EN 13445 Annex C -Primary local membrane stress

[Mihe]

Hi,
I am studying EN 13445 Annex C: Design by analysis - Method based on stress categories.

Regarding primary local membrane stress (PL): ”At discontinuities, primary membrane stresses in shells are classified as local if the equivalent membrane stress exceeds 1,1 times the nominal design stress f and if the region in which this value is exceeded remains within the length of 1,0 (R * ea)^0.5 in the meridional direction.”

Question:
-So, if the equivalent membrane stress is e.g. 1,15 times the nominal design stress f then the stress is local. But if it is e.g. 1.05 times the nominal design stress f would it then be a general primary membrane stress -What is the meaning/purpose of this distinction?
There must be something I am misunderstanding here since if the stress level 1.15*f is categorized as PL then it is within admissible limit (<=1.5f) but if it is 1.05*f and therefore a general membrane stress it is categorized as Pm then it is not within admissible limit (<=f). So for such case the higher stress level would be ok but the lower level would not be ok.

Question:
-Do any of you have a sketch/reference which explains the area ”within the length of 1,0 (R * ea)^0.5 in the meridional direction” ?

-Is the primary local membrane stress PL a bit related to a secondary stress? Secondary stress is defined as for example “stress developed by constraints due to geometric discontinuities”. But is it not also commonthat the reason for a local primary stress being higher than surrounding general primary membrane stress is due to ”geometric discontinuities”?

BR/Micke

 

[Mihe]

Seems my question is similar to this topic:

https://www.eng-tips.com/viewthread.cfm?qid=348605

 

[Mihe]

I will try to answer my own question.

Quote from

https://www.eng-tips.com/viewthread.cfm?qid=348605

:
“I only have ASME Section III handy right now, but the definition it gives (NB-3213.10) is below.

"Cases arise in which a membrane stress produced by pressure or other mechanical loading and associated with a discontinuity would, if not limited, produce excessive distortion in the transfer of load to other portions of the structure. Conservatism requires that such a stress be classified as a local primary membrane stress even though it has some characteristics of a secondary stress. A stressed region may be considered local if the distance over which the membrane stress intensity exceeds 1.1Sm does not extend in the meridional direction more than 1.0*sqrt(Rt), where R is the minimum midsurface radius of curvature and t is the minimum thickness in the region considered..."
“

This quote can maybe help understand the definition of local primary membrane stress in the European pressure vessel code(And for that matter also the definition according to ASME Section VIII).

The quote defines a limit for a “membrane stress” associated with a discontinuity considering that it “has some characteristics” of a secondary stress. It says that if the subjected area is limited**, then the stress is classified as a local primary membrane stress(PL) and therefore should be limited to <1.5*f. Consequently, if the area is larger than the defined limit, then the stress should even though it “has some characteristics” of a secondary stress be regarded as a general primary membrane stress(Pm) and therefore be limited to <1.0*f. Further (my interpretation), if the membrane stress associated with the discontinuity is lower than 1.1*Sm it more has the characteristics of a secondary stress and therefore need not be limited as either a Pm(<1.0f) or a PL(<1.5*f).

**Limited = the distance over which the membrane stress intensity exceeds 1.1Sm does not extend in the meridional direction more than 1.0*sqrt(R*t)


Regaring the question about the "meridional" direction. It often seem to mean the axial direction (hence not the hoop direction). And it is normal to the "Supporting line segment".

Please, add you feedback to this if there is something I have misunderstood.

Remaining questions:
-Why does only the extent in "meridional" direction matter (and not also the extent in the hoop direction)?.
-What is "meridional" direction for a flat end? What is R for a flat end? (or can membrane stresses not be local also for flat components?)

BR/Micke