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

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Mostrando recursos 1 - 20 de 3,911

1. 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 [1] 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...

2. 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...

3. Dynamical behaviour of thermoelastic solids undergoing phase transitions - Knowles, J. K.
This paper summarizes some recent work carried out jointly by the author and R. Abeyaratne of MIT on the continuum modeling of the macroscopic effect of stress-induced phase transitions in thermoelastic solids. Attention is focussed on one-dimensional tensile bars composed of two-phase thermoelastic materials, and emphasis is placed on the manner in which the notions of kinetics and nucleation are imported into continuum mechanics from materials science. Although the response to dynamic loading is the principal objective, the predictive capability of the model with respect to quasi-static experiments involving shape-memory alloys is also discussed. In the case of dynamic loading, the potential relevance of this model for...

4. SAMPEX Observations of Geomagnetically Trapped Anomalous Cosmic Rays - Cummings, J. R.; Cummings, A. C.; Mewaldt, R. A.; Selesnick, R. S.; Stone, E. C.; von Rosenvinge, T. T.
We summarize observations of trapped anomalous cosmic rays made with the Mass Spectrometer Telescope (MASn on the polar-orbiting SAMPEX spacecraft during late 1992 and early 1993. MAST observes a trapped population of heavy ions with ~15 MeV/nuc that includes N, 0, and Ne, but very little C, located in a narrow belt at L=2. The characteristics of this radiation belt are generally consistent with those expected from the mechanism proposed by Blake and Friesen for trapping anomalous cosmic rays in the magnetosphere and with COSMOS observations made during the last solar minimum. We discuss the location, composition, and temporal history of...

5. Fragmentation of Relativistic Krypton and Silver Nuclei - Nilsen, B. S.; Waddington, C. J.; Binns, W. R.; Cummings, J. R.; Garrard, T. L.; Klarmann, J.
In 1990 we exposed a wide range of targets to beams of relativistic krypton and silver nuclei at the LBL Bevalac at energies from 0.5 to 1.5 GeV/nucleon. Incoming and fragmented nuclei were detected in an array of ion chambers, Cherenkov detectors, and multiwire proportional chambers. The total and partial cross sections for charge-changing interactions have been measured, with emphasis on those of cosmic ray interest. These cross sections are compared to those from parametric fits.

6. High-Altitude Time-of-Flight Search for Non-Weakly Interacting Dark Matter in Cosmic Rays - McGuire, P.; Bowen, T.; Barbier, L. M.; Christian, E. R.; Golden, R. L.; Kimbell, B.; Labrador, A. W.; Mewaldt, R. A.; Menn, W.; Mitchell, J. W.; Reimer, O.; Schindler, S. M.; Simon, M.; Stochaj, S. J.; Streitmatter, R. E.
Supermassive electrically- or strongly-charged particles might constitute the missing matter in the galaxy. These particles would be non-relativistic, but nevertheless if these hypothetical particles have high enough mass, they might have enough momentum to be detectable below 5 g/cm^2 of atmosphere. We apply the time-of-flight technique to search for these hypothetical slow-moving, supermassive, highly-interacting particles at balloon altitude. We present un-cut histograms and a scatterplot of actual flight data, showing that we can account for most of the events as accidental coincidences. We can reject most (if not all) of these accidental coincidences by requiring a 4-counter coincidence, and by...

7. Energy Dependence of the Fragmentation of UH-Nuclei - Geer, L. Y.; Klarmann, J.; Nilsen, B. S.; Waddington, C. J.; Binns, W. R.; Cummings, J. R.; Garrard, T. L.
The fragmentation of 10.6 GeV/n Au in CH_2. C, Al, Cu, Sn, and Pb targets has been studied using an array of ion chambers, multi-wire proportional counters (MWPC), and Cherenkov counters. Total charge-changing cross sections were found to be monotonically increasing with target charge over cross sections measured and derived from lower energy data. Partial charge-changing cross sections yielding charge changes less than 1O were depressed from those measured at lower energy.

