Showing papers by "Beihang University published in 2000"

Exact phase transitions in random constraint satisfaction problems

Abstract: In this paper we propose a new type of random CSP model, called Model RB, which is a revision to the standard Model B. It is proved that phase transitions from a region where almost all problems are satisfiable to a region where almost all problems are unsatisfiable do exist for Model RB as the number of variables approaches infinity. Moreover, the critical values at which the phase transitions occur are also known exactly. By relating the hardness of Model RB to Model B, it is shown that there exist a lot of hard instances in Model RB.

Cutting forces and surface finish when machining medium hardness steel using CBN tools

Abstract: Cutting forces generated using CBN tools have been evaluated when cutting steel being hardened to 45–55 HRC. Radial thrust cutting force was the largest among the three cutting force components and was most sensitive to the changes of cutting edge geometry and tool wear. The surface finish produced by CBN tools was compatible with the results of grinding and was affected by cutting speed, tool wear and plastic behaviour of the workpiece material.

Fault detection and diagnosis of permanent-magnet DC motor based on parameter estimation and neural network

Abstract: In this paper, fault detection and diagnosis of a permanent-magnet DC motor is discussed. Parameter estimation based on block-pulse function series is used to estimate the continuous-time model of the motor. The electromechanical parameters of the motor can be obtained from the estimated model parameters. The relative changes of electromechanical parameters are used to detect motor faults. A multilayer perceptron neural network is used to isolate faults based on the patterns of parameter changes. Experiments with a real motor validate the feasibility of the combined use of parameter estimation and neural network classification for fault detection and isolation of the motor.

Wear resistance of a laser surface alloyed Ti–6Al–4V alloy

Abstract: Laser surface alloying with gaseous nitrogen was utilized to improve the wear resistance of a Ti–6Al–4V alloy. Wear-resistant composite coatings reinforced by hard TiN dendrites were produced ‘in-situ’ on a substrate of a Ti–6Al–4V alloy. The hardness and wear resistance of the laser alloyed coating under two-body abrasive and block-on-ring full-sliding wear conditions were significantly enhanced.

Effect of Plate Thickness on Impedance of Perforated Plates with Bias Flow

Abstract: A numerical model is presented of the Rayleigh conductivity of a square-edged orie ce in the presence of e nite plate thickness and bias e ow. Particular attention is paid to the evaluation of the effect of e nite plate thickness on its acoustic impedance. In the presence of bias e ow, the acoustic impedance of a kind of perforated plate, which is constructed with square-edged orie ces in a plate of e nite thickness, is calculated from the Rayleigh conductivity on the assumption that the orie ces are well separated. The numerical analysis shows that, although the acoustic resistance increases with the increasing bias e ow speed on most occasions, it decreases in a certain range of low bias e ow speed, and it can even decrease to negative values for certain combinations of Strouhal number and plate thickness. When the bias e ow speed is near zero, increasing plate thickness leads to an increase of the acoustic reactance. However, the acoustic reactance decreases as the bias e ow speed is increasing. As a result, the effect of plate thickness on the acoustic reactance will disappear at high bias e ow speed. Experimental data are also presented. Qualitatively, the experimental and theoretical results are in good agreement.

Fares and tolls in a competitive system with transit and highway: the case with two groups of commuters

Abstract: This paper deals with pricing and modal split in a competitive mass transit/highway system with heterogeneous commuters. Two groups of commuters that diAer in their disutility from travel time, schedule delay and transit crowding, select the transit or auto mode for traveling from a residential area to a workplace. We compare three pricing schemes: the marginal cost-based transit fare with no-toll (called ‘m’ for short), the average cost-based fare with no-toll (‘a’) and marginal cost-based fare with time-invariant toll for subsidizing transit (‘s’), and derive a socially optimal combination of transit fare and road toll which minimizes the total social cost of the competitive system meanwhile ensuring no deficit to the transit side (‘o’). The main findings from the analytical and numerical results are: (1) the ‘o’ policy generates the most total transit usage, then ‘s’, ‘m’ and ‘a’ in order; (2) the total usage of each mode is independent of the demand composition when group 1 uses both modes; (3) the group 2’s aversion to transit crowding does not aAect total transit usage; (4) group 2 has relatively larger welfare gains from some changes in pricing policy, such as changing ‘m’ to ‘s’ or to ‘o’; (5) the a-policy results in the highest total social cost, then ‘m’, ‘s’ and ‘o’ in that order. ” 2000 Elsevier Science Ltd. All rights reserved.

