Talk:Stress (mechanics)

Text and/or other creative content from this version of Stress (mechanics) was copied or moved into Euler's laws of motion with this edit on 9 September 2010. The former page's history now serves to provide attribution for that content in the latter page, and it must not be deleted as long as the latter page exists.

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This article sucks

for one thing, it focuses exclusively on solid material - what about fluids and gasses? For another, the most important concepts are buried under mountains of crap - the most simple, basic and important concepts, like ductile materials failing under shear and brittle failing under normal, fluids supporting normal but flowing under shear ... that should be in the intro. Instead, the article drones on and on with graduate level tensor math that should be spit into other articles. --32alpha4tango (talk) 20:59, 1 June 2011 (UTC)[reply]

Great ideas. Please add them (with references)! Mgnbar (talk) 22:22, 1 June 2011 (UTC)[reply]

This article is about the concept of stress, not about strength of materials. It is very tempting to start writing in this article about ductile materials, the mechanical behaviour of solids (metals, rock, soil) and fluids under different loads. But this is not what this article is or should be about. sanpaz (talk) 16:46, 10 September 2011 (UTC)[reply]

I agree wholeheartedly, but for different reasons. This article is focused way too much on engineering aspects, as opposed to the basic physics, and spends too much time trying to explain simple concepts in a confusing manner. Stronger emphasis needs to be placed on the general applicability of the stress tensor to continuous media, and on its physical meaning. The overall formatting is crap. Why is the basic equation

\mathbf {T} ^{(\mathbf {n} )}={\boldsymbol {\sigma }}\cdot \mathbf {n}

in the middle of the article? Also,

\mathbf {n}

is not explicitly defined before being used. Furthermore, for an article about the stress tensor, how come there is no link to the Cauchy momentum equation? Also, why is there no mention of the connection to non-mechanical stress tensors like the Maxwell stress tensor? Why, in many of the equations, are the individual components of the tensor written out? Either vector notation, or Einstein notation (with links to the appropriate pages in the intro) should be sufficient. I personally have never written a wikipedia article before, but this one needs some serious revision.198.125.228.16 (talk) 18:21, 2 April 2012 (UTC)[reply]

Can someone please tell me what is T₁^(e1)? — Preceding unsigned comment added by 78.45.152.147 (talk) 16:49, 15 February 2013 (UTC)[reply]

T is the stress vector. T^e1 is the stress vector associated with a plane with normal vector e1. Then, T₁^(e1) is the component (scalar) in the x1 direction of the T vector (associated with the plane with normal vector e1). sanpaz (talk) 17:06, 15 February 2013 (UTC)[reply]

Ah thank you.— Preceding unsigned comment added by 78.45.152.147 (talk) 17:33, 15 February 2013 (UTC)[reply]

In the equation stress tensor is in transpose form actually, as it is evidenced by the matrix members following. Which is same as the stress tensor itself due to its symmetry. Just the symmetry notes should be placed before the equation, not after it. So there would be no initial wondering for those like me, not well familiar with the subject. Twowheelsbg (talk) 17:22, 14 June 2013 (UTC)[reply]

General revision

The "Introduction" section (now "Overview") has gone through a general revision. Hopefully not too many errors were added and the notation was not messed up too much. I plan to work a bit more on this article over the next couple of days. --Jorge Stolfi (talk) 04:03, 7 February 2013 (UTC)[reply]

As your revision included most of the text already in existence, it will take some time to see if some of the text removed is missing in the new revision. At first glance, I noticed you removed the figure that appeared in the introduction. This figure is very useful to show the concept of an infinitesimal element and the stress tensor associated with it. I will include the figure again. sanpaz (talk) 19:42, 7 February 2013 (UTC)[reply]

That figure seemed too detailed for the needs of the head section. Isn't the "double bubble" picture (below the protractor photo) sufficient for that section? --Jorge Stolfi (talk) 03:08, 8 February 2013 (UTC)[reply]

Perhaps it is too detailed for the introduction. However, I think it is very useful in later section. I'll think about where to place it.sanpaz (talk) 16:19, 8 February 2013 (UTC)[reply]

Continuing with the general revision, I noticed that the section "stress (mechanics)#Stress analysis" was almost a verbatim copy of part of the stress analysis article. Some duplication between articles is unavoidable and even good, but verbatim copy is annoying to readers and demands twice as much work from editors. So I trimmed that section of this article, and moved to the stress analysis article some material from stress (mechanics)#Mathematical background that seemed to belong there. I hope I did not do much damage in the process. --Jorge Stolfi (talk) 03:31, 10 February 2013 (UTC)[reply]

