Pressure vessels are mainly of two type first one is thin shells and another is thick shells.
If the ratio of thickness of the wall to the its diameter of the shell is less than 1/10 to 1/15, then shell is called thin shells.
i.e., t/D_i < 1/10 to 1/15
In case of thin shells, it is assumed that the normal stresses, which may be either tensile or compressive are uniformly distributed through the thickness of wall.
Nature of stress in thin cylindrical shell subjected to internal pressure
(i) Hoop stress or circumferential stress will be tensile.
(ii) Longitudinal stress or axial stress will be tensile.
(iii) Radial stress will be compressive.
If the ratio of thickness of the wall of the shell to its diameter is greater than 1/10 to 1/15, then such type of shell is called thick shells.
i.e., t/D_i > 1/10 to 1/15
Radial compressive stress varies from a value at the inner surface equal to pressure ‘P’ to the atmospheric pressure at the outside surface.
If internal pressure in thin cylinders is low, the radial stress is negligible compared with axial stress and hoop stress. This radial stress is neglected.
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