# Stress and strain from atomic point of view

1. Nov 15, 2013

### jayanth nivas

Hi all,
This is a question regarding stress and strain.
Can someone please explain the concepts of stress and strain in atomic level?

My understanding is as follows.Let us consider a metal bar or rod that is subjected to tension.The load applied to the bar will be try to elongate the bar,that is it will try to pull the atoms of the metal apart.The atoms do have a strong force binding them so they will offer resistance to the load.So if i cut the cross section of the bar,the atoms on the surface will try to balance the force applied in order to maintain the equilibrium.So we take the internal resistance as the force applied by the area of the cross section.(But i'm not sure how atomic density affects this).

Now my understanding strain and hence the deformation is as follows.When the load is applied,the distance between the atoms slightly increases before an equilibrium is achieved between internal and external forces.This movement of atoms is converted into stain energy that is stored in the bar.As long as the applied load is retained,the distance of separation and hence strain energy is retained.When the applied load is removed,the strain energy is used to move the atoms back to the original state.

The problem with above statement is I assume the atoms will move before an equilibrium is established.Is that right ? if so can anyone explain this more clearly ?

And also the role of Young's modulus in this.If young's modulus is less,does that mean the atoms will move more before an equilibrium is reached?.If it is more does that mean elasticity is less?

One last question.If a part of the load is getting stored as strain energy,Does that mean that the internal resistance is offered against the remaining load? I know this is a kind of stupid question.But i'm not clear about this so i thought i can ask it anyway..

Please note i'm in high school and i'm not an engineer. I posted the same thread in general physics section,but due to lack of replies i'm posting it here.

Sorry if i have misstated something and thanks in advance

2. Nov 15, 2013

### Simon Bridge

You are imagining a uniform single crystal of a metal bar? That's actually fairly unusual but fair enough.

Higher atomic density means the force binding the atoms are stronger. Think of the atoms being joined by springs - the closer together the shorter the spring, the less give it has.

If you bend a bar - the distances on one side are increased and the other side are decreased.
This compresses the springs on one side and extends the springs on the other.
The strain energy is stored in the springs (IRL electromagnetic fields).

Imagining the bonds as springs should help you understand how the atoms can move before an equilibium is established. It's like putting a load on a spring - energy is stored in the spring, and the spring deforms.
Technically - the behavior of springs is what you are trying to understand - so it is kinda begging the question - but once you can see it in terms of springs, you can replace the spring with the electromagnetic force.

Youngs modulus is another result of the atomic structure.

IRL: there are no pure crystals and most substances are compounds.
The details of their structures at a range of scales gives them their material properties.

Last edited: Nov 15, 2013