The Use of Kalman Filter and Neural Network Methodologies in Gas Turbine Performance Diagnostics: A Comparative Study

Abstract: The goal of Gas Turbine Performance Diagnostics is to accurately detect, isolate and assess the changes in engine module performance, engine system malfunctions and instrumentation problems from knowledge of measured parameters taken along the engine’s gas path. Discernable shifts in engine speeds, temperatures, pressures, fuel flow, etc., provide the requisite information for determining the underlying shift in engine operation from a presumed nominal state. Historically, this type of analysis was performed through the use of a Kalman Filter or one of its derivatives to simultaneously estimate a plurality of engine faults. In the past decade, Artificial Neural Networks (ANN) have been employed as a pattern recognition device to accomplish the same task. Both methods have enjoyed a reasonable success.© 2000 ASME

Development of gradient thermal barrier coatings and their hot-fatigue behavior

Abstract: In this paper, a new type of gradient thermal barrier coating was produced by the co-deposition of a tablet of Al–Al 2 O 3 –YSZ (Yttria Stabilized Zirconia, ZrO 2 +8 wt.% Y 2 O 3 ), or Al 2 O 3 –YSZ and YSZ onto a NiCoCrAlY bond coat by EB-PVD. The analysis of SEM with EDS showed that the composition and microstructure changed continuously across the thickness of the gradient coating. Due to interdiffusion of elements between the bond coat and the gradient zone, a thin layer of a γ′ phase (Ni 3 Al) formed at the surface of the bond coat. The coatings were subjected to a series of tests and characterization studies, including thermal cycling, isothermal oxidation, hot-corrosion and microanalyses. Results showed that the substrate temperature during deposition had great effect on the properties of the coatings. The higher the temperature of the substrate, the lower the microhardness of the gradient coating, which can be considered as the formation of microporosity in the two-phase ZrO 2 +Al 2 O 3 region in the gradient coating, and its epitaxy into the YSZ topcoat. The thermal cyclic tests of specimens were performed by exposure to air at 1323 K for 0.5 h, and then cooled to room temperature within 5 min by forced air cooling, with a lifetime of more than 500 h. In addition, compared with a conventional two-layered coating, this gradient coating exhibited a better resistance to hot-corrosion than a conventional two-layered coating.

A study on aluminide and Cr-modified aluminide coatings on TiAl alloys by pack cementation method

Abstract: The halide-activated pack cementation method was utilized to deposit a suitable coating on TiAl to ensure its oxidation resistance. TiAl alloys were coated by two different pack cementation processes with simple aluminide coating and Cr-modified aluminide coating. The coatings formed on the TiAl alloyed with Nb and Cr were thicker and tougher than those formed on the non-alloyed TiAl. The TiAl3 layer formed on the TiAl alloyed with Nb and Cr had much better cyclic oxidation resistance than that formed on the non-alloyed TiAl. This is attributed to a change in the crystal structure of the TiAl3 from the brittle tetragonal DO22 to the ductile cubic LI2 by addition of a small amount of Cr and Nb. The Cr-modified aluminide coating on the TiAl alloyed with Nb and Cr exhibited excellent cyclic oxidation resistance due to formation of the Al67Ti25Cr8 phase as the coating layer.

The models and economics of carpools

Abstract: For studying carpooling problems, this paper presents two models, namely deterministic and stochastic, and gives the economic explanations to the model solutions. We investigate the jockeying behavior of work commuters between carpooling and driving alone modes through solving each model for both no-toll equilibrium and social optimum. The logit-based stochastic model involves the consideration on preference option of mode choice. Under some assumptions, the paper explains how the amount of carpooling is affected by fuel cost, assembly cost, value of time, preferential or attitudinal factors and traffic congestion. It is found that carpooling is sensitive to traffic congestion reduction only when a congestion externality-based tolling scheme is implemented.

Investigation of the failure mechanism of thermal barrier coatings prepared by electron beam physical vapor deposition

