Modeling Nonlinear Viscoelasticity and Time-Dependent Yielding of Thin-Ply Composites

This project aims at developing a modeling framework capable of accurately predicting the nonlinear viscoelastic and time-dependent yield behavior of thin-ply composite laminates for the reliable design of deployable spacecraft structures. For applications in deployable spacecraft structures, thin-ply composites are currently made from spread-tow carbon fabrics impregnated with epoxy polymer matrices. The polymer matrix exhibits a strongly time-dependent mechanical response, which means that its current stress or strain state is influenced by the loading and temperature histories. Polymers go through several distinct regimes of behavior as the strain level increases, which are characterized by linear viscoelasticity, nonlinear viscoelasticity, time-dependent yielding, viscoplasticity, and time-dependent fracture. A complete constitutive description of all these deformation stages, and incorporation of these behaviors in thin-ply composite laminates, is highly challenging and requires a multi-year effort, but is critical because composite deployable structures are folded to high curvatures for extended periods of time. Previous experiments have shown that an extended stowage time can lead to incomplete deployment, like in the case of the MARSIS instrument in the Mars Express Orbiter mission, and partially recovered deployed shape, resulting in low performance and reliability.nbsp;The proposed effort is a step towards meeting this challenge by focusing on modeling the nonlinear viscoelastic response that precedes permanent deformation as well as the time-dependent yield limits of thin-ply composites. The underlying computational framework is based on the mechanics of structure genome (MSG) [ref msg paper], a recently discovered unified approach for multiscale constitutive modeling of composite structures, and formulated with finite kinematic measures. The material constitutive models are physics-based and informed by appropriate experiments.