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Stress classification of elliptical or conical heads: 1. Top: primary overall membrane stress and primary bending stress； 2. Transition zone or connection area with the housing: primary local membrane stress and secondary bending stress. Image: Primary overall membrane stress – The membrane stress that affects the entire structure, such as the membrane stress caused by equilibrium pressures or distributed loads among various stresses. Bending stress: The linearly distributed bending stress required to balance pressure or other mechanical loads along the thickness direction of a cross-section, such as the bending stress caused by pressure on a flat plate. Local film stress is a stress level that is greater than the overall film stress, but its influence is limited to only a local area of the structure. For example, the membrane stress caused by external loads and moments at the fixed supports or nozzles of the shell. Bending stress: The linearly distributed bending stress required to balance pressure or other mechanical loads along the thickness direction of a cross-section, such as the bending stress caused by pressure on a flat plate. Secondary stress is the normal or shear stress that arises due to the self-restraint of container components or the restraint by adjacent components, or the stress necessary to satisfy the requirements for deformation compatibility (continuity). It also has two characteristics: one is meeting the deformation compatibility condition ； Second, it is self-limiting. That is, local yielding and a small amount of plastic deformation are sufficient to satisfy the constraint conditions or deformation continuity conditions, thereby preventing further increase in deformation. As long as loading is not repeated, the structure remains in a \"stable\" state and no failure occurs. Therefore, the allowable values for secondary stresses should be limited based on the concept of \"stability\", such as thermal stresses and bending stresses at structural discontinuities.