing what appears to me to be the reason that threads strip - the incoming pitch is increased too much to fit rather than it being a simple friction or galling problem, no different than trying to put a 20 TPI bolt thread into a 28 TPI nut.
