Talk:Stress (mechanics)/Archive 1

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Brittle and ductile

I am skeptical about "By definition, brittle materials fail under normal stress, and plastic or ductile materials fail under shear stress." Don't ductile materials undergo plastic yield before failing whereas brittle materials simply break? (I am in the midst of a stress class; if nobody touches this, I'll come back to it later.)

I added this statement, and agree that it is simplified and can be improved. Please do.

However, I don’t believe the statement is false. Its not only that brittle materials simply break; they break in an orientation where the greatest normal stress is seen. And ductile materials do yield before breaking, but in the orientation of maximum shear stress, and plastic flow is a result of shear stress.

By the way, you can sign and date stamp your talk entries with four tildas (~~~~).

Duk 21:30, 16 Oct 2004 (UTC)

I disagree with the anonymous poster. The article is correct, as is. The above statement by Duk is indeed accurate, and very well written. --Simian, 2005-10-03, 06:44 Z

This is an article about stress

This is an article about stress. Stress can occure in solids, liquids and gases so it is unhelpful to turn this into an article about stress in solids. I think that it would be systematic to confine discussions about brittle and ductile fracture to the article fracture which really could do with turning into a decent article. Cutler 12:10, Oct 17, 2004 (UTC)

sounds good. Duk 02:48, 19 Oct 2004 (UTC)

I think the article is excellent, as is. It briefly mentions these terms in one or two sentences, as it should. Then if the reader wants further information, (s)he simply clicks on the links. The stress article should briefly mention this as it does now, and is very well written. --Simian, 2005-10-03, 06:44 Z

Microscopic Interpretation of Stress Force

I am missing any reference to the microscopic interpretation of the stress force in the article, and in particular to the fact that the stress/strain curve is only linear (i.e. obeys Hooke's law) over a much smaller region than one should expect if molecular forces are responsible. I have discussed this issue on my page http://www.physicsmyths.org.uk/hooke.htm and suggested there that in fact plasma polarization fields due to free electrons in the material might actually be responsible for the linear stress/strain curve in the Hooke-region.

Thomas

Cauchy v. Piola–Kirchhoff

There should be some discussion here about the distinction between the Cauchy and the Piola–Kirchoff stress tensors (corresponding, I believe, to true and engineering stress respectively.) —BenFrantzDale 15:59, 11 October 2005 (UTC)

I feel that we should resist the temptation to make this article too complicated and off-putting. Most people who come here will simply want to know the difference between stress and strain without any math. Technical points like this should go to stress tensor. Cutler 18:45, 11 October 2005 (UTC)

If BenFrantzDale is in agreement, this sounds like a good suggestion by Cutler. This does seem to be a perfect fit for the stress tensor article. --Simian, 2005-10-14, 04:22 Z

I agree, except that stress tensor has something to do with relativity. Perhaps there should be a new page for the nitty-gritty details of stress tensors in engineering? —BenFrantzDale 04:48, 14 October 2005 (UTC)

Good point. That page states up front it's devoted to relativity. So I currently retract my previous comment, and my two posts here can be deleted by the next editor. --Simian, 2005-10-14, 13:05 Z

Gonz - I agree.

I think that the discussion of Cauchy stress vs. Piola-Kirchhoff stress does have a place here. If this article is meant to represent the Continuum Theory of stress it is important to make the distinction between the two stress. They are theoretically and physically easy to understand as to not be "off-putting" yet at the same time important to the development of constitutive laws (where Cauchy is most often used) as compared to actual physical measurement (where the 1st P-K is most often used). A statement as to the conditions where these stresses are equal would be helpful as well.71.199.128.182

Title

This title is inaccurate. It should read either Stress (Mechanical Engineering) or Stress (Applied Physics).

