Ductility, strength, toughness, flexing and bending

B

Bo-dacious

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Hey guys, my question is about bending and flexing. How do they relate to strength, toughness and ductility? Brittleness? I read that ductility is the ability to flex and bend without fracturing, but how does that relate to strength? I also read that toughness is the ability to flex and bend without breaking. Are ductility and toughness the same thing? Was what I read wrong? I basically want to know how they all relate to each other and if there is another factor that relates to any of them.

Thanks guys,

Bo
 
Ductility is the ability to be drawn into wires. Strength and toughness are two separate, entirely differen,t things...I always forget which is which..."ability to withstand fracture" vs something else.

Beyond that basically most of what you are asking about are independent metallurgical properties.

Like asking "how is a human's height and hair color related?"
 
marcinek said:
Ductility is the ability to be drawn into wires. Strength and toughness are two separate, entirely differen,t things...I always forget which is which..."ability to withstand fracture" vs something else.

Beyond that basically most of what you are asking about are independent metallurgical properties.

Like asking "how is a human's height and hair color related?"
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Ability to withstand fracture is toughness and ability to withstand deformation from its original shape is strength. (In a nut shell;)).
 
Flexing is related to the elastic yield strength. How far a metal can be bent under load and still return to its original shape when the load force is removed. Bending happens when you have passed the elastic yield point and deformation is permanent.
 
danbot said:
Flexing is related to the elastic yield strength. How far a metal can be bent under load and still return to its original shape when the load force is removed. Bending happens when you have passed the elastic yield point and deformation is permanent.
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Malleability is the ability to be pounded into sheets!

Has nothing to do with the question...but it makes me feel smarter.
 
marcinek said:
Malleability is the ability to be pounded into sheets!

Has nothing to do with the question...but it makes me feel smarter.
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That was probably the next question anyway!;)
Now if we could just figure out what "slicey" means!:p
 
Okay so, and I may have missed something, but do bending and flexing relate to either toughness or strength? Someone said they're all different terms and unrelated, but I thought I read somewhere that either strength or toughness are related to flexing and bending. If "Ability to withstand fracture is toughness and ability to withstand deformation from its original shape is strength" then can those fractures and deformations come from bending or flexing?

Thanks,

Bo
 
Bo-dacious said:
Okay so, and I may have missed something, but do bending and flexing relate to either toughness or strength? Someone said they're all different terms and unrelated, but I thought I read somewhere that either strength or toughness are related to flexing and bending. If "Ability to withstand fracture is toughness and ability to withstand deformation from its original shape is strength" then can those fractures and deformations come from bending or flexing?

Thanks,

Bo
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https://en.wikipedia.org/wiki/Toughness
https://en.wikipedia.org/wiki/Strength_of_materials
https://en.wikipedia.org/wiki/Deformation_(mechanics)
https://www.merriam-webster.com/dictionary/deform
 
As far as I understand...

Strength is how much pushing force it takes to deform (bend or roll).

Ductility is how much pulling force it takes to deform.

Flexibility (as Danbot said) is what happens when it does deform... high flexibility = it can bend a long way and still spring back to its original shape, low flexibility = it can only bend a little bit and return to true, if bent a lot it will remain permanently in the new (bent) shape.

Toughness is also what happens when it does deform. It measures how much (permanent?) bending it can take before fracturing (breaking, chipping). Bend a lot without fracture = tough, will fracture when bent only a little bit = brittle.

So if you use a knife as a prybar, it will not bend if it has high strength. If you apply so much force it does bend, it will flex side to side like a spring but return to normal if it has high flexibility. If you bend it a bit past its strength and flexibility limits, it will break if low toughness; or remain permanently bent if high toughness.

Generally the higher the hardness the higher the strength and the lower the toughness and vice versa.
 
This is a knife blade with lots of strength (resistance to bending, rolling or denting). It's M4 steel at 64 Rc. (Hardness is a proxy for strength). But it's not very tough, so it will break or chip or crack before it bends.

2v2JVngDTxAWtWs.jpg



This is a blade with lots of toughness (resistance to breaking, chipping or cracking). It's cheap stainless steel used by Randall, 440b run soft at 56 Rc. It is unlikely to break, but it will bend or roll or dent easily because it has little strength.

2v2JVnWUSxAWtWs.jpg
 
You should read the links marcinek posted. It'll help you understand that a lot of the words in common diction used to describe materials are often used incorrectly.

For example, on "flexibility": it's not clear if you're referring to strength, which is a material's ability to resist permanent deformation under load (and is very roughly correlated to hardness in metals like steel), or to stiffness, which is a measure of how an object (note: not a material property) resists deformation under loading.
 
From an engineering standpoint steel will withstand a certain amount of stress before it starts to yield and deform. Everybody here has probably bent a piece of wire like a coathanger. The coathanger stays straight up to a point and then bends and stays bent. This steel has a lot of ductility or toughness.

As you make a steel stronger it also gets harder. You have to put more stress on it to reach the point where it will yield, then when it yields it won't deform as much before it breaks.

Make the steel even stronger and you reach a point where it won't yield, it just breaks. This steel has a lot of strength or hardness. If it is so strong that it doesn't yield, it just breaks, then it is considered to be brittle.

As you alloy steel to make it stronger or harder, it has less ductility. There is a tradeoff between strength/hardness and ductility, you have to choose which property you want for a knife and select steel accordingly. Different alloys, different steel production methods, different heat treatments can move the tradeoff point higher up the scale, but there is still a tradeoff.
 
marrenmiller said:
For example, on "flexibility": it's not clear if you're referring to strength, which is a material's ability to resist permanent deformation under load (and is very roughly correlated to hardness in metals like steel), or to stiffness, which is a measure of how an object (note: not a material property) resists deformation under loading.
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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.
 
Bo-dacious said:
Okay so, and I may have missed something, but do bending and flexing relate to either toughness or strength? Someone said they're all different terms and unrelated, but I thought I read somewhere that either strength or toughness are related to flexing and bending. If "Ability to withstand fracture is toughness and ability to withstand deformation from its original shape is strength" then can those fractures and deformations come from bending or flexing?

Thanks,

Bo
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They are all inter-related. Raising hardness reduces toughness. Lowering hardness increases toughness. Flexibility is a balance between hardness and toughness. Bending won't be very successful with high hardness, but will be very successful with low hardness and flexibility will be successful with an even balance between hardness and toughness.
There are always trade offs and compromises between all of these characteristics depending on the application requirements (purposes) for the steel.
Hope this helps!
 
Thank you everybody, good info.

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

Thank you all for the help,

Bo
 
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
 
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