Toughness - Expansion on Charpy C-Notch Values

What surprised me is that CPM154 may have actually come out on top in the stainless category because it is only 2 ft lbs down on S35VN at higher hardness. With that said, the addition of niobium sure seems to have toughened up the old S30V formula a bit. I would like to see the comparable numbers for Elmax, CTS-XHP, Niolox and a few others. The M390 results seem a bit disappointing considering what I have heard about its pricing. As for the effect of having a lot of chrome to make a steel stainless, I think that you can see that result by looking at the numbers for Vanadis 4E. That stuff, like the CPM 4V, supposedly has abrasion resistance like S35VN or S30V which would be at least 25% higher than 3V, but its impact resistance is 20% higher than A2 at similar hardness levels.
 
I tried contacting Bohler to see if they would be willing to divulge the values of some of their popular steels. Waiting a response. If I get one I will be sure to post the info.
 
Here's the PD-1 Charpy data. I don't really understand why it gives two values (example: 60.5HRC = (54) (72)), but perhaps you do!
 
Here's the PD-1 Charpy data. I don't really understand why it gives two values (example: 60.5HRC = (54) (72)), but perhaps you do!

The first value is 54 foot-pounds which converts to ~72 Joules. The proper conversion factor is 1 foot pounds (Ft.Lbs) = 1.35581795 joules (J), so their numbers probably involve some rounding up/down.
 
Thanks for the additional info guys, I'll take look at it and see about adding to the OP.

I'm now trying to figure out whether or not the different Charpy tests are directly related. For example, if a C-notch sample renders a result of 20 ft/lbs, and the same alloy unnotched rendered 30ft/lbs, could another alloy unnotched that gave 30ft/lbs automatically be assumed to result in that same 20ft/lbs C-notch reading? I believe this is the case, but I'm not sure. If it is, there is a wealth of data that could be correlated to expand the list of known values considerably.
 
Thanks for the additional info guys, I'll take look at it and see about adding to the OP.

I'm now trying to figure out whether or not the different Charpy tests are directly related. For example, if a C-notch sample renders a result of 20 ft/lbs, and the same alloy unnotched rendered 30ft/lbs, could another alloy unnotched that gave 30ft/lbs automatically be assumed to result in that same 20ft/lbs C-notch reading? I believe this is the case, but I'm not sure. If it is, there is a wealth of data that could be correlated to expand the list of known values considerably.

I'd be cautious about making those correlations without having data from actual measurements. Where you only have data for one type of test, just be sure to highlight it rather than making unnecessary assumptions.

Also be sure that values were acquired via Charpy rather than Izod impact tests as these are performed in different ways.
 
You can't draw direct correlations between charpy notch and unnotched tests. Because notch sensitivity affects the results. A steel that is more notch sensitive will show a bigger difference than a less notch sensitive one.

I'd say unnotched tests would be better standards for knives than notched ones since we tend to have fairly smooth, unnotched blade profiles.
 
I'd say unnotched tests would be better standards for knives than notched ones since we tend to have fairly smooth, unnotched blade profiles.

Not once they are used! ALL steels demonstrate higher impact resistance when un-notched, and all knives are less prone to fracture if perfectly intact. But many knives have hidden occlusions in the steel or small chips from damage (e.g. on the edge) or a choil-notch or a sydie-hole or a tang-pivot or thumb-stud screw-hole, etc. ALL of these can serve as a "notch" when stressed appropriately. Indeed, I do not understand why anyone would bother using UNnotched standards as they provide no real-world applicability. It is better to know how the steel will perform under less than ideal (i.e. real) circumstances, and notched-samples more closely resemble that.
 
Not once they are used! ALL steels demonstrate higher impact resistance when un-notched, and all knives are less prone to fracture if perfectly intact. But many knives have hidden occlusions in the steel or small chips from damage (e.g. on the edge) or a choil-notch or a sydie-hole or a tang-pivot or thumb-stud screw-hole, etc. ALL of these can serve as a "notch" when stressed appropriately. Indeed, I do not understand why anyone would bother using UNnotched standards as they provide no real-world applicability. It is better to know how the steel will perform under less than ideal (i.e. real) circumstances, and notched-samples more closely resemble that.

Un-notched samples are used when the steel (or other material) isn't tough enough to give a reliable reading when notched, or when a difference isn't evident. Some times when a notched sample is used, all you get is 1-3 ft-lbs. or the data is too scattered to tell anything reliably. Granted, steels that use an un-notched sample generally have very low toughness, so the differences won't be too great anyway.
 
Un-notched samples are used when the steel (or other material) isn't tough enough to give a reliable reading when notched, or when a difference isn't evident. Some times when a notched sample is used, all you get is 1-3 ft-lbs. or the data is too scattered to tell anything reliably. Granted, steels that use an un-notched sample generally have very low toughness, so the differences won't be too great anyway.



what you have said above is right , very professional.:thumbup:
 
Typically all the steels listed would be charpy tested with notched samples. Great pains have to be made to make the test specimens exactly alike, thus making the test specimens not cheap (and even it is difficult to get good test results). There are only a couple industries still using the charpy test to check steel for fracture toughness. Toughness is a property that is difficult to understand imo.

The engineering world has used the charpy test more to find out at what temp a particular steel can experience brittle failure (famous brittle failures with liberty ships, holding tanks) not to measure a steel for toughness. Vary the temp of the specimen and you can determine at what temp a steel transitions to brittle fracture. Izod test is not done much anymore.
 
I'm not a metallurgist or material scientist. But just a quick note that there are many different types of toughness that engineers use. Charpy impact tests are just one category of toughness.