Abstract: Two-layer structure thermal barrier coatings (TBCs) (NiCoCrAlY [bond coat]+[6–8 wt.%] Y2O3-stabilized ZrO2 [YSZ top coat]) were deposited by electron beam physical vapor deposition (EB-PVD) on a Ni-base superalloy. Pre-treatments were carried out in a vacuum to improve the oxidation resistance of the bond coat, and the thermal cyclic life of the TBC system was investigated through thermal cyclic tests. It was found that the pre-treatments in the vacuum at 1273 K increased the thermal cyclic life of the TBCs by improving the oxidation resistance of the bond coat. The results of thermal cyclic tests indicated that the TBCs prepared by EB-PVD were degraded mainly at the interface between the bond coat and ceramic coat, due to the spallation of the YSZ top coat from the bond coat. According to the results of microstructure observation and composition analysis, the mechanism of failure was proposed as follows: micro-cracks are first initiated in the YSZ top coat along columnar boundaries, and then propagate through the whole top coat. The cracks formed at the thermal growth oxide (TGO) layer, due to the growing micro-cracks, are considered to cause abnormal thermally-grown oxides of the bond coat beneath the cracks, and consequently, the build-up stresses due to the volume increase, which are significant in weakening the interface combination of the top coat and bond coat. During the following cooling process, compressive stresses are introduced, which tend to separate the YSZ top coat from the bond coat, and finally cause the occurrence of the failure of TBC system.

‘In-situ’ weld-alloying/laser beam welding of SiCp/6061Al MMC

Abstract: In this paper, ‘in-situ’ weld-alloying/laser beam welding with titanium as alloying element is utilized to join SiCp/6061Al MMC in order to prevent the formation of the needle-like harmful Al4C3 phases in the weld. Microstructure of the weld is characterized as functions of alloying content and processing parameters. Results show that the needle-like harmful aluminum carbides are completely eliminated. A central fusion weld joint reinforced by TiC, Ti5Si3 and Al3Ti is produced by ‘in-situ’ weld-alloying/laser beam welding of SiCp/6061Al MMC, while some large pores are formed in the partly melted zone.

Application of Lade's Criterion to Cam-Clay Model

Abstract: Lade's criterion is one of the best criteria for describing the shear yield and failure behavior of soils in 3D stresses, and the original Cam-clay model is the most popular and fundamental elastoplastic model for normally consolidated clays. In this paper, a transformed stress tensor is proposed for combining Lade's criterion and the Cam-clay model. The transformed stress is deduced from what makes the curved surface of Lade's criterion become a cone with the axis being the space diagonal, i.e., Lade's criterion becomes the extended Mises type criterion in the transformed principal stress space. The Cam-clay model revised by Lade's criterion is capable of describing the mechanical behavior of soils in general stresses. It is presented that the revised model can simulate well the drained and undrained behavior of soils, not only under triaxial compression conditions, but also under plane strain, true triaxial, and hollow cylinder conditions. The elastoplastic models for soils, in which only the first and second stress invariants are used, can be extended simply to the model, including the third stress invariant by adopting Lade's criterion. where sij = deviator stress tensor; and dij = Kronecker's delta. Therefore, the Cam-clay model is generalized by assuming a section of the yield surface to be circular in the p-plane. That is to say, the extended von Mises criterion (q/p = const) was adopted for the shear yield and the shear failure of clay in the Cam-clay model. However, as experimental evidence shows, the extended Mises criterion grossly overestimates strength in triaxial extension, and also results in incorrect intermediate stress ratios in plane strain (Wroth and Houlsby 1985). It is well known that the failure of soils can be explained by the Lade's criterion (Lade and Duncan 1975; Lade 1977; Lade and Musante 1978), and others (Matsuoka and Nakai 1974), which incorporate the third stress invariant. Taking the consistency in the shear deformation and the shear failure into considera- tion, it is reasonable to introduce Lade's criterion or other cri- teria for the shear yield as well as the shear failure of soils. Based on such ideas, some researchers attempted to improve the Cam-clay model to a 3D model for soils (Zienkiewicz and Pande 1977; Randolph 1982). However, Wroth and Houlsby (1985) commented that further study is necessary to improve the Cam-clay model by combining the idea of critical state theory with such failure criteria as Lade and Duncan's (1975)

Construction of multivariate biorthogonal wavelets with arbitrary vanishing moments

Abstract: We present a concrete method to build discrete biorthogonal systems such that the wavelet filters have any number of vanishing moments. Several algorithms are proposed to construct multivariate biorthogonal wavelets with any general dilation matrix and arbitrary order of vanishing moments. Examples are provided to illustrate the general theory and the advantages of the algorithms.

