Process simulation and fabrication of advanced multi-step three-dimensional braided preforms

The fundamentals of multi-step braiding for the fabrication of three-dimensional fibre preforms for composites have been studied. To facilitate the understanding of the complex multi-step braiding processes, a computer simulation algorithm has been developed. The simulation acts as a tool to allow the identification of individual yarn paths, number and location of yarn groups, and braid geometry. It was found that individual control of the rows and columns of yarn carriers on a Cartesian braiding bed allows for the fabrication of advanced multi-step braids; the micro-structural possibilities of three-dimensional braids are thus greatly extended. Some basic relationships of the braiding parameters have been identified. It has been concluded that the traditional four-step and two-step braidings are special cases of multi-step braidings. To verify the feasibility of the structures, experimental investigations have also been carried out. Innovative braid architectures have been designed and fabricated using a prototype multi-step braiding machine.     

Turbo spin-echo sequences in magnetic resonance imaging of the brain: Physics and applications

Fast SE imaging provides considerable measure time reduction, high signal-to-noise ratios as well as similar contrast behavior compared to conventional SE sequences. Besides TR and TE_eff, echo train length (ETL), interecho time , and-space trajectory determine image contrast and image quality in fast SE sequences. True proton density contrast (CSF hypointense) and not too strong T2 contrast are essential requirements in routine brain MRI. A Turbo SE sequence with very short echo train length (ETL=3), short TE_eff and short interecho time (17 ms), and TR=2000 ms was selected for proton density contrast; a Turbo SE sequence with ETL=7, TE_eff=90 ms, =22 ms, and TR=3250 ms was selected for T₂-weighted images. Using both single-echo Turbo SE sequences yielded 50% measure time reduction compared to the conventional SE technique. Conventional SE and optimized Turbo SE sequences were compared in 150 patients resulting in very similar signal and contrast behavior. Furthermore, reduced flow artifacts in proton density—and especially in T₂-weighted Turbo SE images—and better contrast of high-intensity lesions in proton density-weighted Turbo SE images were found. Slightly reduced edge sharpness—mainly in T₂-weighted Turbo SE images—did not reduce diagnostic reliability. Differences between conventional and Turbo SE images concerning image contrast and quality are explained regarding special features of fast SE technique.Address for correspondence: Institut für Röntgendiagnostik, Klinikum der Universität Regensburg, Franz-Josef-Strauß-Allee 11, 93042 Regensburg, Germany. Additional reprints of this chapter may be obtained from the Reprints Department, Chapman & Hall, One Venn Plaza, New York, NY 10119.     

Impact properties of carbon fiber reinforced linear and short-chain branched polyethylenes

Summary An investigation of the influence of short-chain branched length on impact behavior of linear high density polyethylene (HDPE), short-chain branched polyethylenes (SBPEs) and their fiber reinforced samples is reported. The result shows that the total impact energies (E_t) increased with increasing branch length at any given temperature used in this study. Similar trend was found for their fiber reinforced samples. For a given polyethylene resin, E_t increased with fiber content up to 5%, and then decreased consistently with further increase of fiber content. The amount of E_t improved due to the presence of 5% carbon fibers increased significantly with the branch length at temperatures higher than 25°C. In addition, the fracture surface morphology indicated that the adhesion between carbon fibers and PE resins increased with the branch length. However, the adhesion and the amount of E_t improved due to the presence of 5% carbon fibers reduced significantly with decreasing temperature. Finally, it was found that E_t decreased slightly with rising temperature until the temperature reached around 40°C, and then increased sharply with increasing temperature. It is suggested that this transition behavior is related to the molecular motion accounting for the transition of PE resins.     

Standing waves of gaseous chemical reactions in inert porous media

Standing waves of gaseous, exothermic, chemical reactions in inert porous media are investigated in the two-temperature and one-temperature approximations. A semiinfinite reaction zone method is developed to solve the problem analytically. It is shown that the wave solutions obtained by this method form a large set. Traditional solutions with strong heat transfer at the end of the wave make up but a small fraction of this set.LENIIKhIMMASh, St. Petersburg 193167. Translated from Fizika Goreniya i Vzryva, Vol. 30, No. 4, pp. 11–20, July–August, 1994.     

Influence of alloying elements on the structure and wear resistance of chilled cast iron

To work out the optimal chemical composition of wear-resistant cast iron with a pearlitic metallic matrix that is able to restore the relief of a rough surface, we investigated the properties of cast iron with different composition by the method of single factor experiment.Nizhnii Novgorod Branch of the Central Research Institute of Structural Materials Prometei. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 37–39, June, 1994.     

Observations of and applications for self-imaging in optical fibers

Experimental results of self-imaging in optical fibers are reported along with an analytical model that explains the observations. Some implications for sensor design are discussed.     

Pitchfork bifurcation and flip-flop operation with a coupled nonlinear system

A ring cavity system consisting of two hybrid Michelson interferometers coupled together with feedback is constructed. This system shows spatial bifurcation and can be utilized as an all-optical flip-flop device. We demonstrate experimentally a flip-flop operation by use of only a positive pulse in this system.