'Stress (mechanics)' would be better, keeping both physicists and engineers happy. I've changed the first sentence to reflect this and link to mechanics. RDT2 09:49, 15 August 2006 (UTC)

Agree with 'Stress (mechanics)'. And I moved the 'Continuum mechanics' box to the top. --Duk 18:33, 14 January 2007 (UTC)

Agree with 'Stress (mechanics)'. Proposed merger with tensile stress. Katanada (talk) 23:47, 16 December 2007 (UTC)

formatting

why are so many phrases in boldface? this is distracting and makes the article more difficult to read. i suggest that boldface be removed from everything that doesn't need special emphasis (such as the examples of what constitutes a one-dimensional system and every single appearance of the word stress).

force on cube face

The force on the cube face is stress*dA, not dV. I've reverted to the previous version.RDT2 10:25, 19 October 2006 (UTC)

Extending side menu: Continuum mechanics

Hi, I have updated the article about Strain and I think that it should be in this side menu Continuum mechanics, but I don't know how to add it. How do you customise side menus?

Janek Kozicki 13:31, 21 November 2006 (UTC)

I added it to the menu. In the future, you can edit by entering "Template:Continuum mechanics" in the search bar. PAR 17:29, 1 December 2006 (UTC)

great, thanks! Janek Kozicki 21:14, 3 December 2006 (UTC)

Poisson's Ratio

In the section dealing with nominal and engineering stress, it said "its cross-sectional area reduces by an amount that depends on the Poisson's ratio" I changes this to "may change" to reflect a more general condition where the material my have a negative or zero Poisson's ratio. 71.199.128.182

Poisson's ratio will always be greater than zero and less than or equal to 0.5 —Preceding unsigned comment added by 74.60.57.253 (talk) 23:50, 5 August 2009 (UTC)

Residual Stresses

"Press fits are the most common intentional use of residual stress." I think this statement needs to be modified if not removed completely. It is unclear what is meant by "most common" and intentional. Many biologic tissues exhibit residual stresses. This is intentional as it helps the tissue function and is more common in the fact that many animals have tissues with residual stresses. I would propose "In material manufacturing, press fits are the most common intentional use of residual stress."71.199.128.182

Tensor notation

We have three different tensor notations in this article; $\sigma _{ji}$, $\sigma _{ij}$, and $\sigma$ & $\tau$. Should try to be more consistent here ? --Duk

Derivation of the stress vector as a function of the stress tensor

I have made a few changes in this derivation. The argument $h\rightarrow 0$ has been replaced by the idea of the parallelogram shrinking to a point. A note has also been added to Fig.3 explaining the sign convention K.sateesh (talk) 05:56, 18 February 2009 (UTC)

Archive 1

Archive 2

Archive 3

Separate into Intro and Main Page

There seem to be a lot of complaints and problems with this article largely because it's too broad and too dense. I think we should try splitting it the way the entropy and special relativity articles are with a special "intro to" article that discusses a less technical and more conceptual formatting. Thoughts? Vramasub (talk) 19:20, 21 January 2012 (UTC)

Scope of this article

It seems that from time to time this article gets edits related to material engineering, strength of materials, and metal forming processes. I would like to make it clear that the scope of this article should be, and it is, about the treatment of the concept of stress within the field of classical mechanics, i.e. continuum mechanics. All those disciplines use the concept of stress, as explained in this article, to study stresses encountered in the solids under study: during forming of the solid, during shaping of the solid, during use of the solid in a structure, etc. Those findings should go into other articles related to those disciplines, not into this article. sanpaz (talk) 17:55, 10 February 2012 (UTC)

Stress has no meaning outside of its effect on material. The effects of stress on material are needed to describe what stress is.

This article is really bad. The writing is really bad. It's about as exciting as stale dogshit. It drones on and on and on with graduate level tensor math that nobody cares about or will ever use, and which should be moved to its own article. It focuses obsessively on esoteric crap while failing to describe in clear terms what stress is. The article is written like someone who understands stress is talking to themselves, instead of being written for a reader who wants to learn what stress is. How many years have you been working on this article Sanpaz? The whole article should be deleted and re-written from scratch. --Typoheaven (talk) 14:16, 21 February 2012 (UTC)

Some of your criticisms are fair. The article could certainly be improved. This is true of most Wikipedia articles. Perhaps you'd like to help improve it? You could propose a complete rewrite here, on this talk page. Please give us your text.

