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The problem is that I would like to know how strong the creature I have in mind will end up being and if that would be too strong. I know that 15th century plate armor was very good against slashing weapons and that swords would be the last thing one would like to use against an armored opponent, which is why I'm having a hard time getting a clear idea of how much force it would require.

The Scenario: A Humanoid man-made life form enhanced by some of the most advanced magic to enhance it's strength for a short period of time is capable of slicing in half a soldier clad in full-body steel plate.

The creature is aproximately 8 ft tall, 425 pounds and is using a 2.5 meters long Zweihänder which weights about 7.5 kg total.

Skirting around the issue of the sword possibly breaking and how implausible it would seem to cut cleanly and instantly through steel plate in such a fashion, how strong would that creature have to be to achieve such a feat in comparison to a normal human? Or the strongest human, even.

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    $\begingroup$ How sharp is the edge of their super-sword? For example, in David Weber's Safehold the protagonist android has a futuristic high-tech sword made of some sort of stainless unobtainium quite indeformable and with an edge one atom thick; such a sword can indeed cut through just about anything provided it moves sufficiently fast. Said protagonist routinely cuts through armor and vertebrae. $\endgroup$ – AlexP Dec 28 '18 at 14:33
  • $\begingroup$ I was thinking of a sword as sharp as your average sword. It doesn't need to completely survive the impact against the soldier though. $\endgroup$ – Azumentris Dec 28 '18 at 14:38
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    $\begingroup$ The Mythbusters did quite a lot of work on this sort of question - even going so far as to use pig carcasses. It turned out to be remarkably difficult to slice through an entire body. "The Build Team created a razor-sharp, stainless steel fan powered by a lawn mower engine for the "lover's leap" scenario. Even that did not achieve decapitation, but it did cause lethal and horrifying injuries" - deep cuts, en.wikipedia.org/wiki/MythBusters_(2004_season) - Note that the sword might simply break regardless of the strength of its wielder. $\endgroup$ – chasly from UK Dec 28 '18 at 15:51
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    $\begingroup$ Considering the types of materials you are talking about, this video may be a good starting point youtube.com/watch?v=ZCtV_M1n38c (caveat: I haven't checked the math) $\endgroup$ – K Mo Dec 28 '18 at 16:16
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    $\begingroup$ I can't help but ask why you need the answer to this question? The force needed just to stamp a steel plate in half is enormous (many tons of force), and the sword must be stronger than the armor to make that happen. And it's not just strength, it's speed or the metal will bend (not unlike a crushed beer can). There won't be a blievable way to describe the physiology of such a creature - so this is one of those, "make it so!" decisions that's common in storywriting. Why do you need the answer? $\endgroup$ – JBH Dec 28 '18 at 16:35
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For starters, a zweihander was actually more of a thrusting weapon than a cutting weapon, used more like a spear than a sword for the most part, unless the wielder was out of formation in a looting situation. If you look at the martial art manuals for zweihander use, you'll see that their wielders - at least those that survived more than one or two battles - fenced with them rather than winding up to deliver king hits. Their advantage over a regular sword was their length and leverage, with their users hands a couple of feet apart (one grasping the hilt, the other grasping the blunt part of the blade behind the thrust-stop spikes), they could push through a single-handed weapon parry, not with momentum, but with leverage.

So, I can't imagine a professional landsknecht swinging his zweihander around for any purpose other than to frighten non-soldiers.

Secondly, being a largely thrusting weapon, a zweihander is likely to be none too sharp, as the OP has stated. Sharpness is effectively a pressure multiplier. If the wielder of an edged weapon applies a force, F to the hilt of his weapon over a surface area, A1, and applies that force via the edge of that weapon to a target, then the force F is applied over a smaller area, A2. Since pressure is force divided by area, pressure, P at the edge of the weapon is (naively):

P = (F x A2)/(A1^2)

Obviously, minimising A2, by sharpening, allows a higher pressure to be applied. Zweihanders achieve this not so much by having a sharp edge, but by having a sharp point, which is easier to make small than a long edge.

