Repository of works by Caltech published authors.
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On the Propagation of Maximally Dissipative Phase Boundaries in Solids - Abeyaratne, Rohan; Knowles, James K.
This paper is concerned with the kinetics of propagating phase boundaries
in a bar made of a special nonlinearly elastic material. First, it is shown that
there is a kinetic law of the form f = φ(s) relating the driving traction f at a phase
boundary to the phase boundary velocity s that corresponds to a notion of maximum
dissipation analogous to the concept of maximum plastic work. Second, it is shown
that a modified version of the entropy rate admissibility criterion can be described by
a kinetic relation of the above form, but with a different φ. Both kinetic relations
are applied to the Riemann problem...
Shock induced formation of MgAl_2O_4 spinel from oxides - Potter, David K.; Ahrens, Thomas J.
The physics of mineral grain sliding, which occurs upon dynamic compression of rocks, is investigated by shock loading single crystals of corundum (Al_2O_3) and periclase (MgO) in contact obliquely in impact experiments. Energy dispersive X-ray analysis and X-ray diffraction studies of samples recovered from 26–36 GPa, 800 ns experiments indicated that under certain conditions a spinel phase of composition MgAl_2O_4 and thickness ≤20 µm was produced at the interface between the two crystals. Although the computed shock (continuum) temperatures were below those necessary to melt the initial oxides, the spinel nonetheless appears to have formed as a result of localised...
Thermal expansion of mantle and core materials at very high pressures - Duffy, Thomas S.; Ahrens, Thomas J.
The thermal expansivities (α) of MgO and high-pressure phases of CaO, CaMgSi_2O_6, and Fe at ultrahigh pressure are obtained by comparing existing shock compression and temperature measurements to 300 K compression curves constructed from ultrasonic elasticity and static compression data. For MgO, α can be represented by: α = ρ_oγ_oC_V(ρ_o/ρ)^(0.5±0.5)/K_T where γ is the Grüneisen parameter, C_V is the constant volume specific heat, K_T is the isothermal bulk modulus, and ρ is the density. Using this expression, the thermal expansivity of MgO is 28-32×10^(−6)K^(−1) at the pressure of the top of the lower mantle and 10-16×10^(−6)K^(−1) at its base (at...
Impact Jetting of Geological Materials - Yang, Wenbo; Ahrens, Thomas J.
To understand jetting of earth materials, gabbro slabs (5 mm thick) were accelerated to 1.5-2 km sec^(-1) and impacted gabbro (5-10 mm thick), novaculite (10 mm thick), and porous sandstone (12 mm thick) targets at inclination angles of 30°-60°. The ejecta were collected using a catcher box filled with styrofoam and the particles are extracted using chloroform. Jetting angles are determined by the relative positions of the target and the crater produced by the ejecta. The mass of the ejected particles per unit area (∼50 mg cm^(-2)) of the impactor remains almost independent of the impact velocity, inclination angle, thickness...
Comet Shoemaker-Levy 9: Fragment and progenitor impact energy - Takata, Toshiko; Ahrens, Thomas J.; Harris, Alan W.
Initial observational data from the impact of fragments of Comet Shoemaker-Levy 9 (SL9) are compared with smoothed particle hydrodynamic (SPH) calculations to determine their pre-impact diameters and the equivalent diameter of the SL9 progenitor. Diameters (solid ice) of 2.0±0.1, 2.0±0.05, 2.1±0.04 and 1.9±0.05 km for fragments A, E, G1, and W are obtained from impact-induced plume heights from the Hubble Space Telescope (HST) data. Applying these values to scale apparent diameters for the balance of 18 SL fragments in Weaver et al.'s  catalog of 22 objects yields a SL9 progenitor diameter of 5.0±1.8 km. This corresponds to total impact...
Stress wave attenuation in shock-damaged rock - Liu, Cangli; Ahrens, Thomas J.
The velocity and attenuation of ultrasonic stress waves in gabbroic rock samples (San Marcos, California) subjected to shock loading in the 2 GPa range were studied. Prom P wave velocity measurements we determined the damage parameter D_p and crack density ε of the samples and related these to the attenuation coefficient (quality factor) under dynamic strains of 2×10^(−7) and at a frequency of 2 MHz using the ultrasonic pulse-echo method. A fit to the data yields the P wave spatial attenuation coefficient at a frequency of 2 MHz, α_p(D_p) = 1.1 + 28.2D_P (decibels per centimeter). From the relation between...