Some of your criticisms are unfair. The article is not supposed to be exciting; it is an encyclopedia article, not a textbook or lecture. Lots of people care about and use the "graduate level tensor math". The article should include this technical material, as well as less-technical explanations. Mgnbar (talk) 15:26, 21 February 2012 (UTC)

Typoheaven, if you or anyone wants to understand what stress is in simple terms without going into the tensor math then you only need to read the introduction and the first two sections of the article. The rest of the article which has the math to explain stress is needed to really understand what stress is. There is no other way. That is what any book on continuum mechanics have. You cannot avoid going through the tensor math. So for you to suggest that the article should be deleted does not make sense. Not understanding a subject or thinking is too complex is not a reason to delete the content. Then again, the basic explanation of stress is in the introduction.

And to answer your question on how many years I have been working on this article, I say that is irrelevant. That is a personal attack from you that do not have any relevance to the content on the article.

The introduction and the first two sections could be improved, but the content after that will stay as is, because that is what the current knowledge of stress is.sanpaz (talk) 16:38, 21 February 2012 (UTC)

There is also the possibility to reorganize (not delete) the content, perhaps separating the Euler-Cauchy principle into its own article or something like that. But that has to be discussed and thought carefully as to keep the flow of ideas in the article intact.sanpaz (talk) 16:50, 21 February 2012 (UTC)

In my honest opinion, the article is quite good and I have gained lot of information from it. There is, of course, always room for improvement but I think we should respect the work other people are doing and try giving constructive criticism. Aarne Pohjonen (talk) 08:35, 19 December 2012 (UTC)

Scope: stress has no meaning without material. No material, no stress. Describing the effect of stress and its particular components on different materials is many times more informative than graduate level tensor math. At least for someone who wants to learn what stress is. Dump all that esoteric crap into its own article. Where's mohr's circle? That's something that's actually useful as well as illuminating to someone trying to understand stress. Again, this article reads like a graduate student talking to himself. And sorry Sanpaz, I don't mean this as a personal attack. --Typoheaven (talk) 17:04, 21 February 2012 (UTC)

Typoheaven, the reason for not including how stress relates to specific materials is that that is a specific topic on strength of materials, or metal forming, or strength of materials. Those articles should have the information on how stress develops or is present in materials. The article on stress should only include the explanation of the concept of stress, so other articles (strength of materials, metal forming, materials engineering, etc) can use it as reference to describe stress in their own context.

I understand what you are looking for. You are trying to find a space in Wikipedia where the content related to stresses in materials should be located. And when you see this article it seems like a good place to included it. But this article has a very distinctive scope, which I already explained above. One has to frame an article so it does not blow out of proportion and includes everything under the sun. Is the current scope the best? I would not put my hand on the fire for it, but I think the scope is very clear.

What I think you are looking for is a section that links this article with those other articles. A section that talks, without going into to much details, about how stress is important in those fields related to materials so the reader can go there and be more informed on stress as it relates to materials. That can be done (I'll try to do something on this too). If you want to give it a go by all means do.

And again, this article needs to keep the math which is necessary to understand stress in depth. I did not developed the math, that has been done for the last 250 years. There is no way to avoid it. It is a complex math, but that is why the first parts of the article gives and introduction in words to the reader to the concept of stress. That introduction is good enough for understanding stress and applying it to basic strength of materials etc. However, if later the reader wants to go into the details of what stress actually is (a quantity that can be "fully" described by a tensor) then he/she can follow the math.

Good point on the Mohr circle. Actually, Mohr Circle was originally included in the article a year or so ago. However, the section containing the graphical representation of stress (Mohr cirlce) was moved by other users to the article Stress Analysis. There is still a reference to the Morh cirlce in the current stress article.The section on the Mohr circle should have a more prominent location in the article. That's something else that needs to be worked on.

