Supercomputer simulations show that at the atomic level, material stress does not behave symmetrically. Widely-used atomic stress formulae significantly underestimate stress near stress concentrators such as dislocation core, crack tip, or interface, in a material under deformation. Supercomputers simulate force interactions of Lennard-Jones perfect single crystal of 240,000 atoms. Study findings could help scientists design new materials such as glass or metal that does notice up.

That's to a study published in the  Proceedings of the Royal Society A. Study co-author Liming Xiong summarized the two main findings. "The commonly accepted symmetric property of a stress tensor in classical continuum mechanics is based on certain assumptions, and they will not be valid when a material is resolvedat an atomistic resolution. "

Xiong continued that "the widely used atomic Virial stress or Hardy stresses significantly underestimate the stress near a stress concentrator such as a dislocation core, a crack tip, or an interface, in a material under deformation ." Liming Xiong is an Assistant Professor in the Department of Aerospace Engineering at Iowa State University.

Instead, they used the definition by mathematician AL Cauchy of stress as the force per unit area acting on three rectangular plans . With that, they have molecular dynamics simulations to measure the atomic-scale stress tensor of materials with inhomogeneities produced by dislocations, and holes.

Micron-sized sample

The computational challenges, said Xiong, swell up to the limits of what is currently computable when one deals with atomic forces interacting inside a tiny fraction of the space of a raindrop. "The degree of freedom that needs tobe calculated will be huge because even a micron-sized sample will contain billions of atoms. Billions of atomic pairs will require a huge amount of computation resource, "said Xiong.

What's more, added Xiong, is the lack of a well-established computer code that can be used for the local stress calculation at the atomic scale. His team used the open source LAMMPS Molecular Dynamics Simulator, incorporating the Lennard-Jones interatomic potential and modified through the parameters they worked out in the paper. "Basically, we're trying to meet two challenges," Xiong said. "One is redefining stress at an atomic level, the other one is if we have a well-defined stress quantity, can we use supercomputer resources to calculate it?"

"Jetstream is a very suitable platform to develop a computer code, debug it, and test it," Xiong said. "Jetstream is designed for small-scale calculations, not for large-scale ones." Once the code was developed and benchmarked, we ported it to the petascale Comet system to perform large-scale simulations using hundreds to thousands of processors. used XSEDE resources to perform this research, "Xiong explained.

Virtual machine technology

The Jetstream system is a configurable large-scale computing resource that leverages both on-demand and persistent virtual machine technology to support a much wider array of software environments and services than current NSF resources can accommodate.

The simulation work, said Xiong, helps scientists bridge the gap between the micro and the macro scales of reality, in a methodology called multiscale modeling. "Multiscale is trying to bridge the atomistic continuum. In order to develop a methodology for multiscale modeling, we need to have definitions for each quantity at each level … This is very important for the establishment of a self-consistent concurrent atomistic- continuum computational tool.

Xiong and his research group are working on several projects to apply their understanding of stress to design new materials with novel properties. "One of them is de-icing from the surfaces of materials," Xiong explained. "A common phenomenon you can observe is that it forms on a window in cold weather." The force and energy required to remove that ice is related to the stress tensor definition and the interfaces between ice and the car window, the stress, if it is clear at a local scale, it will provide the main guidance to use in our daily life. "

Xiong sees great value in the computational side of science. "Supercomputing is a really powerful way to compute Nowadays, people want to speed up the development of new materials, we want to make and understand the material behavior before putting it into mass production, that will require a predictive simulation tool. , you can see a lot of atoms, and you can see a huge number of atoms.