Electrical conductivity of ion-irradiated carbon - Döbeli, M.; Jones, T. J.; Lee, A.; Livi, R. P.; Tombrello, T. A.
Amorphous carbon films have been irradiated with Cl-ions with energies between 1 and 40 MeV, and the electrical conductivity of the material has been measured as a function of the ion dose. The room temperature conductivity is increased by nearly three orders of magnitude and saturates at a dose of about 10^(15)cm^(−2). The rate of conductivity change vs the ion energy can be explained by an ion track model. The temperature dependence of the conductivity between 100 and 300 K at low doses is in accordance with variable range hopping with a temperature exponent of 1/2. Hopping sites are assumed...
Continuum Modeling of Phase Transitions in Solids - Knowles, James K.
In this paper, we describe a simple phenomenological thermoelastic model for stress-induced
solid-solid phase transitions in a tensile bar. In this model, the bar is treated as a
one-dimensional continuum, and the phase transition is assumed to take place quasi-statically
and isothermally at a temperature θ. By accounting for temperature effects solely in the
kinetics of the phase transition, we show that some of the qualitative features of the experimental
observations in  can be predicted, even though the effects of temperature on the
nucleation of the transition and on the stress-strain relation have been neglected. A purely
mechanical counterpart of the theory described here has been...
Nitrogen detection by means of a broad resonance at 9.3 MeV in ^(14)N(α,α)^(14)N elastic backscattering - Qiu, Yuanxun; Rice, A. P.; Tombrello, T. A.
Because of recent interest in using high energy helium ion backscattering to measure the nitrogen content in heavy substrates, we have determined the differential cross sections for the ^(14)N(α, α)^(14)N non-Rutherford elastic scattering in the energy region 9.1–9.6 MeV at a laboratory scattering angle of 177°. A broad (Γ ∼ 200 keV) resonance around 9.3 MeV was found. The cross section was ∼ 480 mb/sr, about 190 times higher than Rutherford scattering. The resonance, allowing a depth resolution of 40–50 nm, has been used to probe an implanted nitrogen layer in stainless steel and a thick (∼ 4.5 μm) TiN...
Molecular Dynamics Simulations of Cluster Impacts on Metallic Targets - Shapiro, Mark H.; Tombrello, T. A.
A modified version of the multiple-interaction code SPUT2 has been used to simulate impacts of 63-atom Cu clusters on six–layer Cu targets. Simulations were carried out with cutoff times of 100 and 500 fs for an incident cluster energy of 63 keV (1 keV/atom). Significant enhancements were observed in the maximum potential and kinetic energies achieved in the early phase of the collision cascade. Some hard collisions yielded atoms with potential energies as high as 925 eV (in the CM frame). This is almost twice the energy allowed in an isolated two-body collision. The number of hard collisions per time-step...
Shock-induced temperatures of CaMgSi_2O_6 - Svendsen, Bob; Ahrens, Thomas J.
Optical radiation from CaMgSi_2O_6 crystal (diopside) shock-compressed to 145–170 GPa yields shock-induced temperatures of 3500–4800 K, while that from CaMgSi_2O_6 glass, with a density 86% that of CaMgSi_2O_6 crystal, shock-compressed to 96–98 GPa, yields shock-induced temperatures of 3700–3900 K. The observed radiation histories of of the targets containing CaMgSi_2O_6 crystal and glass imply that the shock-compressed states of both are highly absorptive, with effective absorption coefficients of ≥ 500–1000 m^(−1). Calculated shock-compressed states for both CaMgSi_2O_6 crystal and glass, when compared to experimental results, imply the presence of a high-pressure phase (HPP) along both Hugoniots over the respective pressure ranges....