I accept the apology. It seems you like this topic, which is great. I also like this topic, and that is why I work on it. I, and other users, try to present the topic the best we can. We only want other people to benefit from the article. It is never easy to explain things. It is a constant iteration. That is why feedback from users like you is very important, because it shows that there are needs not met. And, I suggest never to use over the top language, such as "dog shit" etc. That does not help at all. It shows to much emotion and not much reason. Always give reasons for changes, but saying something is too complex is not a reason for removing content that is correct and verifiable. A reason for changes may be that the complex content needs better exposition or better introduction. Or that the complex content would be better placed in a separate article. With respect. sanpaz (talk) 17:59, 21 February 2012 (UTC)

Why is this article tagged "very long"?

Is there a point to leave the tag alone? Seriously, there is nothing wrong about this article's length. What are specific issues that brought you into tagging this article as "very long"? --George Ho (talk) 17:39, 11 May 2012 (UTC)

The tag is apparently gone, but the article is indeed to long:

• The Cauchy stress tensor deserves its own article.
• There is some repeated material, in the article and across articles.
• Proofs should be trimmed or moved to a Wikibook.

--Jorge Stolfi (talk) 03:22, 6 February 2013 (UTC)

Built-in stress is stress, sometimes important

The article states that

The stresses considered in continuum mechanics are only those produced during the application of external forces and the consequent deformation of the body; that is to say, relative changes in deformation are considered rather than absolute values. A body is considered stress-free if the only forces present are those inter-atomic forces (ionic, metallic, and van der Waals forces) required to hold the body together and to keep its shape in the absence of all external influences, including gravitational attraction.[1][2] Stresses generated during manufacture of the body to a specific configuration are also excluded.

This is not true. Stress by definition includes any built-in stress. Only in some applications, where one can assume linearity, it is possible (but optional) to leave out built-in stress. In some applications (such as prestressed concrete, polarization, crack propagation, tempered glass) the built-in stress is critical. --Jorge Stolfi (talk) 22:44, 31 January 2013 (UTC)

• Fixed. --Jorge Stolfi (talk) 14:40, 6 February 2013 (UTC)

Topics that are missing

In spite of its length, the article manages to be rather incomplete and narrow. There should be more material about, say:

• The molecular physics of stress
• Historical information
• Pragmatics, e.g.
  • Why heated glass shatters
  • Measuring stress
  • Stress in buildings, structures, furniture
• Simple Bending Stress (The simple Stress due to Compression/Tension and Shear are included, however bending is not.)
• Stress in biology, anatomy, geology
• Stress in composites

Et coetera... --Jorge Stolfi (talk) 03:27, 6 February 2013 (UTC)

• There is now the germ of an History section. If you know more, please contribute... All the best, --Jorge Stolfi (talk) 01:00, 9 February 2013 (UTC)

Stress can exist without deformation

The article also states that

The stresses considered in continuum mechanics are only those produced during the application of external forces and the consequent deformation of the body; that is to say, relative changes in deformation are considered rather than absolute values.

This too is incorrect. Stress can be generated without any deformation or external forces, e.g. by change in temperature or chemical structure. In piezoelectric materials the stress is generated directly by applying an external field, and one can arrange the field so that there is stress but no deformation. Also in flowing viscous liquid or vibrating viscoelastic solid that is momentarily going through its rest (zero-strain) state there will be viscous stress without any deformation. --Jorge Stolfi (talk) 22:50, 31 January 2013 (UTC)

• Fixed. --Jorge Stolfi (talk) 14:41, 6 February 2013 (UTC)

The statement was/is correct. The statement is not saying stress is not present in bodies withoug external loads. What the statement is saying is that for the the field of continuum mechanics what is of interest is stresses produced by loads. sanpaz (talk) 19:38, 7 February 2013 (UTC)

That can't be correct. For example, for the design of prestressed concrete and analysis of fractures in tempered glass the built-in stress is all-important. In woodworking, a straight board becomes bent when humidity changes, or when it is cut in half. A telescope mirror deforms with changes in temperature. Stresses and strains go on dancing in a sound wave even when there is no external load. Shouldn't those phenomena be studied in continuum mechanics?
Perhaps that statement applies to continuum mechanics as it is taught in certain areas of engineering? --Jorge Stolfi (talk) 03:37, 8 February 2013 (UTC)

