G. Ravichandran
John E. Goode, Jr., Professor of Aerospace and Mechanical Engineering;
Otis Booth Leadership Chair, Division of Engineering and Applied Science
G. Ravichandran

Research

High Pressure and High Strain Rate Behavior of Materials

Strength of materials at high-pressures and high-strain-rates is relevant to a number of applications including planetary impact and inertial confinement fusion. Understanding how strength depends on pressure allows for the characterization of materials and validation of constitutive models.

Our approach includes study of shear resistance of glass, melting and solidification, shock compression of metals, dynamic strength and hydrodynamic instabilities of various materials.

Determining strength of materials under dynamic loading conditions using hydrodynamic instabilities

Zachary Sternberger

Hydrodynamic instability experiments allow access to material properties at extreme conditions where the pressure exceeds 100 GPa and the strain rate exceeds 1E6 1/s. Laser ablation dynamically loads a sample material and causes a machined initial perturbation to grow due to hydrodynamic instability. The strength of the material slows the instability, allowing strength to be inferred from instability growth.

Recovery instability targets containing copper and tantalum samples coined with 2D (hill and valley) and 3D (eggcrate) initial perturbations were loaded using the Janus laser at the Jupiter Laser Facility, Lawrence Livermore National Laboratory. The coupling of laser energy into a loading wave is studied with a combination of laser-matter interaction simulations and velocity interferometry data. The development of the instability from initial to final conditions is studied with hydrocode simulations.

A recovered tantalum sample coined with a 2D initial perturbation. The center of the sample is deformed from the high pressure generated in the experiment.

Figure: A recovered tantalum sample coined with a 2D initial perturbation. The center of the sample is deformed from the high pressure generated in the experiment.

Melting and Solidification in Multi-Component Materials

Matthew Neumann

Interior Structure and Dynamics of the Earth

Modeling the interior structure and dynamics of the Earth's core requires knowledge of the equation of state of core materials at the thermodynamic conditions that exist deep within the Earth. The composition of the Earth's core must satisfy several constraints including density, sound speed indicated by seismic data, the general abundance of elements in the solar system, and the appropriate partitioning between solid and liquid phases at Earth core conditions. To satisfy these constraints, the core of the Earth must be a multi-component mixture of iron and light alloying elements. I am studying an iron silicide alloy with 15 weight percent silicon as a first step to understanding how the addition of a light element might affect solidification at the inner core boundary of the Earth.

Thermodynamic and Kinetic Behavior of Materials

Laser driven compression experiments provide a platform to study materials at the pressures and temperatures relevant to the phenomena that occur in the deep interior of the Earth (up to 350 GPa). To generate the shock state in the sample, laser energy is deposited on to a kapton ablator. The ablation process generates an expanding plasma, which drives a shock in the opposite direction of the expansion to conserve momentum.

interface velocity and radiation-hydrodynamic calculation

Figure 1: Left: Target stack used for these experiments. We used a kapton ablator to drive a shock in to the Fe-15Si sample, and a lithium flouride window to observe the interface velocity upon partial release. Right: Radiation-hydrodynamics calculation showing the 1-D wave interactions for this target assembly.

The study of heterogeneous materials like iron silicide is challenging as the number of phases that can exist in equilibrium increases with each material component in the system. Iron silicide is a particularly interesting material as it decomposes in to two chemically distinct structures at high pressure through a process that requires recrystallization. The timescale for this chemical segregation and recrystallization (~ 1 second) is incommensurate with the timescale of laser driven experiments (~ 1 nanosecond), so we are able to observe kinetic phenomena and paths to equilibrium.

In Situ X-Ray Diffraction

To diagnose the phase of the material upon compression we use x-ray diffraction. While the sample is in the shock state, we illuminate a quasi-monocromatic x-ray source for 0.5 to 1 ns, which is generated by laser-induced Heα emission of an iron foil. The diffraction pattern is collected on image plates and used to determine the phase (solid or liquid) as well as the crystal structure of the shocked sample. Efforts are underway to use texture information to determine the orientation relationship between parent and daughter phases for martensitic transformations.

Observed d-spacing and pressure are plotted against their theoretical values for hcp and D03 structures for Fe-15Si upon shock compression.

Figure 2: Observed d-spacing and pressure are plotted against their theoretical values for hcp and D03 structures for Fe-15Si upon shock compression. Due to the texture of the material, the absence of a diffraction peak does not necessarily imply that the structure is not there, it may simply be rotated out of the diffraction condition. The data indicates a sluggish phase transformation from D03 to hcp at pressures ranging from 175 to 300 GPa.

We can diagnose melting indicated by the loss of intense diffraction associated with the strict order of a solid and the onset of diffuse liquid scattering, as well as the complete loss of texture associated with crystallographic orientation.

Representative x-ray diffraction data for shocked Fe-15Si

Figure 3: Representative x-ray diffraction data for shocked Fe-15Si projected in to 2θ versus ϕ space, which correspond to the polar and azimuthal angles about the incident wave vector respectively. The red arrows point to the diffraction peaks associated with the shock compressed sample. Upon shock compression to 318 GPa, we observe the first loss of solid diffraction peaks as well as loss of texture, consistent with melt, shown in panel b.)

Shearing Resistance of Glasses at High Pressures and High Strain Rates

Christian Kettenbeil

[PLACEHOLDER]

Shock compression

Tomoyuki Oniyama

This project studies shock compression of metals such as molybdenum. The focus of the project is on the macroscopic and microscopic behaviors of metals at extremely high pressure. Shock compression is achieved using gas and powder guns, and measurements are taken through optical interferometry and X-ray techniques.

Shock compression research lab