Events Calendar

Astani CEE Department Seminar

Speaker: Jun Li , California Institute of Technology

Talk Title: Time and Temperature Dependent Large Deformation of Membranes for Sustainable Technology Applications

Abstract: Membranes are becoming increasingly important and widely used in sustainable technology including deployable and lightweight structures, environmental protection and purification systems, and energy conversion and storage devices. Understanding the mechanics of membranes that exhibit time and temperature dependent large deformation behavior is critical to ensure long-term viability in applications. However, the nonlinear viscoelasticity of polymers coupled with the thinness of membranes pose challenges in characterization and modeling. In this talk, I will describe a hybrid method (experimental + computational) to develop nonlinear orthotropic viscoelastic models for the linear low density polyethylene (LLDPE) membranes used in the NASA Ultra Long Duration Balloon. I will first discuss their linear viscoelastic characterization through creep tests and harmonic oscillation tests to obtain master curves of creep compliance. I will show that the master curves obtained from these approaches are not equivalent due to the semi-crystalline nature of LLDPE and one has to be careful to choose between the two approaches. A large deformation model based on the free volume theory couples the through-plane strain to the in-plane deformation of the membrane, but in practice the through-plane properties are difficult to measure. I will present a two-step computational scheme combining an evolutionary algorithm and a simplex optimization to extract model parameters through inverse analysis of experimental results. The validity of the model is then demonstrated on tests at different temperatures and strain rates. Finally, I will discuss possible avenues of future development. Effective constitutive models permit the integration with finite element models to study more complicated mechanics problems involving stress concentrations, wrinkling, structural instabilities and failure. The inclusion of other internal parameters will extend the applicability to multiphysics problems of coupled thermo-photo-chemo-electro-magneto mechanics in diverse applications.

Biography: Dr. Jun Li is a postdoctoral scholar in the Graduate Aerospace Laboratories at the California Institute of Technology. He received his Ph.D. in Mechanical Engineering from the University of Illinois at Urbana-Champaign (UIUC) in 2012, as well as M.S. in Mathematics and in Theoretical and Applied Mechanics. At UIUC, he contributed to the mechanics of fractal and random materials resulting in over 10 journal publications. He delivered a keynote at the 10th US National Congress on Computational Mechanics and received the best poster award of “Emerging Researchers in Biomedical Engineering” at 2011 ASME International Mechanical Engineering Congress and Exposition. Prior to UIUC, he obtained B.S. in Mechanical Engineering with a minor in Mathematics from Shanghai Jiaotong University in 2005. His research interest is to develop theoretical analysis tools integrated with computational techniques and experimental characterizations for the design, optimization, and assessment of novel materials and structures.

Host: Astani CEE Department

Location: Kaprielian Hall (KAP) - 209

Audiences: Everyone Is Invited

Contact: Cassie Cremeans