The Super Pressure Balloon is basically a 2-part system.
Zylon tendon
Polyethylene film
The project has extensive component test data. Materials research has clarified critical Super Pressure Balloon material properties.
There are sufficient test data of materials (film cylinder tests, tendon tests, etc.) to determine their strength, and these have been applied to the design and analytical process.
Operational loads are well below material limits.
The current Super Pressure Balloon is a 200 lobed structure, and is straightforward to model and analyze at float at constant pressure.
Project has gone the extra step of using actual fabrication measurements to refine the predicated stresses in the flight balloon.