[PSUBS-MAILIST] sphere construction,

[Sean T. Stevenson via Personal_Submersibles]

Hank, the physics of this doesn't work. Within the shell wall, you have tensile or compressive (hoop) stresses, shear stresses, and bending stresses.  Anytime you stress a material in tension or compression, you also induce stresses in the transverse direction. With thin-walled shells, you can make an effective approximation about the stress distribution being evenly distributed across the shell cross section. With thicker shells, this doesn't hold true. Bending and shear stresses are introduced even in a nominal geometry shell. With a sphere cut into slices, only compressive load which is perpendicular to the seams will carry across the gaps unaffected. At any angle of incidence, shear force is introduced which will not be carried through the interface unless sufficiently precompressed, just as when tensioning a bolt joint. These shear forces will not be borne by glue.

The solution is to make the interface surfaces coincident with the center of the sphere, just as when designing a hatch to replace shell material in a hole. Each ring in such a sphere would consequently have a different conical angle on each side. The difficulty would be in maintaining tight machining tolerances for concentricity and mating angle. You would also require some sort of external strapping / structure in order to both keep it all together, and bear any external loading which is not purely compressive. Also, the glue won't work if there is an extrusion path. Plan on metal to metal sealing between each ring, with an elastomeric backup at the outside of each seam.

Sean



On July 12, 2017 6:50:44 PM MDT, hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Hi all,Today I had an idea that would not have been possible before I
>had a big lathe.  I can buy a series of 4 inch thick rings and machine
>them flat, glue them together to create a sphere.  The rings can be
>machined individually so when glued together I have a sphere.  The
>beauty of this is I can have additional thickness where I need it, like
>the hatch land.  The ring that makes up the hatch land can be
>thicker.Any reason this is a bad idea.Hank
>
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