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Elasticity (understanding elasticity from stress strain curve) 
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#1
Nov2712, 12:24 AM

P: 15

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?? 


#2
Nov2712, 12:29 AM

P: 15

This is a sample curve



#3
Nov2712, 01:36 AM

P: 105

High yield value = strong
Large E = stiff Large strain to failure = ductile 


#4
Nov2712, 02:32 AM

P: 15

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



#5
Nov2712, 05:02 PM

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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?"



#6
Nov2712, 10:56 PM

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#7
Nov2812, 06:03 PM

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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.



#8
Nov2912, 02:20 AM

P: 15

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? 


#9
Nov2912, 02:35 AM

P: 5,462

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 stressstrain 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. 


#10
Nov2912, 02:37 AM

P: 15

So what I finally understand is.. elasticity is not something we can quantify.. so its not a physical quantity right?



#11
Nov2912, 02:54 AM

P: 5,462

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 linearelastic or hookean (which is a sub class). 


#12
Nov2912, 07:18 AM

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#13
Nov2912, 11:13 AM

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The only difference is that I also said linearelastic 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 


#14
Nov2912, 03:21 PM

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Chet 


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