| This thesis involves the design, fabrication and mechanical testing of a bioinspired composite structure with characteristic dimensions of the order of tens of microns. The particular microarchitecture, designed and fabricated using microelectromechanical systems (MEMS) technology, involves two distinct length scales and represents a first attempt at mimicking the crossed-lamellar microstructure of the shell of the Giant Queen Conch Strombus gigas , which contains features the dimensions of which span five distinct length scales. After giving a review of the mechanical properties of mollusks, the detailed design of the microstructure, which approximates the crossed-lamellar arrangement of Strombus gigas, is presented. Fabrication of the microstructure using multi-microfabrication methods is conducted in terms of the designed fabrication flow. The problems encountered during the processes are discussed. The measurements of the flexural strength and toughening of the fabricated microstructure are conducted using a commercially available nanoindenter. Testing results are discussed and conclusions about the mechanical behaviors of the microstructure are drawn to summarize the achievement of this thesis. Finally, future work is outlined to point out the possible directions for improving the mechanical performance of the bioinspired composite.; In parallel with my thesis research, I have developed a theoretical model for the experimentally observed cyclic loading-induced strengthening in MEMS polycrystalline silicon. The model relies on atomistic calculations that predict plastic-like behavior of amorphous silicon, which depending on initial density, is associated with dilatancy or compaction. The amorphous silicon is approximated as a Drucker-Prager plastic material, whose parameters are chosen to match the predictions of the atomistic calculations. The constitutive model is used to simulate the mechanical response to cyclic loads of notched polysilicon MEMS specimens containing deforming amorphous grain boundaries. The results demonstrate that certain combinations of mean stress and alternating stress produce plastic deformation and significant residual compressive stresses at the root of the notch, and in turn an increase in nominal strength. This work is presented in Chapter 6. |
