Creep Behavior of Dispersion-Hardened Aluminum Materials

Dispersion-hardened aluminum materials of pure aluminum with extremely fine oxide and carbide dispersions and very fine grain sizes were creep-deformed under compressive loadings between 573 and 773 K. The creep behavior of the investigated materials is influenced by time, temperature, stress level and microstructure. An increasing content of dispersions causes increasing threshold stresses _thand resistances against creep. The Norton plots of the minimum creep rate versus stress are characterized by extremely high stress exponents n. On the basis of the threshold concept it is demonstrated that the same diffusion process dominates in the dispersion-hardened aluminum materials as in pure aluminum. Their true stress exponents n^*as the slopes of the best fit lines of the are close to 5. The threshold stress decreases considerably with increasing temperature due to the thermally activated recovery of long-range internal back stresses of quasi-planar dislocation structures on the grain boundaries.     

Significance of seismic response spectrum normalization in nuclear power plant design

Peak ground acceleration has been used most commonly to normalize seismic response spectra. Apparently, there is a general consensus that this is not the best parameter for spectrum normalization. Several other parameters have been proposed, such as, peak acceleration-velocity-displacement triad, peak velocity and spectrum intensity. This paper presents statistical studies of response spectra from a number of recorded earthquake motions normalized by several different parameters. The parameters used are peak acceleration, peak velocity, spectrum intensity, spectrum ordinate, etc. Spectrum shapes are generated corresponding to 84 percentile probability level for different normalization parameters. These shapes are compared with the other standrad spectrum shapes used in the nuclear power plant design to investigate significance of different normalization techniques.     

Understanding Web home page perception

A home page is the gateway to an organization's Web site. To design effective Web home pages, it is necessary to understand the fundamental drivers of user's perception of Web pages. Not only do designers have to understand potential users' frame of mind, they also have at their choosing a stupefying array of attributes – including numerous font types, audio, video, and graphics – all of which can be arranged on a page in different ways, compounding the complexity of the design task. A theoretical model capable of explaining user reactions at a molar level should be invaluable to Web designers as a complement to prevalent intuitive and heuristic approaches. Such a model transcends piecemeal page attributes to focus on overall Web page perceptions of users. Reasoning that people perceive the cyberspace of Web pages in ways similar to their perception of physical places, we use Kaplan and Kaplan's informational model of place perception from the field of environmental psychology to predict that only two dimensions: understanding of information on a Web page, and the involvement potential of a Web page, should adequately capture Web page perception at a molar level. We empirically verify the existence of these dimensions and develop valid scales for measuring them. Using a home page as a stimulus in a lab experiment, we find that understanding and involvement together account for a significant amount of the variance in the attitude toward the Web page and in the intention to browse the underlying Web site. We show that the informational model is a parsimonious and powerful theoretical framework to measure users' perceptions of Web home pages and it could potentially serve as a guide to Web page design and testing efforts.     

Antiferromagnetic, Neutral, and Superconducting Band in La2CuO4

The symmetry of the Bloch functions in the conduction band of tetragonal and orthorhombic La₂CuO₄ is examined for the existence of symmetry-adapted and optimally localizable (usual or spin-dependent) Wannier functions. It turns out that such Wannier functions do not exist in the tetragonal phase. In the orthorhombic phase, on the other hand, the Bloch functions can be unitarily transformed in three different ways into optimally localizable Wannier functions: they can be chosen to be adapted to each of the three phases observed in the pure or doped material, that is, to the antiferromagnetic phase, to the superconducting phase or to the phase evincing neither magnetism nor superconductivity. This group-theoretical result is proposed to be interpreted within a nonadiabatic extension of the Heisenberg model. Within this model, atomic-like states represented by these Wannier functions are responsible for the stability of each of the three phases. However, all the three atomic-like states cannot exist in the tetragonal phase, but are stabilized by the orthorhombic distortion of the crystal. A simple model is proposed which may explain the physical properties of La_2-x Sr _x CuO₄ as a function of the Sr concentration x.     

Anytime clausal reasoning

Given any incomplete clausal propositional reasoner satisfying certain properties, we extend it to a family of increasingly‐complete, sound, and tractable reasoners. Our technique for generating these reasoners is based on restricting the length of the clauses used in chaining (Modus Ponens). Such a family of reasoners constitutes an anytime reasoner, since each propositional theory has a complete reasoner in the family. We provide an alternative characterization, based on a fixed‐point construction, of the reasoners in our anytime families. This fixed‐point characterization is then used to define a transformation of propositional theories into logically equivalent theories for which the base reasoner is complete; such theories are called “vivid”. Developing appropriate notions of vividness and techniques for compiling theories into vivid theories has already generated considerable interest in the KR community. We illustrate our approach by developing an anytime family based on Boolean constraint propagation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Convergence problems in the theory of random elastic media

A main result of the rigorous theory of random, linearly elastic media consists in the representation of the tensor of effective elastic moduli as a Neumann type infinite series which contains the infinite set of correlation functions of the distribution of the local elastic moduli. Under the restriction to statistically homogeneous and isotropic finite media it is proved that convergent series can always be obtained provided the local elastic moduli remain finite everywhere in the medium. This means that the mentioned theory cannot be applied in the above mentioned form to media with pores and/or rigid inclusions. It also means that the theory is not restricted to media with small fluctuations of the elastic parameter.