Understanding the way that a wave moves through granular materials has vast implications for modern science. After all, scientists use such stress wave propagation to detect the magnitude of earthquakes, locate oil and gas reservoirs, design acoustic insulation, and develop materials for compacting powders.
A study by a team that includes Ryan Hurley, an assistant professor of mechanical engineering, offers important insights into the way stress wave propagation unfolds. Using X-ray measurements and analyses, the team has shown that velocity scaling and dispersion in wave transmission is based on particle arrangements and chains of force between them, while reduction of wave intensity is caused mainly from particle arrangements alone.
“Our study provides a better understanding of how the fine-scale structure of a granular material is related to the behavior of waves propagating through them,” says Hurley. “This knowledge is of fundamental importance in the study of seismic signals from landslides and earthquakes, in the nondestructive evaluation of soils in civil engineering, and in the fabrication of materials with desired wave properties in materials science.”
Hurley conceived of this line of research while a postdoc at Lawrence Livermore National Laboratory, collaborating with a team that included physicist Eric Herbold. The experiments and analysis were later performed by Hurley and Whiting School postdoc Chongpu Zhai after Hurley moved to Johns Hopkins, with experimental assistance and continued discussions with Herbold. Their study appeared recently in the online edition of the journal Proceedings of the National Academy of Sciences.
Research from the late 1950s describes what may be happening to material underlying wave propagation, but the new research provides evidence for why.
“The novel experimental aspect of this work is the use of in situ X-ray measurements to obtain packing structure, particle stress, and interparticle forces throughout a granular material during the simultaneous measurement of ultrasound transmission,” says Hurley. “These measurements are the highest-fidelity dataset to date investigating ultrasound, forces, and structure in granular materials.”
Adds Zhai, the study’s lead author: “These experiments, along with the supporting simulations, allow us to reveal why wave speeds in granular materials change as a function of pressure and to quantify the effects of particular particle-scale phenomena on macroscopic wave behavior.”