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Although the pronghorn ( Antilocapra americana ) resembles an antelope, its nearest relatives are the giraffe and okapi. In this study we have examined the placentae of 6 pronghorns using lectin- and immunocytochemistry to identify giraffid and bovid features. Binucleate cells (BNC) of the placenta exhibited features intermediate between those of the giraffe and bovine; Dolichos biflorus agglutinin binding – strong in the bovine BNC and absent in the giraffe – was evident in only a subpopulation of BNC while binding to blood vessels, as in the giraffe. Binding of Phytolacca americana agglutinin resembled that of the giraffe and okapi whereas many other glycans were found in all four clades. PAG antigens were similar to bovine and okapi but not giraffe. In summary, although the pronghorn outwardly resembles an antelope, placental BNC show giraffid features. Although each clade has its own individual characteristics, there are far more similarities than differences between them, emphasizing the common ancestry of all four clades.

The superplastic deformation characteristics of a modified aluminum alloy were investigated at temperature range from 500 to 550°C. The alloy chemical composition was 4.69% Mg, 1.56% Mn, 0.177% Cr, 0.03% Si, and 0.07% Fe by weight. Total tensile elongations as high as 750% were observed at lower strain rates. A non-isothermal constitutive equation, that includes the evolution of grain size, was established and implemented in a finite element code. Numerical simulations of the uniaxial tension specimen were performed, and good agreement between the testing and the modeling results was obtained. A pressure control algorithm for the blow forming model was developed based on an averaging scheme that considered a subset of finite elements with the highest strain rate. A pressure history was generated from the finite element simulations for a constant target strain rate of 1×10 −3 s −1 . When these pressure histories were used in tray forming experiments, good agreement was obtained between the forming and modeling results.

The study illustrates the importance of the distribution and alignment of holes as a factor in determining tensile elongations in Pb-Sn alloys. It is possible for a sample with more holes to give a higher elongation than ones with fewer holes. This seemingly counter-intuitive result can occur due to void distribution effects, with strain localization occurring at more cross-sections. A modification of the earlier model to incorporate simultaneous deformation of non-cavitated and cavitated sections resulted in predictions of failure elongations which are in fair agreement with experimental trends and results. This can also be used to explain the higher elongations obtained in some Al-Mg-Mn alloys with elongated cracked stringers than those without, under otherwise similar conditions.

In this work, we investigate the mechanical behavior of superplastic materials under various loading conditions and examine the nature of anisotropy. This and other phenomena are modeled within a continuum theory of viscoplasticity which includes an overstress function, ‘yield surface’ and internal variables. Anisotropic hardening is represented by an internal stress tensor whose evolution (including plastic spin) consists of hardening, dynamic recovery, and static recovery terms. The ‘dynamic’ yield surface (within a viscoplasticity formulation) is assumed to be isotropic and depends on both J2 and J3. Various experiments on a PbSn superplastic alloy including tension, jump test, stress relaxation, fixed-end torsion, and combined tension-torsion tests were performed and the results are used to calibrate and verify the theoretical model. It is shown that the isotropic dynamic surface can successfully model the uniaxial behavior of superplastic deformation, including stress relaxation, and produces axial stresses in fixed-end torsion.

The instability of biaxial stretching of thin sheets of viscoplastic metals under plane stress conditions is investigated using a linear stability analysis. An instability criterion for biaxial stretching is developed based on the assumption that localized necking initiates along the direction perpendicular to the major principal stress direction. Various “optimum” variable strain rate paths, which ensure a stable deformation of the sheet without neck formation, are computed for different strain ratios based on the instability analysis. The variable strain rate paths are applied in the finite element modeling of the superplastic uniaxial extension of a tabular specimen and supe´rplastic blow-forming of a hemisphere. A reduction of forming time is achieved compared with the established constant strain rate forming method, while uniformity in the thickness distribution of the formed parts are maintained.

