HEMI Fellow Jaafar El-Awady Identifies New Method to Predict Cracks in Metal Before They Are Visible

Congratulations to Jaafar El-Awady, HEMI Fellow and associate professor in the Department of Mechanical Engineering, for discovering a new method to predict cracks in metal before they appear. His results were published in Science.

His paper, titled “The heterogeneity of persistent slip band nucleation and evolution in metals at the micrometer scale,” identifies a new method to test metals and predict vulnerabilities before visible cracks are formed. This has implications in understanding and predicting the life of a metal mechanism, especially to help prevent fatigue failure and to avoid discarding parts that may still be useful.

“We’re able now to have a more fundamental understanding about what leads up to cracks,” El-Awady said. “The practical implication is that it will allow us to understand and predict when or how the material is going to fail.”

Read El-Awady’s paper here.

HEMI Fellow Ryan Hurley Leads Research Team to Determine How Forces Control Granular Material Properties

Understanding the way a wave moves through granular materials—after an earthquake, for example—has vast implications for modern science. After all, scientists use stress wave propagation through granular materials to detect the magnitude of earthquakes, locate oil and gas reservoirs, design acoustic insulation, and develop materials for compacting powders.

Using X-ray measurements and analyses, a team of researchers led by a Johns Hopkins mechanical engineering professor have 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. The research appeared yesterday in the online edition of the journal 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,” said Ryan Hurley, assistant professor of mechanical engineering at Johns Hopkins Whiting School of Engineering and fellow of the Hopkins Extreme Materials Institute. “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 JHU, with experimental assistance and continued discussions with Herbold.

Structure-property relations of granular materials are governed by the arrangement of particles and the chains of forces between them. These relations enable the design of wave damping materials and non-destructive testing technologies. Wave transmission in granular materials has been extensively studied and demonstrates unique features: 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 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 was originally posted on the Hub in June 2020.

The image is a combination of two sets of data from X-ray scans of single crystal sapphire spheres.
The image is a combination of two sets of data from X-ray scans of single crystal sapphire spheres. The reconstructed X-ray Computed Tomography (XRCT) data defines the surface of all 621 grains in the load frame. The far-field X-ray Diffraction (ff-XRD or 3DXRD) data provides a strain tensor that is mapped to each grain center. The combination and colorization of this data shows the distribution of stresses for each grain under load. This information was used as initial conditions for ultrasonic transmission measurements, where structure-property relationships were measured in-situ.

This research was funded by the Johns Hopkins Whiting School of Engineering and LLNL’s Laboratory Directed Research and Development program, and was carried out at the Advanced Photon Source, an Office of Science User Facility, operated by Argonne National Laboratory.

HEMI Fellow Sarah Hörst Published in Sky and Telescope Magazine

Congratulations to HEMI Fellow Sarah Hörst, an assistant professor in the Dept. of Earth and Planetary Sciences whose article, “Titan’s Veil,” is featured in the February issue of Sky & Telescope magazine.

The eight-page article details the chemical ingredients found within the each region of atmosphere of Saturn’s largest moon, Titan, and likens that complex atmospheric makeup to that of early Earth.  In doing so, Hörst makes a point that, by studying Titan, we might learn enough to identify markers that will allow us to recognize habitable planets surrounding other stars.

Hörst’s primary research interest is atmospheric chemistry. She is particularly interested in the complex organic chemistry occurring in the atmosphere of Titan, but is also interested in complex organics elsewhere in the solar system and universe, whether they are produced in an atmosphere or on a surface.

Sky & Telescope is the essential guide to astronomy, showcasing each month a wide array of celestial events and astronomy news to a highly-engaged audience that includes astronomy practitioners of all levels – from novices with their first telescope, to intermediate and advanced backyard astronomers, to professionals.



HEMI Fellow June Wicks Authors Paper Containing Record-breaking Data for Use in Simulating Core Conditions for ‘Super-Earth’ Exoplanets

Congratulations to June K. Wicks, a HEMI Fellow and an assistant professor in the Department of Earth and Planetary Sciences, who published a paper today in Science Advances reporting X-ray diffraction data on iron-silicon alloys at core conditions of “super-Earth” exoplanets.

Using high-powered laser beams, Wicks and her colleagues at Princeton University, Lawrence Livermore National Laboratory, and the University of Rochester have developed techniques to extend X-ray diffraction of solid materials to TPa pressures (1 TPa = 10 million atmospheres). This work represents the highest-pressure X-ray diffraction data ever reported.

