Showing papers by "Northwestern Polytechnical University published in 2000"

Numerical methods for solving the optimal problem of contrast enhancement

Abstract: This paper proposes two effective numerical methods to solve the optimal problem of contrast enhancement for the coherent and incoherent cases, respectively. For the coherent case, the objective function of the optimal problem is transformed into a bilinear form. Then a numerical method is presented by using the idea of the Sequential Unconstrained Minimization Technique (SUMT). For the incoherent case, a cross-iterative method is proposed for solving the optimal problem of contrast enhancement, based on the formula of the optimal contrast polarization state in the matched-polarized channel. Both the proposed methods are convergent and straightforward for programming. In addition, the proposed methods can be used for the bistatic radar case because in this paper, it is unnecessary to restrict the symmetry of the scattering matrix and the Kennaugh matrix. For showing the effectiveness of the proposed methods, the authors give three examples. The results of the calculation are completely identical with other papers', showing the validity of the proposed methods. In these three examples, if the receiving polarization state is independent of the transmitting's, the power ratios may be much larger than those in the cases of the co-polarized, the cross-polarized, and the matched-polarized channels, showing the importance of the considered models. In addition, the computation costs are estimated by these illustrative examples, illustrating that the proposed methods are very effective.

Electro-Rheological Multi-layer Squeeze Film Damper and Its Application to Vibration Control of Rotor System

Abstract: In this paper, a new electro-rheological multi-layer squeeze film damper (ERMSFD in short) is designed first and the constitutional Reynolds equation is established. Then the behavior of the rotor system is analyzed, the vibration around the first critical speed is suppressed and an on/off control is proposed to control the large amplitude around the first critical speed. A control method is used to suppress the sudden unbalance response. Finally, experiments are carried out to investigate the behavior of the rotor system to prove the effectiveness of the ER damper to suppress the vibration around the critical speed and the sudden unbalance response.

Microstructure and oxidation resistance of carbon/silicon carbide composites infiltrated with chromium silicide

Abstract: In order to improve the mechanical properties and oxidation resistance of three-dimensional carbon fiber-reinforced silicon carbide composites prepared by Low-pressure chemical vapor infiltration (LCVI), electrodeposition chromium combined with silicon melt infiltration method (CSI) is developed for filling the pores of CVI C/SiC composites with chromium silicide. By this method, not only the large pores between bundles but also the small pores between fibers in a bundle can be filled. In the chemical vapor infiltration process, pyrolytic carbon interfacial layer and silicon carbide matrix layer are deposited on the surface of the carbon fiber. The open porosity of as-received CVI C/SiC is as high as 20%. Then the porous composite is infiltrated with chromium. Finally, the composite is infiltrated with silicon melt to react with chromium in the pores, thereby chromium silicide is in-situ formed and C/SiC–Cr 3 Si composites with open porosity of 5% is obtained. Compared with CVI C/SiC composites, C/SiC–Cr 3 Si composites exhibit better mechanical property and oxidation resistance. The room-temperature strength of the C/SiC–Cr 3 Si is 586MPa, much higher than that of the CVI C/SiC. After the CVI C/SiC–Cr 3 Si composite is exposed in the flame of burner rig for 20 h at 1300°C, the flexural strength of composite decreases only by 4.3%.

Prediction of intrinsic fracture toughness for brittle materials from the apparent toughness of notched-crack specimen

Abstract: In this investigation, fracture process zone model is used to establish a new relationship to predict the intrinsic fracture toughness from the apparent fracture toughness of a notched-crack specimen. The parameters needed in the proposed model are very rare, such as, the fracture process zone size of materials, the notch radius. Specimens made up of two kinds of polycrystalline alumina and one soda-lime glass with notch radii as small as a few micrometer are used to verify the predictions of this model. Besides, the results also show that fracture toughness of ceramics decreases with the decreasing of notch root radius. Under condition of the radius of crack tip is not greater than the averaged grain size, the apparent toughness can be approximately regarded as the fracture toughness of the materials.

Study of workability limits of porous materials under different upsetting conditions by compressible rigid plastic finite element method

Abstract: Workability limits must be considered when designing powder metallurgy (PM) forging processes. This research successfully applied the general upsetting experiment method to the deformation of porous materials. Based on the plastic theory of porous materials, the compressible rigid plastic finite element method is used to simulate the deformation processes of cold upsetting of disks and rings for porous metal materials with a full account of contact friction boundary conditions, the height-to-diameter ratio, the initial relative density, and the die and workpiece geometry. Furthermore, a successful analysis of the cold forging process results in the prediction of the stress, the strain, and the density field. By coupling with the ductile fracture criterion, which is a strain-based criterion obtained by Lee and Kuhn, possible defects leading to material failure have been checked. This research reveals that larger height to diameter and a lesser friction factor can delay the local strain locus to intersect with the Lee and Kuhn’s fracture line and restrain formation of the surface crack. Meanwhile, it reveals that the initial relative density has only a very small influence on the strain to fracture in compression, and it shows the forming behavior of the ring and disk with the curved die. According to Lee and Kuhn’s results, the calculated results agree well with the experimental results.

