Shape Memory Alloy Cellular Solids.

Nitinol (NiTi) shape memory alloy honeycombs, fabricated in low densities using a new brazing method, recently demonstrated enhanced shape memory and superelastic properties by exploiting kinematic amplification of thin-walled deformations. The realization of such adaptive, light-weight cellular structures opens interesting possibilities for design and novel applications. This dissertation addresses the consequent need for design and simulation tools for engineers to make effective use of such structures. The focus of the initial portion of the work is the analysis of the response and stability of superelastic honeycombs with a hexagonal unit cell. A hysteretic, rate-independent pseudoelastic material model is implemented in a research finite element code (FEAP), along with a small strain - large rotation beam element. The Bloch wave representation theory is used to efficiently predict the onset of instability during compression of an infinite honeycomb. A parameter study is performed to investigate the influence of different material laws on the behavior of an infinite and finite honeycomb. It is demonstrated that the response and stability of the infinite case gives insight to the behavior of a finite perfect and finite imperfect honeycomb. Subsequently, employing a generalized hexagonal unit cell, the exact dimensions of which are varied, and adopting the methods developed in the earlier part of this work, the kinematic amplification of the thin walled structure is exploited in the design of reusable kinetic energy absorbers. Contour plots are provided, that allow to obtain the highest absorbed energy to honeycomb weight ratio for a given maximum allowable reaction force of the compressed honeycomb. Finally, a constitutive model that demonstrates both superelasticity and shape memory effect (SME), still focusing on the rate-independent case, is described and implemented. It is determined that simulated honeycombs credibly capture the essential characteristics of the SME, while they exhibit bifurcated paths during both low and high temperature compressive cycles.Ph.D.Aerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/64596/1/pamich_1.pd

Shape memory alloy honeycombs: experiments & simulation

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76238/1/AIAA-2007-1739-156.pd

Adaptive optimal control for large-scale nonlinear systems

In this paper, we present an adaptive optimal control approach applicable to a wide class of large-scale nonlinear systems. The proposed approach avoids the so-called loss-of-stabilizability problem and the problem of poor transient performance that are typically associated with adaptive control designs. Moreover, it does not require the system model to be in a certain parameterized form, and most importantly, it is able to efficiently handle systems of large dimensions. Theoretical analysis establishes that the proposed methodology guarantees stability and exponential convergence to state trajectories that can be made as close as desired to the optimal ones. A numerical example demonstrates the capability of the proposed approach to overcome loss-of-stabilizability problems. Moreover, simulation experiments for energy-efficient climate control performed on a ten-office building demonstrate the effectiveness of the proposed approach in large-scale nonlinear applications.Team DeSchutte

Molecular Epidemiology of SARS-CoV-2 in Greece Reveals Low Rates of Onward Virus Transmission after Lifting of Travel Restrictions Based on Risk Assessment during Summer 2020

The novel coronavirus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spread rapidly during the first months of 2020 and continues to expand in multiple areas across the globe. Molecular epidemiology has provided an added value to traditional public health tools by identifying SARS-CoV-2 clusters or providing evidence that clusters based on virus sequences and contact tracing are highly concordant. Our aim was to infer the levels of virus importation and to esti-mate the impact of public health measures related to travel restrictions to local transmission in Greece. Our phylogenetic and phylogeographic analyses included 389 full-genome SARS-CoV-2 sequences collected during the first 7 months of the pandemic in Greece and a random collection in five replicates of 3,000 sequences sampled globally, as well as the best hits to our data set identified by BLAST. Phylogenetic trees were reconstructed by the maximum likelihood method, and the putative source of SARS-CoV-2 infections was inferred by phylogeographic analysis. Phylogenetic analyses revealed the presence of 89 genetically distinct viruses identified as independent introductions into Greece. The proportion of imported strains was 41%, 11.5%, and 8.8% during the three periods of sampling, namely, March (no travel restrictions), April to June (strict travel restrictions), and July to September (lifting of travel restrictions based on thorough risk assessment), respectively. The results of phylogeographic analysis were confirmed by a Bayesian approach. Our findings reveal low levels of onward transmission from imported cases during summer and underscore the importance of targeted public health measures that can increase the safety of international travel during a pandemic. IMPORTANCE Our study based on current state-of-the-art molecular epidemiology methods suggests that virus screening and public health measures after the lifting of travel restrictions prevented SARS-CoV-2 onward transmission from imported cases during summer 2020 in Greece. These findings provide important data on the efficacy of targeted public health measures and have important implications regarding the safety of international travel during a pandemic. Our results can provide a roadmap about prevention policy in the future regarding the reopening of borders in the presence of differences in vaccination coverage, the circulation of the virus, and the presence of newly emergent variants across the globe