Stress - acting force or reacting force??

[ninjaz]

Please forgive me if this questions seems very silly, But i doubt there might be different perspective here.
My understanding is that, the Stress is the internal resistance offered by an object which is equal to the external force acting on it. So stress should be an reacting force.

But when i go through some basic dynamic problems in websites and books, they are using the phrase."stress acting on the point is...".How can stress acts?

 

[TGS4]

In my opinion, the phraseology that you are seeing is sloppy. Is should be either "the stresses acting at the point", "the stresses occurring on the point", or "the stresses occurring at the point".

 

[rb1957]

stress in the internal reaction to an applied load. if a force is applied to a body, the body reacts this external force with an equal and opposite internal force that is the integral of the internal stresses acting over an area.

if you push on the table, the table pushes back.

Quando Omni Flunkus Moritati

 

[IDS]

Both the body resisting an applied force, and the body applying the force have a stress over the area of contact. Why should the term stress only apply to the resisting body?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rb1957]

the applied force acting over an area is more pressure than stress (IMHO)

Quando Omni Flunkus Moritati

 

[Drej]

If pressure is force acting over an area then how would you define stress?

Differentiating between the two - or rather, whether to use one or the other - for me has always been down to the loadcase being assessed. For example, a gas exerts a pressure on a body (say internally within a pressure vessel) and stresses in the body (the vessel) are set up as a result of this pressure. Pressure for me is usually specified when dealing with fluids generally speaking.


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[IceBreakerSours]

Stress = lim[sub]A->0[/sub] F/A. Stress has direction associated with it (S tensor = Force vector/Area vector). Pressure does not; "mean pressure" is simply non-hydrostatic stress.

A point, conceptually, is a very small cube within which there is a homogeneous strain/stress state (immaterial of material properties, time-dependent effects, external/body forces, etc.) and still satisfy the conditions of continuum mechanics. It is not supposed to be a molecule, atom or smaller.

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[IDS]

Pressure and stress are different names for the same thing. They tend to be used in different circumstances, but they are both F/A. If pressure is applied to a surface it has a direction, just as it would if you called it stress.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[jhardy1]

As IDS says:
Stress / Pressure: Two names for the same thing, differing only in how they are commonly used. (Stress tends to be used in structural mechanics, pressure tends to be used in fluid mechanics.) If you apply a hydrostatic or wind "pressure" onto a component, the normal stress at the impinged surface is equal and opposite to the applied "pressure". If two parts act on each other, the contact stress on the "resisting" body is equal and opposite to the contact stress on the "acting" body.

We will be debating how many angels can dance on the head of a pin next!

(It's 42, by the way, if anyone is wondering ...)

http://julianh72.blogspot.com

 

[rb1957]

stress is the internal effect of an external, applied, pressure.

just because the terms have the same unit doesn't mean they're interchangable.

Quando Omni Flunkus Moritati

 

[IDS]

stress is the internal effect of an external, applied, pressure.

just because the terms have the same unit doesn't mean they're interchangable.

No, but when they have the same definition they are interchangeable unless their usage is clearly defined. In the case of stress and pressure the usage is not clearly defined, and there is nothing wrong with using the term "contact stress", for instance, or to talk about the stress acting at an interface.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[dangermx]

Please note that using both therms, pressure and stress, like the two of them were the exact same thing is not correct.

rb1957 is right by saying that just because the two of them have the same units doesn't mean they are exchangables, stress is defined by a pressure applied to an specific area, sigma equals F/A.

However I think the others guys answered correctly to the topic in this post

Regards

 

[IDS]

I don't want to labour the point, but they are the exact same thing. The definition of pressure is F/A and the definition of stress is F/A.

Perhaps those who insist they are different can point to a case where it would make a difference if you used pressure, when you should have used stress, or vice versa.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rb1957]

well since you are laboring the point ... I guess you could say that an area A reacts a force F by exerting a pressure but I suggest everyone else would call that a stress. similarly, you might say that you stress your tires to 34psi but I think nearly everyone would call that a pressure.

getting back to the OP ... a "stress acting at a point" means either that there are a bunch of points that make up an area or that at some point (geometrical, not mathematical) in the structure the local stress is ...

Quando Omni Flunkus Moritati

 

[IDS]

well since you are laboring the point ... I guess you could say that an area A reacts a force F by exerting a pressure but I suggest everyone else would call that a stress.

So you have never heard anyone referring to the stress in the soil under a footing as a "bearing pressure"?

similarly, you might say that you stress your tires to 34psi but I think nearly everyone would call that a pressure.

So what do we call F/A in the rubber where it meets the road, stress of pressure?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rb1957]

depends ... sometimes it's pressure, sometimes it's stress, they're very similar in meaning but have developed different usage (you don't stress a tire to 34psi) ... i don't deal with soil bearing, i'll take your experience as knowing whether it's "stress" (i doubt) or "pressure" (more likely). if i'm analyzing a piece of structure then P/A is IMHO stress and not pressure; if i'm applying a load to this strcuture then P/A is pressure (again, IMVHO)

Quando Omni Flunkus Moritati

 

[IDS]

depends ... sometimes it's pressure, sometimes it's stress, they're very similar in meaning but have developed different usage (you don't stress a tire to 34psi) ...

That's just what I am saying. They are, by definition, the same thing, but they are used in different contexts. The problem is that the contexts are not well defined and are not consistent. For instance "the internal resistance per unit area offered by an object resulting from* the external force acting on it" will usually be called a stress, but if the object is a soil it will usually be called a pressure, and if it is a fluid it will always (as far as I know) be called a pressure. The different words don't mean we are talking about something different, it's just convention.

* Italics indicate The statement in the OP amended to avoid talking about a stress or pressure being equal to a force.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[jhardy1]

@rb1957
"you don't stress a tire to 34psi"
No - you inflate it! (Here in Oz, we don't "pressure" tyres, but we might "pressurise" them.)

http://julianh72.blogspot.com

 

[IceBreakerSours]

Perhaps a more theoretical approach might help resolve the debate:
[ul]
[li]Stress is a tensor (usually, denoted by σ) and most often used in solid mechanics. It has several components (σ[sub]11[/sub], σ[sub]12[/sub], σ[sub]13[/sub], σ[sub]21[/sub], σ[sub]22[/sub], σ[sub]23[/sub], etc.). Each of these components has a physical meaning associated with it.[/li]

[li]A stress tensor may be expressed by its invariants (such as, I[sub]I[/sub], I[sub]II[/sub], I[sub]III[/sub], and so on), which may be used to define constitutive relations (for example, isotropic elasticity, anisotropic hyperelasticity, etc.).[/li]

[li]Equilibrium conditions and conservation equations in mechanics (continuum, solid, fluid, elasticity, plasticity, .. FEA, CFD, etc.) use the averaged stress over an infinitesimally small unit cube of material as the starting point in establishing a theoretical foundation.[/li]
[/ul]

What about pressure?
[ul]
[li]The term pressure is *most* often used when fluids are under consideration.[/li]

[li]Pressure is (-1/3)*(σ[sub]11[/sub]+ σ[sub]22[/sub]+ σ[sub]33[/sub]). It is an algebraic sum of the normal components of the stress tensor; or, a scalar because area vector is normal to the force vector.[/li]

[li]Not all mathematical operations permitted for scalars are permitted for tensors.[/li]

[li]Pressure is hydrostatic causing volumetric deformation.[/li]
[/ul]
I welcome any corrections to my understanding. Certainly, nomenclature from different areas or non-rigorous nature of grade/school physics must contribute to some apparent confusion.

Anyone counting the number of angels?

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