Membrane stress local primary

Local primary membrane stress, Pl. Local primary membrane stress is not technically a classification of stress but a stress category, since it is a combination of two stresses. The combination it represents is primary membrane stress, P, , plus secondary membrane stress, Qn produced from sustained loads. These have been grouped together in order to limit the allowable stress for this particular combination to a level lower than allowed for other primary and secondary stress apphcations. It was felt that local stress from sustained (unrelenting) loads presented a great enough hazard for the combination to be classified as a primaiy stress. 

Examples of local primary membrane stresses exist ... 

Within the context of local primary membrane stress, Pl, as well as secondary stress, Q, the discontinuity effects need not be elaborated. The structural discontinuity can be either gross or local. Gross structural discontinuity is a region where a source of stress and strain intensification affects a relatively large portion of the structure and has a significant effect on the overall stress or strain pattern. Some of the examples are head-to-shell and flange-to-shell junctions, nozzles, and junctions between shells of different diameters or thicknesses. 

Primary stresses, including general primary membrane stress, local primary membrane stress, and primary bending stress Secondary stresses Peak stresses... 

On the other hand, the stresses from the inward radial load could be either a primary stress or secondary stress. It is a primary stress if it is produced from an unrelenting load or a secondary stress if produced by a relenting load. A general primary membrane stress will not redistribute upon yielding, whereas a primary local membrane stress will, and for a secondary stress the load will relax once slight deformation occurs. 

Specifically for the ASME Code, the primary membrane stress intensity. Pm, and the combined membrane plus bending stress intensity. Pm + Pb, (also the local membrane plus bending stress intensity, Pl + Pb in some cases) for the various loading conditions are shown below. 

Primary local membrane stresses are a combination of membrane stresses only. Thus only the membrane stresses from a local load are combined with primary general membrane stresses, not the bending stresses. The bending stresses associated with a local loading are secondary stresses. Therefore, the membrane stresses from a WRC-lOT-ri pe analysis must be broken out separately and combined with primary general stresses. The same is true for discontinuity membrane stresses at head-shell junctures, cone-cylinder junctures, and nozzle-shell junctures. The bending stresses would be secondary stresses. 

Therefore, Pl = Pm -I- Q,n, where Qm is a local stress from a sustiiined or unrelenting load. Examples of primary local membrane stresses are ... 

This procedure determines the bending stress in the stiffener only. The stresses in the vessel shell should be checked by an appropriate local load procedure. These local stresses are secondary bending stresses and should be combined with primary membrane and bending stresses. 

Primary local membrane stress, Pl- A primary local membrane stress is produced either by design pressure alone or by other mechanical loads. Primary local membrane stresses have some self-limiting characteristics like secondary stresses. Since they are localized, once the yield strength of the material is reached, the load is redistributed to stiffer portions of the vessel. However, since any deformation associated with yielding would be unacceptable, an allowable stress lower than a secondary stress is assigned. The ability of primary local membrane stresses to redistribute after the material yields allows for a higher allowable stress but only in a local area. 

The bending stresses associated with a local loading are almost always classified as secondary stresses. Therefore, the membrane stresses from a WRC-107-type analysis must be broken out separately and combined with general primary stresses due to internal pressure, for example. 

Primary plus secondary stress intensity. The maximum stress intensity S as based on the primary or local membrane stresses plus the primary bending stress plus the secondary stress (cr m or ctl + (7b -f X) 2) cannot exceed the... 

Pl = primary local membrane stress as defined in Table 8.2... 

Peak stress, F. Peak stresses are the additional stresses due to stress intensification in highly localized areas. They apply to both sustained loads and self-limiting loads. Tliere are no significant distortions associated with peak stresses. Peak stresses are additive to primary and secondary stresses present at the point of the stress conc-entration. Peak stresses are only significant in fatigue conditions or brittle materials. Peak stresses are sources of fatigue cracks and apply to membrane, bending, and shear stresses. Examples are ... 

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