An Extension Problem For Discrete-Time Periodically Correlated Stochastic Processes

In the context of wide-sense stationary processes, the so-called Caratheodory–Fejer problem of extending a finite non-negative sequence of matrices has been much studied. We here investigate a similar extension problem in the setting of wide-sense periodically correlated processes: given the first N coefficients of T scalar-valued sequences, we study under which condition(s) it is possible to find T extensions which are the cyclocorrelaion sequences of a periodically correlated process with period T . Using a result of Gladysev, the problem is shifted to a Caratheodory–Fejer problem with symmetry constraints. The existence of extensions is proved. In nondegenerate cases, the set of all solutions is given in terms of a homographic transformation of some Schur function G . The choice G =0 leads to the maximum entropy solution. The associated Gaussian processes are then proved to have a periodic autoregressive structure.     

Discontinuous precipitation of β-Ru phase in Ni–18Ru alloys

The precipitation of β-Ru phase upon aging of a solutionized Ni–18 at.% Ru alloy was studied by X-ray diffraction and electron microscopy. It was shown that: the β phase forms by discontinuous lamellar decomposition from nucleation sites on α grain boundaries; the phases exhibit well-defined orientation relationships; and the hardness of the alloy correlates strongly with the extent of decomposition. The implications for the use of heat treatment to control the microstructure and mechanical properties of the alloy are discussed.     

锻造和热处理对Al-Si和Al-Pb轴承合金在油润滑条件下耐磨性的影响

研究了锻造态和热处理态不同成分铝基轴承合金在油润滑条件下的摩擦行为，以及材料的硬度、热处理制度和锻造对材料耐磨性的影响。所用材料是 Al-8.5Si-3.5Cu 和 Al-15Pb-3.7Cu-1.5Si-1.1Fe。在锻造过程中，应变范围为10%~20%。并对材料进行T6热处理。在销-盘式磨损试验机上对材料的摩擦性能进行测试。结果表明，锻造过程使材料的硬度得到增加；铸造应变在10%~20%的范围内对材料的摩擦磨损影响不明显。     

Elastic Coefficients of Zn1?x Be x O Solid Solutions: a First-Principles Study

ZnO-BeO solid solutions (Zn_1?x Be _x O, ZBO) are wide-band semiconductors that also display a strong spontaneous polarization. We present here the results of density functional theory calculations to determine the elastic stiffness coefficients as a function of the Be concentration x. We construct three distinct supercells for ZBO solid solutions, including the prototypical wurtzite (P6₃ mc) structure and orthorhombic supercells with Pmn2₁/Pna2₁ or P2₁ symmetry. The components of the elastic stiffness tensor and the bulk modulus of ZBO in the three constructs are almost identical. The bulk modulus of ZBO solid solutions varies from 139.5?GPa to 211.6?GPa for x?=?0 and x?=?1, respectively, with bowing parameter of 41.9?GPa.     

Heat‐damage assessment of carbon‐fiber‐reinforced polymer composites by diffuse reflectance infrared spectroscopy

Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was used to assess the effects of heat damage on carbon‐fiber‐reinforced polymer composites. Moisture‐saturated graphite–epoxy laminates with a quasi‐isotropic lay‐up were heat‐damaged above their upper service temperatures. The loss of matrix‐dominated mechanical properties due to heat exposure was investigated in the laboratory under environmental testing conditions with mechanical tests, ultrasonic C‐scanning, and DRIFT spectroscopy. The reduction of the mechanical strength of the composite materials was accompanied by an increase in the carbonyl band integral and a decrease in the phenyl ratio and hydroxyl and hydrocarbon band integrals, as shown by the DRIFT spectra. DRIFT was confirmed to be more effective than ultrasonic inspection in evaluating the extent of heat damage, and a good correlation was found between the mechanical test results and DRIFT spectra. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1222–1230, 2005     

Challenges for delivering healthcare education through telematics

Within the context of the education of professional healthcare providers, the authors give an overview of the challenges faced by those wishing to introduce telematics as both a mechanism for content delivery and as subject content itself. After presenting a brief overview of the current state of telematics applications to healthcare education, focusing on the European sphere, the authors outline the challenges before discussing the collaborative and communicative issues in detail. The authors conclude by suggesting that, while the authors believe that telematics is a necessary direction for the future development of healthcare education for professionals, the collaboration and communication challenges are of greater importance than the technical and policy challenges and that there is a need to educate the majority of educators, based in the experiences of the enthusiasts.     

Strain engineering of piezoelectric properties of strontium titanate thin films

The in-plane and out-of-plane piezoelectric properties of (001) strontium titanate (SrTiO₃, STO) epitaxial thin films on pseudo-cubic (001) substrates are computed as a function of in-plane misfit strain. A nonlinear thermodynamic model is employed, which takes into account the appropriate mechanical boundary conditions, the electromechanical coupling between the polarization and the in-plane lattice mismatch, and the self-strains of the ferroelastic and ferroelectric phase transformations. The piezoelectric behavior of epitaxial STO films is described in various strain-induced ferroelectric phase fields in a temperature range from ?50 to 50 °C. The calculations show that by carefully tailoring in-plane misfit strains in both tensile and compressive ranges, piezoelectric coefficients that are of the order of prototypical lead zirconate titanate and other lead-based piezoceramics can be realized. These results indicate that strain engineered STO films may be employed in a variety of sensor and actuator applications as well as surface acoustic wave devices and thin-film bulk acoustic resonators.     

Planning and design of industrial engineering education quality

A Quality Function Deployment (QFD) approach to improve industrial engineering (IE) education quality at the Middle East Technical University (METU) is presented. Major stakeholders of IE education are identified as students, faculty members and future employers of the students. Requirements of these groups from an IE graduate are determined by surveys and interviews with them. These requirements are prioritized. Main education requirements are identified to meet the stakeholders' requirements. Finally, the results and plans for future studies are given.     

On response of initially stressed structures to random excitations

This work is intended to provide a tool to the design engineer who is dealing with the design of structures subjected to random excitations. In order to increase the usefulness of the tool, other than linearly elastic behavior, no practical limitation is imposed as far as the type, geometry, support conditions, and material are concerned. The dynamic equilibrium equations of the initially stressed structures are obtained in discrete form with the finite element method and the associated Hamiltonian. These equations may be solved in the usual way by finding first the eigenvalues and the eigenvectors of the “free vibrations”, within the frequency range of the expected excitations, and then uncoupling the equations by the normalized eigenvectors. The damping may be represented by the percent of critical damping of the related modeshapes. The uncoupled equations may be handled in the frequency domain by finding the power spectral density functions associated with the random excitations. For this purpose an improved version of the ELAS program is taken to generate stiffness matrices. The program is modified to produce mass and geometric matrices. For the eigenvalue extraction problem, a new algorithm is developed which uses a combination of the inverse iteration with shifts method in conjunction with the Sturm Sequence property. The power spectral density functions of the random, ergodic stationary excitations may be either input or they may be obtained from the histograms of the excitations. The program exploits fully, the symmetry and variable bandwidth property of the assembled matrices, both during the decomposition, and also during the eigenvalue extraction. The input format of the ELAS program is almost left intact. With this software, it is also possible to handle linear buckling and equilibrium problems of the structures. The term structures here means two or three-dimensional trusses, two or three-dimensional frames, gridworks plates, shells (with or without bending), two or three-dimensional elasticity problems, shells of revolution (with or without bending), and solids or revolution.