Pressure test of a pressure vessel with glass dome and cylinder of other material

Glass dome is a very good component for pressure vessel. A sphere by two domes becomes very strong pressure vessel. However, combination with cylindrical housing of the other material is not good. Young's modulus of glass is 64GPa and is very low compared with conventional steel, titanium alloy, aluminum alloy or ceramics, which means that a glass dome shrinks larger than a cylinder made of the other material. The glass dome is very strong against outer pressure when it shrinks evenly, but when it shrinks unevenly by any reasons, tensile stress or stress concentration on edge of the dome may come out, which leads to crack and to collapse of the glass dome. The author tested a glass dome with ceramic cylinder, but it collapsed at a little higher pressure than the planned operational pressure. Analysis showed that the collapse was due to the uneven shrinkage of glass dome. In order to avoid uneven shrinkage of glass dome, the friction at the rim of glass dome with the supporting structure must be zero or very small. However, conventional low-friction material like Teflon sheet is not good for this purpose because of the plastic flow by the large contact stress. Thus, the author introduced a special coating technology, Diamond-Like Carbon (DLC) coating. This coating requires very smooth surface with Ra<;0.02μm, and the coated surface becomes very hard like diamond, which makes the friction coefficient as 0.05~0.15 at the manufacturer's data. The pressure test was conducted for a pressure vessel of a steel cylinder with two glass dome caps. The glass domes were supported by DLC-coated seats at both ends of the steel cylinder. The maximum test pressure was targeted to 110% of the operational pressure, but the pressure vessel collapsed at a little lower pressure than the planned operational pressure. However, the analysis of this collapse gave us new knowledge on the structural strength of the pressure vessel with glass dome. This paper discusses about the design philosophy of a pressure vessel with glass dome and a cylinder.