Current experimental methods are influenced by the end effects that cause non-uniform strain rates in the gauge section and material flow within the grips. A series of tension tests and finite element models confirm this for an Al-5083 alloy. Both the tests and the finite element simulations predict that the actual strain rate begins at about 60 percent of the desired strain rate and increases gradually with strain. Material flow from the grips into the gauge effectively ``slows`` the strain rate at the initial stages of the test. As the test proceeds thinning of the gauge section occurs and most of the strain occurs in the gauge section due to the relative cross-sectional areas of the grip and gauge section. Testing and models were also run comparing specimens with and without alignment holes in the grips. It was shown that alignment holes increase flow from the grips and thus introduce additional error in the tests. Further modeling was performed to evaluate the improved accuracy of specimens with increased length-to-width ratios. This work showed that a specimen with 50% reduction in the standard gauge width and double the standard gauge length (4:1 increase in length-to-width ratio) gave strain rates within 10%more » of the desired value throughout the test.« less

The superplastic behavior of the 5083 aluminum alloy with additions of Mn and Zr was studied by uniaxial tensile testing and microstructural evaluations. Additions of up to 0.2% Zr and 0.8% Mn were made to a base 5083 aluminum alloy to decrease the grain size and improve superplastic behavior. Constant strain-rate tensile test data were used to determine strain-rate sensitivity (m values) and elongations-to-failure for the alloys at strain rates ranging from 4 {times} 10{sup {minus}4} to 1 {times} 10{sup {minus}1} s{sup {minus}1} at temperatures of 450 to 550C. Elongations-to-failure of up to 400% at 1 {times} 10{sup {minus}2} s{sup {minus}1} were achieved for the modified alloys. The strain-rate sensitivity for the alloys as a function of strain was determined and two distinct behaviors were observed. For the alloys having composition close to the base 5083 alloy, the m value steadily decreased with increasing strain; however, in alloys with higher levels of Zr, the m value remained stable. A maximum m value of 0.65 was achieved at 0.7 strain for the 1.6% Mn and 0.2% Zr alloy at 1 {times} 10{sup {minus}3} s{sup {minus}1}.

When considering the forming and post-forming properties required of a superplastic material, attractive candidates are commercial Al-Mg-Mn weldable alloys such as AA5083. There have been several investigations of hot deformation of 5083-type alloys in the literature. Only two studies evaluated commercial-purity 5083 and they achieved tensile elongations of 150% and 200%. Alloy modification has produced improved behavior in three 5083-type alloys developed specifically for SPF. Two were deemed high-purity 5083 (low Fe and Si) and achieved elongations of 450% and 630%. Engineering strains up to 700% were measured by Watanabe et al. in a 5083-based alloy with the addition of 0.6% Cu as a grain refiner. These results suggest that alloy modifications such as reduced Fe and Si contents or Cu additions may be required to improve superplastic response. Unfortunately, specific SPF-grade 5083 alloys are substantially more expensive than the commercial grade, and the addition of Cu decreases the corrosion resistance of the base material. The purpose of this work is to examine the effect of prior degrees of cold work on the SPF behavior of a standard-grade 5083 alloy. Superplastic behavior of this material at 510[degree]C is assessed and compared to published results for the SPF-grade alloys.

In this paper, we report a novel heuristic for requantizing JPEG images. The resulting images are generally smaller and often have improved perceptual image quality over a "blind" requantization approach, that is, one that does not consider the properties of the quantization matrices. The heuristic is supported by a detailed mathematical treatment which incorporates the well-known Laplacian distribution of the AC discrete cosine transform (DCT) coefficients with an analysis of the error introduced by requantization. We note that the technique is applicable to any image compression method which employs discrete cosine transforms and quantization.

This article discusses many of the processes and related considerations involved in the forming of superplastic sheet metal parts. It reviews the requirements for superplasticity and describes the characteristics of superplastic metals. The characterization of superplastic behavior includes the characterization of plastic flow, internal cavitation, and fracture behavior. Processing variables needed for the overall characterization of superplastic behavior are summarized. The article discusses the superplastic forming methods, namely, blow forming, vacuum forming, thermoforming, deep drawing, superplastic forming/diffusion bonding, forging, extrusion, and dieless drawing. It provides information on superelastic forming equipment and tooling. The article explains the thinning characteristics and quick plastic forming and its technological elements. It describes the manufacturing practice of the process. The article concludes with a discussion on the superplastic behavior in iron-base alloys.

We propose a new method for requantizing JPEG images, based on the well-known Laplacian distribution of the AC discrete cosine transform (DCT) coefficients and on an analysis of the error introduced by requantization. The resulting images have improved perceptual image quality over a “blind” requantization approach, that is, one that does not consider the properties of the quantization matrices.

