Unstable rational function approximation

The problem is considered of approximating a transfer function with stable and unstable poles by a lower-order transfer function with the same number of unstable poles. A method is suggested and compared with alternative known methods based on Hankel-norm approximation. Examples suggest that no one method always outperforms other methods in terms of minimizing the approximation error.     

High-Temperature Stress Corrosion Cracking in Ceramics

Single-phase ceramics are shown to be susceptible to stress corrosion cracking at elevated temperatures in the presence of a wetting amorphous deposit. Crack arrest occurs when the crack is depleted of amorphous material, provided that the stress intensity is below a "blunting" threshold. A preliminary model of such cracking has been developed. The model considers stress corrosion as a process wherein the corrosive medium, by virtue of its wetting characteristics, allows the crack to propagate along grain boundaries. A crack velocity that depends sensitively on the dihedral angle is predicted. Furthermore, since the corrosive medium acts as a conduit for rapid atom transport to the crack tip, the crack velocity also exhibits a strong dependence on the viscosity of the fluid medium. Implications for such stress corrosion processes on premature failure are discussed.     

Cryptotype,Overgeneralization and Competition: A Connectionist Model of the Learning of English Reversive Prefixes

This study examined the role of covert semantic classes or 'cryptotypes' in determining children's overgeneralizations of reversive prefixes such as un - in *unsqueeze or *unpress . A training corpus of 160 English verbs was presented incrementally to a backpropagation network. In three simulations, we showed that the network developed structured representations for the semantic cryptotype associated with the use of the reversive prefix un- . Overgeneralizations produced by the network, such as *unbury or *unpress , match up well with actual overgeneralizations observed in human children, showing that structured cryptotypic semantic representations underlie this overgeneralization behaviour. Simulation 2 points towards a role of lexical competition in morphological acquisition and overgeneralizations. Simulation 3 provides insight into the relationship between plasticity in network learning and the ability to recover from overgeneralizations. Together, these analyses paint a dynamic picture in which competing morphological devices work together to provide the best possible match to underlying covert semantic structures.     

THERE AND THERE - A COLUMN

Abstract

Henry L. Stimson is best known to readers of this journal for his quote, “Gentlemen do not read each other's mail,” and his shutting down of Herbert O. Yardley's black chamber. The present piece reveals a second cryptologic episode from his life. During WWII, Stimson made a request of hundreds of libraries that they withdraw all material on explosives, secret inks, and ciphers from circulation and report the names of anyone requesting these materials to the local office of the Federal Bureau of Investigation (FBI). A pair of original documents is reproduced for the first time along with some references. A list of libraries contacted is also provided.     

Stress and Fracture Behavior of Monolithic Fuel Cell Tapes

This paper summarizes the stress and fracture analyses of a three-layer ceramic composite which is a fundamental building block for the monolithic solid oxide fuel tell currently being developed at Argonne National Laboratory. A small difference in the coefficients of thermal expansion of different layers can result in large stresses and cause cracking parallel to the plane of the layers. A simple fracture model for a three-layer composite has been developed. The cracking behavior predicted by the model is in reasonably good agreement with the fabrication experience to date.     

Theory of Fatigue for Brittle Flaws Originating from Residual Stress Concentrations

A theory is formulated for the general fatigue response of brittle flaws which experience residual stress concentrations. The indentation crack is taken as a model flaw system for the purpose of setting up the basic fracture mechanics equations, but the essential results are expected to have a wider range of applicability in the strength characterization of ceramics. A starting fatigue differential equation is first set up by combining an appropriate stress intensity factor for point- or line-contact flaws with a power-law crack velocity function. Analytical solutions are then obtained for the case of static fatigue. The resulting relation between lifetime and failure stress is shown to have exactly the same power-law form as the conventional solution for Griffith (residual-stress-free) flaws. This "equivalence" is used as a basis for extending the results to dynamic fatigue. A comparison of these analytical solutions with numerical counterparts defines the limits of accuracy of the theoretical procedure. However, while the form of the lifetime relation remains invariant, the values of the exponent and coefficient differ significantly for flaws with and without residual stress. Accordingly, the application of conventional fatigue theory to evaluate crack velocity parameters, without due regard for the nature of the critical flaw, can lead to serious errors. Explicit conversion formulas are given for transforming "apparent" velocity parameters for indentation flaws directly into "true" parameters. The implications of these results concerning the use of the indentation method for materials evaluation are discussed.     

原子力显微镜评价奥氏体不锈钢晶间腐蚀敏感性的研究

由13Cr可焊马氏体不锈钢(WMSGSS)制造的油气管线由于价格便宜,相对于昂贵的耐腐蚀合金来说是一个很有竞争力的选择。但是实验室试验和现场经历表明,13Cr马氏体不锈钢焊接接头热影响区发生了沿晶应力腐蚀开裂(IGSCC)。对于低等级的马氏体不锈钢来说,沿晶应力腐蚀开裂机理被认为是在晶界的碳化物附近存在贫铬区。对于中等级和高等级马氏体不锈钢来说,只在实验室试验发现有应力腐蚀开裂。它们的应力腐蚀开裂机理还没有明确,虽然最可能的应力腐蚀开裂机理也被认为和低等级马氏体不锈钢的应力腐蚀开裂机理类似。原子力显微镜(AFM)已开始广泛用于材料显微组织和环境敏感断裂的研究中。高等级马氏体不锈钢焊接接头对晶间腐蚀/应力腐蚀开裂敏感性很有希望由原子力显微镜检测到。在原子力显微镜技术应用于这项研究之前,用由304不锈钢准备的不同敏化度的试样来确定它的适用性是十分有益的。本文用原子力显微镜研究了不同敏化程度的304不锈钢的晶间腐蚀敏感性并和SEM和EDS结果进行了对比。