The Physical Reality of Forceful Edge-to-Edge Impacts
By Kevin Cashen
Can a piece of steel with a sharpened leading edge impact another similar steel with no effect or consequences? Can it be done repetitively? What are the effects, short term and long term?
The ability of any material to penetrate and displace another is heavily dependant on hardness. Harder materials will resist deformation to greater degree than softer more malleable ones. This concept is so elementary that it renders many aspects of the above questions academic. Yet, as with many things regarding swords, common sense is far too easily thrown out the window in order to better suit our preconceived fantasies about historical blades.
Although there are some specific complications and variables to the opening question above, when applied to heat treated steel in the form of an edged tool, the same basic principles apply here as do apply anywhere in the physical universe. So, in order to get down to the very simple crux of this basic question I will address some of these complications and variables that many sword enthusiasts may cling to in order to maintain the stability of their delusions about forceful edge-to-edge impacts.
The two greatest determining factors in the question of a metal edge to penetrate (cut) without itself being deformed in some way would have to be heat treatment and edge geometry. Heat treatment in blades can almost be distilled down to the simple concept of the perfect compromise of softness versus brittleness for the given application. The ability to deform without failure is due to ductility. The ability to withstand deformation is due to hardness. In extremes, the softness of excessive ductility renders the tool as useless as the brittleness of excessive hardness.
A decidedly soft blade will have substantial amounts of metal displaced when penetrative stress is applied, but will merely deform before any fracture. An overly hard blade will not plastically deform at all yet will readily fracture along regular crystalline planes within the metal, or to put it simply "something's got to give." When the atomic arrangement is such as to not allow deformation via a slip system the crystalline structure will simply come apart at the seams.
These situations can be observed in two types of edge damage. Notches in the edge resembling the cross section of the object struck are clearly the result of plastic deformation as the metal moved and conformed to the object. Chips missing from the edge as if some kind of ravenous little steel gremlin had taken hungry bites are highly indicative of brittle fracture and blowout at the point of impact. The metal could not deform and the applied stress produced a fracture that found a path to travel through the crystalline interfaces around the point of highest introduced energy.
The question is almost unavoidable; is one better or capable of defeating another? The answer would most certainly be yes, for both! This is where the influence of edge geometry comes into play with the heat treatment. All things being equal, a dead soft metal will be penetrated by a very hard one. But if a very soft metal of greater mass and a strong geometry is impacted by one with a very thin and hard cross section, the brittleness of the thinner piece will be the greater weakness and catastrophic failure may occur. Chisels can be made to cut steel all day so long as the material being cut is of equal or less hardness and the edge of the chisel is obtuse enough to provide the reinforcement required.
Now that we have some of those basics out of the way let's look at what impact is all about with steel. Simply put, like so much else in our universe, it is energy transfer. The kinetic energy of a moving blade is focused to an incredibly concentrated point by the blade edge. For softer materials (i.e. flesh) very little resistance is offered to that energy and the blade is required to absorb much less of its own impact via Newton's pesky little laws. Have you ever noticed how a really well executed sword cut feels like it hit nothing at all as the most efficient use of that concentrated energy was achieved? Objects with increasingly more resistance will force the blade to absorb increasingly more of its own energy in impact. The object that forces the blade to take more of its own energy than that object itself has to deal with could end up the winner.
Where does that energy go? In steel that is soft enough to deform, the energy will be used for that deformation in a process known as slip. Stacks or rows of atoms in the crystalline structures will slip over one another and allow for the plastic deformation of the material. But in our universe energy simply doesn't disappear it is always transferred instead. In this case the kinetic energy of impact will be stored in many concentrated points within the crystalline array as strain. Strain is the driving force for multitudes of changes that can occur in steel, not all of them good.
The really nasty part about strain energy is that you can't easily see it or feel it and there are only a few ways to get rid of it in steel. A notch on a blade edge can be removed by filing the entire edge down to the bottom of that notch and everything will "appear" as good as new, but the deformation will have displaced material all around the notch and the structure of the metal in that area (deeper than the notch itself) will still have been permanently altered in ways that include a much higher concentration of strain energy.
What of it? This part is frustrating in that we have to actually point out the obvious to so many normally intelligent people, but simply consider the following: Take a piece of wire and lightly tap it with a chisel and then begin bending it over an area that includes the notch. Where will the stress automatically concentrate and result in the failure of the previously tough wire? Or instead, simply score a piece of glass and then apply pressure from the back. Where will a crack unavoidably propagate and travel on the piece? When material scientists or engineers wish to perform Izod or Charpy type impact tests on any material from dead soft to brittle hard how do they insure the fracture will occur in a predictable place and fashion? They notch it!
Notches in a sword blade are points of concentrated opportunity for catastrophic failure; this is not opinion or belief, it is material science fact based upon the way our universe works.
A quick side note on flexing: flexing a blade shows nothing about how brittle it may be in impact situations, as these are entirely different properties of a material. A piece of glass can be heated and drawn out until it will flex like a monofilament line, but the slightest tap will reveal that it still has the same low impact of brittle glass. In my opinion, people who insist on flexing swords to demonstrate the quality of heat treatment are either purposefully, or unknowingly, practicing smoke and mirror gimmickry to sustain a complete falsehood. No matter how far a person flexes a blade, the same flex can be achieved in another blade of any hardness, simply by grinding it thinner. Flexing is only important to people who are used to blades that snap like glass or ones that bend easily with very little effort.
