Stress concentration on a curve
Stress concentration on a curve
(OP)
Hello All,
Let's assume that an angle iron's sides are being squeezed towards each other and pivoting at the elbow. On the stress plot you can see the stresses on the outside and inside part of the elbow. Now, some time in the past I do remember reading an article about stress concentration in FEAs. That the nodes on the curve, because of the math, will calculate higher stress than what might be really happening, it also said that to get a good estimate of "true" stress to read the surrounding nodes near the curve. So in the end, the highest stress in the legend is not what you want to take as max, but the next level down from the top.
Can somebody tell me if I'm imaging this, because I can not find any info on this on the web?
Also, I use Pro Mechanica if that helps.
Thanks
Let's assume that an angle iron's sides are being squeezed towards each other and pivoting at the elbow. On the stress plot you can see the stresses on the outside and inside part of the elbow. Now, some time in the past I do remember reading an article about stress concentration in FEAs. That the nodes on the curve, because of the math, will calculate higher stress than what might be really happening, it also said that to get a good estimate of "true" stress to read the surrounding nodes near the curve. So in the end, the highest stress in the legend is not what you want to take as max, but the next level down from the top.
Can somebody tell me if I'm imaging this, because I can not find any info on this on the web?
Also, I use Pro Mechanica if that helps.
Thanks
Tobalcane
"If you avoid failure, you also avoid success."





RE: Stress concentration on a curve
corus
RE: Stress concentration on a curve
RE: Stress concentration on a curve
RE: Stress concentration on a curve
If the stress is at a corner then high stresses at that location don't necessarily mean the structure fails even if the stresses are above yield. Localised stresses should be considered as causing damage through stress cycling, leading to fatigue damage.
corus
RE: Stress concentration on a curve
This depends on your model. From the sound of it you modeled the angle with 3D elements. If you had modeled with shell elements what you say would be true because the corner where the two legs come together is a singularity of sorts.
But if you did a solid model using 3D element (tets or bricks) then all other things being equal your model is showing the correct stresses. Of course if you are yielding you will have to account for that with a non-linear analysis.
You have to pay attention to the sense of the stress. Is it compressive or tensile? vonMises won't tell you that.
BTW, many times FEA is used to calculate stress concentrations. Just remember that FEA does not give engineering stress, it gives true stress.
KTOP
RE: Stress concentration on a curve
RE: Stress concentration on a curve
I guess I picked up a bad habit, but I'm not alone:
h
I could of sworn that I read somewhere that the peak localized stress can be neglected due to the math. Where stress is calculated through out the nodes and once it starts to concentrate onto one or two nodes at the curve the stress goes up exponentially. So the nodes closes to the peak stress are acutely close to actual stress. In ProM we use Von Mises. When I do my Von Mises hand calcs, it always comes lower than what I have as max in ProM. However, in any case, I will take the max stress as actual stress and make sure the geometry and mesh are appropriate.
Tobalcane
"If you avoid failure, you also avoid success."
RE: Stress concentration on a curve
"When I do my Von Mises hand calcs, it always comes lower than what I have as max in ProM." ... doesn't that sound like a problem ? mind you the von Mises output could be derived from an intermediate stress result, and it could be inconsistent with the principal stresses (or the nodal stresses) you're using to hand calc it.
RE: Stress concentration on a curve
RE: Stress concentration on a curve
if the principals are all +ve then vM < s1 (the max. principal)
i agree about preferring max principal over vM as a failure criteria, partially 'cause vM is a yield (linear) criteria. that said i don't think it that "dangerous" ... it's probably conservative 'cause it doesn't account for plastic strain energy.
RE: Stress concentration on a curve
Tobalcane
"If you avoid failure, you also avoid success."
RE: Stress concentration on a curve
TOP
CSWP
BSSE
www.engtran.com
www.niswug.org
"Node news is good news."
RE: Stress concentration on a curve
RE: Stress concentration on a curve
Tobalcane
"If you avoid failure, you also avoid success."
RE: Stress concentration on a curve
TOP
CSWP
BSSE
www.engtran.com
www.niswug.org
"Node news is good news."
RE: Stress concentration on a curve
In the op, this was an assumption for an example to speak too. I typically do solid elements when I do ProM. My question was about peak stress and what to do with them. But, I want to thank you for your time participating on this thread. What is your opinion on peak stress?
Tobalcane
"If you avoid failure, you also avoid success."
RE: Stress concentration on a curve
TOP
CSWP
BSSE
www.engtran.com
www.niswug.org
"Node news is good news."