Porous materials are typically heterogeneous containing pores ranging from nanoscale to macroscale. These vast variation in pore structure in nano and micro scale will influence the mechanical performance of the overall macroscopic behavior of the material. In continuum scales, mechanical properties of porous materials are estimated by considering porosity, while the underlying variations in nano and micro pore structures are ignored. 

This presentation numerically investigates the effect of nanopore morphology in addition to porosity as the first-order term, on overall mechanical performance of amorphous silica (a-SiO~2~). To explore the influence of the higher-order terms, different pore sizes, shapes, and orientations were investigated.

Circular, square, and triangular pores were studied. 0- and 45-degree orientations were used for square shape pores, while 0- and 90-degree orientations were used for triangular ones. The pore sizes and shapes were selected based on the experimental observation of pores in SiO~2~-based materials. Three porosity values between 2-5% were selected.

Molecular dynamics (MD) simulations coupled with the-state-of-the-art reactive force field (ReaxFF) was adopted for this investigation. LAMMPS, an open-source MD tool, developed at Sandia National Laboratories was used for the analysis. A simulation box of \~30 x 30 x 1.5 nm^3^ were set up. The simulation domain was carefully selected to ensure that the extracted properties do not have any domain size dependency and also there is no boundary effect on the fracture behavior of the pores located at the center of the domain. The total number of atoms were between 85,000 to 86,400.  Tensile loading combined with periodic boundary conditions were imposed to each simulation box.

The results revealed that the presence of pore significantly influences both Young’s modulus and critical energy release rate (G~IC~) of a-SiO~2~. Specifically, porosity lowered both Young’s modulus and G~IC~ values. Furthermore, for each constant porosity value, the change in the nanopore morphology impacts both mechanical and fracture parameters (e.g., Young’s modulus and G~IC~). The influence of higher-order terms such as pore shapes and orientations are linked to the potential energy and ligament length defined as the distance between the pore and the edge of the simulation box, in another word, the wall thickness. While circular pore leads to the highest value of Young’s modulus, the square pore corresponds to the highest strength value.

Our finding suggests that we can design nonporous materials with desired mechanical properties by controlling the pore morphology of the materials at the nanoscale.


Investigation of Nanomechanics of Amorphous Porous Silica Using Reactive Molecular Dynamic Simulations

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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