A numerical study of various plastically unstable behaviors in tension and compression

Various types of behavior due to plastic instability under uniaxial tension and compression are numerically investigated regarding sheet materials, plane strain blocks, and cylindrical bars and hollow cylinders. The code GOLDA for analysis of large elastic-plastic deformation previously developed by the author is used, which is one of quasi-static explicit FEM programs. Both of diffuse type and localized type of instability are concerned with. The role of vertex-hardening in plastic instability is payed attention, by using the J2G (J2-Gotoh’s corner theory) as the plasticity constitutive equation, which was proposed previously by the author and is a kind of vertex-hardening theory. Following results are mainly derived. 1) In plane strain tension, shear-type strain localization is realized by the use of J2G, but not by the conventional J2F (J2-flow theory). In axisymmetric tension of a cylindrical solid bar, however, such strain localization would never appear even by J2G, as expected by the experiment. 2) In axisymmetric tension of a hollow cylinder under the condition of no contractlion of its bore (therefore, in almost plane strain state with no circumferential strain), it is found that shear-type strain localization could occur. This is realized again by the use of J2G, whereas J2F never allows such strain localization. 3) In compression under the embedded edges condition, it is found that a sheet with initial aspect ratio of 2:1 (in height:breadth) yields double barreling, whereas a plane strain block yields double barreling at the initial aspect ratio of 3:1, not at 2:1 though expected from the conventional slip-line theory. As for a cylindrical solid bar, double barreling appears for the initial aspect ratio of 2:1 but only for higher n-vaiue (the strain hardening exponent), say n=0.5. 4) As for barreling mode, when thick-walled axisymmetric tubes with the initial ratio of thickness to inner diameter 1/5 to 1/3 are compressed axially, corresponding to the initial aspect ratios of 3:1, 2:1 and 1:1, triple barreling, double barreling and single barreling appear, respectively. 5) It is found that thin-walled tubes buckle in a progressive periodic mode with almost stationary compressive load under axial compression. Thus it can be used as a simplified model for buckling of more complex structures such as the honeycomb.     

Effect of the addition of silver compounds during the pyrolysis of manganese nitrate on tantalum anodic oxide film

A study of the pyrolysis of an aqueous solution of manganese nitrate in the presence of silver compounds has been carried out. Thermal analysis showed that the MnO₂ formation temperature and the transformation temperature from MnO₂ to Mn₂O₃ shifted towards a lower temperature in the presence of silver acetate. A large particle-size and high crystallinity MnO₂ was formed; this may be a useful method of making an excellent tantalum capacitor with high capacitance.     

Mesophase structure discovered through in-situ X-ray measurement in drawing process of poly(ethylene 2,6-naphthalene dicarboxylate) fiber

The structure development in the continuous laser-heated drawing process of poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) fiber was analyzed by in-situ X-ray diffraction measurement. Because of the rapid and uniform laser heating, and the resultant steady-state nature of the necking-drawing, the structure development after the on-set of necking could be measured in the time resolution of several hundred microseconds. We found for the first time the temporal appearance of meridional (001′) diffraction at several milliseconds after the on-set of necking indicating that the mesophase structure similar to the one reported for poly(ethylene terephthalate) was also formed in the initial stage of fiber structure development of PEN. The d-spacing of the (001′) diffraction 1.230 ± 0.003 nm was shorter than the c-axis lengths of both α and β crystals.     

Molecular dynamics simulation of 〈<Emphasis Type="Bold">c</Emphasis>+<Emphasis Type="Bold">a</Emphasis>〉 dislocation core structure in hexagonal-close-packed metals

The core structures of 〈c+a〉 dislocations in hexagonal-close-packed (hcp) metals have been investigated by molecular dynamics (MD) simulation using a Lennard-Jones-type pair potential. The 〈c + a〉 edge dislocation has two types of core at 0 K; one is a perfect dislocation (type A), and the other has two 1/2 〈c+a〉 partials (type B). Type A transforms to type B by abruptly increasing temperature from 0 K to 293 K, while type B is stable in temperature range from 0 K to 293 K. In contrast, type A extends parallel to (0001) at 30 K, and this extended core is still stable at 293 K. These results suggest that the 〈c+a〉 edge dislocation glides on the {11 2} as two 1/2 〈c+a〉 partial dislocations and becomes sessile due to changes of the core structure. The 〈c+a〉 screw dislocation spreads over two {10 1} planes at 0 K. The core transforms into a unsymmetrical structure at 293 K, which is spread over {11 2} and {10 1}, and core spreading occurs parallel to {11 2} at 1000 K. A critical strain to move screw dislocations depends on the sense of shear strain. The dependence of the yield stress on the shear direction can be explained in terms of these core structures. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

