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rss_1.0 Recursos de colección

Caltech Authors (93,451 recursos)
Repository of works by Caltech published authors.

Type = Article

Mostrando recursos 1 - 20 de 36,372

1. 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...

2. On an integrodifferential equation arising in a theory of phase transitions in solids - Abeyaratne, Rohan; Knowles, James K.
This note is concerned with some properties of an integrodifferential equation a rising in a continuum model of solid-solid phase transitions.

3. 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...

4. 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...

5. 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...

6. 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 [1995] catalog of 22 objects yields a SL9 progenitor diameter of 5.0±1.8 km. This corresponds to total impact...

7. 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...

8. On the Representation of the Elasticity Tensor for Isotropic Materials - Knowles, James K.
This note provides a different proof of the theorem concerning the representation of the elasticity 4-tensor for isotropic materials. Other relatively recent proofs may be found in [1]-[3].

9. 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...

10. 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...

11. 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....

12. 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...

13. 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...

14. 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...

15. 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...

16. 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...

17. 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...

18. Complex craters: Relationship of stratigraphy and rings to impact conditions - O'Keefe, John D.; Ahrens, Thomas J.
One of the key issues associated with the understanding of large scale impacts is how the observable complex crater structural features (e.g., central peaks and pits, flat floors, ring shaped ridges and depressions, stratigraphic modifications, and faults) relate to the impactor's parameters (e.g., radius, velocity, and density) and the nonobservable transient crater measures (e.g., depth of penetration and diameter at maximum penetration). We have numerically modeled large-scale impacts on planets for a range of impactor parameters, gravity and planetary material strengths. From these we found that the collapse of the transient cavity results in the development of a tall, transient...

19. Tumor Volume Delineation in Head and Neck Cancer With Imaging Modalities: CT, PET, MRI, Compared With Pathological Tumor Volume - Huang, Z.; Feng, Y.; Lo, S.; Ju, A. W.; Yuh, K.; Wang, R.; Grecula, J .C.; Mayr, N. A.; Yuh, W.
Accurate delineation of the gross tumor volume is crucial in radiation treatment planning. It has been reported that the gross tumor volume determined by CT (GTVCT) or MRI (GTVMRI) is larger than that by PET (GTVPET). The tumor volume determined by a standardized uptake value (SUV) has been reported not to correlate well with tumor volume as judged by the surgically resected pathological specimen (GTVPA). In order to more accurately guide radiation therapy, correlation between the pathological tumor volumes and various imaging-based tumor volume delineations is essential to validate the efficacy of these imaging methodologies.

20. Universal spectrum of 2d conformal field theory in the large c limit - Hartman, Thomas; Keller, Christoph A.; Stoica, Bogdan
Two-dimensional conformal field theories exhibit a universal free energy in the high temperature limit T → ∞, and a universal spectrum in the Cardy regime, Δ → ∞. We show that a much stronger form of universality holds in theories with a large central charge c and a sparse light spectrum. In these theories, the free energy is universal at all values of the temperature, and the microscopic spectrum matches the Cardy entropy for all Δ≥c/6 . The same is true of three-dimensional quantum gravity; therefore our results provide simple necessary and sufficient criteria for 2d CFTs to behave holographically...

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