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751.
Abstract— This paper introduces an analytical procedure for predicting engineering crack formation life of high temperature turbine components. This procedure is based upon elastic-plastic fracture mechanics rather than linear elastic fracture mechanics. State of the art, some experimental results and future developments are discussed. 相似文献
752.
Erkamp RQ Emelianov SY Skovoroda AR O'Donnell M 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》2004,51(5):532-539
In tissue the Young's modulus cannot be assumed constant over a wide deformation range. For example, direct mechanical measurements on human prostate show up to a threefold increase in Young's modulus over a 10% deformation. In conventional elasticity imaging, these effects produce strain-dependent elastic contrast. Ignoring these effects generally leads to suboptimal contrast (stiffer tissues at lower strain are contrasted against softer tissues at higher strain), but measuring the nonlinear behavior results in enhanced tissue differentiation. To demonstrate the methods extracting nonlinear elastic properties, both simulations and measurements were performed on an agar-gelatin phantom. Multiple frames of phase-sensitive ultrasound data are acquired as the phantom is deformed by 12%. All interframe displacement data are brought back to the geometry of the first frame to form a three-dimensional (3-D) data set (depth, lateral, and preload dimensions). Data are fit to a 3-D second order polynomial model for each pixel that adjusts for deformation irregularities. For the phantom geometry and elastic properties considered in this paper, reconstructed frame-to-frame strain images using this model result in improved contrast to noise ratios (CNR) at all preload levels, without any sacrifice in spatial resolution. From the same model, strain hardening at all preload levels can be extracted. This is an independent contrast mechanism. Its maximum CNR occurs at 5.13% preload, and it is a 54% improvement over the best case (preload 10.6%) CNR for frame-to-frame strain reconstruction. Actual phantom measurements confirm the essential features of the simulation. Results show that modeling of the nonlinear elastic behavior has the potential to both increase detectability in elasticity imaging and provide a new independent mechanism for tissue differentiation. 相似文献
753.
H. W. Liu C. Q. Feng H. Tang L. Song K. L. Zhang 《Journal of Materials Science: Materials in Electronics》2004,15(8):495-497
In this paper, spinel LiMn2–x
In
x
O4 powder were prepared by rheological phase reaction method with the CH3COOLi·2H2O, (CH3CO2)2Mn·4H2O, and In2O3. The structures were characterized by X-ray diffraction and the electrochemical cycling performance were also observed. The results showed that the spinel LiMn2–x
In
x
O4 were pure phase with very excellent performance. As a cathode material for lithium secondary batteries, LiMn1.98In2O4 had a high initial discharge-capacity with 132.42 mAh/g and a good cyclic reversibility with 98.9%. Even after 60 cycles, the discharge capacity still kept 126.29 mAh/g. 相似文献
754.
Koratkar N Modi A Kim J Wei BQ Vajtai R Talapatra S Ajayan PM 《Journal of nanoscience and nanotechnology》2004,4(1-2):69-71
The influence of electric fields on carbon nanotubes is experimentally demonstrated. Alignment of nanotubes along field lines, directed motion of nanotubes between electrodes separated by several thousand micrometers, and impressive solid-state actuation behavior of nanotube-embedded structures are demonstrated, taking into account the polarization and charging of the nanotubes. These effects are reported for long strands of nanotubes, nanotubes dispersed on substrates, and nanotube-embedded polymer strips. The relative magnitude of the field responsible for polarization and directed motion was found to be dependent on the morphology of the nanotubes used. These observations may foreshadow novel electromechanical applications for nanotube elements. 相似文献
755.
We study electron wave packet propagation and scattering in ZnO nanorods. By solving the time-dependent three-dimensional Schr?dinger equation, we were able to describe propagation and dynamics of the scattering process of the wave packet by ionized impurities. Scattering behavior of the wave packet by an attractive/repulsive scattering center is clearly demonstrated. 相似文献
756.
Wells JC Noid DW Sumpter BG Wood RF Zhang Q 《Journal of nanoscience and nanotechnology》2004,4(4):414-422
Several first-principles surface and bulk electronic structure calculations relating to the nucleation and growth of single-wall carbon nanotubes are described. Density-functional theory in various forms is used throughout. In the surface-related calculations, a 38-atom Ni cluster and several low-index Ni surfaces are investigated using pseudopotentials and plane-wave expansions. The energetic ordering of the sites for C atom adsorption is found to be the same, with the Ni(100) facet favored. The bulk diffusion coefficient of C in Ni as a function of cluster size and temperature is calculated from various molecular dynamics approaches. In another group of bulk-related calculations, Gaussian orbital basis sets are used to study a cluster or "flake" containing 14 C atoms. The flake is a segment of three hexagons from an "unrolled" carbon nanotube, with an armchair termination. The binding energies of C, Ni, Co, Fe, Cu, and Au atoms to it were calculated in an effort to gain insight into the mechanism for the high catalytic activity of Ni, Co, and Fe and the lack of it in Cu and Au. The binding energies of Cu and Au are about 1 eV less than those of the three catalytic elements. Similar methods are used to study the initial stages of nanotube growth within the context of classical nucleation theory. Finally, issues relating to the establishment of a fundamental catalytic mechanism are addressed. 相似文献
757.
