How to draw trajectories of stress lines without calculation

[feynman1]

as title

 

[Baluncore]

There is no question mark in the title.
Are you going to explain it, or are you asking a question.

 

[FEAnalyst]

That’s what photoelasticity is meant for. You can also use some software to evaluate stresses with FEA.

 

[feynman1]

I'm asking the question as in the title under the condition that no numerics (including FEA) is used.

 

[Baluncore]

Do you want to draw the stress lines by hand? or somehow project them onto a screen? Either way you will need an analogue of the geometry and stress.

 

[Tom.G]

See:
https://www.physicsforums.com/posts/6348445/

 

[feynman1]

how to draw those lines by hand or intuition?

 

[Tom.G]

how to draw those lines by hand or intuition?

Aren't those identical in this application? If you want to use 'intuition', that implies you have some, perhaps unconscious, idea of what is going on. The typical way to gain this 'intuition' is by examining many instances of a phenomina and drawing conclusions as to how various factors affect the result.

Anyhow, stress concentrations occur where forces are either applied or forced to change direction.

For instance if you have a sheet of plastic and you apply a uniform bending force along an edge, that will be the origin of stress, with the stress lines distributed and radiating from the edge.

If you apply the same force at a point along an edge, all that stress will be at the point of application, and the stress lines will originate there.

You get a similar result where a force is redirected due to non-uniformity in the material. If you cut a diamond shaped hole in the sheet of plastic, you will see concentration of stress especially at the corners, and less so, but still significant, along the straight edges of the hole.

Your intial question reminds me of an old saying: "How do you describe color to a blind person?" I have yet to hear a satisfactory answer.

Get some polarizing material, some clear plastic, a light source and play with it.

Cheers,
Tom

 

[feynman1]

thanks a lot tom! i wonder from which papers/textbooks one can get such intuitive explanations of stress lines?

 

[Tom.G]

No idea. I would start with some Google searchs, text and images, for stress lines or stress concentration.

Paging @jrmichler. He may know of some sources; he is one of the more knowledgeable mechanical folks on the site.

But you will learn much by doing some experiments yourself with the polarized light trick.

Cheers,
Tom

 

[jrmichler]

I do not recall ever reading anything about hand drawing stress lines. The advice in Posts 3, 5, 6, 8, and 10 above is all good advice.

You could study the theory behind standard cases of stress concentrations such as shoulders, notches, round holes, and elliptical holes. Use that knowledge to recognize situations that require more in depth analysis, such as FEA. But the ability to hand draw stress lines has little, if any, value in the real world.

 

[Baluncore]

https://en.wikipedia.org/wiki/Photoelasticity

Google 'Photoelasticity'