With the continued discovery of thousands of planets outside our solar system, an effort that will be extended with instruments such as the James Webb Space Telescope, scientists are eager to learn more about interior structures of these exoplanets. However, in this quest researchers have encountered two major limitations: they have no direct measurements of our own planetary core from which to extrapolate, and interior pressures in super-Earths can reach more than 10 times the pressure at the center of the Earth, well beyond the range of conventional experimental techniques. By using groundbreaking techniques, Wicks and her colleagues have generated the first experimental data on likely constituent core materials at pressures exceeding 1.3 TPa, about three times higher than previous experiments. These data are directly useful for modeling the interior structure of large, rocky exoplanets, which in turn provides insights into the types of planetary architectures that may exist in our galaxy.

Wicks and her colleagues directed a short but intense laser beam onto two iron samples: one alloyed with 7 weight-percent silicon, similar to the modeled composition of Earth’s core, and another with 15 weight-percent silicon, relevant to more silicon-rich planets. They compressed these samples for only a few billionths of a second, but it was just long enough to probe the atomic structure using a pulse of bright X-rays. The resulting diffraction pattern provided information on the density and crystal structure of the iron-silicon alloys, revealing that the crystal structure changed with higher silicon content.

The researchers found that at ultrahigh pressures, the lower-silicon alloy organized into a hexagonal close-packed crystal structure, while the higher-silicon alloy adopted body-centered cubic packing.

“This atomic difference has enormous implications,” said Wicks. “Knowledge of the crystal structure is the most fundamental piece of information about the material making up the interior of a planet, as all other physical and chemical properties follow from the crystal structure.”

Future research will investigate how other light elements, such as carbon or sulfur, affect the structure and density of iron at ultrahigh pressure conditions. The researchers also hope to measure other key physical properties of iron alloys, to further constrain models of exoplanets’ interiors.

Crystal structure and equation of state of Fe-Si alloys at super-Earth core conditions,” by June Wicks, Raymond Smith, Dayne Fratanduono, Federica Coppari, Richard Kraus, Matthew Newman, J. Ryan Rygg, Jon Eggert and Thomas Duffy was published April 25 in Science Advances. The research was funded by the National Nuclear Security Administration through the National Laser Users’ Facility Program (contract nos. DE-NA0002154 and DE-NA0002720) and the Laboratory Directed Research and Development Program at Lawrence Livermore National Laboratory (project no. 15-ERD-012).

HEMI Fellow June Wicks Contributes to the Formulation of New Mass-Radius Relationship for a Hypothetical Pure Iron Planet at Super-Earth Core Conditions

Congratulations to HEMI Fellow June K. Wicks (Dept. of Earth and Planetary Sciences), who, along with collaborators from Lawrence Livermore National Laboratories, Princeton, and the University of Rochester, has published the first experimentally-based mass radius relationship for a hypothetical pure iron planet at super-Earth core conditions.

The discovery, published earlier this week by Nature Astronomy, is hoped to be used to evaluate plausible composition of large, rocky exoplanets and aid with the formation of interior planetary models.

They measured the first absolute equation of state of iron to 1.4 TPa (14 million atmospheres) on the National Ignition Facility, the world’s largest and most energetic laser.

“The high-pressure behavior of iron metal is the anchor point for interpreting exoplanet observations,” Prof. Wicks says. “Now that we know the equation of state of iron, we can probe the effect of light element alloying on observable properties and explore the diversity of planetary cores in our universe.”

To read the paper in its entirety, visit www.nature.com/articles/s41550-018-0437-9.

Harvesting Wind Energy with a Piezo-Flag: Kang Group Published in Applied Energy

HEMI fellow Sung Hoon Kang and his team were recently published in Applied Energy, a journal  focusing on the modern state of research and development for energy conversion and conservation. Titled “Harvesting Ambient Wind Energy With an Inverted Piezoelectric Flag,” the publication showcases a novel way to enhance the wind-harvesting capabilities of piezoelectric materials.

Pictured above, HEMI postdoctoral fellow Santiago Orrego demonstrates the technology; optimizing the flexible and conductive properties of piezoelectric material to make a nano-generator that could provide energy for time-spans extending over months, and even years. Dr. Kang’s group has since refined this innovative energy source by changing the flag orientation, allowing the “piezo-flag” to be tuned to the most efficient flapping direction for ambient wind conditions.


The paper describes an experimental study of wind energy harvesting by self-sustained oscillations (flutter) of a flexible piezoelectric membrane fixed in a novel orientation called the ‘‘inverted flag.”


The flutter and piezo-leaf design was experimentally tested using a wind tunnel simulator, as shown in the video here. The design was also mounted onto the roof of Garland Hall at Johns Hopkins University to test ambient wind conditions on the nano-generator. In this case, the generator powered a sustainable temperature sensor.

Orrego is working on this project in conjunction with Assistant Professor Sung Hoon Kang from the The Kang Group and the Department of Mechanical Engineering and mechanical engineering Professor Rajat Mittal from the Flow Physics & Computational Lab.

