The damage evolution and structural failure in trees and other plants are mainly originated from the plastic yielding in the wood cell wall at the microstructural level, which consists of cellulose fibrils embedded in a polymer matrix of hemicellulose and either lignin or pectin. Understanding the mechanical behavior of wood cell wall at the plastic regime is critical to the investigation of the fracture characteristics of trees at macro-scale. In this research work, the wood cell wall, which consists of cellulose fibrils, hemicellulose chains and lignin macromolecules, is modeled at the mesoscale to investigate the mechanical responses under deformation. By examining the force-strain relationship, the mechanical behaviors of the wood cell wall at the plastic yielding range are obtained, which are initiated from the slippage between the fibrils and polymer matrix. The simulation results are compared with experimental measurements and theoretical predictions to provide a bottom-up description of micromechanics of the wood cell wall, and to explain the damage evolution and structural failure occurred at the larger scales. The wood cell wall investigated here can be applied to the construction of wood hierarchical structure as a basic unit and adapted to the studies of different natural materials.

Bulk GaN substrates are of significant interest because they offer both high quality and low dislocation densities. Our group has previously reported the formation and movement of dislocations in high quality bulk GaN in response to nano-indentation. We have also proposed a mechanism involving an r-plane (-1012) slip initiated by plastic deformation during a pop-in event, a theory that was supported by molecular dynamics simulations. Herein, we present experimental evidence for this r-plane (-1012) slip mechanism in an indented GaN surface using nano-indentation with an indenter having a smaller radius (∼100 nm) and imparting a lower pop-in load (∼400 μN) compared to the values applied in our previous studies. In addition, this study included TEM observations immediately after the plastic deformation, such that cross-sectional TEM images of the indented surface of the c-plane bulk GaN were acquired just after the pop-in event. The pyramidal dislocation line of an r-plane slip was clearly observed and was inclined by 43° relative to the c-plane surface. Neither a basal c-plane slip nor a prism m-plane slip occurred as a result of dislocation multiplication as secondary or tertiary slip systems, even though these slips had been identified when employing a larger radius indenter and a higher pop-in load. From these experimental results, we were able to confirm that plastic deformation in bulk GaN is initiated via an r-plane slip.

The dynamic evolution and interaction of defects under the conditions of shock loading in nanocrystalline Al with an average grain size of 20 nm is investigated using molecular dynamics simulations for an impact velocity of 1 km/s. Four stages of defect evolution are identified during shock deformation and failure that correspond to the initial shock compression (I), the propagation of the compression wave (II), the propagation and interaction of the reflected tensile waves (III), and the nucleation, growth, and coalescence of voids (IV). The results suggest that the spall strength of the nanocrystalline Al system is attributed to a high density of Shockley partials and a slightly lower density of twinning partials (twins) in the material experiencing the peak tensile pressures.

Large scale molecular dynamics (MD) simulations are carried out to investigate the failure response of nanocrystalline Mg using the EAM potential under conditions of uniaxial tensile stress and uniaxial tensile strain loading. The MD simulations are carried out at a strain rate of 109s-1 for grain sizes in the range of 10 nm to 30 nm. The effect of grain size on the strength of the metal is investigated and the critical grain size for transition to inverse Hall-Petch regime is identified.

A severe plastic deformation (SPD) technique was applied to an Al-7075 alloy reinforced with 10 vol.% Al2O3. This processing method of high-pressure torsion (HPT) was performed at room temperature under a pressure of 6.0 GPa through a total number of up to 20 turns. The metal matrix composite (MMC) showed a significant grain refinement from an initial average grain size of ∼8 μm to ∼300 nm after processing by HPT through 20 turns which led to an increase in the average values of Vickers microhardness at room temperature.

Novel multi-stage adaptive morphing of a hydrogel cube has been achieved by combining multi-metal ionoprinting and redox chemistry of iron. A demonstration of the two-stage deployment has been shown for (1) the selective opening and closing of the cube’s lid, where the hinge point has been ionoprinted with iron, and (2) the full unfolding and folding of the cube into its cruciform net, with remaining hinges ionoprinted with vanadium. The selective unfolding and folding is achieved by alternating the oxidation state of iron between +2 and +3. This is achieved using redox chemistry selective for iron. This approach could be applied, in principle, to more degrees of staging by adding additional redox responsive ionoprinted cations and appropriate selection of reducing agents.