| The need for a comprehensive understanding of thin polymer film load-deformation behavior is evident as an increasing number of inflatable and deployable spacecraft use thin films as structural elements. Pristine thin film behavior is often well characterized by Hooke's law. However, the packaging process inevitably causes material wrinkles, which drastically change the thin film's constitutive behavior. Material wrinkles cause extreme nonlinearities, reduce tangent Young's modulus by as much as 98%, give rise to a negative Poisson's ratio and uniformly shrink the material. This thesis, for the first time, quantifies these effects. The research herein is limited to random material wrinkles, which are permanent, uniformly distributed material creases and bends created during the fabrication, packaging and deployment processes.; Both empirical and analytical constitutive behavior characterization methods are investigated. An experimental method is developed for measuring the general in-plane constitutive behavior of wrinkled thin films. The method is implemented for four different wrinkled states each in 0.5 and 1.0 mil Kapton® TAB-E. Investigation of deformation mechanisms on the scale of a single wrinkle reveals thin films to be developable surfaces. As a consequence, wrinkled thin films primarily experience bending deformations. A deterministic elastic constitutive model is developed based on bending and uses effective wrinkle amplitude and wavelength for parameters. The model is verified using biaxial stress-strain tensile test data.; Various methods for estimating model parameters are investigated. For preliminary design purposes, a tensile test does not need to be performed. Parameters can be calculated from direct measurements of wrinkle wavelength and initial material shrinkage. Parameter estimates that better represent material behavior require a test. A uniaxial tensile test is recommended in cases where moderate accuracy is required and an equibiaxial tensile test is recommended for applications requiring the greatest accuracy. |
