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71.
S.R. Beissel C.A. Gerlach G.R. Johnson 《International Journal of Impact Engineering》2006,33(1-12):80-90
The ability of computations to model characteristics of hypervelocity impact is demonstrated using an algorithm for the automatic conversion of distorted finite elements to meshfree particles. The Lagrangian formulation tracks material boundaries and properties without the errors typical in an Eulerian formulation as the material traverses large distances. A computation of a sphere impacting a bumper is shown to reproduce three regions in the debris cloud that are observed in tests: a front region composed of droplets of melted projectile and target, a middle region of fragmented projectile, and a back region of spalled projectile. Additional computations reproduce the observed traits that result from variations in the projectile shape and obliquity. The computation of a projectile impacting spaced plates demonstrates the ability of the method to model the damage to the rear plate of a Whipple shield for spacecraft protection. 相似文献
72.
Allen M. Johnson Michael A. Schoenfelder David J. Lebold 《Quality and Reliability Engineering International》1993,9(1):55-62
The Rainbow net simulation technique is applied to modelling the impact of system load and fault handling on the availability of a fault-tolerant multiprocessor architecture. Rainbow nets are described along with the motivation for creating this modelling technique. A Rainbow net fault-handling model is created for the fault-tolerant multiprocessor architecture and the topology is shown to remain constant in size, independent of the number of processor, memory and I/O elements configured in the system. Simulation is performed with a varying load in terms of the number of active jobs the system must support. Results are given showing how the fault-tolerant capability varies with load. Two new metrics for evaluating fault tolerance are introduced; namely full fault-tolerability and partial fault-tolerability. They are based on simple observations in the model. 相似文献
73.
A series of polyesters was prepared to evaluate hydrolytic stability as a function of cyclohexyl dibasic acid content. The
three cyclohexyl dibasic acids: 1,2; 1,3; and 1,4 were formulated into polyesters with two glycols. The proportion of cis
and trans isomers was evaluated via 1H NMR. The hydrolytic stability of short chain polyesters was evaluated in an acetone/water mixture which solubilized the
polyesters to mimic oligoester behavior within a thermosetting polyester coating environment. The rate of hydrolysis was monitored
by acid titration and corroborated by GPC. Surprisingly, 1,2-cylohexyl diacid-based polyesters were robust, and 1,3-cyclohexyl
diacid-based polyesters were the most susceptible to hydrolysis. Evidently, a 1,2-anchimeric effect for cyclohexyl dibasic
acid polyesters was not an important consideration, while the 1,3-cyclohexyl ester interaction was. Consequently, an anomeric
effect was proposed.
Presented at the 81st Annual Meeting of the Federation of Societies for Coatings Technology, November 12–14, 2003, in Philadelphia,
PA. 相似文献
74.
J. Johnson 《Artificial Life and Robotics》2003,7(1-2):16-21
Popular interest in robotics has increased astonishingly in the last few years. Robotics is seen by many as offering major new benefits in education at all levels. Before rushing to exploit this popularity, educators should ask serious questions about the universality and longevity of the robotics phenomenon. Is it a fashion? To be useful, the energy released by robotics must be sustained and universal, and the means of exploiting it must be systematic. Universities define their own robotics curriculum, but most schools lack both the resources and the freedom to do this, and must work with a national curriculum. If it can be shown that robotics has sustained potential in education, it seems inevitable that new ways need to be found to integrate it into the school curriculum. 相似文献
75.
76.
F. G. Johnson B. L. Olmsted Samuel Chen G. W. Wicks 《Journal of Electronic Materials》1993,22(3):331-334
The composition profile of an (AlAs)1/2(GaAs)1/2 tilted superlattice is characterized for the first time. The tilted superlattice sample is thermally disordered, and the
energy of the direct band gap photoluminescence peak is measured as a function of increasing layer interdiffusion. The shift
in the photoluminescence peak energy after completely disordering the tilted superlattice is 39 meV. A theoretical model is
used to simulate the change in band gap as a function of layer interdiffusion for several composition profiles. The profile
that gives the best fit to the experimental data is chosen. The tilted superlattice composition profile is found to be sinusoidal,
varying from Al0.40Ga0.60As to Al0.60Ga0.40As. 相似文献
77.
78.
Johnson R.A. de la Houssaye P.R. Chang C.E. Pin-Fan Chen Wood M.E. Garcia G.A. Lagnado I. Asbeck P.M. 《Electron Devices, IEEE Transactions on》1998,45(5):1047-1054
This paper reviews the prospects of thin-film silicon-on-sapphire (TFSOS) CMOS technology in microwave applications in the 1-5 GHz regime and beyond and presents the first demonstration of microwave integrated circuits based on this technology, MOSFET's optimized for microwave use, with 0.5-μm optically defined gate lengths and a T-gate structure, have ft values of 25 GHz (14 GHz) and fmax values of 66 GHz (41 GHz) for n-channel (p-channel) devices and have noise figure values below 1 db at 2 GHz, some of the best reported performance characteristics of any silicon-based MOSFET's to date. On-chip spiral inductors exhibit quality factors above ten. Circuit performance compares favorably with that of other CMOS-based technologies and approach performance levels similar to those obtained by silicon bipolar technologies. The results demonstrate the significant potential of this technology for microwave applications 相似文献
79.
SL Barker MJ McNicholas D Kader P Abdon T Adalberth D McGurty DI Rowley CM Walker 《Canadian Metallurgical Quarterly》1998,43(6):400-403
A cohort of 39 patients (28 male, 11 female) that had undergone total meniscectomy as adolescents (mean age 16 years) underwent FISP 3D Magnetic Resonance Imaging at a mean follow up of 30 years. The presence of meniscal tissue was assessed by two independent observers blinded to the operation details. The volume of any meniscal tissue present was calculated. A posterior horn remnant was seen in 57% of medial and 45% of lateral meniscectomy cases. The mean volume of an operated medial meniscal remnant was 0.29 mL compared with a mean volume of 1.15 mL for an intact medial meniscus. The mean volume of an operated lateral meniscal remnant was 0.30 mL compared with 1.07 mL for an intact lateral meniscus. We have shown that the incidence of incomplete excision of the posterior horn is more common after total medial meniscectomy, and that at a mean follow up of 30 years there is no convincing in vivo MRI evidence of long-term meniscal regeneration. 相似文献
80.
Pigmentation phenotypes of variant extension locus alleles result from point mutations that alter MSH receptor function 总被引:2,自引:0,他引:2
LS Robbins JH Nadeau KR Johnson MA Kelly L Roselli-Rehfuss E Baack KG Mountjoy RD Cone 《Canadian Metallurgical Quarterly》1993,72(6):827-834
Coat colors in the chestnut horse, the yellow Labrador retriever, the red fox, and one type of yellow mouse are due to recessive alleles at the extension locus. Similarly, dominant alleles at this locus are often responsible for dark coat colors in mammals, such as the melanic form of the leopard, Panthera pardus. We show here that the murine extension locus encodes the melanocyte-stimulating hormone (MSH) receptor. In mice, the recessive yellow allele (e) results from a frameshift that produces a prematurely terminated, nonfunctioning receptor. The sombre (Eso and Eso-3J) and tobacco darkening (Etob) alleles, which both have dominant melanizing effects, results from point mutations that produce hyperactive MSH receptors. The Eso-3J receptor is constitutively activated, while the Etob receptor remains hormone responsive and produces a greater activation of its effector, adenylyl cyclase, than does the wild-type allele. 相似文献