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# Determining Stress and Predicting Rupture (LMP)

## Determining Stress and Predicting Rupture (LMP)

(OP)
I know from having done PV code calcs that wall thickness is, in almost all cases, governed by hoop (circumferential) stress in thin wall cylindrical shells. In the case of ASME pressure vessel calcs, the hoop stress and longitudinal stresses are considered mutually exclusive and the wall thickness is determined by the higher stress and not a combination of the two stresses, which I don't fully understand.

I'm currently looking at a slightly different application where I need to determine time to rupture using the Larson Miller Parameter. For this calculation, I have to determine stress in a thin wall cylinder under internal pressure and experiencing an external longitudinal load. I can calculate the hoop stress and the longitudinal stress. However, I can't help but think the total stress would be a combination of hoop and longitudinal. I can't find any references to understand why they would or would not be considered additive or similar to an equivalent Von Mises type stress equation. Which I understand is a combination of normal and shear (not considered in thin wall cylinders) or biaxial stresses.

What can the smart people offer me in advice for this one?

Eric W

Benevolent Order of the Blue Wrench

If you can't fix it with a hammer, it's an electrical problem.

Replies continue below

### RE: Determining Stress and Predicting Rupture (LMP)

Hi Eric

Have a look at this link https://wp.optics.arizona.edu/optomech/wp-content/...

I would look at the Mohr circle to get my resultant stressses.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

### RE: Determining Stress and Predicting Rupture (LMP)

you need something like von Mises stress ... in school you may remember them talking about a stress element with two normal stresses (stress_x and stress_y) and shear stress (stress_xy) ... and I'm not trying to be a d!ck about this (but maybe it comes naturally). This gives you two principal stresses, and then you combine these with some failure criteria (like von Mises, which is probably appropriate for pressure vessels). Your "Larson Miller" calculation "should" tell you how to do this.

when you say "time to rupture" do you mean pressure cycles to failure ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

### RE: Determining Stress and Predicting Rupture (LMP)

And if your external longitudinal load is compressive, you better check buckling!

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