Of course, since weapons are swung or thrust from some point distant from their intended target, the force applied to the target via the edge/point is dependent upon the force the wielder applies to the weapon over the time that the weapon is swung prior to and during impact. The blow then becomes a question of impact energy.

So, if the OP's homunculus is to swing a zweihander (against all common sense, unless it is immune or higly resistant to potential counterattacks) and cleave a fully armoured soldier in two, then a number of factors come into play:

  1. Unless the zweihander has been sharpened to a degree more akin to the sharpness of a scalpel or a Japanese samurai's katana, this will mean that a higher amount of input force will be required.

  2. Oiling or greasing the blade - or potentially even plating it with lead - will assist by reducing the friction between the blade and the cut edges of the opponent's armour.

  3. Plate armour came in several grades, from thin and even rusty iron or mild steel, to thick high-carbon steel proofed against bullets. The latter obviously being far more expensive than the former, with many variations in between. A 'fully plate armoured opponent' may have been wearing thin mild steel that a blow from a merely human opponent could be expected to penetrate if delivered with strength and skill by a sharp pointed weapon, or they may have been wearing thick, high-carbon steel siege plate, against which almost any blow could be expected to be more likely to knock the wearer down than to penetrate. We will assume cheap, thin mild steel plate armour that has suffered from rust during its career, and is now effectively thinner than originally manufactured, even if it is now polished.

  4. The capabilities of the wielder of the zweihander now come into play. Being big and muscular helps in applying force to the hilt of the zweihander, but being big and strong is not all there is to it. Muscles can only contract so fast, and the closer they approach their maximum contraction speed, tbe less force they can apply. Animals achieve high strength by greater leverage - a higher joint-to-muscle-length to limb-length ratio, and achieve speed by making this ratio smaller. Both can be achieved by having big muscles and a small joint-to-muscle-length to limb-length ratio. If magically enhanced, the big muscles may contract faster and more forcefully than usual too.

  5. The technique with which the blow is delivered also plays a part. By "winding up" for the blow, bringing the zweihander around behind the wielder's body and then delivering the blow by applying muscular force not only from the arms, but also the waist and hips, the greatest force can be applied to the hilt of the weapon over the greatest time, resulting in a higher velocity.

  6. The grip the wielder of the zweihander has on the ground also plays a part. Apply more force to the blade than the grip between the wielder's feet and the ground can take, and the wielder's feet will slip, reducing the overall force over time applied to the blade.

  7. Just where the victim of the blow is hit will affect the blow's effectiveness. Obviously, a hit to the abdomen, with the blade passing through one of the intervertebral disks rather than bone, and the blade also hitting the armour at a joint so that the metal need be less cut than merely bent out of the way would make the end result easier to achieve. This would be best achieved with the attacker's shoulders level with the victim's waist given the typical design of plate armour waist articulation.

  8. Obviously, the victim failing to block or parry the blow is very important. Perhaps they are engaged with multiple foes, or have dropped their weapons, or are simply caught off-guard.

So, to ask "how much stronger" obviously depends on all these factors, and can't be answered naively. However, assuming that all else is optimal, the wielder would not need to be greatly stronger than a very strong human, and as other factors become less optimal, the strength required increases to the point where it is impractically high.

To attempt to give a naive figure, if all else is optimal, perhaps twice as strong as a dedicated bodybuilder would be the right figure, but the creation would also be faster and heavier, and may even have clawed feet.

Whether or not the zweihander survives unbroken during this event would be dependent upon the negative factors I have already mentioned, as well as the quality of its metallurgy, construction and its sharpness being points in its favor.

I base this on historical records of merely mortal samurai cutting unarmoured prisoners in two at pretty much every place and angle imaginable, the most difficult successful cut being recorded on the blade as proof of its quality. Famously, one prisoner, being informed that he was to be executed by a cut at waist level, rather than exhibiting fear, quipped that he wished that he had been given more notice, so that he could have swallowed rocks that would have chipped the samurai's precious blade.

Additionally, Japanese records of no-dachi wielding samurai in battle record said samurai cutting horses in two through the body on occasion, the no-dachi being an oversized curved sword like a katana that were in actuality used in the manner that it is only imagined that zweihanders were used.