To read the full abstract and publication, click here.

Follow the project and check out a sustainable temperature reading: https://www.youtube.com/channel/UCDxonH3QS1GNzysYly6hsTw/live

Prof. Kevin Hemker, Dr. Kelvin Xie, and Collaborators Publish Groundbreaking Research: “Breaking the Icosahedra in Boron Carbide”


Dr. Kelvin Xie (Left) & Dr. Kevin Hemker (Right)

Join us in congratulating HEMI fellow Prof. Kevin Hemker and his collaborators for their latest publication: “Breaking the Icosahedra in Boron Carbide.” The paper discusses their revolutionary use of Laser-assisted Atom Probe Tomography to study the atomic structure and bonding of boron carbide ceramics.

Hemker and Dr. Kelvin Xie (Assistant Research Scientist, JHU) discovered that the 12-atom icosahedra disintegrated during their experiments. They worked in conjunction with Professor William A. Goddard III from Caltech, and Professor Julie Cairney from The University of Sydney.

“About ten years ago, we discovered that boron carbide undergoes amorphization and suffers a dramatic loss of strength during ballistic impact, but a fundamental understanding of the underlying mechanism for this amorphization has not been fully established.” says Hemker. Cairney adds that, “Atom probe has been generally used to identify the chemical and spatial information of individual atoms and clusters. In this work, the temporal, spatial and compositional information provided by atom probe tomography makes it a unique platform for elucidating the relative stability and interactions of primary building blocks in hierarchically crystalline materials”.

These would be the first atom probe experiments on boron carbide (to our knowledge). The research was supported by grants from the Army Research Laboratory, Defense Advanced Research Projects Agency, and the National Science Foundation. Click here to view the full article.

Hemker was recently elected a fellow of the American Association for the Advancement of Science “for discoveries in underlying atomic-scale processes governing mechanical behavior of advanced materials systems, including nanocrystalline, micro-lattice, thermal barrier, and high-temperature materials.” At HEMI, he and his students have employed atomic resolution electron microscopy and microscale mechanical testing to change the way the materials community thinks about and understands the mechanical behavior of advanced materials.






Paper by Professor Sung Hoon Kang Published in Advanced Materials Introducing a New Class of Energy-Absorbing Material

Congratulations to Professor Sung Hoon Kang who, along with colleagues at Harvard University, published a paper in Advanced Materials introducing their new class of reusable energy-absorbing material.

In the paper, which is co-authored by Kang’s incoming Ph.D. student Lichen Fang, Prof. Kang and his colleagues at Harvard introduce a material-independent mechanism for energy absorption that can be used to make reusable, energy-absorbing materials.  The team combined numerical modeling and 3D printing to design an elastic metamaterial (a material with properties unavailable in nature) where the energy inserted into the system during loading is trapped in the elastic deformation of a large number of bistable elastic elements.

This new structural mechanism gives the material the ability to absorb energy when needed, but to also return to its original state afterward. It also drastically improves the impact protection and offers a unique range of advantages such as scalability and loading rate and history independence. This new structural design could be combined with different classes of materials and produce stimuli-responsive structures that are capable of recovering when exposed to environmental cues such as temperature and solvent.

While the collapse of structures is commonly associated with dramatic structural failure, the research transforms this association into new opportunities for generating novel and useful materials. The below video shows mechanism in action and illustrates how the energy-absorbing material can protect a fragile object.

To read the published paper in Advanced Materials, please click here.


Nature Communications Publishes Article by Jaafar El-Awady on Unravelling the Physics of Size-Dependent Dislocation-mediated Plasticity

Size-affected dislocation-mediated plasticity is important in a wide range of materials and technologies. The question of how to explain and predict the effect of size on the properties and response of materials has been at the forefront of mechanics and materials research. In a recent article in Nature Communications, HEMI Professor Jaafar A. El-Awady addresses this by developing an experimentally validated generalized size-dependent dislocation-based model from discrete dislocation dynamics simulations. The model is shown to predict the single and polycrystalline strength as a function of crystal/grain size and the dislocation density. In the article titled “Unravelling the physics of size-dependent dislocation-mediated plasticity”, Prof. El-Awady also developed a new deformation mechanism map for single crystals based on these simulations.

This work presents a micro-mechanistic framework to predict and interpret strength size-scale effects, and provides an avenue towards performing multiscale simulations without ad hoc assumptions. Because of the applicability of the model to an extremely large set of crystal/grain sizes (that is, bulk to tens of nanometers) and its portability for predicting the strength of both single and polycrystals, the proposed model is expected to have further applications in constitutive law development and multiscale methods. This model can also be extended to address problems where the high strain rate sensitivity of mobile dislocation density is important.