Publisher Summary This chapter draws a comparison between the effects of near-field seismic motion on bridge structures and those of far-field seismic motion. To do so, it was necessary to develop loading protocols to simulate each of these conditions. Using quasi-static and pseudo-dynamic displacement protocols to simulate near-fault loading conditions, the performance of a series of reinforced concrete columns is assessed. The results of these tests and a related series of tests at UC San Diego are used to calibrate a finite element model for near-fault structural response of bridge columns. Models of simple bridge structures are under construction using this information for the assessment of system response to near-fault ground motion records. Preliminary conclusions include a likely reduction in plastic hinge length under pulse-type ground motions and little change in ultimate load and displacement capacities for ductile-detailed columns.

A number of alloys based on the composition of Al-Li-Zr have been shown to be superplastic under at least one of two different microstructural conditions: 1. fully recrystallized to a fine, stable grain size, and 2. warm- or cold-worked and unrecrystallized prior to superplastic deformation. For the latter case, static recrystallization was impaired by the presence of fine Al/sub 3/Zr particles, and dynamic recrystallization was observed to occur during superplastic deformation in which the heavily worked microstructure evolved into a fine grained fully recrystallized microstructure. This process is observed in other Al alloys as well, such as the Al-Cu-Zr alloys (Supral alloys), Al-Zn-Mg-Zr alloys, Al-Mn-Zr alloys, and Al-Mg-Mn alloys where the dynamic recrystallization has been suggested to be a continuous reaction in which recrystallization occurs by a gradual and homogeneous process during deformation rather than by the more common nucleation and growth process. Experimental observations of continuous recrystallization show development of a subgrain structure which coarsens continuously while deformation proceeds, with a concurrent increase in the misorientation angle between adjacent subgrains which ultimately approaches that of a high-angle boundary, characteristic of a fully- recrystallized microstructure. During the first 50 to 300% deformation, the microstructure evolves from the heavily worked tomore » a fully recrystallized microstructure after which the fully recrystallized microstructure apparently exhibits the typical micro-grain superplastic characteristics. Superplasticity under continuous dynamic recrystallization is of interest both from scientific and technological standpoints since the rates at which superplastic deformation can be obtained are often higher than those for the fully recrystallized microstructures.« less

An analytical model was developed for the elastic-ideally plastic behavior of concentric cylindrical composites under uniaxial tension. The model predicted the creation of transverse stresses in the composite during axial loading. The existence of these stresses was experimentally verified in a 4140 steel core-18% Ni maraging steel case composite system by examining the effects of the trans verse stresses on the composite axial stress-strain curve and the composite tangential surface strain ratio curve.

Deformation and fracture of fine and coarse grain Al-Zn-Mg alloy sheets have been studied near solution-treatment temperature by conducting constant strain-rate tensile tests. Deformation is found to be controlled by a combination of grain deformation and grain boundary processes. Even though fine grain size yields mild superplasticity, strain hardening occurs continuously, and leads to the development of defect structure. Thus, flow stress and rate-sensitivity for large plastic deformation has been found to be dependent on both strain and strain-rate. Metallographic observation of internal cavity formation and growth during deformation is quantified by measurements of density change. Identification of void sites and the void growth kinetics have been considered in conjunction with the flow behavior so that the extent of tensile ductility obtained in these alloys can be understood.

The paper describes a compact L-band duplexer capable of handling peak powers in the megawatt range. It uses a hybrid waveguide/coaxial construction in which the transmitter power is fed through the waveguide section and the received signal is coupled out by a coaxial probe. A simplified theoretical analysis is included in the paper which predicts the loss and power-handling capacity of the circuit. Measured results are presented for an experimental device operating in the 1.25-1.35 GHz which indicate a transmitter-channel loss of less than 0.2 dB, and a receiver-channel loss of less than 0.5 dB for a 28 dB isolation level. Zusammenfassung: Es wird die Entwicklung eines kompakten L-Band Duplexers beschrieben, bei dem die Sendeleistung durch einen Hohlleiter, die Empfangsleistung aber durch ein Koaxteil geschaltet wird. Eine vereinfachte theoretische Betrachtung zeigt eine gute Ubereinstimmung mit den gemessenen Daten. Messungen an einem Labor-Muster zeigen eine Durchlasdampfung von ca. 0.5 dB und eine Sperrdampfung von ca. 28 dB uber eine Bandbreite von ca. 10% bei 1.3 GHz. Die Leistungsfestigkeit betrug uber 700 kW bei einer Pulslange von 5 μs und einem Tastverhaltnis von 1: 500.

An electronically tuned microwave filter employing p-i-n diodes is described. Experimental results for a 2-section device tuning from 0.8 to 1.2 GHz are given.