What's the difference between inelastic and plastic? 1

[Alaazaki]

Hello,

Is there a difference between inelastic and plastic, I know that they are the same literally but i ask if they have a different definition in structural analysis.

 

[KootK]

I struggle to think of a difference in our field. In Venn diagram lingo, I think that all of "plastic" fits within the container representing "inelastic" and very nearly fills it. The unfilled bits would be arcane material science phenomena like strain rate dependent hysteresis and the like.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rb1957]

i use the terms interchangably, both are non-linear material response to load/stress.

another day in paradise, or is paradise one day closer ?

 

[SlideRuleEra]

IMHO, an example of an inelastic, but not plastic, structural material would be cast iron, such as used is building columns from 100+ years ago. Of course, steel would be a typical example of a material that can exhibit plastic properties.

http://www.SlideRuleEra.net

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[rb1957]

cast iron is brittle, no? ... elastic only ? no significant post-yield straining ?

another day in paradise, or is paradise one day closer ?

 

[woodman88]

It depends on who taught the person using the word. Which is why a standard writen by several people will use both words. IMHO

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[cooperDBM]

I would sum it up as inelastic simply means not elastic. It doesn't necessarily mean plastic which specifically requires significant post-yielding strain capability (i.e. not brittle). So in structural engineering steel and cast iron, or concrete, highlight the difference.

 

[KootK]

I'm going with a dissenting opinion on this one. I think that meaningful post-yield strain capacity is necessary for inelasticity. With regard to particular materials:

1) Cast iron. Neither plastic nor inelastic. Rather, I would characterize it as non-linear elastic up to brittle fracture.

2) Unreinforced concrete. Again, non-linear elastic up to fracture/crushing.

3) Reinforced concrete. Quite plastic and inelastic for most applications.

4) Steel. Plastic and inelastic so long as buckling is prevented.

Of course, few materials fit any model perfectly.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rb1957]

what's the difference between "inelastic" and "non-linear elastic" ?

actually, i thought you'd cracked it with inelastic is non-linear elastic as opposed to plastic (with permanent deformation).

another day in paradise, or is paradise one day closer ?

 

[btrueblood]

Oy. Linear elasticity I think we all understand. Nonlinear elastic is what a rubber band does (the stress-strain curve is not linear, but it is more or less repeatable from one loading application to the next). Inelastic says that the stress-strain curve depends on variables other than stress and strain, i.e. strain rate or prior stress history, time, etc., and that those variables shift the stress-strain curve left/right or up/down (permanent deformations take place), and thus the stress-strain curve of one loading may not be duplicated by the next loading/unloading cycle. Mostly we use time-dependent behaviors to describe inelastic materials. Thus plastic, elastic/plastic with strain hardening, viscoelastic, etc. Hysteresis in rubber would be an example of inelastic behavior (even though the curve repeats, you need to know which direction the loading is going to know which curve to follow in the stress-strain diagram).

My $.02...

 

[KootK]

Interested readers might also want to check out this recent thread where I got schooled some on my material science: Link. Specifically, a poster explained the interesting case of a rubber band.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Alaazaki]

I am very grateful to you for your replying and trying to answer my question

 

[rn14]

In structural engineering I think it really depends on context. For example, if talking about a member "going inelastic" inelastic would refer to a ANY stresses in the member cross-section exceeding the yield stress where as the member is plastic once the ENTIRE cross-section reaches yield stress.
If you are talking about materials it can take on different meaning ie. a material modeled as elasto-plastic has a stress-strain curve that is linear in the elastic range and flat in the inelastic range (meaning additional strain/deformation without an increase in stress).

 

[IDS]

Just goes to show that we need to define what we mean, even when using words that everybody knows what they mean.

My usage is:
Linear elastic: does what Robert Hooke said it should, or at least very nearly, since nothing has perfectly linear behaviour when loads are applied over a finite time.

Non-linear elastic: Non-linear stress-strain, but close to zero permanent strain when loads are removed.

Inelastic: Any behaviour that has significant permanent strain after removal of loads.

Plastic: Any permanent strain due to stresses exceeding a well defined yield stress.

Examples of inelastic behaviour that I would not call plastic are soil under loading and unloading, and reinforced concrete at first cracking.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[btrueblood]

I dislike the term "inelastic" only because it is not very specific.

" if talking about a member "going inelastic" inelastic would refer to a ANY stresses in the member cross-section exceeding the yield stress where as the member is plastic once the ENTIRE cross-section reaches yield stress."

Why wouldn't you just say the member yielded, or exceeded its yield stress? Call a pig by another name, it's still a pig.

 

[Compositepro]

The problem here is that "inelastic" is not usually used in a technical sense. To most people it simply means that "it does not stretch very much". In materials science elasticity is recoverable strain due to stress and plasticity is the unrecoverable strain due to yielding. Add viscosity and you can describe the behavior of most any material under stress.
Visco-elasticity is often described using the spring and dashpot model. The dashpot can be in parallel and or in series with the spring. Plastics have a relatively large viscous component whereas metals do not.

When "inelastic" is used in a scientific paper it means that the material does not have an elastic region in a stress/strain curve. So water would be "inelastic".

 

[IDS]

When "inelastic" is used in a scientific paper it means that the material does not have an elastic region in a stress/strain curve.

Except when it means something else.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Compositepro]

Very true. Context helps a lot, and when it does not, there is a communication problem.

 

[rn14]

Why wouldn't you just say the member yielded, or exceeded its yield stress? Call a pig by another name, it's still a pig.

Because sometimes that's not all you are interested in. If your are limiting your analysis to first yield then, yes, it doesn't matter. If you are, for example, designing a plastic hinge, then it makes a difference as the capacity at first yield and at full yield are not the same.

 

[btrueblood]

IF we were designing beyond the elastic limit of a steel beam, we'd discuss plastic deformation and work hardening, etc., not "inelasticity", no? A partially yielded beam is not truly "inelastic", it only has some plastic deformation occurring simultaneously occuring elastic deformations, i.e. there is a combination of behavior occurring. We would want to be precise about how much plastic vs. elastic strain has occurred. My point is that your example is complex, and must be described by complex language, and not by the misleading term "inelastic".