Steel vs flesh & bone: comparative strength

I’m trying to get a sense of how much stronger I would be were I a “man of steel”. IANA physicist or structural engineer, so please bear with with my less than accurate terminology. I can think of at least three categories if comparison. though the same forces may be at work in more than one of them.

First would be puncture resistance. How much more force does it take to drive the same object through a 1/16" thick piece of steel than the same thickness of human flesh? I’m guessing there’s not enough difference among the soft tissues to worry about whether it’s skin, fat, muscle or something else, but maybe I’m wrong.

Second would be sheer strength, if that is the correct term. Basically, fix one end of a sample in place let the other hang down and start loading on weight until the sample pulls apart. .

Is rigidity the correct term for this last one? Again, fix one end of a rigid sample, and stick out the other end sideways and load on weight. Eventually, lateral forces will snap or deform the sample. One would need to test this using bone, or conceivably tooth enamel.

Are there more strength comparisons that I’m missing?

Being a man of steel would not be as advantageous as you might think. Good engineering is not merely choosing the strongest material but the one most appropriate for the task. For example cold rolled alloy steek l skin would be more puncture resistant than the kind you have now but there’s the small problem that it doesn’t flex very well. Steel is not yet self repairing and self replenishing, another disadvantage over flesh.

So much for the IMHO part. Y ou can find engineering specs for pretty much any steel alloy you choose from mild, low carbon steel to heat treatable tool steel.

The “springiness” property is called modulus of elasticity. That basically tells you how much material temporarily deforms under a given force. Rigidity is a $64k question. It involves the modulus of elasticity but also the dimensions, particularly the cross section size, of the part. If I have a one foot long rod and a one foot long tube which have the same amount of material the tube will be more rigid provided it does not collapse. This is seen often in bicycles where an aluminum bike will be more rigid than a steel one even with a lighter frame and more elastic metal because the tubes are larger in diameter.

The force needed to permanently bend something is called yield strength and expressed usually in pounds per square inch cross section area. The force required to break something is tensile strength. Hardness is measured on a couple of difference scales, Rockwell and brinell by denting material with a ball bearing under a given force and measuring how big the resulting crater is.

The Riddle of Steel — “Steel is stronger than flesh, but what is stronger than steel?”

Answer — “Flesh is stronger! What is the blade compared to the hand which wields it?”

Here are some figures* for the strength of bone, for comparitive purposes (someone else will have to supply the numbers for steel):

Compressive strength: 1330 - 2100 kg/cm[sup]2[/sup]
Tensile strength: 620 - 1050 kg/cm[sup]2[/sup]
Shear resistance: ~500 kg/cm[sup]2[/sup] directed parallel to the grain, perhaps as high as ~1176 kg/cm[sup]2[/sup] for forces directed perpendicular to the grain

(* from Analysis of Vertebrate Structure, Milton Hildebrand, 3rd Ed.)

Let’s see; kg/cm[sup]2[/sup] is kind of a non-standard engineering unit, at least where I come from, MPa being preferred. 1 kg/cm[sup]2[/sup] = 0.1 MPa, roughly, so let’s say that, for bone:

Tensile strength: 62 - 105 MPa
Compressive strength: 133 - 210 MPa
Shear resistance: ~50 MPa directed parallel to the grain

For a cold-drawn 1040 steel, for comparison:

Tensile strength: 669 MPa
Compressive strength: 669 MPa (assumed to be the same as tensile strength for ductile isotropic homogeneous materials)
Shear strength: ~386 MPa (assumed to be a factor of 1/sqrt(3) times the tensile strength for ductile isotropic homogeneous materials)

However, these could be much higher, depending on the steel you use. 4340 steel can have a tensile strength of ~1700 MPa, and cold-drawn 302 stainless up to 2400 MPa. However, higher strength comes at the expense of the steel becoming more brittle (particularly, I suspect, in the case of the stainless; 1/4 hard 302 is shown with a tensile strength of only 862 MPa).

Note also fourthings:

  1. as brittleness increases, materials may tend to have different compressive and tensile strengths
  2. as brittleness increases, the shear strength calculation changes
  3. ductile material like steel also have a yield strength which is less than the tensile strength. Tensile strength is where the material breaks; yeild is where it starts to permanently bend. Yield for 1040 is 568 MPa, yield for 4340 is 1586 MPa, yield for annealed 302 is 517 MPa.
  4. Yield strength of very soft steels is probably as low as ~250 MPa as a lower bound.

All numbers from Mark’s Handbook.

I remember seeing somewhere that ounce for ounce, bone is stronger than steel, this is due to the structure of the bone itself. I think I saw this on the discovery documentary about building the Eifel Tower.

The numbers seem to belie that fact. I guess it wouldn’t be the first time that the Discovery channel got something wrong.

