Total Membrane Stress Intensity

[Newbie]

I just started working as a graduated engineer. Never heard of total membrane stress intensity during my studies and yet have to solve the total membrane stress intensity. Anyone can teach me about membrane stress intensity and how do I calculated for the total?

 

[TGS4]

Can you please provide a little context to your question?

 

[Newbie]

I'm trying to solve the total membrane stress intensity for the pipe-line tube in a flange adapter.
Rated working pressure = 10,000 psi
3 1/16'' x 4 1/16'' Flange Adapter.
I'm sorry I really have no clue regarding total membrane stress intensity(never encounter this term before). Could you explain to me what is this term really, why do you need to compute this for pressure vessels, what are other methods aside from calculation to solve for this and anything else you think is important for a rookie like me to understand this subject. Really appreciate your help!

 

[TGS4]

I was hoping for a little more information with regards to the Code of Construction, and perhaps the paragraph that requires such a quantity to be calculated...

Which parts do you not understand?

Stress intensity generally refers to the Tresca method of calculating the invariant of the component stresses.

Membrane generally refers to an average across a cross-section.

I need the full context to understand how total comes into the picture...

 

[Newbie]

Hope these images would help and thank you so much for keeping up with me.

 

[Gilmiril]

The 2004 version of ASME BPVC Section VIII Division 2 had the following definitions in its Appendix 4:

Membrane Stress. The component of normal stress which is uniformly distributed and equal to the average value of stress across the thickness of the section under consideration.

Stress Intensity. The equivalent intensity of combined stress, or in short, the stress intensity, is defined as twice the maximum shear stress. In other words, the stress intensity is the difference between the algebraically largest principal stress and the algebraically smallest principal stress at a given point. Tension stresses are considered positive and compression stresses are considered negative.

That Appendix 4 (Design Based on Stress Analysis) was based on the Maximum Shear Stress Theory of Failure which predicts failure when the maximum shear stress exceeds one half the yield strength of the material (for ductile materials). So stress intensity is a convenient short hand for canceling out the "one half" term since you're really just multiplying both sides of the equation by "2".

Note that Section VIII Division 2 was drastically rewritten in 2007, but some design codes (e.g. API 6A) are explicitly based on the 2004 version.

 

[TGS4]

You appear to beer using an internally generated calculation sheet for your company's pressure fittings. Whomever wrote that calculation sheet had (generally, in my opinion) poor English. Therefore, I can deduce that the terminology that they used is inaccurate. Furthermore, the calculation is not actually to find a stress intensity, but rather a hoop-direction stress component.

The calculation that is being done (p*r/t) is a thin-shell approach to calculating the average hoop stress in a cylinder. Based on the drawing that you included, I would definitely not say that your geometry is thin-shell. Furthermore, these calculations refer to an Edition of the ASME Code that is now 9 years old. As gilmiril indicated, that Division if the ASME Code was significantly rewritten in 2007.

Does what gilmiril and I wrote about membrane stresses and stress intensity make sense to you?

 

[Newbie]

Hi Gilmiril and TGS4.

Thank you so much for the answers. To start with, the design report or rather the design package was originally written by the previous engineer before I commenced, my job (as been instructed to me) is to find any mistakes in the calculations and to correct them. I do recognized that the p*r/t calculation won't be applicable for the design and that the phrase "total membrane stress intensity" is totally misleading here. Your responds do make a lot of sense now.

To conclude for now, can we say that the "total membrane stress intensity" isn't an appropriate value to be solved for? Or that "total membrane stress intensity" is just a typing/translation error?

 

[TGS4]

It is definitely a typing/translation issue.

It may be appropriate to calculate the membrane stress (principal or equivalent) for comparison to the allowable stress.

 

[Swlamech]

Looks like that part should be evaluated using "Thick" wall formulas. John Fowler @ online resources has a book on designing to API-6A/16A/etcc... that covers parts like this.