Alloying enhanced negative Poisson's ratio in two-dimensional aluminum gallium nitride (AlxGa1-xN).

The negative Poisson's ratio (NPR) effect usually endows materials with promising ductility and shear resistance, facilitating a wider range of applications. It has been generally acknowledged that alloys show strong advantages in manipulating material properties. Thus, a thought-provoking question arises: how does alloying affect the NPR? In this paper, based on first-principles calculations, we systematically study the NPR in two-dimensional (2D) GaN and AlN, and their alloy of AlxGa1-xN. It is intriguing to find that the NPR in AlxGa1-xN is significantly enhanced compared to the parent materials of GaN and AlN. The underlying mechanism mainly originates from a counter-intuitive increase of the bond angle θ. We further study the microscopic origin of the anomalies by electron orbital analysis as well as electron localization functions. It is revealed that the distribution and movement of electrons change with the applied strain, providing a fundamental view on the effect of strain on lattice parameters and the NPR. The physical origin as revealed in this study deepens the understanding of the NPR and shed light on the future design of modern nanoscale electromechanical devices with fantastic functions based on the auxetic nanomaterials and nanostructures.

Practical back-analysis methodology to predict underground train-induced building vibrations.

The vehicle-track interaction leads to the vibrating source for the prediction of train-induced building vibrations. To prevent modeling difficulties in the source part, this study proposes a practical back-analysis methodology for calculating underground train-induced building vibrations. The methodology combines the advantages of field measurements and numerical simulations. The fundamental concept of the hybrid methodology is to first create a virtualized moving source at the rail surface and subsequently modify it until the numerical predictions are consistent with the field measurements at the same locations. These locations are frequently selected at the ground surface or near the building foundation. Finally, this imaginary force can be used to predict the vibrations of buildings. The practicality of the hybrid methodology is verified by comparing the predicted vibrations of buildings with field test results. As an application of the proposed method, the transmission laws and characteristics of the vibrations in buildings are analyzed.