Uniaxial mechanical properties of multi-layer thin films in use for scientific balloons

Essential and accurate mechanical parameters of multi-layer thin films for scientific balloons are indispensable for performing structural analysis. This study focuses on the determination of uniaxial mechanical properties of a new multi-layer thin film and the understanding of the effects of mechanical properties on structural behavior and altitude with three-dimensional numerical models. For uniaxial monotonic mechanical properties, a method to determine yield stress is proposed in combination of geometrical and energetic principles. It is found that average yield stresses are 20.5 MPa and 15.0 MPa for machine and transverse directions. The cyclic mechanical properties in terms of elastic modulus and ratcheting strain tend to be stable after certain cycles. Moreover, the Poisson's ratios of 0.31 and 0.15 in machine and transverse directions are determined with digital image correlation (DIC) technique. Furthermore, these mechanical properties are utilized to analyze a 12 m diameter spherical superpressure balloons floating at the altitude of 20 km. A corresponding three-dimensional numerical model is developed to understand the effects of mechanical properties on structural behavior and altitude. The numerical results in terms of elastic moduli show that the effects of mechanical properties on structural behavior and altitude are relatively apparent and slight, respectively.