8. Detection Efficiency of the CRIS Scintillating Optical Fiber Hodoscope - Binns, W. R.; Crary, D. J.; Cummings, A. C.; Geer, L. Y.; Klarmann, J.; Lawrence, David J.; Mewaldt, R. A.
Measurements of the detection efficiency of a Scintillating Optical Fiber Trajectory (SOFT) detector for 660 MeV/n Carbon, Boron, Beryllium and Lithium nuclei and for 300 MeV/n Fe nuclei are described. Detection efficiencies of 99.8%, 98.7%, 97.5%, and 93.8% for C, B, Be, and Li respectively were obtained for a single layer of fibers at an angle of incidence of 30°. These fibers had a black coating of extramural absorber (EMA) to optically decouple the fibers. Somewhat better detection efficiencies were obtained for optically coupled fibers (no EMA). The detection efficiencies for iron nuclei were ~99.7%. We have used these results to...

9. Recent Faulting in Northern Turkey - Allen, Clarence R.
The North Anatolian fault of Turkey is almost completely analogous to the San Andreas and other active regional strike-slip faults. Convincing evidence of extensive faulting can be traced continuously from more than 900 km in a broad arc from west of Abant to east of Karhova, except for two areas of en echelon offset near Niksar and Erzincan. Distinctive rift topography and abundant physiographic features of Quaternary faulting such as scarps and abundant physiographic features of Quaternary faulting such as scarps and sag-ponds mark the fault throughout; they are particularly well exposed at Gerede, north of Havza and Ladik, at Destek, and east of Suşehri. Consistent dextralstream...

10. Wave generations from confined explosions in rocks - Liu, C. L.; Ahrens, Thomas J.
In order to record P- and S-waves generated from confined explosions in rocks in the laboratory, a method is developed based on the interactions between incident P- and SV-waves and free-surfaces of rocks. The relations between particle displacements of incident P- and SV-waves, and the strains measured using strain gauges attached on free-surfaces of rocks are analytically derived. P- and SV-waves generated from confined explosions in Bedford limestone are recorded.

11. Shock temperatures and the melting point of iron - Ahrens, Thomas J.; Holland, Kathleen G.; Chen, George Q.
New measurements of the ratio of Fe to LiF and Al_2O_3 anvil thermal diffusivities are used to obtain revised shock temperatures for Fe. New results match Brown and McQueen’s (1) calculations of the temperatures of 5000 and 5800K at the 200 and 243 GPa transitions in Fe. New sound speed measurements along the Hugoniot of γ-Fe, centered at 1573K, demonstrate that this phase melts at ∼70 GPa and ∼2800 K and the γ phase does not occur above ∼93 GPa. At higher pressures, perhaps over the entire pressure range of the Earth’s molten outer core (132 to 330 GPa), the...

12. Shock wave equations of state of chondritic meteorites - Anderson, William W.; Ahrens, Thomas J.
We have obtained shock compression data for Murchison and Bruderheim chondritic meteorites. Data for Murchison suggest that the Hugoniot states are described by a smooth curve to ≥90 GPa, having ρ_0=2.656 Mg/m^3, K_(S0)=24.2±.7 GPa, K′=4.17±.10, and constant γ=1.0. The data for Bruderheim suggest more complicated behavior. A mineral mixture model consistent with the Bruderheim data suggests that the Hugoniot state is a low pressure phase below 25 GPa, with ρ_0=3.555 Mg/m^3, K_(S0)=146 GPa, K′=2.53, and constant ργ=7.11 Mg/m^3; and a high pressure phase above 65 GPa, with ρ_0=4.40 Mg/m^3, K_(S0)=225 GPa, K′=3.25, and constant ργ=7.485 Mg/m^3.

13. Shock compression and release of polycrystalline magnesium oxide - Duffy, Thomas S.; Ahrens, Thomas J.
Release wave profile and Hugoniot equation of state data for low‐porosity polycrystalline MgO are reported to 36 and 133 GPa, respectively. At low pressure, the Hugoniot lies above single‐crystal data, reflecting material strength of 3.1±0.8 GPa. Unloading wave velocities to 27 GPa are consistent with extrapolated compressional sound velocities. The unloading profiles can be fit using a viscoplastic model, but there are significant differences between waveforms for different experimental geometries.

