I was testing the durability and overall quality of a sword by whacking the broad side of the blade on an object to see if it kept it's shape and stayed straight. I probably hit each side about 30 times with a strong amount of force and I might be mistaken because I have no way to actually prove it but it seems to me like the entire sword is slightly more flexible, especially where I thought it was too stiff in the middle. The sword did not bend, at least to the naked eye.

Can you actually loosen the steel up by just putting it under stress, or I am just imagining this?

I hope it is just your imagination, otherwise it is possible the increased flexibility is due to stress micro cracking. I don't know of any way that the heat treat would change otherwise. Maybe the more metalurgically inclined could give other ideas.

Unless you hit hard enough to flatten a mid-blade ridge (or similar), then cracks. Or imagination.

Next time, measure the flexibility before and after. Clamp the base of the blade, and hang a weight from the tip, and see how far the blade bends under the load. Whack-test, and repeat. Expect no difference, and if you see one, suspect cracks. This is a real-life crack-detection technique; see https://biblio.ugent.be/publication/3140766 and http://www.sciencedirect.com/science/article/...8306005580

You could load the blade like this, and feel whether the upper surface is smooth when bent. I don't know whether cracks would open up enough to feel, but you could try. At least a negative result might reassure.

I don't think you really cracked your blade, but I would advice against such rigorous tests next time. A few whacks on each edge and flat against an object is ok to see if there is a weak spot in the blade, but 30 strikes each side is maybe a bit too abusive.

Sounds like metal fatigue to me... I could wildly speculate that repeated rapid flexing, due to impact, in a very restricted area could create enough heat to alter the temper in that zone. But my money is on metal fatigue, stress micro-fractures.

I don't think you'll find much info on that related directly to sword blades but you might find some stuff on this internet thing about car leaf springs, written in more-or-less laypersons' terms, by people with some material science/engineering/metallurgy pedigree. You'll find some parallel insights perhaps.

Stress micro-fractures aren't the end of a piece of metal's life but many will argue that it is the beginning of the end. What this really means to you depends on what you really use the sword for - if you are into test cutting heavy targets I would have serious concerns.

I think that an instinctive understanding of this is why there aren't more historical blades out there to be found - a certain amount of hard use/ abuse and a fighting man might start to worry about how much life was left in a blade.

Ah, excellent Timo. Thank you for that explanation, that is something new to me. So relative stiffness / wiggly-ness is determined by other factors (material density, or cross section?), heat treating and tempering mostly effecting failure resistance and the nature of the failure?

Basically, yes. Heat treatment determines failure point and mode of failure.

Density for various steel alloys and iron doesn't vary much; it's the cross-section that matters for stiffness. For side-to-side stiffness, the stiffness is proportional to the width, and the cube of the thickness. That cubic dependence is why diamond-section blades, especially hollow diamond with high ridges, are so stiff.

IIRC, there's less than 10% variation in elastic modulus (Young's modulus) over most steels, including stainless steels with about 30% of the alloy being other than iron. (Cast iron can have a much lower elastic modulus.)

A handy table: http://www.engineeringtoolbox.com/young-modulus-d_773.html