Why are graphical algorithms used in the design of externally pressurized cylinders?

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Why are graphical methods used in the design of pressure vessels subjected to external pressures? Explain the meaning of A and B in the pressure calculation diagrams specified by GB/T150, as well as the origin and scope of application of these diagrams. Calculating the wall thickness using the elastic theory formula for the critical pressure of a cylinder under external pressure is rather complicated, and repeated calculations are required; moreover, the elastic theory formula is not applicable in the case of inelastic instability. The use of graph algorithms simplifies the design process, and they can be applied to the design of both long and short cylinders, as well as in cases of elastic and inelastic instability, and for both thin-walled and thick-walled cylinders. The graphs in GB/T150 are derived from cylinders subjected to uniformly distributed external pressure; they are calculated and plotted using simplified critical pressure formulas for long and short cylinders based on the Mason’s formula. In the figure, A equals the circumferential strain of the cylinder at the critical pressure, which can also be referred to as the circumferential critical strain; B equals twice the allowable circumferential stress of the cylinder at the critical pressure.