Ductility, strength, toughness, flexing and bending

[danbot]

Also, if toughness is the ability to withstand plastic deformations without fracturing? What is the name for the ability to withstand flexing (elastic deformations?) without suffering plastic deformations?

Thanks,

Bo

Flexing (or flexibility) is by definition withstanding permanent deformation. Maybe you are referring to yield strength?
Once the steel is flexed past the yield point, it will bend and permanently deform.

 

[marrenmiller]

It is not intuitive but all steel has virtually the same stiffness. The measure of steel stiffness is youngs modulus, also called modulus of elasticity. Whether it is low carbon, high carbon, stainless, it all has the same stiffness up to the point where it reaches first yield.

It sure does! That's one of the things that often confuses those who havent taken materials engineering classes. I considered mentioning that but I didn't want to get too far into the weeds.

What I'm referring to here is stiffness rather than elastic modulus, as in an object property (dependent on object shape and size) rather than a material property. Stiffness =\= elastic modulus.

 

[marrenmiller]

Flexing (or flexibility) is by definition withstanding permanent deformation. Maybe you are referring to yield strength?
Once the steel is flexed past the yield point, it will bend and permanently deform.

Could he be asking about resilience? That would be the energy absorbed by a material before permanently deforming, just like toughness is the energy absorbed by a material prior to fracture.

 

[danbot]

Could he be asking about resilience? That would be the energy absorbed by a material before permanently deforming, just like toughness is the energy absorbed by a material prior to fracture.

Now you're getting into the weeds!

 

[bdmicarta]

is yielding the same as flexing or bending?
Is ductility and toughness the same thing? (I think I missed that)

Also, if toughness is the ability to withstand plastic deformations without fracturing? What is the name for the ability to withstand flexing (elastic deformations?) without suffering plastic deformations?

"flexing" and "bending" are somewhat generic terms. You can flex or bend a spring for instance, and it "springs" right back, or you can flex or bend a piece of aluminum foil and it doesn't spring back. The terms are not specific enough.

For steel that you are bending, ductility and toughness are somewhat the same thing.

Toughness is the ability to withstand plastic deformations, but we almost need then to define what a plastic deformation is. The ability to withstand bending without suffering permanent deformation is basically high yield strength.

Everybody wants to make it too complicated. Lets go back to the coathanger wire that I mentioned previously. Surely everybody has played with that material. Clamp a long piece of the wire in a vise. Pull a little bit on the end of it, it moves as the wire bends. Let go of the wire and it springs back to straight. It didn't undergo permanent deformation so the bending was "elastic". A piece of spring wire the same diameter would have behaved exactly the same way. Now lets pull it again but pull a little harder. It moves a certain amount, then you let go and it only springs back partway. It has undergone permanent deformation, which is defined as "plastic" deformation. There is some point in there where you reach the limit of how far you can pull it and it springs back completely, this point is called the yield strength of the material. Below the yield strength it is completely elastic, pulled beyond the yield strength it undergoes permanent deformation. Yield strength is one of the basic properties of any given alloy of steel. From the softest steel used to make wire or cans or whatever, to the strongest steel is something like a factor of 20. Another property, the stiffness (technically youngs modulus or modulus of elasticity) tells how much the steel moves elastically under a given amount of stress. Reasonable ranges of steel alloys all have about the same modulus of elasticity. Another property of steel that is important to us is the ductility- how far the material will undergo plastic deformation before it fractures. As steel is made stronger, the ductility is reduced to the point where at the extreme the yield point and fracture point pretty much coincide and we consider that material to be brittle. It doesn't undergo any plastic deformation, is just reaches a point where it breaks. Some knife steels are like this. So the softest steels- you pull on them, they deform elastically up to a relatively low stress, then they deform plastically up to the point where they fracture. The hardest steels- you pull on them, they deform elastically, you have to keep pulling harder and harder and they deform a little bit more, elastically, up to a point where they just fracture. This point will be very much higher than the point where the soft steel starts to have permanent deformation. There are steel alloys spread out at all points between these 2 extremes. Think in terms of what it does before it reaches the yield point, and what it does after it reaches the yield point, and you can understand steel behavior better. I skipped some of the intricacies but this is the basic behavior.

