Frank,

When there are "conical male/female shapes at the failure point" it generally indicates tensile overload failure (caused by exceeding the materials tensile strength). A giveaway would be necking on either side of these shapes, which might be what you're calling failure "at the weld taper".

Fatigue looks very different - the fracture surface shows a smaller smooth portion surrounding the origin, with the remainder of the fracture surface appearing granular (perhaps what you're calling calling "crystallization". Fatigue failure is caused by thousands, perhaps tens of thousands, of load variation in the presence of a small defect in the part (usually on the surface).

These are two completely different failure modes, and have two completely different causes. It would be interesting to see pictures of the actual failed ring and the fracture surface.

-John

On Tuesday, June 6, 2017 at 4:26:34 PM UTC-4, Frank Whiteley wrote:
We had a TOST large ring failure while winch launching at a CAP Flight Academy a few years ago. It failed at the weld taper and the ring also broke opposite the weldment with the appearance of fatigue crystallization at the break. So, it's possible several launches were done with the failed weld before the ring failed at the second point. It was reported back to the vendor and also to TOST IIRC. Locally we discussed if perhaps some counterfeit supplies were getting into the channel. The academy leader passed away some time after the academy so I don't know the result of the report but perhaps the vendor will recall the event. I'll check.

Frank Whiteley

I'll amend my post to say that it was not evident that the failed ring was welded, but that separation had conical male/female shapes at the failure point. Looking for images in the archives, but nothing so far.

Frank Whiteley