Attaining the rate-independent limit of a rate-dependent strain gradient plasticity theory

The existence of characteristic strain rates in rate-dependent material models, corresponding to rate-independent model behavior, is studied within a back stress based rate-dependent higher order strain gradient crystal plasticity model. Such characteristic rates have recently been observed for steady-state processes, and the present study aims to demonstrate that the observations in fact unearth a more widespread phenomenon. In this work, two newly proposed back stress formulations are adopted to account for the strain gradient effects in the single slip simple shear case, and characteristic rates for a selected quantity are identified through numerical analysis. Evidently, the concept of a characteristic rate, within the rate-dependent material models, may help unlock an otherwise inaccessible parameter space

Embryonic regulation and post-embryonic function of the single-minded gene in the Drosophila central nervous system

The single-minded (sim) gene in Drosophila melanogaster has long been known to play important roles in specifying the mesectodermal cell fate in the embryonic central nervous system (CNS). Mesectoderm cells differentiate into CNS midline cells by mid-embryogenesis. CNS midline cells contribute both neurons and glia to the developing nervous system, and are a source of both attractive and repulsive axonal guidance cues that combinatorially pattern the bilateral CNS. Removal of sim function leads to the failure of midline cell formation, and concomitantly a lack of instructive signal presentation to pathfinding axons from the lateral CNS. As a result, commissural axon tracts that cross the midline are largely absent and parallel longitudinal axon tracts that flank the midline appear fused as a single connective at the embryonic mid-plane. Due to this patterning defect, sim mutants are late embryonic lethal. In addition to the CNS midline, Sim can also be found in the developing foregut, posterior terminal structures, and a subset of myoblasts, although its roles in these compartments are less well understood. In collaboration with others, we demonstrated functions for sim in developing posterior terminal structures and gonads, patterning the larval cuticle, organizing the adult brain, and in adult behavior and locomotion. Using the MARCM strategy for positively marking sim mutant cell clones, we demonstrated that in contrast to its role in neurogenesis in the CNS mesectoderm, sim functions to pattern axon fascicles in the larval central brain, a region known to be important in the interhemispheric communication and the coordination of leg movement. Using RT-PCR, we showed that the sim locus yields a third, previously unidentified transcript that is the primary isoform used post-embryonically. Genetic dissection of inter- and intragenic regions from the sim locus revealed locations of enhancers that drive expression in the CNS midline, myoblasts, and foregut. Taken together, these results have broadened our understanding of sim, an important regulator of development with complex regulation

Understanding the Effect of Body Weight on Muscle Activity During Unilateral Hopping

Running can be described as a series of bilateral single-legged jumps and landings. Interestingly, single-legged jumps and landings are qualitatively more difficult unilaterally than bilaterally known as bilateral deficit syndrome (BDS). For example, one leg will produce less mechanical energy while jump squatting unilaterally than it would produce if both legs worked synchronously. The analysis of dynamic conditions under different levels of body weight support of the hopping movement may prove useful to understanding the bilateral deficit. Purpose: To compare muscle activity of the lower extremity during single-legged hopping at different levels of body weight support. Methods: The research study was recently approved by the Institutional Review Board and data collection has begun. Therefore, no data are presented in the abstract at this time. Subjects will be equipped with electromyography (EMG) leads to measure muscle activity of the rectus femoris, semitendinosus, medial gastrocnemius, and tibialis anterior. Subjects will be asked to perform 5 trials of hopping forwards at a preferred speed (PS) for 1 minute at varying levels of body weight (80%, 70%, 60%, 50%, 40%). EMG data of all four muscles will be compared across the separate body weight control conditions

Initial results from the Caltech/DRSI balloon-borne isotope experiment

The Caltech/DSRI balloonborne High Energy Isotope Spectrometer Telescope (HEIST) was flown successfully from Palestine, Texas on 14 May, 1984. The experiment was designed to measure cosmic ray isotopic abundances from neon through iron, with incident particle energies from approx. 1.5 to 2.2 GeV/nucleon depending on the element. During approximately 38 hours at float altitude, 100,000 events were recorded with Z or = 6 and incident energies approx. 1.5 GeV/nucleon. We present results from the ongoing data analysis associated with both the preflight Bevalac calibration and the flight data