Jorge, are you planning on moving a lot of the content of stress (mechanics) to other articles (stress analysis, Euler-Cauchy stress principle) and leave this article dealing with stress in a general sense? I think that would be a good idea.sanpaz (talk) 18:21, 11 February 2013 (UTC)[reply]

I am tempted to do that, but I do not know whether I will have the energy. 8-) --Jorge Stolfi (talk) 01:25, 12 February 2013 (UTC)[reply]

What does it mean here stress tensor is type (0-2) - covariant ? Because in article for Caushi stress tensor is stated contravariant tensor, which I interpret as type (2-0). Twowheelsbg (talk) 18:58, 14 June 2013 (UTC)[reply]

Theory sections have been split off

The main theoretical sections were way too long and dense. They have been split off to Cauchy stress tensor and Euler-Cauchy stress principle, where they will be hopefully easier to read and edit. Of course, summaries of those sections should remain here with wikilinks to those articles. I will try to do that in the coming days, but please help. --Jorge Stolfi (talk) 22:14, 23 February 2013 (UTC)[reply]

"Further reading" section needs trimming

The "Further reading" list seems way too long, and some of the entries seem rather specialized (e.g. specific to soil mechanics and geology). Some entries have been already moved to the split-off articles Cauchy stress tensor and Euler-Cauchy stress principle, but the list still needs some trimming IMHO. Perhaps move some enrties to continuum mechanics? --Jorge Stolfi (talk) 22:19, 23 February 2013 (UTC)[reply]

This (previously excellent) article has REALLY deteriorated in quality, rigor, completeness, accuracy and organization

Key ideas such as the Euler-Cauchy stress principle have been banished to other pages. The definition of the Cauchy stress tensor is gone. The excellent figures demonstrating the balance of internal forces are gone. There is a new and rather confused section on stress analysis. There is a considerable amount of redundant material. There are absurd references to "particles" midway in the article, which defeats the whole purpose of introducing a continuum framework to define stresses, to begin with. And what purpose does that picture of a tank car serve?

Dr. Stolfi, no doubt your edits were made with the intention of improving things but excuse me when I say that they have really hurt the article. The previous version was excellent, rigorous, comprehensive and of high quality overall (thanks to the tireless efforts of editors Sanpaz and Bbanerje, who are both experts in continuum mechanics). Watching it over the years, it had also reached a degree of stability. Unfortunately, I have neither the time nor the energy to make the extensive edits this article needs. Commutator (talk) 05:34, 25 March 2013 (UTC)[reply]

Please provide diffs. Or just revert back to the version you preferred. KatieBoundary (talk) 10:50, 25 March 2013 (UTC)[reply]

I think it is clear that this article has changed in scope. It went from addressing stress from the continuum mechanics perspective, to a addressing stress in a more general sense. I do not think this is a bad approach. What has happened is that the old article got the Cauchy stress tensor portion (continuum mechanics component) removed from it and sent to a separate article, leaving the current article with the a general explanation of stress. What perhaps needs to be done in this article to further improve it is to make sure it covers all aspects of stress and also highlights the links to the most relevant related articles (Cauchy stress tensor, Stress analysis, Mohr's circle, Cauchy's equations of motion, Euler's laws of motion etc.)sanpaz (talk) 01:54, 12 June 2013 (UTC)[reply]

Sign convention?

Is there a convention for the sign of simple uniaxial stress σ, or did I miss it in my reading of the article? Although I didn't see it stated anywhere, it looked to me from the direction of the force vectors that in the equation F/A = σ, tensile stress is positive (σ > 0) while compressive stress is negative (σ < 0). --Chetvorno^TALK 20:38, 17 April 2013 (UTC)[reply]

Confusing and Ambiguous Notation and Pictorial Discriptions

No fault to the authors. This seems to be an historical problem guaranteed to confuse initiates to the subject.

One of the first things I ask is, "What are the units of the quantities involved?"

Stress is identified as having units of pressure in the Units section. "The dimension of stress is that of pressure...". Within the Cauchy Stress Tensor section, the unit vector is said to have units of length. Therefore the stress tensor has units of force per unit volume.

