The effects of laser-induced modification of surface roughness of Al2O3-based ceramics on fluid contact angle

Laser surface treatment of Al₂O₃-based refractory ceramics, by melting and re-solidification, can be successfully applied to producing surfaces that are pore-free, homogeneous and crack-free. Such treated surfaces can lead to an increase in the corrosion and erosion resistances of the materials, due to lower permeability to corrosive species and higher surface hardness, respectively. Furthermore, the corrosion resistance can be influenced by the wetting characteristics of the treated surfaces in service environment. Therefore, it is important to investigate the effect of laser treatment of ceramic materials on the interaction of the surface with the various environmental elements. This work is concerned with an experimental investigation of the effects of laser surface treatment, by melting and re-solidification, on the fluid contact angles of Al₂O₃-based refractory ceramics. These effects are examined by the modification of the surface roughness characteristics induced by laser treatment. Laser-treated surfaces, both containing cracks and crack-free, are compared with untreated surfaces and the results are reported. The untreated surfaces demonstrated considerable non-uniformity in wetting, in contrast to the treated surfaces. The extent of wetting of the laser-treated surfaces containing cracks was proportional to laser power density. This is due to wetting being enhanced, among other factors, by surface roughness, which increased with power density. The crack-free surfaces were the most smooth and, thereby, exhibited the smallest extent of wettability variations. The reduction in wettability after the laser treatment (crack-free) may have an advantage for corrosion resistance.     

Numerical analysis of jacking operations for a self-elevated jack-up unit

Localized failure and collapse of a jack-up unit may take place during installation on site, especially under severe sea state conditions. To minimize the loss, structural behavior is predicted before construction. When the hull of a unit is elevated from sea water level, the structure system varies with time. This means that normal structural analysis is difficult to satisfy the relevant requirements. To capture the system status at different times, an innovative method is proposed and incorporated into Finite Element Analysis (FEA). With a full model of a jack-up unit having been built and with the techniques developed, behavior such as Rack phase difference (RPD), vertical reaction forces of the pinions and footing reactions, etc., under different loads and boundary conditions can be determined by FEA in the time domain. These valuable data would be helpful in re-designing the unit, and also provide guidance for site installation. To determine the general characteristics of a jack-up unit, casestudies on a practical design have been done, with some important findings concluded.     

The AC losses measurement and analysis of superconducting NbTi CICC for HT-TU superconducting Tokamak

Superconducting TF and PF coils have been measured in SULTAN test facility. Segregated copper strands are included in four NbTi CICC and this is a technical innovation. Two AC losses measurement methods, calorimetric and electromagnetic methods, have been used in the experiments, and a broad frequency range (from 0.05 Hz to 6 Hz) is covered in sample test. The purpose of this experiment was to investigate AC losses of TF and PF CICC conductor including segregated copper and to check the design of PF and TF CICC coated with different resistive barriers (Pb-30Sn-2Sb and Ni plating on NbTi strands).     

A Type of Triangular Ball Surface and its Properties

1IntroductionIntheCONSURFsystemll--3]developedbyA.A.Ball,thebasisfunctionsforcubicpolynomialsweredefinedbyandhencecubicBallcurveswithcontrolpointsPi(i=0,1,2,3)canberepresentedasp(u)=Z?=.B!(u)Pi,0Su51.TwotypesofhigherdegreegeneralizedBallbasisfunctionsandcorrespondingcurveshavebeenderivedbyWangl4]andSaidls]independently.InWang'spaper,generalizedBailbasisfunctionsofarbitrarydegreeweredefinedasfollows'.ifnisanevennumber.ifnisanoddnumberwherew~2u(1--u),05u51.Aparametriccurveofdegreentcall…     

计算机视觉在球类机器人和行为理解研究中的应用与发展

智能机器人是当今机器人研究的热点，并逐渐由制造领域向非制造领域发展。本文以几种国内外近年研制的新型球类运动机器人为例，介绍了计算机视觉在智能机器人研究中的应用情况。另外，有关人与机器人之间的视觉行为理解研究是计算机视觉研究与智能机器人研究领域的新发展，本文对此也进行了介绍。     

Low-velocity impact analysis of composite laminates using linearized contact law

Usually many researchers have used the modified Hertzian contact law or experimental static indentation law to analyze impact response of composite laminates subjected to low-velocity impact. In this study, physical meaning of the analytical method using the laws was investigated and the difference between the analytical results obtained using the two laws was also investigated. Furthermore parametric study on contact coefficient and exponent of the contact law was performed. Finally it could be shown that linearized contact law could be well applied to the low-velocity impact analysis of composite laminates. If this concept is used, any general-purpose finite element method software can be used to solve impact problem without direct developing any FEM code by each researcher. In this paper, some analytical results analyzed using a general-purpose commercial FEM software were also presented.     