 

[l1ranger]

That's one of the things that often confuses those who havent taken materials engineering classes.

it still confuses some of us that have.
its all confused even more when we bring in 'non engineering' terms and their meanings; which differ sometimes from person to person.

 

[danbot]

"flexing" and "bending" are somewhat generic terms. You can flex or bend a spring for instance, and it "springs" right back, or you can flex or bend a piece of aluminum foil and it doesn't spring back. The terms are not specific enough.

For steel that you are bending, ductility and toughness are somewhat the same thing.

Toughness is the ability to withstand plastic deformations, but we almost need then to define what a plastic deformation is. The ability to withstand bending without suffering permanent deformation is basically high yield strength.

Everybody wants to make it too complicated. Lets go back to the coathanger wire that I mentioned previously. Surely everybody has played with that material. Clamp a long piece of the wire in a vise. Pull a little bit on the end of it, it moves as the wire bends. Let go of the wire and it springs back to straight. It didn't undergo permanent deformation so the bending was "elastic". A piece of spring wire the same diameter would have behaved exactly the same way. Now lets pull it again but pull a little harder. It moves a certain amount, then you let go and it only springs back partway. It has undergone permanent deformation, which is defined as "plastic" deformation. There is some point in there where you reach the limit of how far you can pull it and it springs back completely, this point is called the yield strength of the material. Below the yield strength it is completely elastic, pulled beyond the yield strength it undergoes permanent deformation. Yield strength is one of the basic properties of any given alloy of steel. From the softest steel used to make wire or cans or whatever, to the strongest steel is something like a factor of 20. Another property, the stiffness (technically youngs modulus or modulus of elasticity) tells how much the steel moves elastically under a given amount of stress. Reasonable ranges of steel alloys all have about the same modulus of elasticity. Another property of steel that is important to us is the ductility- how far the material will undergo plastic deformation before it fractures. As steel is made stronger, the ductility is reduced to the point where at the extreme the yield point and fracture point pretty much coincide and we consider that material to be brittle. It doesn't undergo any plastic deformation, is just reaches a point where it breaks. Some knife steels are like this. So the softest steels- you pull on them, they deform elastically up to a relatively low stress, then they deform plastically up to the point where they fracture. The hardest steels- you pull on them, they deform elastically, you have to keep pulling harder and harder and they deform a little bit more, elastically, up to a point where they just fracture. This point will be very much higher than the point where the soft steel starts to have permanent deformation. There are steel alloys spread out at all points between these 2 extremes. Think in terms of what it does before it reaches the yield point, and what it does after it reaches the yield point, and you can understand steel behavior better. I skipped some of the intricacies but this is the basic behavior.

it still confuses some of us that have.
its all confused even more when we bring in 'non engineering' terms and their meanings; which differ sometimes from person to person.

I don't think there is too much about this stuff that is really simple.
Except for "Don't use your knife as a pry-bar"!

 

[Bo-dacious]

Okay, I'm still a bit confused but this is all helping. There's one thing in particular that I need clarification on: So a harder material will be able to flex more before permanently or plastically deforming? Or is that wrong?

Can somebody give me all the terms reletive to this topic and their opposites so I know which ones go with which ones?

Thanks everybody for the help,

Bo

 

[Larrin]

Here is an article on flexing/bending: https://knifesteelnerds.com/2018/03/13/why-doesnt-heat-treating-affect-steel-flex/

 

[chiral.grolim]

I will give it a shot:

Hardness and strength are essentially the same and refer to how resistant the material is to bending, stretching, squashing, denting, rolling, etc. If you have two blades of the same thickness and subject each to the same stress, the harder/stronger blade is the one that bends/deforms (elastically or plastically) less.

Stiffness is similar but depends heavily (or rather cubically) on material thickness. In other words, if you take a piece of steel of a given hardness and slice it such that one piece is 2X thicker than the other, that thicker piece will require 8X more force to bend/flex than the thinner piece, although both are at the same hardness. Similarly, a thin piece of harder steel can be strong enough to resist deformation to the same extent as a thicker piece of softer/weaker steel.

Consider how important this is for the very thin edge of a knife blade.

Tough vs Brittle comes into play AFTER strength has yielded. As was mentioned above, "tough" material will bend more before breaking than "brittle" material at the same thickness.

