Primary bending stress
Primary bending stress
(OP)
Dear All
Can anybody help me understand the occasions that primary bending stresses occur?
I tried table 5.6 in ASME BPVC VIII-2 but it seemed a little paradoxical in some ways. Try the highlighted cases in the image.

Can anybody help me understand the occasions that primary bending stresses occur?
I tried table 5.6 in ASME BPVC VIII-2 but it seemed a little paradoxical in some ways. Try the highlighted cases in the image.






RE: Primary bending stress
RE: Primary bending stress
What I do not understand is the contents of table 5.6. As I have shown the first highlighted case says gradient through thickness for any shell, remote from discontinuity is secondary. However, in the second highlighted case it is obvious that for cylindrical shell there is bending stress Pb.
RE: Primary bending stress
RE: Primary bending stress
On the whole, in the beginning I thought that primary bending stress is very easy to understand. The more I try to understand the occasions where this stress occurs the more I feel difficulty dealing with this stress.
RE: Primary bending stress
RE: Primary bending stress
And I don't clearly understand you when you sayDo you agree that a purely hinged flat head will only have (general) primary bending stresses? IMHO there are two cases: if you calculate the primary bending stress as if the flat head was purely hinged (often primary stresses are calculated by formula), then all other bending stresses due to the restraint at the periphery will be secondary, but if you calculate the bending with a model of the actual boundary restraint, then you'll have to classify the bending at the periphery as primary general.
This shows that the stress classification is not carved in the stone, in other words is not purely objective, it is instead partly subjective, as it depends on how you build your classification model.
Also, as another example of difficult classification, the fraction of the bending stress in a flat head (even if purely hinged), that is caused by a non zero Poisson's ratio, should be classified as secondary, as it is deformation related. However this is not very important, as that fraction of stress is generally low.
These examples show indeed that the primary bending stress is not 'very easy to understand', but you'll be able to sort it out in the end
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