Stress transmission through three-dimensional granular crystals with stacking faults

We explore the effect of stacking fault defects on the transmission of forces in three-dimensional face-centered-cubic granular crystals. An external force is applied to a small area at the top surface of a crystalline packing of granular beads containing one or two stacking faults at various depths. The response forces at the bottom surface are measured and found to correspond to predictions based on vector force balance within the geometry of the defects. We identify the elementary stacking fault as a boundary between two pure face-centered-cubic crystals with different stacking orders. Other stacking faults produce response force patterns that can be viewed as resulting from repetitions of this basic defect. As the number of stacking faults increases, the intensity pattern evolves toward that of an hexagonal-close-packed crystal. This leads to the conclusion that the force pattern of that crystal structure can be viewed as the extreme limit of a face-centered-cubic crystal with a stacking fault at every layer.This work was supported by NSF-CTS 0090490 and by the NSF MRSEC Program under DMR-0213745. MJS acknowledges support by the University of Chicago MRSEC Summer 2002 REU program.     

Mode of zero wear in mechanical systems with dry contact

A theoretical and experimental analysis of a mechanical system with dry contact and subject to harmonic excitation is made. The adopted model is a two-degree-of-freedom system representing two sub-structures in dry contacts from their flat ground surfaces. It is theoretically shown that the two sub-structures move in phase with equal amplitudes at a particular frequency of oscillation. This particular frequency is called the frequency of virtual sticking and depends upon the physical properties of the two contacting sub-structures as well as the ratio between the amplitudes of external forces applied on these two structures in the direction of their friction sliding. Zero wear of contact surfaces can thus be established since relative velocity between encountered asperities of sub-structure surfaces becomes zero. It is shown, theoretically and experimentally, that when one of the applied harmonic forces is zero, the frequency of virtual sticking is equal to the natural frequency of the unforced sub-structure. The displacement ratio of the two contacting structures under the effect of harmonic excitation in their sliding direction is investigated within a frequency range up to 400 Hz. The tangential contact stiffness and the dynamic contact rigidity are theoretically determined and experimentally predicted from the measured forced displacement ratio.     

新型钢包车用角型减速机的开发设计

介绍了武汉钢铁公司1^#RH精炼装置钢包车关键配套件－角型减速机的开发背景，工作特点及其结构特点。     

Fretting fatigue crack initiation behavior using process volume approach and finite element analysis

This study investigates crack initiation behavior by incorporating fretting fatigue process volume. Three critical plane based fretting fatigue crack initiation parameters are characterized by computing their averaged values over the process volume and then comparing with their counterparts obtained from the localized approach. Two approaches are used: first one involves the computation of parameter at several points over a rectangular region and then its average, and second approach computes the average state of stress/strain over a radial region from which the averaged parameter is calculated. Both approaches require pre-determination of a critical location at or around which the process volume needs to be placed. Effects of size and location of process volume on the averaged value of parameters are studied in detail. Two radii of cylindrical pad are analyzed to investigate the effect of severity of stress gradient on process volume approach. Effects of finite element mesh refinement are also investigated. Averaged value of parameter decreases with the increase of process volume size. This decrease is higher when the process volume is located in the region that is away from the contact zone. Further, a parameter based on normal stress on the critical plane shows more dependence on the size of process volume than that based on shear stress or on a combination of both shear and normal stresses. Orientation of crack initiation changes within a range that is well within the scatter band of experimental observations as the process volume size increases. Averaged value of parameter for a pad with higher stress gradient has a larger reduction with the increase of process volume size than that with a lower stress gradient. Process volume size has less effect on the averaged value of parameter with coarser finite element mesh. Finally, the localized approach provides a conservative value of fretting fatigue crack initiation parameter compared to its counterpart based on the process volume.