Here is the twist: a hard/strong steel may resist bending/deformation such that it requires more force to get it to "take a bend" than it takes to bend a "tough" softer steel to the point of fracture! You can take a piece of soft steel and bend it until it snaps, then take a knife blade and fail to bend it at all using the same amount of force, but this does not make the fractured steel "brittle" in comparison to the strong steel, it only makes it weaker. In order to determine which is more "brittle", you need to get BOTH to bend to the point of fracture and determine which steel bent less before breaking regardless of the force load required. Remember, resistance to deformation-force is strength, resistance to fracture AFTER deformation is toughness.

 

[danbot]

Here is an article on flexing/bending: https://knifesteelnerds.com/2018/03/13/why-doesnt-heat-treating-affect-steel-flex/

Great article Larrin! I'm glad you dropped in here!
You have a nice (although somewhat technical) way of explaining this stuff!

 

[chiral.grolim]

Okay, I'm still a bit confused but this is all helping. There's one thing in particular that I need clarification on: So a harder material will be able to flex more before permanently or plastically deforming? Or is that wrong?

Strength is not about how much (or how far) it can bend/flex before taking permanent (plastic) deformation, rather it is about how much force is required to get it to bend to the point of permanent deformation. The harder/stronger material will require more force to bend, but it may break readily rather than bending a given amount. A weaker material will require less force to bend and may bend a LOT before taking a permanent bend or fracturing.

Consider two springs of equally thick metal under the same amount of stress - one stretches 1" and the other stretches only 1/2". The second spring is stronger/harder. You keep applying stress and the first spring stretches 2" but the other fractures before reaching 1". When you release the stress on the intact spring, it does not return to true. The first spring was stronger but more brittle than the second spring.

 

[Larrin]

Great article Larrin! I'm glad you dropped in here!
You have a nice (although somewhat technical) way of explaining this stuff!

I can explain things in fewer words, but not when I have a whole article on one thing.

 

[Larrin]

Strength is not about how much it can bend/flex before taking permanent (plastic) deformation, rather it is about how much force is required to get it to bend elastically.

The force required to bend elastically is generally referred to as the modulus (i.e. Young's modulus or elastic modulus) rather than strength.

Stiffness is a combination of elastic modulus and the geometry (length and cross section) of a part.

 

[marrenmiller]

Okay, I'm still a bit confused but this is all helping. There's one thing in particular that I need clarification on: So a harder material will be able to flex more before permanently or plastically deforming? Or is that wrong?

Can somebody give me all the terms reletive to this topic and their opposites so I know which ones go with which ones?

Thanks everybody for the help,

Bo

Does this help?

Strength [material property]: a measure of how much stress (applied load distributed throughout a material) a material can withstand. This is completely independent of geometry of the object, as it's a material property. Generally we refer to yield strength, or the load a material can take without permanently deforming; if exceeded, the material has plastically deformed and will not return to it's original shape. There's also ultimate strength, which is the maximum stress a material can take before complete fracture.

Hardness [material property]: roughly speaking is a measure of how difficult it is to dent or scratch a material. There's lots of ways to test this and standards to define this with (Rockwell C scale is a popular one used in this industry). Hardness is positively correlated with strength on most steels, but they are not the same thing, and there are steels that are strong but not hard and vice versa. For a rough estimate, though, there are charts that you can use to estimate strength based on hardness for certain kinds of steels.

Elastic Modulus [material property]: a measure of how much a material strains, or changes shape, when stress is applied to it. The higher the modulus, the less the material will change shape when under load. This is only applicable for elastic materials and only until a material exceeds its yield strength.

Stiffness [object property]: a measure of how much an object changes shape when a load is applied to it. This depends on the object geometry and the elastic modulus

In your example, a stronger/harder steel object will take more load to deform permanently than a weaker one. A stiffer object will deflect less when loaded than an object with lower stiffness. However, how much an object deflects and how much load an object can sustain are governed by different properties entirely, so it's impossible to answer your original question.

For example: Let's say you built identical rods out of 6Al-4V titanium and 202 grade stainless steel, and clamped one end of each in a vice. If you put weight on the end of both, you should expect the 6Al-4V titanium rod to deflect more, because it has a lower elastic modulus than 202 stainless (about 30% lower). However, 6Al-4V titanium is has a higher yield strength than 202 stainless; therefor, if you continued to load the rods with more weight, the 6Al-4V rod would hold substantially more weight than 202 stainless before it developed any permanent bend (plastic deformation).