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  • $\begingroup$ Also, of course, making a cutting edge sharper (and with a smaller edge angle) makes it more fragile. This may not be an overwhelming problem in, for instance, an executioner's blade, but on a battlefield where repeated uses should be considered the norm it's a real problem. $\endgroup$ – WhatRoughBeast Dec 29 '18 at 15:47
  • $\begingroup$ @WhatRoughBeast, I agree, keeping a blade sharp through a battle would have been a problem. However, Hollywood notwithstanding, I don't believe that trained swordsmen parried edge to edge, a parry would have placed the side of the blade in the way of an incoming blow, and an attacker, seeing that his blow would be parried, would turn his blade if he could to protect its edge. So, any impact that might blunt a blade would likely be with an enemy's armour, if the swordsman could help it. $\endgroup$ – Monty Wild Dec 30 '18 at 1:34
  • $\begingroup$ Also, in Japan, a sword polisher would ask his customer what use he expected the sword to be put next before he started working, since the answer would affect how he sharpened the blade - use against armour in battle would require a blade with a edge sharpened to a larger angle than use against unarmoured peasants and silk-wearing samurai and ronin would require. $\endgroup$ – Monty Wild Dec 30 '18 at 1:38
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Turn your sword from a slashing weapon (like a katana) to a chopping weapon (a grand shamsheer, perhaps, with a lot of mass in the tip and a more axe-like edge contour) and it ought not to take even superhuman strength to make the cut you describe. Get a good swing (say, blade velocity around 30 m/s, about like a good fastball, well under the club head speed of a professional golfer), and a blade weighing 3-4 kg can readily cleave through plate light enough for the wearer to walk around -- along with the body inside.

Cutting through the abdomen, FWIW, is easier than cutting the legs or ribs -- much less bone in cutting one vertebra, than in cutting a femur, multiple ribs, or even a single humerus.

Now, your cutter will be taking a "mighty swing" to do this, so he'll leave himself open for stop hits, but if the victim is busy cowering instead of trying to kill as he dies, he'll fall just the same.

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  • $\begingroup$ Alright, i'll just have to adjust it then. It will make things a bit easier as well if the creature doesn't end up being too inhumanly strong either. I'm asuming about 5-10 times the strength of the strongest human male would be a good enough point to make it terrifying enough. That should be enough to cleave a plated foe, then. $\endgroup$ – Azumentris Dec 28 '18 at 22:54
  • $\begingroup$ This is a rather naive analysis. $\endgroup$ – Monty Wild Dec 29 '18 at 9:07
  • $\begingroup$ I would really like to see some justification on your claims. Simple arithmetic says that a 1 meter sword with a velocity of 30 m/s 1/3 of the way back from the tip (which is a close approximation for the best strike point) means an angular speed of 20 rad/s, or about 7 rps. And that simply isn't going to happen. Furthermore, since the target is not fixed in place it will tend to move with the impact, severely diminishing the cut's effectiveness. I would need some real-life demos to accept your argument. If nothing else, swords are very bad at dealing with solid armor. Cutting steel is hard. $\endgroup$ – WhatRoughBeast Dec 29 '18 at 17:51
  • $\begingroup$ ... There is a very good reason war hammers were invented. $\endgroup$ – WhatRoughBeast Dec 29 '18 at 17:52
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I am afraid no amount of strength will really help; only a radically different construction of the "sword" can do anything to it.

The main point is that the sword is stopped by the dense matter it is cutting through. The depth it can reach is determined by the densities of the sword and the matter it cuts, and by the width of the sword blade. To cut through iron (or steel) that has approximately the same density as the sword, you need a rather large blade; as mentioned in the answer of @ZeissIkon an axe-shape weapon will be better for this purpose.

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  • $\begingroup$ Okay, I suppose the weapon will have to be different. I suppose making it just 5-10 stronger than a peak human would be more than enough with the right (speciallized) weapon to cleave through someone. Thanks. $\endgroup$ – Azumentris Dec 28 '18 at 22:57
  • $\begingroup$ Another rather naive analysis. The cutting power of a sword depends on more than Newton's impact depth approximation. $\endgroup$ – Monty Wild Dec 29 '18 at 9:08

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