Kevin M. Crosby

Case-study experiments in the introductory physics curriculum

Carthage College added inquiry-based case study activities to the traditional introductory physics laboratory. Student teams designed, constructed, and executed their own experiments to study real-world phenomena, through which they gained understanding both of physic principles and methods of physics research. Assessment results and student feedback through teacher evaluations indicate that these activities improved student attitudes about physics as well as their ability to solve physics problems relative to previous course offerings that did not include case study.

Simulations of tensile fracture in thin films bonded to solid substrates

Abstract We perform simulations of crack growth in a brittle-elastic two-dimensional film bonded to a rigid substrate and subject to isotropic tensile stress. The motion of the film sites is dissipative and we consider both purely dissipative dynamics and dynamics with finite dissipation. In both cases, we find a threshold stress for crack growth below which the lattice is stable against further breaking. Effective fractal dimensions are computed for crack clusters nucleated by a single defect in an otherwise undamaged lattice with both finite and infinite dissipation. When the dissipation is infinite, the crack clusters have asymptotic fractal dimensions of 2-0 and exhibit a cross-over from a fractal dimension of 1 -0 to a fractal dimension of 2-0 as the stress in the lattice is increased from the threshold. When the dissipation is finite, all crack clusters grown with stress equal to or greater than the threshold stress have fractal dimensions of 2-0. These results contrast with those found in crack mod...

Reduced Pressure Cyclone Separation Studies using Synthetic Lunar Regolith

In order to provide a safe and sustainable astronaut crew environment at a lunar or extraterrestrial outpost, dust mitigation techniques appropriate for the crew vehicle or space environment must be developed. Cyclonic separation is an attractive method because of equipment durability and maintainability. Further dust mitigation studies are required to optimize cyclone separator performance under the reduced air pressures anticipated in crew habitat and air-lock environments. In this paper we examine the collection efficiency of several cyclonic separators under ambient and reduced pressures. Performance testing of a commercial cyclone separator is used as a baseline for comparison with a custom-designed cyclone. Details of the experimental test system along with the design, performance, fabrication, and implementation of a custom cyclone separator are examined. We also describe the customization and implementation of specialized optical diagnostic instrumentation providing particle size, count, and flow data. Test results are compared with cyclone computational flow dynamic modeling to evaluate model parameters and optimize collection efficiency under anticipated flow conditions.

A mean-field theory of athermal martensite growth

Pattern formation in acicular martensite growth has recently been modelled. Numerical simulations of the growth process reveal that the martensite grain size distribution is governed by an unstable fixed point of a renormalization group. This fixed point corresponds to a limit of the model in which the martensite grains nucleate simultaneously and grow with a common finite velocity. We present a mean-field theory of the martensite transformation in this limit and obtain size distributions which agree well with the published simulation data.

Modal Propellant Gauging in Low Gravity

We present results from parabolic flight testing of a low-gravity propellant gauging technology that relies on detection of resonant acoustic mode shifts due to changes in the tanks effective mass as liquid propellant is removed from the tank. The modal propellant gauging (MPG) method is shown to be (1) relatively robust against sloshing, (2) of comparable effective resolution to existing gauging methods for settled propellant, and (3) particularly robust at low fill-fractions where conventional gauging methods are known to be inaccurate. Lab (1-g) measurements of gauging resolution show no more than a 1.0% error for fill-fractions between 10-50% of total tank volume. Data for sloshing liquids in zero-gravity suggest an error of no greater than 1.5% over the same range of fill fractions. An algorithm for the real-time identification of modal response behaviors in zero-g is presented and demonstrated to accurately associate zero-g frequency response functions with the equivalent 1-g FRF, allowing for the automated assignment of fill levels. Gauging applications for small satellite missions and for propellant transfer operations in orbital fuel depot concepts are considered.

Hands-on demonstrations and teaching tools for optics and adaptive optics

A set of demonstrations suitable for use in classrooms at the secondary, undergraduate and graduate level, and for public use in museums and science centers were developed to exhibit basic optics principles, vision science, and adaptive optics techniques. This paper will exhibit these demonstrations, and how they can be used to promote understanding of optics principles in a wide range of applicable areas. Demonstrations include units showing image formation, orientation, and scale; a unit showing 3-dimensional ray tracing through optical systems using a scattering medium and laser diodes; a unit allowing users to directly observe the color sensitivity of their eyes; and a demonstration directly demonstrating wavefront errors, image distortion, and a Shack-Hartmann sensor for wavefront measurement. Together, these bring real-world optical principles into the hands-on regime, and demystify optics principles that are difficult to visualize in three dimensions. These demonstrations are currently in use at the Keck Observatory in Hawaii, at Nauticus in Virginia, at the Yerkes Observatory and at Carthage College in Wisconsin, as well as at several middle and high schools in Illinois and Wisconsin. They have also been integrated into a unit in the Hands on Universe program published by the Lawrence Hall of Science. This work was supported by the National Science Foundation through the Center for Adaptive Optics.

Optical properties and laser damage measurements of inorganic polymer films

Linear phosphorus-nitrogen bases polymers have been synthesized and deposited as thin films from solution on silica or silicon substrates using dip-coating or spin-casting methods. Most materials transmit well into the ultraviolet region of the spectrum and have refractive indices (1.5 to 1.7 at 500 nm) which are controlled by the functionality of chemical substituents introduced at the phosphorus atom. Films exposed to pulsed laser irradiation (1064 nm, 8ns pulsewidth) exhibit damage morphologies ranging from severe cratering to delamination. The chloro-substituted polymer shows a spheroidal surface morphology after exposure to a 25 J/cm{sup 2} pulse suggesting melting or condensation of ablated material. The relative stability of these materials to laser irradiation will be discussed in terms of chemical bond ionicity which is influenced by atom electronegativity and the nature of substituent groups. Preliminary measurements of the non-linear optical response observed in several of these materials (SHG) will also be discussed.