Thesis Defense: Sirui Bi, “On the Processing, Microstructure and Strength of Brittle Foams”

August 18, 2020





Doctoral Candidate


Tuesday, August 25, 2020

3 PM

Contact Elena Shichkova for access to this presentation.

“On the Processing, Microstructure and Strength of Brittle Foams”

This dissertation focuses on the nonlinear mechanics of open-cell foams and in particular the connection between their cellular microstructure and their macroscopic strength. We first examine the quasi-brittle failure of Reticulated Vitreous Carbon (RVC) foams under compression. The carbon foam microstructure is analyzed using microcomputed tomography. It is shown to follow closely the polyhedral structure of the precursor polymer foam, that is pyrolyzed to produce its carbon counterpart. Scanning single foam-ligaments reveals that their cross-sectional area is a hypocycloid with a non-uniform distribution across the ligament length. X-ray tomography also shows several processing-induced defects in the form of anisotropy and remaining closed-cell faces. Specimens of different geometries and dimensions are crushed between rigid surfaces in order to examine the effect of load distribution, specimen size, relative density and cell-size on the resulting response and the associated crushing strength of the foam. In-situ testing and image analysis is utilized to observe the failure mechanism and associate it with the recorded force-displacement response.

In the second part of this thesis, additive manufacturing is employed to examine the strength of brittle foams with controlled microstructural characteristics. Tessellation-based topologies are used to generate realistic microstructures of open-cell foams that are subsequently 3D-printed by stereolithography. The stress-strain curve and fracture strength of the base photopolymer are measured using tensile tests on small dog-bone specimens with the dimensions of foam ligaments. Synthesized foams are scanned by microcomputed tomography and manufacturing-induced variations are quantified through image analysis. Characterization shows that there is a small amount of volume shrinkage of the material caused by the additive manufacturing process, but all other microstructural features are accurately reproduced. We then perform a series of experiments to measure the compressive response and strength of the 3D-printed foams and connect it to load-transferring conditions, the strength of the base solid material and the foam relative density.

Finally, we examine the compressive response of open-cell aluminum foams under high temperatures. Foam specimens with different cell-sizes are compressed within an environmental chamber under a range of temperatures from 20 °C to 300 °C. The localization and evolution of collapse are monitored and analyzed using Digital Image Correlation (DIC) and the overall force-displacement response is measured. The results indicate that high temperatures significantly affect all mechanical properties of aluminum foams. Both the limit and plateau stresses were found to decrease linearly with temperature. More importantly, their drop is not proportional to the corresponding one in the base material’s yielding stress. The densification strain, that is a measure of the plateau extension, also follows a linear trend albeit in an increasing manner. This increase is attributed to changes of the collapse mechanism, which at high temperatures involves localization in different zones within the foam, as well as increased compaction at the cell-level caused by the base material softening. Finally, we measure the reduction in the strain energy absorption capacity of the foam caused by high temperatures.

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