This image is a combination of two sets of data from X-ray scans of single crystal sapphire spheres. The reconstructed X-ray Computed Tomography data defines the surface of all 621 grains in the load frame. The combination and colorization of this data shows the distribution of stresses for each grain under load. Image: Johns Hopkins University.Better understanding of stress wave propagation through grainy, or granular, materials is important for detecting the magnitude of earthquakes, locating oil and gas reservoirs, and designing acoustic insulation and materials for compacting powders.
A team of researchers led by a mechanical engineering professor at Johns Hopkins University has now used X-ray measurements and analyses to show that velocity scaling and dispersion in wave transmission is based on the arrangements of particles and the chains of force between them. Whereas the reduction of wave intensity is caused mainly by particle arrangements alone. The team reports its findings in a paper in the Proceedings of the National Academy of Sciences.
"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," explained Ryan Hurley, assistant professor of mechanical engineering at Johns Hopkins Whiting School of Engineering. "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 research while a postdoc at Lawrence Livermore National Laboratory (LLNL), collaborating with a team that included LLNL physicist Eric Herbold. The experiments and analysis were later performed by Hurley and Whiting School postdoc Chongpu Zhai after Hurley moved to Johns Hopkins University, with experimental assistance from Herbold.
The structure-property relations of granular materials are governed by the arrangement of particles and the chains of forces between them. These relations allow the design of wave-damping materials and non-destructive testing technologies. Wave transmission in granular materials has been extensively studied and demonstrates unique features, including power-law velocity scaling, dispersion and attenuation (the reduction of the amplitude of a signal, electric current or other oscillation).
Earlier research, dating back to the late 1950s described ‘what’ may be happening to the 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 inter-particle forces throughout a granular material during the simultaneous measurement of ultrasound transmission," said Hurley. "These measurements are the highest fidelity dataset to-date investigating ultrasound, forces and structure in granular materials."
"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," said Zhai, who led the data analysis efforts and was that paper's first author.
The research provides new insight into time- and frequency-domain features of wave propagation in randomly packed grainy materials, shedding light on the fundamental mechanisms controlling wave velocities, dispersion and attenuation in these systems.
This story is adapted from material from Johns Hopkins University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.