 

[bdmicarta]

Wow, where to start?

>Okay, I'm still a bit confused but this is all helping. There's one thing in particular that I need clarification on: So a harder material will be able to flex more before permanently or plastically deforming? Or is that wrong?
Yes the harder material, being the stronger material, will deform farther elastically before it reaches the stress level where it will undergo permanent deformation.


>Here is an article on flexing/bending: https://knifesteelnerds.com/2018/03/13/why-doesnt-heat-treating-affect-steel-flex/
The article is correct, the best thing about it are the stress/strain graphs which show the behavior- how some steels go to higher stresses before plastic deformation and how they have less ductility to fracture once they do get to plastic deformation.


>Hardness and strength are essentially the same and refer to how resistant the material is to bending, stretching, squashing, denting, rolling, etc. If you have two blades of the same thickness and subject each to the same stress, the harder/stronger blade is the one that bends/deforms (elastically or plastically) less.
Somewhat incorrect- the 2 blades will deform the same amount as long as you don't reach the yield point of either one, this is elastic deformation. Once you reach the yield point of the less hard blade it will deform more under plastic deformation.

>Stiffness is similar but depends heavily (or rather cubically) on material thickness. In other words, if you take a piece of steel of a given hardness and slice it such that one piece is 2X thicker than the other, that thicker piece will require 8X more force to bend/flex than the thinner piece, although both are at the same hardness. Similarly, a thin piece of harder steel can be strong enough to resist deformation to the same extent as a thicker piece of softer/weaker steel.
Stiffness is a function of material thickness but varies almost none due to hardness/strength of the steel.

>Here is the twist: a hard/strong steel may resist bending/deformation such that it requires more force to get it to "take a bend" than it takes to bend a "tough" softer steel to the point of fracture! You can take a piece of soft steel and bend it until it snaps, then take a knife blade and fail to bend it at all using the same amount of force, but this does not make the fractured steel "brittle" in comparison to the strong steel, it only makes it weaker. In order to determine which is more "brittle", you need to get BOTH to bend to the point of fracture and determine which steel bent less before breaking regardless of the force load required. Remember, resistance to deformation-force is strength, resistance to fracture AFTER deformation is toughness.
It is somewhat universal that the stronger steel will be less tough, but slight differences in alloying, differences in the production method of the steel, and differences in heat treat will all affect this. There is the general tradeoff between strength and toughness. All of the things we do to improve the alloy, production method and heat treat will move the tradeoff point slightly up and down the scale.


>Strength is not about how much (or how far) it can bend/flex before taking permanent (plastic) deformation, rather it is about how much force is required to get it to bend to the point of permanent deformation. The harder/stronger material will require more force to bend, but it may break readily rather than bending a given amount. A weaker material will require less force to bend and may bend a LOT before taking a permanent bend or fracturing.
Flexural behavior, stress and deflection, are interrelated. The equations in the article previously linked will show that. A higher strength material of the same thickness will bend farther while it is reaching the higher stress level required to cause permanent deformation. Below the point where permanent deformation starts, there is no difference in the stiffness of the 2 steels.

>Consider two springs of equally thick metal under the same amount of stress - one stretches 1" and the other stretches only 1/2"
This is not possible. Same stress and same thickness gives same deflection. We're talking about steel here. Someone mentioned titanium vs. steel and that will change the results because as stated the 2 materials have different modulus of elasticity.

>The second spring is stronger/harder. You keep applying stress and the first spring stretches 2" but the other fractures before reaching 1". When you release the stress on the intact spring, it does not return to true. The first spring was stronger but more brittle than the second spring.
This is correct- the intact string reached yield and underwent plastic deformation.

 

[Bo-dacious]

Marren: that's great information, thank you. But what does deflect mean here, Bending?

Thank you everybody. That's a lot of good info. I will read those articles later today.

Thanks again,

Bo

 

[BluntCut MetalWorks]

 

[Larrin]

That is an impressively bad reading of the chart.

 

[BluntCut MetalWorks]

I interpreted chart as: 'plastic deformation' line/curve corresponding to yield strength and 'total deflection' to fracture strength. Perhaps you have a better or right way to interpret the chart? Please just state your opinion rather than being young about it.

That is an impressively bad reading of the chart.