Young's Modulus - what is it?

phyzzy_physh

Hi everyone,

Apologies if this is in the wrong section, I'm still relatively new to the forum.

I'm an engineer studying Young's modulus in building materials. I have a passing interest in physics, but let's say my knowledge of the subject is far from exhaustive!

I'm interested in finding out the physics behind Young's modulus, and what causes/changes it. The books I've found so far detail how to measure it, and what it is as a physical property (i.e. stress over strain), but not the intrinsic meaning of it or where it comes from.

If anyone could offer any physics wisdom on this, or even point me in the direction of texts/papers discussing it, you'd really be helping me out; I've hit a bit of a dead end at the moment.

Cheers,

Phyzzy Physh

Mapes

You'd probably enjoy looking at a mechanics of materials book (like Callister, Dowling, or Courtney). Young's elastic modulus is related to the stiffness of a material's individual atomic bonds. Stronger bonds generally result in higher stiffness. (Stronger bonds also generally result in higher melting temperatures, so more refractory materials are frequently stiffer.)

EDIT: Ah, I should also cover polymers in case you're considering these. The above explanation applies well to ceramics and metals, whose atomic structure is essentially fixed, but polymers are long macromolecular chains that can slide past each other. Each chain contains stiff covalent bonds, but polymers are relatively compliant (soft). Why? Because for small strains, you're just straightening out the chains rather than stretching the covalent bonds. When you let go, the chains because disorganized again (like a necklace on a vibrating surface, for example). The technical way of saying this is that the Young's elastic modulus of metals and ceramics is enthalpy-based (bond lengths recovering a minimum-energy distance), while that of polymers is entropy-based (polymer chains recovering a disorganized state).

Dadface

Suppose I asked you the following question:By how much does copper a copper wire stretch when it is subjected to a certain force?You might go on to explain the stretch in terms of the forces needed to pull the copper atoms further apart but you wouldn't be able to give me a numerical answer because the question is incomplete,more information is needed.The stretch depends not only on the material but also on its physical dimensions,the longer the wire the more it stretches and the smaller the cross sectional area of the wire the more it stretches.for a given load a long thin wire stretches more than a short thick wire.I could answer my own question if I looked up the Youngs modulus(E) of copper this being a property of the material and independant of its dimensions.Knowing the equation I could then plug the numbers in to work the answer out.

E=(Fl)/(Ae) F=force l= length A= area e= extension(I remember it as E= flea)

phyzzy_physh

Thanks for the replies guys, much appreciated