Roughly speaking, toughness is the energy required to break a sample. So typically (but not always), toughness has units of energy/volume or energy/area.
https://en.wikipedia.org/wiki/Toughness

Energy = work = force*distance

Tensile Toughness: Under increasing tensile load, integrate stress*strain until the sample breaks. Conceptually, you can think of stress as force, and strain as distance. So by integrating their product, you basically get energy.

Compressive Toughness: Same, as tensile toughness, but instead of a tensile load, you instead apply a compressive load.

Fracture Toughness: Lots of stuff here that I don't know here. But conceptually, it how resistant a material is to propagating a crack through a sample. I don't really understand this one (for example, why does the stress intensity factor (K_ic) have units [Pa*sqrt(m)]? I'm having trouble understanding why there is a square root of meters...). On the other hand, work-of-fracture makes more sense to me because it has units of [J/(m^2)].
https://en.wikipedia.org/wiki/Fracture_toughness

Impact Toughness: Charpy

I also think there are various types of sheer toughness, etc.

It would be interesting to learn what are all the different types of toughness that engineers use, and then see which ones are the most related to knife performance.
 
Tensile strength

Compressive strength

Fracture toughness

It's important to keep the terms distinct.

As you mentioned, the strength tests feature increasing load and measure stress/strain. In impact toughness tests, the load (stress, which is generally tensile in nature as the impact pendulum stretches the material apart) is sudden and static (not increasing) and the strain is minimized by the suddenness of the stress - the material is not given much time to flex/endure before fracture occurs. The ability of a material to absorb impact stress (toughness) differs dramatically from its ability to absorb more slowly increasing compressive or tensile stress. Taffy presents a clear exaggerated difference, wherein slow stretch or compression of the material results in a lot of strain but no fracture, but a sudden (impact) stretch of sufficient force results in minimal strain and sudden fracture. Although the sample eventually fractures in strength tests as well, the observed phenomenon is quite distinct from impact fracture.
 
Strength = force required to break a sample.
Toughness = energy required to break a sample.

Because force is not the same as energy, strength is not the same as toughness.

As for tensile, compressive, sheer, impact, etc. these are different modes for breaking a sample. So we have a huge combination of possible quantities, many of which are used by engineers. Here is the Cartesian product of most of the terms possible, followed by some examples

"Failure Mode" x "Force or Energy"
{Tensile, Compressive, Sheer, Impact, Fracture, etc.} x {Strength, Toughness}

Examples include:

Tensile Strength, Tensile Toughnes,
Compressive Strength, Compressive Toughness,
Fracture Toughness, etc.

So I was specifically talking about tensile toughness, which is the energy required to break a sample as load is slowly increased (as you described). Similarly, there is tensile strength, of which there are several kinds. For example, the tension at which the sample begins to deform plastically is the yield strength, and the maximum force over the entire stress-strain curve is the ultimate tensile strength.

Basically, you have a bunch of different ways the sample can break, and you want to analyze both the forces and the energies involved. Based on the material and application, some of these quantities are more relevant than others. So, some things are (almost?) never used, like impact strength.


I'm not a material scientist, but this is my understanding based on what I've read.
Most of my understanding comes from the following sources:

My undergraduate degree in physics, plus:

_Why Things Break_ by Mark Eberhard (2007)
http://www.amazon.com/Why-Things-Br...d=1393260688&sr=8-1&keywords=How+Things+Break

_The New Science of Strong Materials_ by J. E. Gordon (2006)
http://www.amazon.com/Science-Mater...tmm_pap_title_0?ie=UTF8&qid=1393260432&sr=1-3

Wikipedia

Undergraduate lectures such as on Prof. Bhadeshia's YouTube channel:
https://www.youtube.com/channel/UC8gl2omRFMCaStGqiXqNMoQ

Forum members who are material science engineers or students.

Sincerely,
--Lagrangian
 
Yes, I realize that this thread is 6 years old.

high-alloy-toughness-2-3-2020.jpg


low-alloy-toughness-2-3-2020.jpg


XHP-toughness-chart.jpg


Testing method is about halfway down on this page:

https://knifesteelnerds.com/how-you-can-help/
 
Please help me understand. I though "toughness" was about resistance to breaking from impact - as distinct from "strength: being resistance to deformation from force.

Would he ability to resist being "cut" by another tool reflect "toughness" or "strength" - or both?

[...] Toughness is a property that is difficult to understand imo.[...]

Great thread; above my paygrade in metallurgy, physics and engineering but great thread nonetheless. Because I am unencumbered by technical knowledge and am but a simple knife user, I base my simple opinions on "toughness" from the ability to use (and perhaps moderately abuse) knives without failure or serious damage. granted, this s not a scientific testing method by any means, but it works for me forming my opinions, purchasing decisions and choice(s) of what to grab when I go afield. I like 1095 (and 52100) and have enjoyed good service our of KaBar/Becker 1095 CroVan. I may not have elaborate testing equipment results to back up my opinion(s) but have done many things - things generally within the realm or "normal" knife usage with 1095, 52100 and (especially) 1095 CroVan without failure, substantial damage or even so much as pronounced degradation of performance. Again, I'm a simple guy and have simple opinions based upon simple available data. As for quality, heat treat and the like - certainly these factors enter the equation, but I have some cheap beater/loaner 1095 Schrade knives, admittedly with fat grinds, that have had the snot beat out of them yet still perform. My testing "methods" may need meet scientific standards and the analysis may be generalized, but I like 1095; it is serviceable and provides value IMO.

The latter criteria is a whole different conversation. If a 6" field knife in 1095 costs $X and a similar knife in say 3V costs $Y (Y=3X) then query whether or not the 3V knife is three times better or, conversely, would three 1095 knives be better than one 3V knife? There are those here in both camps. Then there are those who may purchase the $X knife and use the cost difference for more beer or better whiskey! I decline to divulge which theory I subscribe to.
 
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