External pressure deformation and stress analysis of the head The instability of a pressure vessel refers to the phenomenon that the original geometric shape is suddenly lost when the load on the vessel exceeds a certain critical value. Different forms of container structures, as well as the geometric shapes after instability caused by different forms of loads are different. For dish-shaped heads, under uniform external pressure, compressive film stress is generated in the head, and its magnitude is the same as the tensile film stress under equal internal pressure. When the external pressure load increases to a certain value, the head will suddenly lose its original shape, be squashed or wavy wrinkles, and cannot be restored to its original shape after the load is removed. This phenomenon is when the head bears the external pressure load. Buckling or instability. The structure of the dish-shaped head consists of a spherical shell, a transition section and a cylindrical part.

　　According to the axisymmetric characteristics of the dish-shaped head, the ANSYS finite element analysis software is used to conduct a two-dimensional finite element static analysis.

　　To observe the internal stress distribution. Under the condition that the dish-shaped head bears uniform external pressure, the transition section (ring shell) swells to the outside, and the spherical shell and cylinder part deflate to the inside. In addition, the analysis shows that the circumferential stress of the transition section is tensile stress, while the circumferential stress of the spherical shell and cylinder part is compressive stress. Because the structure may buckle under compressive stress, it can be seen that the transition section of the dish-shaped head will not be buckled. As for the spherical shell part of the head, the circumferential compressive stress reaches the maximum at a distance of about 800 units of length from the top of the head, so the dish-shaped head is very likely to buckle instability at this position.

　　Through the above analysis, it can be seen that the cylindrical part has little effect on the circumferential stress of the head shell and the transition section.