No; I’d agree with Discovery. At least, depending on how you define “strength”.

Using the compressive strength, which is reasonable, 1040 steel is to 3.2 to 5.0 times as strong as bone, while 4240 steel is 8.1 to 12.8 times as strong.

However, bone is say, 0.8 - 1.0 gm/cm[sup]3[/sup], while steel is 7.8 gm/cm[sup]3[/sup], giving a weight ratio of 7.8 to 9.8.

So bone, by weight, is stronger than a typical, middle-of-the-road cold-worked 1040 steel, and is actually close to the very strong 4240 steel. Thus, some steels are stronger, ounce for ounce, than bone, but I would venture to say the most steels are not. Plus, remember that, with the stronger steels, you give up some toughness (they’re more brittle), so I’m not sure you’d want to use a very strong steel as a substitute for bone, anyway.

Well, my surgery team used chrome steel to replace my femur and tibia back in 1987. But I don’t know how strong it is.

This is related to what I was talking about with the rigidity of a rod vs a tube. A bone is even more complex with a structure that is more like a fiber composite with varying density and marrow cavities in long bones. I don’t know if that can be replicated in steel with any current technology. There is often a vast difference in the strength of a material compared to the strength of a constructed part.

I was considering steel skin vs. bio skin. The regular kind has moderate abrasion and puncture resistance. A steel skin that was just as light would have to be foil thin so its puncture resistance would be poor. Abrasion resistance would be better on an initial test but it would be quickly worn away as it isn’t being constantly replaced.

So are you thinking of keeping the same mass or same dimsensions when you get transformed into metal-man?

Do you want to remain the same size (as in height/girth) and replace your bones with steel like the Terminator and add a steel skin? If so you’d probably weigh around 1600lbs or so, and you’d be a lot stronger overall.

Or do you want to remain the same size (as in mass) at say 200lbs? In that case you’d probably stand about 3 feet tall. In either case I think you’d be proportionaly weaker… for your mass you wouldn’t have as much power or toughness. Although compared to someone the same height you would be stronger/tougher.

“Man of Steel” is catchy, and you could probably use some supernickel steel alloys for joints, but I would replace the long segments of the bones with hollow titanium tubes. If I were going to go really exotic, I’d do a carbon-carbon composite casting or lay-up of the skeleton. I have no idea what to replace your skin with, though. Genetically modified elephant skin? Armadillo hide?

By the way, some of the terminology you were looking for:

Ultimate strength is the amount of force (or force per area, depending on context) you can put on an object before it breaks. The object may or may not yield before this. There are a few different kinds of strength, depending on how you apply force: Compressive strength applies to forces pusing on the object, tensile strength applies to forces pulling the object apart, and shear strength applies when you’re trying to slide the object apart.

You can also combine the yield of an object in all different directions, in an object called the stress-strain tensor. Basically, given how much stress (force per area) you put on an object in any direction, this will tell you how much strain (deformation) the object will undergo, and in what directions. High strain is not necessarily a bad thing, depending on how elastic the object is. A spring, for instance, or a rubber band, might be able to easily double its length without damage, but a solid steel rod would probably break if you tried to strain it that far.

Keep the same dimensions, increase the mass as necessary. Being a two foot tall man of steel just wouldn’t be too impressive, even if I could kick anybody’s ass. People would think I was a *boy *of steel, and I’d get lots of r2d2 jokes – never twice from the same source, however :stuck_out_tongue:

With all this talk about strutrual materials we’ve neglected to give our man of metal alloys and carbon fiber composites a motive force. If it don’t move onit’s own it’s just a statue of a metal man. Too bad we don’t have a material that can perform work and serve as part of the structure like muscles.

Actually, there is a stress tensor (which can be defined in several ways) and a strain tensor (which can also be defined in several ways). To get strain from stress or vice versa, you need a “constitutive equation”, which is different for different materials. The most common constitutive equation is that of linear elasticity, which states that strain is directly proportional to stress, when everything is happening in one direction.

Bone is orthotropic, meaning it has different stiffnesses in three mutually perpendicular directions (because of its porosity and how it grows in plates). It also has different strengths in different directions. Bone is still pretty much linear elastic despite its directional dependence. Metals are generally assumed to be isotropic and linear elastic for small strains. Muscle and skin are viscoelastic, meaning they dissipate as well as store energy when stretched (I know there is a more precise mathematical definition of viscoelasticity but let’s not go there unless someone really wants to).

I also note that “strength” has a different meaning in mechanics from what it means in comic books or the gym. When we say “Superman is strong”, we usually mean that he can exert large forces with his muscles. But when we say that steel is strong, we mean it can withstand large forces before breaking.

And Padeye, there are materials that can both exert forces and supply structural stiffness. One is Nitinol