The unfortunately named stress tensor does not have units of stress if we are to believe the references.

It should be most important to clarify this to new travelers.

In places, pressure is identified with the letter T. In other text and pictorials it has the symbol sigma, confusing it with the stress tensor. This should not occur within a single article.

In pictorials the two index stress tensor is drawn as a vector as if it has a direction and magnitude! This is sure to befuzzle anyone.

I am new to this subject and having a difficult time untangling the, apparently traditional, mess. Anyone attempting to understand this subject must carefully read each line of text, checking for errors, inconsistency and misdirection. Craigde (talk) 15:32, 18 January 2015 (UTC)[reply]

I believe that the author of the "Cauchy stress tensor" section means that the normal vector has a length of 1 ("unit length") in dimensionless units. The rest of the article has been modified arbitrarily by several people over time and needs to be re-examined by someone who understands the subject. Bbanerje (talk) 08:58, 20 January 2015 (UTC)[reply]

Thank you. This addresses one point. However, I don't think the author(s) gave it much thought at all or they would not have used the term "unit length" to describe something you say might be unitless.

To keep it simple, the stress tensor is only a multilinar map from from an area element to a vector valued pressure. It should not be confused with pressure itself. More, I find the mathematical modeling, even in tensor notation, an ill marriage with the physics. It cannot survive in any other then 3 spatial dimensions. The reason for this is the area normal obtained through an implicit cross product. I'm still looking for the mathematics that will properly model stress in a coordinate and dimension free way. 2001:5B0:2BFF:3EF0:0:0:0:36 (talk) 11:50, 20 January 2015 (UTC)[reply]

I'm not an expert on this subject, but I think that you need to carefully distinguish between the stress tensor and the traction vector. You get the latter by applying the former to a surface element, or equivalently its scaled normal vector.

You are right that a lot of this subject is specific to three (or two) dimensions. For example, the crucial Cauchy momentum equation is derived using the divergence theorem, which relates three-dimensional bodies to their two-dimensional surfaces. The divergence theorem has analogues in every dimension (see Stokes theorem), but the notation changes, and I've never seen the topic presented in that generality. Anyway, maybe this gives you an idea of how to generalize? Mgnbar (talk) 14:19, 20 January 2015 (UTC)[reply]

One possibility: Differential k-forms using the wedge (outer product) and Hodge star and sans inner products, with the unit area 2-from replacing the unit (or unitless) area normal vector. But this would be going too far engineers and students of classical physics, even if it could be done. Furthermore, no one would like it and there are no reference sources. Consider it a mathematical challenge. — Preceding unsigned comment added by 2001:5B0:2BFF:EF0:0:0:0:3C (talk) 01:51, 22 January 2015 (UTC)[reply]

For an old reference that uses the differential geometry perspective, see Mathmatical Foundations of Elasticity by Marsden and Hughes. A lot of recent research also takes a geometric approach. However, there are numerous unsolved problems in 3 and 3.5 dimensions and k (smooth) dimensions is not a priority from an engineering perspective. Bbanerje (talk) 07:22, 24 January 2015 (UTC)[reply]

Thanks for that. It is for that sort of perspective, that might be generalized to relativity, that I came to this article. Craigde (talk) 19:56, 24 January 2015 (UTC)[reply]

Redundant Picture

I think that the picture of the car window is not helpful for the article and should be removed. The resons are: 1. At the beginning of the article, there is already a picture clearly showing the visual effect of stressed materials when viewed through a polarizing filter 2. The most dramatic changes between the two views of the car window are not caused by the stress internal to the material, but by the reflection properties of its surface. With this I refer mostly to the transparency of the window when viewed with the polarizing filter. The finer structure caused by the material intrinsic stress is only visible when the picture is view in screen size and is much fainter than the one described above. Don't get me wrong, I think the picture is technically advanced , but I also think it illustrates the change of transparency much better than the effect of stress.
132.187.199.137 (talk) 11:25, 6 July 2015 (UTC)[reply]

The stress zones of the protractor are not intentional but inevitable result of the production process, while the stress matrix of the car window is an outcome of a purposeful design and very exact planning. It is of course my subjective and biased view and I humbly wonder whether other wikipedians approve the deletion of my photo. Etan J. Tal^(talk) 12:07, 6 July 2015 (UTC)[reply]

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Stress

Stress 106.196.15.64 (talk) 13:23, 28 August 2022 (UTC)[reply]