James and Dan -
Thank you for the info that is based on your experience with actual impact. The only experience I have even slightly close to that which you describe is a crazy period in one dojo where, at the instructor's behest (and our idiotic acquiesence), we did our kata with relatively hard impact (say about 1/3 force), using those cheap pot-metal sword replicas. All the replicas quickly became saws, and the single practice ended within a 1/2 hour when one was hit on the shinogi and snapped, one piece nearly spearing someone on the other end of the dojo. And that, thank God, allowed a return to bokken, and the much more acceptable, under the circumstances, risk of cracked knuckles and other broken bones. BTW, I'm not using this as evidence regarding nihonto and shinogi uke - even I know that pot metal aint folded steel - it's just a lead in to wondering how impossible it would be to do "stress tests" with two people doing kata or even one move with impact at full force. A sword blade helicoptering across the room or a chip lodging in someone's eye would be a high price to pay in proving once and for all how swords perform in the kind of impact. and then, of course, you'd have to repeat it lots of times to get any reliable data. Much less the possibility that one really did attack with integrity and the other person couldn't block - whoops, I mean parry. Anyway, great discussion. I always look forward to reading what you guys have to say about an instrument I love to swing, but know all too little about in terms of its structure and capabilities.
With respect
Ellis