Help : a conservative value to membrane plus bending stresses 1

[ss1017878]

Hey folks,

I would like to do a quick verification of the design of a clamp (longitudinal guide). It is subjected to forces generated by a pipe.

The clamp is made from a bended steel plate and the design has to be validated according to ASME code.

One of the criteria is to verify that :

Sigma_m < S

Sigma_m + Sigma_b < 1.5*S

where Sigma_m is the membrane stress linearised on a section
Sigma_b is the bending stress linearised on the same section

If I understood well what are membrane and bending stresses, their sum at each point of the section is equal to the normal stress to the section. Am I right?

My calculation use a 3D finite element method to calculate stresses on the clamp. So my reasioning is that the maximum principal stress (highest eigenvalue of the Stress tensor) is in always superior or equal to the normal stress. Am I right on this point too?

So my final question is : can I simplify the entire problem to :

max value of maximum principal stress on the clamp < S ??


please ask me questions if I am not clear

Kind regards

 

[TGS4]

What you are proposing may be conservative. The answers to your question can be found in ASME Section VIII, Division 2, Part 5.

 

[ss1017878]

I am sorry but I didn't find part 5 in the code. I only have AM, AF ,AD divisions...

I have tried to figure out how to maximize the normal stress, using Mohr's circles. So for me it might be an other value in specific case Absolute value of minimum principal stress.

Is the stress intensity (difference of max and min principal stresses) can be used too?

 

[prex]

What you are checking are primary stresses, as those limits are for primaries.
Normally primary stresses may be easily calculated by formula: normal stress is N/A and bending stress is M/Z, where the load characteristics N and M are given by the imposed load.
So you shouldn't need a FEM model to check you stresses. FEM results would be required to check secondary stresses (where the stress intensity comes into play) or peak stresses for fatigue purposes.

prex

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[TGS4]

OK - let's try this again - I'm talking ASME Section VIII, Division 2, something more recent than 2006. You have an OLD version of the Code, and you need to see the new one, as there are MANY improvements.

That said - prex has put you on the right path - do the hand calcs. They will get you further, with less complication, than an FEA in a simple situation.

If you really think that you need to do an FEA, then I will give you the same advice that I have given multiple times before - hire the experience. Find a consultant who has the experience and expertise to do this and pay them. (Think of it this way - if something really screws up in your design, and somebody gets killed, do you want to be on the witness stand with what you have above? With an actual expert on the other side ready to tear you to pieces? I didn't think so...)