Hypervelocity Impacts and Magnetization of Small Bodies in the Solar System - Chen, Guangqing; Ahrens, Thomas J.; Hide, Raymond
The observed magnetism of asteroids such as Gaspra and Ida (and other small bodies in the solar system including the Moon and meteorites) may have resulted from an impact-induced shock wave producing a thermodynamic state in which iron-nickel alloy, dispersed in a silicate matrix, is driven from the usual low-temperature, low-pressure, α, kaemacite, phase to the paramagnetic, epsilon (hcp), phase. The magnetization was acquired upon rarefaction and reentry into the ferromagnetic, α, structure. The degree of remagnetization depends on the strength of the ambient field, which may have been associated with a Solar-System-wide magnetic field. A transient held induced by...
Scaling craters in carbonates: Electron paramagnetic resonance analysis of shock damage - Polanskey, Carol A.; Ahrens, Thomas J.
Carbonate samples from the 8.9-Mt nuclear (near-surface explosion) crater, OAK, and a terrestrial impact crater, Meteor Crater, were analyzed for shock damage using electron paramagnetic resonance (EPR). Samples from below the OAK apparent crater floor were obtained from six boreholes, as well as ejecta recovered from the crater floor. The degree of shock damage in the carbonate material was assessed by comparing the sample spectra to spectra of Solenhofen and Kaibab limestone, which had been shocked to known pressures. Analysis of the OAK Crater borehole samples has identified a thin zone of allocthonous highly shocked (10–13 GPa) carbonate material underneath...
Physics of interplanetary dust capture via impact into organic polymer foams - Anderson, William W.; Ahrens, Thomas J.
The physics of hypervelocity impacts into foams is of interest because of the possible application to interplanetary dust particle (IDP) capture by spacecraft. We present a model for the phenomena occurring in such impacts into low-density organic polymer foams. Particles smaller than foam cells behave as if the foam is a series of solid slabs and are fragmented and, at higher velocities, thermally altered. Particles much larger than the foam cells behave as if the foam were a continuum, allowing the use of a continuum mechanics model to describe the effects of drag and ablation. Fragmentation is expected to be...
A model for “dynamic roughness” in turbulent channel flow - McKeon, B. J.
A simple model for roughness elements with a time-varying height was used to investigate the effect of time-dependent, “dynamic” roughness on wall-bounded flow. Temporally varying wall velocities were specified in a turbulent channel flow simulation in order
to model the effect of introducing a roughness time scale in addition to a distribution of
roughness length scales.
Emplacement of penetrators into planetary surfaces - Anderson, William W.; Ahrens, Thomas J.; Gibson, Andrew; Scott, Ronald; Suzuki, Kojiro
We present experimental data and a model for the low-velocity (subsonic, 0–1000 m/s) penetration of brittle materials by both solid and hollow (i.e., coring) penetrators. The experiments show that penetration is proportional to momentum/frontal area of the penetrator. Because of the buildup of a cap in front of blunt penetrators, the presence or absence of a streamlined or sharp front end usually has a negligible effect for impact into targets with strength. The model accurately predicts the dependence of penetration depth on the various parameters of the target-penetrator system, as well as the qualitative condition of the target material ingested...
Planetary Cratering Mechanics - O'Keefe, John D.; Ahrens, Thomas J.
The objective of this study was to obtain a quantitative understanding of the cratering process over a broad range of conditions. Our approach was to numerically compute the evolution of impact induced flow fields and calculate the time histories of the key measures of crater geometry (e.g. depth, diameter, lip height) for variations in planetary gravity (0 to 10^9 cm/s^2), material strength (0 to 2400 kbar), and impactor radius (0.05 to 5000 km). These results were used to establish the values of the open parameters in the scaling laws of Holsapple and Schmidt (1987). We describe the impact process in...
Impact-induced tensional failure in rock - Ahrens, Thomas J.; Rubin, Allan M.
Planar impact experiments were employed to induce dynamic tensile failure in Bedford limestone. Rock discs were impacted with aluminum and polymethyl methacralate flyer plates at velocities of 10 to 25 m/s. This resulted in tensile stresses in the range of ∼11 to 160 MPa. Tensile stress durations of 0.5 and 1.3 μs induced microcrack growth which in many experiments were insufficient to cause complete spalling of the samples. Ultrasonic P and S wave velocities of recovered targets were compared to the velocities prior to impact. Velocity reduction, and by inference microcrack production, occurred in samples subjected to stresses above 35...