An old bayonet in my collection
Many, if not most, had fullers and their function was to create a stiff stout blade that wouldn't fold without weighing a ton.
A blade of identical length and width and weight would be thin and flimsy.
Yes. Like I said earlier, 2 dimensions and weight need to be identical for the comparison, not a single dimension and weight, which would as Rick said, allow one to be thinner with a fuller which of course would not be stiffer in the lateral direction than the thicker, unfullered blade. I think you're mostly right that we all are comprehending this and speaking past each other a little bit, or, in my case being a little pedantic about assuming someone means 2 dimensions when they only say one. I'm not trying to be pedantic about it for the sake of argument, only because it's important to understanding and communicating clearly what a fuller does, how it does it, and why it can be as you said earlier, an advantageous design feature.
Here is a bowie blade I modeled (and made if it matters), by dimension:
Constrained at the tang with the constant force I used for all simulations applied it deflects 1.994" at the tip.
Constrained at the tang with the force applied to the edge it deflects .101" at the tip.
Now, cut a healthy fuller into that blade, which coincidentally reduced the mass exactly 1 oz by luck or poor simulation tolerance, it of course deflects more:
Only slightly more in the edge direction.
So at this point - I think most of us agree, removing material from a blade to create a fuller, by whatever means, without retaining that mass somewhere else in the blade (same weight) does not stiffen it. What I take issue with is when people get lazy and say, as Rick posted originally, "Now I'm putting a fuller in to stiffen it." No, it's stiffer now than it will be when you put the fuller in. You're putting a fuller in to reduce the weight while retaining as much strength and stiffness as possible.
Then I added an ounce of mass back into the blade. I retained the same thickness of the spine, and simply made it taller.
And applied the load again, in the same points relative to the shoulders and edge which stayed constant:
This is what I was questioning about last night. It still deflects more in this axis than the non-fullered blade. It matters
where you add the mass that would normally reside in the fuller, for example by creating a "T" cross section that's thicker at the top of the T than the original spine thickness. But putting these fullers in still weakened this blade when the .200" cross section max thickness did not change.
Apply the force to the edge:
And you see that making the blade taller DID make it stiffer in that axis.
So my conclusion is this: If you're building a knife and you are working within weight and size constraints, fullers are an optimal way to make weight without getting too small. If you can reuse that mass in either the width of the blade or the thickness of the spine, you can create a knife of the same weight and length, that acts stiffer, but where you use that mass is important in how the blade is intended to be used. A bayonet I would want to increase the "T" width thickness of the spine to resist the type of back and forth action that might occur when stabbing and withdrawing a blade. A chopper I would want to make taller to resist flexion in the other axis.
I'm not an engineer. My models and simulations are likely not perfect. I'm doing this to the best of my limited ability because it's within my ability where destructive testing is not, it's easy to share the information and that's my only goal is to share the best information I have because it's an interesting topic. If someone with more knowledge in FEA simulations can tell me I'm doing something wrong I'd love to know what it is because I don't have any formal training. Because of this I've tried to keep everything constant in the simulations as I possibly could, so they would be like a dial test indicator, maybe not tell us exactly how far this blade will bend with a given force, but only indicate the difference changing the shape and mass of the blade might cause.