The stress classification is one of the main differences between the conventional design and analysis design of the vessel.
Conventional design: only limit the film stress in the wall of the vessel, but it does not conform to the actual situation. There are both film stress and other stresses in the container, such as the discontinuity of the head and the body: bending stress, local film stress, etc.
Analysis and design: Classify the stresses in the vessel wall, and use different allowable stresses to limit them according to the different effects of these stresses on the damage.
Analytical design allows local plasticity, infinite life of the structure as a whole and limited life in the part. This method is a perfect combination of engineering and mechanical analysis, reflecting the advanced design level.
According to the cause of stress and the area where the stress exists, the stress is divided into: primary stress, secondary stress and peak stress
1. Primary stress Primary stress:
Stresses necessary to balance pressure or other applied mechanical loads. It satisfies the static balance relationship between external, internal and internal moments.
The primary stress is non-self-limiting, increasing with the increase of external force, and will not be self-limiting due to plastic deformation of the vessel wall until the container bursts.
There are three types of primary stress:
a. The overall primary film stress Pm exists far away from the discontinuous zone of the structure and is considered to be uniformly distributed along the wall thickness.
For example, the film stress generated by the internal pressure of a thin-walled cylinder or spherical shell, and the axial stress and circumferential stress generated by the internal pressure of a thick-walled container along the wall thickness all belong to the primary overall film stress.
b. A local film stress