More Like this

1. New Methodologies for Grain Boundary Detection in EBSD Data of Microstructures

   https://doi.org/10.2514/6.2022-1424  

   Catania, Richard K.; Senthilnathan, Arulmurugan; Sions, John; Snyder, Kyle; Al-Ghaib, Huda; Zimmerman, Ben; Acar, Pinar (December 2021, New Methodologies for Grain Boundary Detection in EBSD Data of Microstructures)

   This work discusses new methodologies for identifying the grain boundaries in color images of metallic microstructures and the quantification of their grain topology. Grain boundaries have a large impact on the macro-scale material properties. Particularly, this work employs the experimental microstructure data of Titanium-Aluminum alloys, which can be used for various aerospace components owing to their outstanding mechanical performance in elevated temperatures. The grain topology of these metallic microstructures is quantified using the concept of shape moment invariants. In order to capture the grains using the shape moment invariants, it is necessary to identify the grain boundaries and separate them from their respective grains. We present two methodologies to detect the grain boundaries. The first method is the tolerance-based neighbor analysis. The second method focuses on creating three-dimensional space of pixel intensity values based on the three color channels and measuring the Euclidean distance to separate different grains. Additionally, since the grain boundaries may not possess the same material properties as the grain itself, this work investigates the effect of including the grain boundaries when determining the homogenized material properties of the given microstructure. To generate adequate statistical information, microstructures are reconstructed from the experimental data using the Markov Random Field (MRF) method. Upon separating the grains, we use the shape moment invariants to quantify the shapes of different grains. Using the shape moment invariants and the experimental material property values, three neural network functions are developed to investigate the effects of grain boundaries on material property predictions. 

   more » « less
2. Crustal Structure of Southern California from Velocity and Attenuation Tomography

   https://doi.org/10.4401/ag-9149  

   Jordan, Thomas H (January 2024, Annals of Geophysics)

   Recent studies of Southern California have employed velocity and attenuation tomography to elucidate two aspects of crustal structure. Low-frequency velocity tomography reveals large-scale heterogeneity that can be approximated by a discrete set (𝐾 ≤ 10) of tectonic regions, each characterized by an isostatically balanced lithospheric column reflecting the composition and tectonic history of the region. The boundaries between tectonic regions are typically localized structures expressed at the surface by major faults, topographic fronts, and geochemical transitions. The efficacy with which tomographic regionalization (TR) represents the subsurface geography of major tectonic units in Southern California indicates that the TR idealization works surprisingly well in this part of the Pacific‐North America plate boundary. High‐frequency attenuation tomography in Southern California supports the hypothesis that the decay of P and S waves propagating through the upper and middle crust is dominated by elastic scattering from small-scale heterogeneities of high fractal dimension, rather than by anelastic dissipation. The amplitude of the scattering varies systematically among the tectonic regions and correlates with the degree of recent faulting and deformation within a region. These results motivate speculation that small-scale crustal heterogeneities responsible for high-frequency attenuation are generated within the seismogenic zone through a dynamic interplay between distributed deformation and localized fracturing. 

   more » « less
3. Coupled Brittle and Viscous Micromechanisms Produce Semibrittle Flow, Grain‐Boundary Sliding, and Anelasticity in Salt‐Rock

   https://doi.org/10.1029/2020JB021261  

   Ding, J.; Chester, F_M; Chester, J_S; Shen, X.; Arson, C. (February 2021, Journal of Geophysical Research: Solid Earth)

