Microarchitectured Materials: Current Status and Future Prospects
The Department presents our Fall 2019 Distinguished Colloquium speaker:
Dr. Norman A. Fleck
Professor, Engineering Department
University of Cambridge, United Kingdom
When: September 17, 2019 | 11 AM
Where: CEME 2202| 6250 Applied Science Lane
Refreshments served at 10:45 AM.
Abstract: Lattice materials are emerging with a broad range of properties due to their combination of microarchitecture, length scale and composition. They range from periodic lattices to nanofoams, and can possess impressive properties: as bulk materials, as the cores of sandwich panels and as engineered surfaces. Their toughness knows no bounds, and high toughness can be achieved by the successive renucleation of fracture in the lattice ahead of a macroscopic crack tip. Likewise, there are no theoretical bounds for tensile ductility. Extreme properties, such as high strength are evident with a diminishing length scale, but new challenges (such as low toughness) also appear.
An overview is presented on the role of microarchitecture in dictating the properties of lattice materials. They can exhibit unusual mechanical properties ranging from actuation (pre-compressed cellulose in the presence of water) to extreme thermal properties such as low thermal conductivity at small scale (PMMA nanofoams). For example, the mechanisms of fluid infiltration (water, glycerol or ethanol) into a cellulose foam is explored experimentally: it is found that capillary flow is followed by cell wall diffusion. Fluid infiltration can give rise to a large actuation stretch ratio, by a factor of x10 or more. An interesting pumping mechanism exists such that actuation stimulates fluid-infiltration.
Nanofoams can be generated by the controlled expansion of carbon dioxide bubbles within PMMA. This has high pay-off in the development of thin insulating panels. However, it is difficult to achieve high porosity at small cell size. In order to gain insight into the mechanics of this solid-state foaming process, a model is developed for the foaming process based on a deformation mechanism map for PMMA with dissolved carbon dioxide present. Nanofoaming experiments suggest that the cell wall ductility is reduced when the cell wall thickness is on the order of the end-to-end distance of the PMMA molecules. Molecular dynamic simulations have been performed to predict the observed sensitivity of biaxial ductility to cell wall thickness.
The advent of additive manufacture has opened up the opportunity of varying the topology and material choice within a shaped part. However, current manufacturing methods introduce defects into the cell walls that reduce the strength of the component. The sensitivity of strength to asmanufactured and as-designed defects are explored for an idealised lattice made by laser cutting. Additive manufacture also allows for the opportunity to tailor microstructure in order to give a stress versus strain curve upon demand. A related opportunity exists in the development of Li ion batteries. There is a challenge to develop composite cathodes that comprises both a solid state electrolyte and an active cathode material that undergoes large swelling. A novel 2D and 3D microstructure has been analysed with potential for this application.
Biography: Prof. Fleck received all his degrees from University of Cambridge, UK. There, he is now a professor of mechanics of materials and director of Cambridge Center for Micromechanics. His internationally acclaimed research has influenced many areas of mechanics of materials. He is a fellow of several learned societies notably, the Royal Society (UK), the Royal Academy of Engineering (UK) and the European Mechanics Society. He received numerous awards and prizes, including the Koiter medal from the American Society of Mechanical Engineers. His expertise is sought by several industries worldwide. He chairs Manufacturing and Structures Scientific Advisory Board of Rolls- Royce.