We derive limit values of high-order derivatives of the Cauchy integrals, which are extensions of the Plemelj-Sokhotskyi formula. We then use them to develop the Taylor expansion of the logarithmic potentials at the normal direction. Based on the Taylor expansion and numerical integration methods for weekly singular functions using grid points, we design fast algorithms for computing the logarithmic potentials. We prove that these methods have an optimal order of convergence with a linear computational complexity. Numerical examples are included to confirm the theoretical estimates for the methods. 相似文献
758.
Microcantilever-based biosensors have been found increasing applications in physical, chemical, and biological fields in recent years. When biosensors are used in those fields, surface stress and mass variations due to bio-molecular binding can cause the microcantilever deform or the shift of frequency. These simple biosensors allow biologists to study surface biochemistry on a micro or nano scale and offer new opportunities in developing microscopic biomedical analysis with unique characteristics. To compare and illustrate the influence of the surface stress on the frequency and avoid unnecessary and complicated numerical solution of the resonance frequency, some dimensionless numbers are derived in this paper by making governing equations dimensionless. Meanwhile, in order to analyze the influence of the general surface stress on the frequency, a new model is put forward, and the frequency of the microcantilever is calculated by using the subspace iteration method and the Rayleigh method. The sensitivity of microcantilever is also discussed.The supports from the Key Project from the Chinese Academy of Sciences (No. KJCX2-SW-L2), projects from the National Natural Science Foundation of China (No. 10225209, No. 19928205, No. 50131160739 and No. 10072068), and the National 973 project (No. G1999033103) are gratefully acknowledged. 相似文献
759.
Lee KH Cheng MC Chan KC Wong KK Yeung SS Lee KC Ma QY Yang ES 《IEEE transactions on bio-medical engineering》2004,51(11):2024-2030
A high-temperature superconductor (HTS) was used on magnetic resonance imaging (MRI) receiver coils to improve image quality because of its intrinsic low electrical resistivity. Typical HTS coils are surface coils made of HTS thin-film wafers. Their applications are severely limited by the field of view (FOV) of the surface coil configuration, and the improvement in image quality by HTS coil is also reduced as the ratio of sample noise to coil noise increases. Therefore, previous HTS coils are usually used to image small in vitro samples, small animals, or peripheral human anatomies. We used large-size HTS coils (2.5-, 3.5-, and 5.5-in mean diameter) to enhance the FOV and we evaluated their performance through phantom and human MR images. Comparisons were made among HTS surface coils, copper surface coils, and cool copper surface coils in terms of the signal-to-noise ratio (SNR) and sensitivity profile of the images. A theoretical model prediction was also used to compare against the experimental result. We then selected several human body parts, including the wrist, feet, and head, to illustrate the advantage of HTS coil over copper coil when used in human imaging. The results show an SNR gain of 200% for 5.5-in HTS coil versus same size copper coils, while for 2.5- and 3.5-in coils it is 250%. We also address the various factors that affect the performance of large size HTS coils, including the coil-to-sample spacing due to cryogenic probe and the coil-loading effect. 相似文献
760.
Chau Q Lahaye T Ménard S Donadille L Bolognese T Rannou A 《Radiation protection dosimetry》2004,110(1-4):297-300
Tissue-equivalent proportional counters (TEPC) allow the measurements of the absorbed dose and the ambient dose equivalent for neutron fields. A device based on this approach, called NAUSICAA((1,2)), has already been developed by IRSN to be used in high energy neutron fields for space applications. The response of this detector underestimates significantly the dose equivalent at low energies (several hundred keV) which represent the major component of neutron fields at workplaces in the nuclear industry. A counter with a similar geometry (cylindrical detector) and a lower gas pressure was studied in order to simulate a 1 microm biological site. In 2003, the performance of the device was further improved by adding a small amount of 3He to the tissue-equivalent gas (propane based) in order to increase the response for the lower energies of neutrons. Three amplification circuits were used to cover lineal energy range from 10(-1) to 10(4) keV microm(-1). Tests were performed in monoenergetic neutron and source fields. This paper presents the experimental results obtained with this change. 相似文献