   Abstract The operation of fracture, diffusion, and intracrystalline‐plastic micromechanisms during semibrittle deformation of rock is directly relevant to understanding mechanical behavior across the brittle‐plastic transition in the crust. An outstanding question is whether (1) the micromechanisms of semibrittle flow can be considered to operate independently, as represented in typical crustal strength profiles across the brittle to plastic transition, or (2) the micromechanisms are coupled such that the transition is represented by a distinct rheology with dependency on effective pressure, temperature, and strain rate. We employ triaxial stress‐cycling experiments to investigate elastic‐plastic and viscoelastic behaviors during semibrittle flow in two distinctly different monomineralic, polycrystalline, synthetic salt‐rocks. During semibrittle flow at high differential stress, granular, low‐porosity, work‐hardened salt‐rocks deform predominantly by grain‐boundary sliding and wing‐crack opening accompanied by minor intragranular dislocation glide. In contrast, fully annealed, near‐zero porosity salt‐rocks flow at lower differential stress by intragranular dislocation glide accompanied by grain‐boundary sliding and opening. Grain‐boundary sliding is frictional during semibrittle flow at higher strain rates, but the associated dispersal of water from fluid inclusions along boundaries can activate fluid‐assisted diffusional sliding at lower strain rates. Changes in elastic properties with semibrittle flow largely reflect activation of sliding along closed grain boundaries. Observed microstructures, pronounced hysteresis and anelasticity during cyclic stressing after semibrittle flow, and stress relaxation behaviors indicate coupled operation of micromechanisms leading to a distinct rheology (hypothesis 2 above). 

   more » « less
4. Spurious Rayleigh-wave apparent anisotropy near major structural boundaries: a numerical and theoretical investigation

   https://doi.org/10.1093/gji/ggae305  

   Zeng, Qicheng; Lin, Fan-Chi; Tsai, Victor_C (August 2024, Geophysical Journal International)

   SUMMARY The recent developments in array-based surface-wave tomography have made it possible to directly measure apparent phase velocities through wave front tracking. While directionally dependent measurements have been used to infer intrinsic $$2\psi $$ azimuthal anisotropy (with a 180° periodicity), a few studies have also demonstrated strong but spurious $$1\psi $$ azimuthal anisotropy (360° periodicity) near major structure boundaries particularly for long period surface waves. In such observations, Rayleigh waves propagating in the direction perpendicular to the boundary from the slow to the fast side persistently show a higher apparent velocity compared to waves propagating in the opposite direction. In this study, we conduct numerical and theoretical investigations to explore the effect of scattering on the apparent Rayleigh-wave phase velocity measurement. Using 2-D spectral-element numerical wavefield simulations, we first reproduce the observation that waves propagating in opposite directions show different apparent phase velocities when passing through a major velocity contrast. Based on mode coupling theory and the locked mode approximation, we then investigate the effect of the scattered fundamental-mode Rayleigh wave and body waves interfering with the incident Rayleigh wave separately. We show that scattered fundamental-mode Rayleigh waves, while dominating the scattered wavefield, mostly cause short wavelength apparent phase velocity variations that could only be studied if the station spacing is less than about one tenth of the surface wave wavelength. Scattered body waves, on the other hand, cause longer wavelength velocity variations that correspond to the existing real data observations. Because of the sensitivity of the $$1\psi $$ apparent anisotropy to velocity contrasts, incorporating such measurements in surface wave tomography could improve the resolution and sharpen the structural boundaries of the inverted model. 

   more » « less
5. Whistler Mode Waves in the Compressional Boundary of Foreshock Transients

   https://doi.org/10.1029/2019JA027758  

   Shi, Xiaofei; Liu, Terry Z.; Angelopoulos, Vassilis; Zhang, Xiao‐Jia (July 2020, Journal of Geophysical Research: Space Physics)

   Abstract Earth's foreshock is filled with backstreaming particles that can generate a variety of waves and foreshock transients. According to recent studies, these particles can be further accelerated while being scattered by field fluctuations, including waves, inside foreshock transients, contributing to particle acceleration at the parent bow shock. The properties of these waves and how they interact with particles and affect particle acceleration inside foreshock transients are still unclear, however. Here we take the first step to study one important type of these waves, whistler waves. We use Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations and employ multiple case studies to investigate the properties of whistler waves in the compressional boundaries of foreshock transients where THEMIS wave burst mode is triggered. We show that the whistler waves are quasi parallel propagating with bidirectional Poynting vectors, suggesting that they are locally generated. We focus on how they interact with electrons. We show that the diffusion surfaces for these waves in the electron velocity space match the observed electron phase space density distribution contours better when the modeled pitch angle diffusion coefficients from these waves are higher. We also demonstrate that higher‐energy electrons are more likely to be scattered by whistler waves. Our results suggest that whistler waves are important for scattering tens to hundreds of electronvolt electrons inside foreshock transients and elucidate electron dynamics and whistler wave properties in such environments. 

   more » « less