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-   -   Elasticity (understanding elasticity from stress strain curve) (http://www.physicsforums.com/showthread.php?t=655121)

maheshshenoy Nov27-12 12:24 AM

Elasticity (understanding elasticity from stress strain curve)
 
Hello .. I have problem understanding how to decide which material is more elastic based on stress strain curve.. my understanding is as follows
1)if a material has big youngs modulus.. then it is more stiff

2)a material with a big youngs modulus may be or may not be very elastic (elasticity is independent of youngs modulus.. it only tells how stiff something is)

3)if a material has a very big yield point.. then is it more elastic?? what if a material can deform more (so less youngs modulus) but has lesser yield point.. then is it more elastic??

maheshshenoy Nov27-12 12:29 AM

Re: Elasticity (understanding elasticity from stress strain curve)
 
1 Attachment(s)
This is a sample curve

afreiden Nov27-12 01:36 AM

Re: Elasticity (understanding elasticity from stress strain curve)
 
High yield value = strong
Large E = stiff
Large strain to failure = ductile

maheshshenoy Nov27-12 02:32 AM

Re: Elasticity (understanding elasticity from stress strain curve)
 
Well.. i got that.. but how do find which is more ELASTIC?

Chestermiller Nov27-12 05:02 PM

Re: Elasticity (understanding elasticity from stress strain curve)
 
Most people would quantity it by the maximum amount of recoverable tensile strain. But why do you feel compelled to have a definition of "more elastic?"

maheshshenoy Nov27-12 10:56 PM

Re: Elasticity (understanding elasticity from stress strain curve)
 
Quote:

Quote by Chestermiller (Post 4175619)
Most people would quantity it by the maximum amount of recoverable tensile strain. But why do you feel compelled to have a definition of "more elastic?"

So you mean to say.. we don't have to really define which is more elastic ? its not relevant in the real world application?

Chestermiller Nov28-12 06:03 PM

Re: Elasticity (understanding elasticity from stress strain curve)
 
I guess it helps your intuitive mind set to be aware that elastomers/rubbers exhibit large recoverable strains, and metals and ceramics exhibit small recoverable strains. But I think you already knew that.

maheshshenoy Nov29-12 02:20 AM

Re: Elasticity (understanding elasticity from stress strain curve)
 
Yes.. and we still say steel is more elastic compared to those elastomers.. so basically elasticity is not just a measure of how much recoverable strain its can exhibit right?..
So thats what i wanted to know exactly when we say something is elastic.. what do we mean.. it must have high yield point and must exhibit high recoverable strains.. correct?

Studiot Nov29-12 02:35 AM

Re: Elasticity (understanding elasticity from stress strain curve)
 
Quote:

So thats what i wanted to know exactly when we say something is elastic.. what do we mean.. it must have high yield point and must exhibit high recoverable strains.. correct?
Technically, elasticity is the property of a material to recover its strain after removal of stress.

Nothing more.

It does not imply that the strain be large or that any particular mathematical relationship exists between stress and strain or that all the strain be recovered. It does refer to all the stress induced strain that is recovered, however.

The stress - strain relationship may be linear or non linear.
The elastic relationship for steel is particularly linear over its elastic range which is why people say (loosely) that steel is 'more elastic' than a rubber band which does not have a linear stress-strain relationship.

Strain can also occur by other agencies eg thermal strain which can be stress free.

Elasticity may appear in conjunction with other effects such as hysteresis or plasticity.

In ordinary English elasticity is often mixed up with these other effects and the definition is not so precise. The ordinary English definition should not be used in technical forums.

maheshshenoy Nov29-12 02:37 AM

Re: Elasticity (understanding elasticity from stress strain curve)
 
So what I finally understand is.. elasticity is not something we can quantify.. so its not a physical quantity right?

Studiot Nov29-12 02:54 AM

Re: Elasticity (understanding elasticity from stress strain curve)
 
Quote:

Quote by maheshshenoy (Post 4177510)
So what I finally understand is.. elasticity is not something we can quantify.. so its not a physical quantity right?



You are getting there.

No, even in technical English 'elasticity' is not mathematically quantifiable in general.

However for obvious reasons we have developed several definite mathematical (quatifiable) relationships, the most important being linear-elastic or hookean (which is a sub class).

Chestermiller Nov29-12 07:18 AM

Re: Elasticity (understanding elasticity from stress strain curve)
 
Quote:

Quote by Studiot (Post 4177520)
You are getting there.

No, even in technical English 'elasticity' is not mathematically quantifiable in general.

However for obvious reasons we have developed several definite mathematical (quatifiable) relationships, the most important being linear-elastic or hookean (which is a sub class).

To me, as a guy with lots of experience in materials engineering, linearity of stress-strain behavior is not a requirement for a material to be considered very elastic. The key requirement, as I said before, is how much recoverable strain the material is capable of exhibiting. Elastomers display non-linear behavior from the get-go even over large deformations, but yet, they are capable of a high degree of recoverable strain. Where do we think the "elast" of elastomers comes from?

Studiot Nov29-12 11:13 AM

Re: Elasticity (understanding elasticity from stress strain curve)
 
Quote:

To me, as a guy with lots of experience in materials engineering, linearity of stress-strain behavior is not a requirement for a material to be considered very elastic. The key requirement, as I said before, is how much recoverable strain the material is capable of exhibiting. Elastomers display non-linear behavior from the get-go even over large deformations, but yet, they are capable of a high degree of recoverable strain. Where do we think the "elast" of elastomers comes from?
That's almost exactly what I said so what is your angle?

The only difference is that I also said linear-elastic is the most important. This is simply because linear theory is still by far the most solvable in almost all disciplines and we try to linearise or create linear approximations wherever possible.

go well

Chestermiller Nov29-12 03:21 PM

Re: Elasticity (understanding elasticity from stress strain curve)
 
Quote:

Quote by Studiot (Post 4177904)
That's almost exactly what I said so what is your angle?

The only difference is that I also said linear-elastic is the most important. This is simply because linear theory is still by far the most solvable in almost all disciplines and we try to linearise or create linear approximations wherever possible.

go well

Thanks Studoit. In the case of polymeric materials, man-made fibers, and rubber products (e.g., automobile tires) which constitute a major part of my experience base, the deformations are typically large and the behavior is typically non-linear and often viscoelastic. So this is basically my angle.

Chet


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