Aerodynamic forces and flow structures of an airfoil in some unsteady motions at small Reynolds number

Abstract: The aerodynamic forces and flow structures of a NACA 0012 airfoil in some unsteady motions at small Reynolds number (Re=100) are studied by numerically solving the Navier-Stokes equations. These motions include airfoil acceleration and deceleration from one translational speed to another and rapidly pitching up in constant freestream (equivalent to pitching up during translational motion at constant speed). It is shown that at small Reynolds number (Re=100), when the airfoil is performing fast acceleration or deceleration from one speed to another, a large aerodynamic force can be generated during and for a time period after the acceleration or deceleration; a large aerodynamic force can also be generated when the airfoil is performing a fast pitching motion in a constant freestream. In these fast unsteady motions, an airfoil in low Re flow can produce a large aerodynamic force as effective as in large Re flow, or the effect of unsteady motion dominates the Reynolds number effect. During the fast unsteady motion of the airfoil, new layers of strong vorticity are formed near the upper and lower surfaces of the airfoil under the previously existing thick vorticity layers, and it is the generation and motion of the new vorticity layers that is mainly responsible for the generation of the large aerodynamic force; the large-scale structure and movement of the newly produced vorticity layers are similar to that of high Re flow.

Oxidation behavior of TiAl/TiAl–SiC gradient coatings on gamma titanium aluminides

Abstract: Gradient TiAl/TiAl+SiC coatings on the gamma titanium aluminide were fabricated by using electron beam physical vapor deposition (EB-PVD) technique, and the oxidation resistance behavior was investigated. The coating surface was in the structure containing mainly TiAl and SiC identified by the X-ray diffractometer. The oxidation resistance depends on the ratio of the surfacial content of SiC to TiAl. Only 1.03 mg/cm 2 weight gain was recorded after isothermal oxidation at 1173 K in air for 200 h, and no spallation was recognized after cyclic oxidation at 1173 K in air for 100 h for the coating with the ratio of approximately 8:3 TiAl/SiC. The contribution of SiC to the improvement of the oxidation resistance can be considered as the formation of amorphous SiO 2 .

Martensitic transformation of the Ti50Ni48Fe2 alloy deformed at different temperatures

Abstract: The Ti50Ni48Fe2 alloy was deformed at low temperature (−70°C) and ambient temperature, where the initial states of the samples were pure martensitic phase or austenitic, respectively. The shape memory effect and the reverse transformation temperature were studied on the samples deformed at different temperatures. A maximum shape recovery strain of 5.6% was obtained and the As′ point of the samples deformed at low temperature was raised by about 40°, while a recovery strain of 4% was obtained and the As″ was raised by 55°, rising up to 35°C, in the samples deformed at ambient temperature. An access was approached to the pre-deformation, storage, transportation and installation of components made of Ti50Ni48Fe2 alloy at ambient temperature.

In-situ TEM study of fracture mechanisms of polysynthetically twinned (PST) crystals of TiAl alloys

Abstract: The fracture mechanisms of polysynthetically twinned (PST) crystals of Ti–49at.% Al in different lamellar orientations have been investigated by using in-situ transmission electron microscopy (TEM). The results indicated that the fracture mechanisms and crack propagation behavior depended strongly on the lamellar orientation of PST crystals. When the tensile axis was nearly parallel to the lamellae, the main crack propagated discontinuously by nucleation, growth and linkage of microcracks formed ahead of crack tip. Slip band decohesion, fracture along prism plane of α2 plates and twinning-induced microcracks are three modes of nucleation of microcracks. When the loading axis was perpendicular to the lamellae, the crack parallel to the lamellae propagated by tearing and shear of shear ligament. Interfacial microcrack usually occurred ahead of the main crack tip. Sometimes, the main crack propagated along (111) cleavage plane within γ lamellae.

Aging behavior of Li containing Al–Zn–Mg–Cu alloys

Abstract: The aging behaviors of three Al–Li–Zn–Mg–Cu alloys were systemically studied including one-step aging, two-step aging and pre-straining aging. It is shown that the addition of Li suppressed the homogenous precipitation of MgZn 2 phases in Al–Zn–Mg–Cu alloys, and the Zn-rich phases precipitate primarily on grain boundaries in the form of large particles. The two-step aging plays a key role in stimulating nucleation in the Li-free alloy, but has little effect on the hardening efficiency of Li containing alloys. The pre-deformation promotes the precipitation of Zn-rich phases in the matrix, and improves the yield strength greatly. These results are interpreted in terms of the effect of preferential binding of Li atoms and vacancies on the nucleation mechanism. Since Li atoms possess a high concentration and significant vacancy-binding energy, they preferentially trap the quench-in vacancies, then retard the diffusion of Zn, Mg atoms, and suppress the homogenous nucleation of Zn-rich phases in the matrix. The two-step aging shows little benefit on the precipitation of Zn-rich phases in the matrix because of the retarding effect of Li atom. While pre-deformation increases the defect concentration in the matrix, thereby causes the modification of size and distribution of Zn-rich precipitates.