Oxidation behavior of three-dimensional SiC/SiC composites in air and combustion environment

Abstract: A three-dimensional Hi-Nicalon/SiC composite was prepared, the mechanical properties and weight changes before and after oxidation in air and combustion environment were investigated and compared in the present paper. The combustion gas with a high flow rate and a larger amount of H 2 O accelerated the silica formation on the fibers and matrix, the oxidation channel could be sealed much earlier in combustion than in air, and the PyC interlayer was oxidized less. Weight loss of the composite in air was larger than that in combustion. The flexural strength did not decrease remarkably, in both air and combustion. Oxidation took place along and weakened the two interfaces of the interlayer with the fibers and matrix besides removal of the interlayer, the fracture displacement was greatly increased after oxidation in combustion compared with those before and after oxidation in air. It was shown that the composite had an excellent oxidation resistance in a short period of exposure. This result was of very important significance in the development of a self-adaptable interlayer.

Optical Second-Harmonic Generation in Benzophenone

Abstract: Introduction Versatile and efficient sources of blue light are of fundamental importance for various applications including optical data storage [1,2], ophthalmologic or other medical techniques [3], and laser displays [4 to 6]. In spite of the rapid development of blue laser diodes [7,8] and concurrent physical principles like optical upconversion[9], the optical second-harmonic generation [10] (SHG) still is one of the most important methods to achieve intense coherent blue light with a good optical beam quality [11]. From symmetry aspects, SHG is only possible in crystals belonging to one of the 20 point groups which lack a center of symmetry [12]. Furthermore, to meet phase matching aspects in homogeneous crystals, refractive indices for the fundamental and the generated harmonic wavelength must be of identical size [13,14], optical birefringence is necessary. Here we discuss the properties of benzophenone (C 13 H 10 O), an organic crystal, which excellently meets the described conditions. Crystal Properties Crystals of benzophenone can be grown from the melt (melting point approximately 48°C) [15]. The crystal structure is orthorhombic with point symmetry 222. This forces the material to be optically biaxial with the axes of the indicatrix fixed to the three mutually perpendicular symmetry axes of the crystal. Theoretical studies revealed - due to the conjugated bonds - a good second order nonlinear optical polarizability [16]. Optical Absorption For optical applications in general - and especially for SHG - the material considered must be transparent in the wavelength region of interest. Measurements of the optical absorption in benzophenone show a region of excellent transparency spanning from 390 to 1700 nm. Depending on light polarization - parallel or perpendicular to the crystallographic c-axis - two typical spectra are found. The experimental results are sketched in Fig. 1.

The influence of microstructure on dwell sensitive fatigue in Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy

Abstract: The fatigue behaviour of titanium alloy Ti–6.5Al–3.5Mo–1.5Zr–0.3Si (TC11) was examined at 520°C to study the effects of microstructural variation on the dwell sensitivity. Three microstructures (equiaxed, tri-modal and basketweave) were used in this study. When a 3-min dwell time was imposed at the peak of each cycle a significant fatigue life reduction was observed for all microstructures tested. Among the three microstructures, equiaxed microstructure showed the strongest fatigue life reduction. The basketweave microstructure had a little higher dwell-time fatigue life than tri-modal microstructure at low maximum stress levels. In all cases, extensive quasi-cleavage facets and planar slips with track-like dislocations have been intimately linked with the dwell sensitive fatigue response. The amount of quasi-cleavage facets and planar slips decreased with a decrease of the α phase content. A rationalization for planar slip was proposed based on the mechanism of dislocations shearing α2 particles. It is believed that α2 particle formation and oxidization effects played an important role in dislocation planar slip.

Solidification behavior of undercooled Cu70Ni30 alloy melt

Abstract: The solidification behavior of undercooled Cu70Ni30 alloy in cylindrical crucible was investigated. By controlling the nucleation point at the top of the specimen, a directionally solidified dendrite structure in the undercooling range of 90–185 K has been obtained even if no longitudinal temperature gradient existed in the melt prior to nucleation. The dendrite growth velocity as a function of undercooling was measured by high speed cinematography. The results indicate that the dendrite growth velocity increases slowly with increasing undercooling when the directional solidification occurs. However, as the directional solidification breaks down above 185 K, the dendrite growth velocity rises up quickly. It is demonstrated that the breakdown of the directional dendrite growth at high undercoolings results from the destabilization of the dendrite growth, while this destabilization is caused by the solidification contraction stress occurring in the rapid dendrite growth. At higher undercoolings the destabilization even leads to the formation of a number of new crystals, and the recrystallization of the deformed solid during or immediately after solidification can make the grain size decrease further. On the assumption that only the thermal diffusion fields around the dendrites overlap, and that the dendrite tip can be represented by a paraboloid of revolution, a semi-quantitative theoretical model has been established.