The statement was taken from the book of Atanackovic (see reference section). The cases you suggested are all examples of materials being affected by loads: the stresses the pre-stressed concrete has are reached by applying a load (tension tendons) on the concrete; when you cut a piece of wood you are unloading the wood. The statement is trying to make it clear that stresses produced during manufacturing are not the scope of continuum mechanics (that is the scope of other fields). The mathematical body of continuum mechanics does not have the tools to analyze those inter-atomic forces (ionic, metallic, and van der Waals forces) required to hold the body together and to keep its shape in the absence of all external influences, including gravitational attraction. The statement is correct, it comes from a reference, and frames the study of stress from the perspective of continuum mechanics, which is the treatment of this article.sanpaz (talk) 16:33, 8 February 2013 (UTC)

Not everything that comes from a reference is correct 8-).
I am confused: since the internal stress in the wood example is "unloaded" by cutting, then isn't it to be considered a "load"?
Another example of stress without external load: a throbbing blob of water floating in the Space Station. The viscous forces inside the blob are definitely within the scope of C.M. but are not due to any external load. C.M. does not even assume that there were any external forces in the past that set the blob in motion.
Perhaps what the author really wanted to say is: continuum mechanics studies all the forces and stresses in a material, internal or external, whatever their causes; but is not concerted with the causes themselves? In particular, C.M. does cover built-in stress (and must do so when things get non-linear), but not its physical causes? All the best, --Jorge Stolfi (talk) 00:54, 9 February 2013 (UTC)

Another sentence that gives an overly narrow definition of CM:

Following classical Newtonian and Eulerian dynamics, the motion of a material body is produced by the action of externally applied forces.

Again, continuous materials can have motion without external force; for example, sound waves, thermal expansion, viscous relaxation of compressed memory foam These are squarely within the domain on CM. Perhaps this too applies to stress analysis in engineering, specifically, rather than CM? --Jorge Stolfi (talk) 04:20, 10 February 2013 (UTC)

CM can handle rigid materials too

This sentence was taken from from stress (mechanics)#Mathematical background:

Continuum mechanics deals with deformable bodies, as opposed to rigid bodies.

This may be a phylosophical quibble; but fluid dynamics has no problem dealing with pressure in flows of incompressible liquids. Now, pressure is to stress like is change of volume is to strain. So it seems quite reasonable to study "rigid" materials in CM Those would be materials so stiff that the strain can be considered zero, even though the stress is not. Makes sense? --Jorge Stolfi (talk) 04:14, 10 February 2013 (UTC)

Be careful with what "rigid body" means. In physics, a rigid body is that which is assumed not deforming when loads are applied. It is not talking about actual objects (there is no such thing as a pure rigid body). Newtonian mechanics and analytical mechanics deals with the kinematics and kinetics of rigid bodies. CM, however, deals with deformable bodies, not "rigid bodies". The statement is correct.sanpaz (talk) 18:20, 10 February 2013 (UTC)

I understand the physics sense of "rigid", but it is just like "incompressible" for fluids. No real fluid is incompressible, yet in CM one can and does model them as such --- and still have well-defined stress fields in them, including stress waves, etc.. The point is that stress can be modeled in CM without assuming the existence of strain. But I will now quibble more about that. --Jorge Stolfi (talk) 03:38, 11 February 2013 (UTC)

Incompressible fluids still have flow and motion, they still deform(?) (change configuration/placement). The intent of the statement is to show that CM does not deal with the rigid body motion of objects, that is the scope, like I said before, of Newtonian mechanics and analytical mechanics. For example, the rotation of a rigid body around an axis is not a problem for CM. It is true that rigid bodies have built-in stresses, but when a load is applied on a 'rigid body' the state of stresses does not change. If they do change, then it is not a rigid body. I would prefer to have a statement in the lines of: Continuum mechanics deals with the motion of deformable bodies, as opposed to the motion of rigid bodies. But perhaps this statement is more suited for the CM article. The statement is not a deal-breaker for this article on stress, though. sanpaz (talk) 18:19, 11 February 2013 (UTC)