Understanding Young's Modulus

Young's modulus is a crucial mechanical property in engineering, as it defines the stiffness of a material and tells us how much it will deform for an applied stress.
In this video I take a detailed look at Young's modulus, starting with tensile tests and stress-strain curves, all the way through to what is happening at the atomic scale.
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ERRATA:
- At 05:27 steel is incorrectly shown as being a substitutional alloy. It is actually an interstitial alloy, where the carbon atoms are located between the iron atoms.
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The Efficient Engineer is a channel aimed at mechanical and civil engineers. The mission is to simplify engineering concepts, one video at a time!
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[@MFDOOMgr]

i have a question. I understand everything about Young's Modulus but, when they say a material has for example 210000 N/mm^2 , what do they mean? that it can handle 210000N/mm^2 in the elastic region? and then it goes to the plastic?

[@TheEfficientEngineer]

Young's modulus, yield strength (the stress at which a material goes plastic) and ultimate strength (the stress at which a material fractures) all have the same units. So it doesn't make sense to say "a material has 210000 N/mm^2", without specifying which parameter we are talking about. 210 GPa is a typical Young's modulus value for steel, so it is likely that in this case the 210000 N/mm^2 is Young's modulus.

[@MFDOOMgr]

@@TheEfficientEngineer and practically this means? that this kind of material can take up to 210000 N / mm^2 and then breaks?

[@TheEfficientEngineer]

No - it means that the slope of this material's stress-strain curve in the elastic region is equal to 210000 N/mm^2. So for example for an applied stress of 210 MPa, we would get a strain of 0.1%.

[@whitelight32]

@@TheEfficientEngineer Doesn't that also mean that we need 2.1 MN of force to change the materials area by 1 mm^2 ?

[@gieaudio8762]

@@whitelight32 no, it means that you need 210 GPa stress in material to deform it by 100%, of course it will fail because Young modulus is only appropriate (linear) in elastic range of the material. Simply saying, Young modulus is the number that helps you transform stresses to strains and vice versa but only in the elastic range of the material, for concrete it is 0,20% for compression, for reinforcing steel it is up to ~0.24% in tension

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[@jongeiser7569]

This is a really great straight forward video. As a Metallurgist, this was a really good introduction. You explained it way better than my professors did. I don't wanna be that guy that tells you why your video is wrong. But around 5:30, you show that carbon replaces the iron atoms in your model. In reality, carbon goes in between the iron atoms in the interstitial space. This is hopefully a video that you could do in the future talking about until cells and Crystal structures. Keep up the good work!

[@TheEfficientEngineer]

Thank you for your kind comments Jon. You are of course correct about the interstitial nature of steel - my mistake. Hopefully the animation still illustrates the point without being too misleading. A video on unit cells would be really interesting - thanks for the idea!

[@a1mforthetop]

​@pyropulse As an engineer with quite some work experience i must say the following: The stuff with the atoms is nice and everything but it should have been left out of a beginners introduction video entirely. The only thing that has to stick in the head of an efficient engineer is that E is a material constant that represents the slope of sigma and epsilon and is different for different materials. It is also commonly used in combinations like EI and EA. For the advanced theoretical engineer the atom part is important of course ;)

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[@TheEfficientEngineer]

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[@TheEfficientEngineer]

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[@KirillBelov92]

Hello The Efficient Engineer! Thank you for your videos! They are great! I have one question. Why did you show on graphic on 2:38 that wood (pependicular to grain) is stiffer than wood (parallel to grain). I think it must be contrary because if load direction is parallel to grain than grains are tensed by all their length. But if load is pependicular to grains, so only part of grain and the space between grains are strained. Isn't the second case lesss stiff than the first one?

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[@TheEfficientEngineer]

Probably best if you send me an email to [e-mail] with specifics.

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Great video :) I have a question. In your opinion, is the young's modulus more important than the bending resistance in parquets? or is there a difference between them ? thanks

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[@aadharshram4975]

Awesome video! Btw, just a question. So assuming that stiffness in polymeric material is caused by the intermolecular forces. So the stress-strain curve for polymeric materials flatter in higher stresses cause the molecules are farther apart and the intermolecular forces are weaker and less stress is required to pull the molecules apart. Is that right?

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Just found your page tonight I find it interesting so far. I’m a dual ticket Red Seal Ironworker and Welder and I’ve performed tensile tests both in school and at work. What you covered is very informative but you could have added more about quenching and tempering and how much tensile strength it can add. How it increases brittleness and ductility. I had a weld test on mild steel with 7018 SMAW welding electrode(rated for 70000 psi per square inch) heated red hot and quenched immediately. It sheared at 138,000 psi on the tensile test which I found very interesting.

[@paulcarroll5602]

I meant lowered ductility, sorry it’s 1am

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[@mitch6808]

I would calculate the movement about a plastic lever; so how much it move before the break. movement = constant * (yield strain * lenght^2 / thickness)

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Thanks for this.

[@eriktempelman2097]

Good video that I can recommend to my students. But be careful: in your stress-strain curve, you have greatly overestimated the elastic strain (it's just 0.1-0.5% for most steels) as compared to the plastic strains. Also, while many engineering materials indeed follow Hooke's law, this is by no means generic behaviour. Many plastics, foams, and biological matter are very different :-)

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[@feynstein1004]

Great video. Wish it were a bit longer. I especially wanted to see a comparison of various materials, including graphene, which has the highest Young's modulus as far as we know.

[@TheEfficientEngineer]

Thanks Feynstein! Graphene would have been a good one to discuss. I'll try and mention it in a future video.

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[@Kimmiesambapiet]

Very instructive video. Thanks a lot for sharing. I have one small remark regarding the position of the carbon atoms in the steel lattice. In the animation, it looks like they substitute for Fe atoms, while in reality they are present in the interstitials. Maybe this might confuse people. Besides that, I am wondering what the exact reason for the slightly lower modulus of the high carbon steel (vs the low carbon steel) is. I can imagine that the presence of interstitial carbon slightly modifies the equilibrium bond length/strength between two Fe atoms. I suppose that the average Fe-Fe bond length increases with increasing Carbon content. How is the Fe-Fe bond strength affected and how does this overall lead to a slightly lower modulus with increasing bond strength? Can we assume that the bond strength remains the same and the bond length increases so that the strength/strain ratio decreases with increasing carbon content? Thank you!

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At around 2:30, i hear wood and composites as an isotropic material. I somehow remember them to be orthotropic. Correct me if i am wrong. Nice videos: this one and others on this channel. I sometime stream them on TV as well. Thanks for putting such info in concise form. :)

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