Combination of load cases and stress linearization 1

[VibFrank]

Hello,

we often have the typical situation of a pressure vessel with a nozzle. At the nozzle there are given 6 max allowable loads (3 forces, 3 moments), but those loads are absolute values, so the direction is not known. The stress distribution in the pressure vessel should be checked with ANSYS and the membrane/membrane+bending concept. Up to now, we calculate 6 individual load cases. Then for each load case, the Tresca-Stress is calculated at each node of the volume. Then the absolute value of the Tresca stress at each node is summed up with respect to the load cases. Then the highest stress value of SINT (that is peak!) is compared to the allowable value for membrane: 1.0Sm. This is very conservative...
Is it ok and still conservative, to make scl's, estimate the SINT membrane and membrane+bending stresses at each scl and add at each scl the membrane stesses (absolut) of all load cases and also to add the membrane+bending of all load cases absolut and then comprare them with 1.0Sm respective 1.5Sm ?

Thank you for your comments

 

[rb1957]

i would not normally use "allowable" for "applied" ... that is how i read your first sentence.

i would not superimpose tresca stresses. these are (i believe) unsigned combinations of normal and shear stresses, like von mises. superimposing these would be conservative and if you can live with it then ok. i think it is better to superimpose the individual loadcases (any FEA would it's salt can do this) and recalc tresca for the combined loadcase.

 

[corus]

The problem as I saw it, is that Tresca is calculated on the absolute maximum difference of principal stresses, so the worst case may be when a combination of stresses gives you a negative principal stress with a positive value. I'm not certain that simply combining the Tresca from each load case would give you that worst case and as such you may be left having to combine each permutation of +/- load cases. Is that 2 x 6 possible cases assuming that all the 6 load cases act at the same time?

corus

 

[rb1957]

yeah, you're right corus ... there are 12 individual loadcases (3 forces, 3 moments, +/-) ... something like 2^6 combinations (++++++,+++++-,++++-+,++++--, ...) but then superpoisition is easy inside FEA, as is combining all these cases into a max/min envelope.

 

[TGS4]

Hold on a minute!!

If you are performing any calculation with any of the ASME Codes, then you should realize that you have made a grave mistake by performing ANY of yoru calculations using the scalar quantity (SINT, von Mises, etc). Any and all summations (with respect to superposition) must be made at the component level.

I take it that you are the pressure vessel manufacturer, and you are being provided with these nozzle loads. First thing that I would do is go back to your client and ask whether or not the loads are to be taken as occurring ALL simultaneously, or not. Also, ask if you are to assume the worst-case combination of the loads. Once that's done, then proceed with your analysis.

The process that you should be following is to evaluate the results along stress classification lines (SCLs). However, the worst-case SCL for a combined-loading condition will most certainly not be the same location for each individual load evaluated separately.

Finally, you should be comparing the calculated stresses to the allowable values in, for example, ASME Section VIII, Division2, Appendix 4 (2006 Edition and prior) or Part 5 (2007 Edition and later). This would be limiting the local membrane stress intensity to 1.5*Sm, provided that the extent to which the membrane stress intensity exceeds 1.1*Sm is within 1*sqrt(R*t) in the meridional direction. Plus, you would limit the membrane-plus-bending stress intensity to 3*Sm. Al of this within the guidance of creating appropriate stress classification lines that is given in WRC 429, or in the 2007 Edition (and later) of Division 2, in Annex 5.A.

These issues have been discussed numerous times in the Boiler and Pressure Vessel forum. Check the archives.