From aid to development partnership

노트 : This publication has been made possible by the generous support of the Korea Foundation and Friends of the Asia Foundation Korea. This is the most frequently used resource during 20140721-20140727

Effects of prestrain on the ductile-to-brittle transition of ice

AbstractThe ductile-to-brittle transition was investigated in prestrained columnar ice at −10 °C. Laboratory-grown specimens of freshwater and saline ice were prestrained under uniaxial across-column compression (to levels from εp = 0.003 to εp = 0.20, at constant strain rates in the ductile regime) and likewise reloaded (at rates from 1 × 10−6s−1 to 3 × 10−2s−1). Prestrain caused solid-state recrystallization as well as damage in the form of non-propagating microcracks. The ductile-to-brittle transition strain rate ε˙D/B increased by a factor of 3–10 after prestrain of εp = 0.035 in both freshwater and saline ice, compared to that of initially undamaged ice of the same type. Additional prestrain had little further effect on ε˙D/B. The results are interpreted within the framework of a model (proposed by Schulson, 1990, and Renshaw and Schulson, 2001) that predicts the transition strain rate based on the micromechanical boundary between creep and fracture processes. Model parameters primarily affected by prestrain were the power-law creep coefficient B (more so than the creep exponent n), Young's modulus E and, by extension, the fracture toughness KIc

The Role of Damage and Recrystallization in the Elastic Properties of Columnar Ice

Effects of damage on elastic properties were studied in columnar-grained specimens of freshwater and saline ice, subjected, at −10°C, to varying levels of inelastic strain. The ice was compressed uniaxially at constant strain rates up to 0.20 strain, which caused localized recrystallization and imparted damage in the form of non-propagating cracks. Damage was quantified in terms of dimensionless crack density, which, along with recrystallized area fraction, was determined from thin sections. The change in porosity due to stress-induced cracks served as another indicator of damage. Elastic properties were derived using P-wave and S-wave ultrasonic transmission velocities measured in across-column directions through the damaged ice, either parallel (x 1) or perpendicular (x 2) to the initial loading direction. In general, as damage increased with greater strain, the ice became more compliant and (particularly freshwater ice) more anisotropic. Furthermore, with increasing strain rate, the magnitude of these effects and crack density tended to increase, in contrast to the recrystallized area fraction, which tended to decrease. We observed compliance to correspond closely with porosity and with dimensionless crack density, for strains up to 0.10. At greater levels of strain these correspondences became less clear due, in part, to the different character of the damage

Role of The Cortex in Visuomotor Control of Arm Stability

Whereas numerous motor control theories describe the control of arm trajectory during reach, the control of stabilization in a constant arm position (i.e., visuomotor control of arm posture) is less clear. Three potential mechanisms have been proposed for visuomotor control of arm posture: 1) increased impedance of the arm through co-contraction of antagonistic muscles, 2) corrective muscle activity via spinal/supraspinal reflex circuits, and/or 3) intermittent voluntary corrections to errors in position. We examined the cortical mechanisms of visuomotor control of arm posture and tested the hypothesis that cortical error networks contribute to arm stabilization. We collected electroencephalography (EEG) data from 10 young healthy participants across four experimental planar movement tasks. We examined brain activity associated with intermittent voluntary corrections of position error and antagonist co-contraction during stabilization. EEG beta-band (13–26 Hz) power fluctuations were used as indicators of brain activity, and coherence between EEG electrodes was used as a measure of functional connectivity between brain regions. Cortical activity in the sensory, motor, and visual areas during arm stabilization was similar to activity during volitional arm movements and was larger than activity during co-contraction of the arm. However, cortical connectivity between the sensorimotor and visual regions was higher during arm stabilization compared with volitional arm movements and co-contraction of the arm. The difference in cortical activity and connectivity between tasks might be attributed to an underlying visuomotor error network used to update motor commands for visuomotor control of arm posture

Electric Arc and Electrochemical Surface Texturing Technologies

Surface texturing of conductive materials can readily be accomplished by means of a moving electric arc which produces a plasma from the environmental gases as well as from the vaporized substrate and arc electrode materials. As the arc is forced to move across the substrate surface, a condensate from the plasma re-deposits an extremely rough surface which is intimately mixed and attached to the substrate material. The arc textured surfaces produce greatly enhanced thermal emittance and hold potential for use as high temperature radiator surfaces in space, as well as in systems which use radiative heat dissipation such as computer assisted tomography (CAT) scan systems. Electrochemical texturing of titanium alloys can be accomplished by using sodium chloride solutions along with ultrasonic agitation to produce a random distribution of craters on the surface. The crater size and density can be controlled to produce surface craters appropriately sized for direct bone in-growth of orthopaedic implants. Electric arc texturing and electrochemical texturing techniques, surface properties and potential applications will be presented

Home-Based Programs: Recommendations to Improve Program Quality

Recommendations for practices that strengthen early childhood programs are proposed based on findings from two studies. The Family Day Care Rating Scale is widely used to assess program quality. A total of 223 home-based programs in Missouri were observed for these studies and provide the basis for information in this recommended practices document. Items from the Environment Rating Scale that were consistently rated as below average were divided into two program components: health and safety issues and programming. Specific recommendations for improving programs are outlined

Preschool Classrooms: Recommendations to Improve the Quality of Early Childhood Programs

Recommendations for practices that strengthen early childhood programs are proposed based on findings from two studies. The Early Childhood Environment Rating Scale-Revised is widely used to assess preschool classrooms. A total of 191 preschool classrooms in Missouri were observed for these studies and provide the basis for information in this recommended practices document. Items from the Environment Rating Scale that were consistently rated as below average were divided into two program components: health and safety issues and programming/curriculum. Specific recommendations for improving programs are outlined. are outlined

Infant and Toddler Programs: Recommendations for a Stimulating Learning Environment

Recommendations for practices that strengthen early childhood programs are proposed based on findings from two studies. The Infant-Toddler Environment Rating Scale-Revised is widely used to assess infant a toddler classrooms. A total of 102 infant and toddler classrooms in Missouri were observed for these studies and provide the basis for information in this recommended practices document. Items from the Environment Rating Scale that were consistently rated as below average were divided into three program components: room/furniture design, displayed materials, and play materials. Specific recommendations for improving programs are outlined

Infant and Toddler Programs: Recommendations for Quality Child-Teacher Interactions

Recommendations for practices that strengthen early childhood programs are proposed based on findings from two studies. In one study, researchers in the Midwest Child Care Research Consortium conducted a study to assess program quality. In the second study, observations were conducted using the Environment Rating Scales as part of the Missouri Quality Rating System Pilot Study. The Infant-Toddler Environment Rating Scale-Revised is widely used to assess infant and toddler classrooms. Items from the Environment Rating Scale that were consistently rated as below average are divided into four program components: displayed materials, block play, pretend play, and free play. Specific recommendations for improving programs are outlined