OK guys. I don't want to rain on your parade, but if you want to make a composite casing that works, don't reinvent the wheel. Learn how the pros do it. DoD, NASA and the aerospace industry have spent billions learning how to do it right, so do some research of your own and find out how it's done.
0.) Motor casing just don't happen, they are designed and engineered. You need to do some mechanical and thermal calculations to see what properties and materials you need for your casing.
1.) You only use a composite case to save weight. Otherwise there is no advantage to them and they cost up to 10x as much as a metal casing.
2.) Professional composite casings are single use. They are not designed to be reused no matter what you have read here. The heat soak after the burn is likely to permanently change the mechanical properties and render the casing useless. You could make a stronger composite casing, but then you could have made it from metal and saved a bundle.
3.) Standard composite casings are no more resistant to heat than aluminum casings. Composite casings are insulated from the propellant grain by rubber or case bonding with unpolymerized rubber. You could make them thicker however both steel and titanium maintain their mechanical properties at much higher temperatures which is why they are used instead of composited in many SRM systems.
4.) Virtually all composite motor casings are wound on a 4(+) axis winding machine and many smaller composite casings are filament wound around the propellant grain and possibly around a nozzle assembly, or a flange to allow attachment of a nozzle assembly. They are then heat cured in an oven to a known temperature to provide characterized mechanical and thermal properties. Checkout
https://www.xwinder.com/ for a $3000 4-axis winding machine.
5.) Flanges are not pinned or bolted to a composite tube. As you observed, you will get a shear failure at the pinning points due to stress concentration unless you reinforce the area which would make the casing heavier which defeats the reason why you used a composite casing. Composite casings and components are always bonded by resin which is heat cured to maintain mechanical properties of the composites or overwound with a filament winding machine.
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I'm not saying you can't hand make a composite motor casing. I've done it, but you have to understand what you're doing and you're not there yet.
Bob