One final point for this topic that should not be ignored is the effects of repeated flexing. If there is any ductility to the blade at all, every flex will indeed store some amount of strain energy in the steel, and while it may take many flexes for any manifestations of this strain to even be noticeable, the blade you have immediately after the flex will not be the exact same pristine blade you took out the box. And oddly enough, the way most modern sword makers would instinctively deal with this is to make the blade softer so that it has all the more storage space for strain before fatigue effects show. It is kind of like adding more hard drive space because of all of the viruses that are slowing down your computer; with enough gigabytes your computer can absorb 10 times more malicious code. But if the blade does not deform plastically, the strain energy does not build up, so increasing hardness could eliminate the fatigue issues--a good firewall makes a whole lot more sense than more hard drive space. Think about it, a good truck spring doesn't fail by bending, rather it eventually snaps when it load is exceeded. Bending is the exact opposite of what you want in a spring.
Now back to two edges coming together. If both blades have the same edge geometry and one is harder than the other, the harder blade will invariably be the winner, until the brittle limits in impact strength are reached. When a blade reaches a certain level of hardness, impact will simply result in catastrophic failure through the entire cross section.
I previously mentioned the process of deformation known as slip. Slip is the motion of one stacked plain of atoms sliding over another. Imagine the metal atoms stacked in an orderly pile like ping-pong balls or oranges at a fruit stand. If one whole layer could slide independently of the other, the atoms, like the oranges, would slide over top of the lower ones until they came to rest in the next space over, created by the stacking. This would result in permanent deformation and the accumulation of strain energy. If the stacking resisted enough that the atoms only slid half way, without falling into the next space over, and then return to their original position upon removal of the stress, the deformation would be elastic in nature and one would have a spring like flex instead of a bend, gouge, or dent.
A well-hardened and tempered blade will tend to flex and release the energy, as opposed to deforming and storing it like a blade that is too soft. Yet at the other extreme, there is so much resistance to any deformation, either plastic or elastic, that complete failure is virtually the only option left to resolve the impact energy dilemma. All of this kind of lays the groundwork for the next question: Can one whack the sharp edge of one sword (regardless of quality) against the sharp edge of another and expect to have either remain in the same pristine condition in which they first obtained it new?
This depends, but on the average the answer will be a resounding NO! If one has nice beefy edge geometry and only comes up against a blade of lesser hardness, it could do very well, but this will be at the expense of the other impacted blade edge; that energy simply has to go somewhere. But on the whole, most blades will be of similar hardness and will see some level of permanent traumatic effect virtually anytime their edges are brought together with even moderate force.
Very few swords blades on the market today are expected to see Rockwell hardness in excess of the 55 "C" scale so the permanent effects will undoubtedly be plastic deformation in nature with accumulated strain energy. The instant that blade contacted another object with enough force to deform to any degree at all it will not be the exact blade that was originally purchased. Indeed even heavily flexing the blade could impart enough strain energy to say that it is not any longer equal to the original condition.
Over time, cars wear out, clothing gets threadbare and frayed, even the human body, no matter how well maintained, expires. A sword can see much rougher use than these yet is expected by so many naive and unrealistic minds to be a tool most indestructible and everlasting.
As to the nature of the strain energy in the blade resulting from impact; all of my studies in metallurgy and all of my experience in a lifetime of bladesmithing have taught me one very firm a reliable rule: The secret to improving the overall toughness and strength properties of steel resides in evenness. A completely even heat treatment will result in a straight blade that will have excellent strength versus toughness, with no irregular points of stress concentrators. It could be said that plastic deformation (in lay terms "cold working") could result in a "stiffer" or harder blade. Indeed an area that has been heavily deformed will resist further deformation more than adjacent untouched areas due to the accumulated strain (higher dislocation density), but in order for this to be at all useful for improving the blade as a whole it would have to be done completely and evenly throughout the entire structure of the blade, otherwise we are simply back to the notched wire we discussed earlier. And if the blade was subjected to such an even deformation (say, by a rolling mill), the results of strain hardening are always anisotropic in the direction of the deformation. Any testing or reading transverse to this would show quite the opposite.
I am a professional swordsmith, not a swordsman, but common sense and the basic laws of physics tells me that a glancing blow will be much easier on both sword and wielder than an edge to edge one. To take a forceful edge blow in a direct 90-degree angle of attack requires all of the energy of impact to be taken by the blade's edge and the eventually the hand wielding it. A glancing blow will simply redirect the energy away and makes infinitely more sense both scientifically and tactically. Furthermore, the physical advantage to be had by spreading that concentrated energy of an impacting edge over a wider area simply cannot be ignored. Taking the edge into a blade flat (or a wide domed area of plate armor) makes such sense according to the simple laws of physics that it just cannot be ignored.
If a sword fighter purposefully takes all of the cutting energy concentrated to that fine planar point of the edge with an equally fine point, according to the laws of physics, according to very basic metallurgical principles, and according to tactical wisdom, they are flirting with disaster.
See also: On Damaged Edge