14. Free surface velocity profiles in molybdenum shock compressed at 1400 °C - Duffy, Thomas S.; Ahrens, Thomas J.
The equation of state, constitutive properties and unloading wave velocities of molybdenum have been determined from free surface velocity profiles on samples shock compressed from a 1400 °C initial state. The equation of state of 1400 °C molybdenum agrees with previous streak camera measurements and the combined equation of state between 12 and 96 GPa is: U_S =4.78 (0.02)+1.42 (0.02)u_p . Unloading wave velocities measured between 12 and 81 GPa range from 6.30 to 7.91 km/s and are 4–8% below extrapolated 25 °C compressional velocities. The yield strength, Y, was found to be 0.79–0.94 GPa, compared with values of 1.3–1.6 GPa from ambient‐temperature...

15. Oblique 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/s and impacted inclined gabbro (5–10 mm thick), novaculite (10 mm thick) and porous sandstone (12 mm thick) targets at angles of 30°–60°. The ejecta were collected using a catcher box filled with styrofoam and the particles were extracted using chloroform. The mass of the ejected particles per unit area (∼50 mg/cm^2) remains almost independent of impact velocity, inclination angle, thickness of the target and sample mineralogy and density. Based on this result, we predict that a 500 m diameter asteroid will produce only ∼10^7...

16. Stress wave attenuation in shock damaged rock - Luo, Cangli; Ahrens, Thomas J.
The attenuation of ultrasonic stress waves in samples of gabbroic rock subjected to shock loading in the 11 Gpa range were studied. We determined the damage deficits, D_p, and attenuation coefficients, α_p, for the samples with different damage deficits under dynamic strains of 2×10^(−7) and at frequencies around 2 MHz using ultrasonic pulse-echo method. A fit to the data yields the P-wave spatial attenuation coefficient versus damage deficit: α_p=40.9D_p−30.5D^2_p (db/cm). Basing on the O’Connell-Budiansky theory, the relation between attenuation coefficient and crack density is given. The predictions of α_p from Walsh’s theory agrees well with the experiment results for the samples...

17. Application of shock compression science to Earth and planetary physics - Ahrens, Thomas J.
After the development of shock compression methods for obtaining pressure-density Hugoniot curves, it became clear that these could be applied to both determining the equations-of-state and investigation of polymorphic phase changes in silicate minerals of planetary mantles and crusts, as well as, the iron alloys of the metallic cores of terrestrial planets. These data, when taken with seismological models of the Earth, yield constraints on the composition of the Earth’s mantle and core. Shock data for molten silicates provide a basis for understanding the initial layering of a cooling terrestrial magma ocean. Application of shock-wave data is critical to delineating...

18. Characteristic of dynamic tensile fracture in augite-peridotite - He, Hongliang; Jin, Xiaogang; Jing, Fuqian; Ahrens, Thomas J.
Planar impact experiments were carried out to induce controlled dynamic tensile fracture in augite-peridotite. Samples, backed with PMMA buffer and windows, were impacted with PMMA impactor at velocities of 30 to 160 m/s. This resulted in maximum tensile stresses were in the range of ∼50 to 290 MPa. Spall strength was determined to be ∼58.1 MPa from a particle velocity profile measurement. The spall strength/HEL ratios for augite-peridotite and several other rocks were discussed based on the Griffith’s yield criterion and the experimental measurements.

19. Shock Temperature of Stainless Steel and a High Pressure - High Temperature Constraint on Thermal Diffusivity of Al_2O_3 - Gallagher, Kathleen G.; Bass, Jay D.; Ahrens, Thomas J.; Fitzner, M.; Abelson, J. R.
Time dependent shock temperatures were measured for stainless steel (SS) films in contact with transparent anvils. The anvil/window material was the same as the driver material so that there would be symmetric heat flow from the sample. Inferred Hugoniot temperatures, T_h , of 5800–7500 K at 232–321 GPa are consistent with previous measurements in SS. Temperatures at the film‐anvil interface (T_i ), which are more directly measured than T_h , indicate that T_i did not decrease measurably during the approximately 250 ns that the shock wave was in Al_2O_3 or LiF anvils. Thus an upper bound is obtained for the...

20. Theory of shock magnetization of asteroids Gaspra and Ida - Chen, George Q.; 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, ε (hcp), phase. The magnetization was acquired upon rarefaction and reentry into the ferromagnetic, α, structure. The degree of re-magnetization depends on the strength of the ambient field, which may have been associated with a solar-system-wide magnetic field. A transient field induced by...

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