High-precision continuous-flow measurement of delta13C and deltaD of atmospheric CH4

We describe our development of a CH4 preconcentration system for use with continuous-flow gas chromatograph combustion isotope ratio mass spectrometry (GC/C/IRMS). Precision of measurement of delta13C-CH4 is 0.05/1000 (1sigma) on multiple 60-mL aliquots of the same ambient air sample. The same front-end on-line CH4 preconcentration system allows us to measure deltaD of CH4 by gas chromatography IRMS when the combustion furnace is replaced with a pyrolysis oven (GC/P/IRMS). Precision of measurement for deltaD-CH4 is 1.5/1000 (1sigma) using 120 mL of ambient air based on multiple aliquots of the same air sample. These are the first reported measurements of atmospheric CH4 using GC/P/IRMS methodology. Each isotope analysis can be made much more rapidly (30-40 min) than they could using off-line combustion of an air sample (1-6 h) followed by conventional dual-inlet IRMS measurements (12-20 min), while requiring much less total volume and retaining a comparable level of precision and accuracy. To illustrate the capabilities of our preconcentration GC/C/IRMS system, we compare the results of measurement of 24 background air samples made using both GC/C/IRMS and conventional vacuum line/dual-inlet IRMS methodology. The air samples were collected on a shipboard air sampling transect made across the Pacific Ocean in July 2000 and are part of an ongoing atmospheric CH4 research program. The average difference between the two methods of IRMS analyses on these 24 samples is 0.01 +/- 0.03/1000 (95% confidence interval) for delta3C-CH4. These are the first measurements to be reported of air samples directly intercompared for delta13C-CH4 using both GC/C/IRMS and dual-inlet IRMS measurement methodology. Measurement of deltaD-CH4 of these air samples is also presented as an illustration of the ability of this system to resolve small isotopic differences in remote air. High-precision measurement of delta13C and deltaD of atmospheric CH4 made using our coupled preconcentration GC/IRMS system will greatly improve our ability to utilize isotopic data in understanding spatial and temporal changes in atmospheric CH4 and the biogeochemistry of its sources and sinks.     

Tensile creep behavior of 3-D woven Si-Ti-C-O fiber/SiC-based matrix composite with glass sealant

The present work investigates the tensile creep behavior (deformation and rupture) at 1100–1300°C in air of a 3-D woven Si-Ti-C-O (Tyranno) fiber/SiC-based matrix composite with and without glass sealant. The composite contained Si-Ti-C-O fibers with an additional surface modification in order to improve interface properties. Although a significant decrease in tensile strength was observed in the unsealed composite beyond 1000°C in air (and attributed to oxidation of the fiber/matrix interface), the composite with glass sealant possessed excellent mechanical properties for short-term (<1 hr.) exposure in air. In this study, tensile creep testing was conducted at 1100–1300°C in air and the effect of glass sealant on medium- and long-term strength was investigated. In addition, chemical stability of the glass sealant was evaluated by X-ray diffraction analysis (XRD) and energy dispersive X-ray spectrometer (EDS). The creep rupture behavior of the composite with glass sealant under long-term exposure is suggested to depend on several factors including decomposition, evaporation, and crystallization of the glass sealant material, in addition to the applied stress.     

Cermet-type hydrogen separation membrane obtained from fine particles of high temperature proton-conductive oxide and palladium

A mixed ionic and electronic conducting hydrogen separation membrane, which consisted of proton-conductive oxide and metallic palladium, was fabricated. A porous alumina tube was employed as a support, and proton-conductive oxide particles were introduced into a microporous top layer of the support by an impregnation method. Palladium particles were deposited into the same porous layer by chemical vapor deposition. Hydrogen permeated preferentially via the membrane thus obtained with a hydrogen permeance (PH₂) of 1.2 × 10^− 9 mol·m^− 2·s^− 1·Pa^− 1 at 873 K. Selectivity for hydrogen (PH₂/PN₂) increased with the operating temperature due to an increase in proton conductivity of the membrane, and PH₂/PN₂ = 5.7 was attained at 873 K.     

A Passive UHF RF Identification CMOS Tag IC Using Ferroelectric RAM in 0.35-μm Technology

A passive UHF RF identification (RFID) tag IC with embedded 2-KB ferroelectric RAM (FeRAM) for rewritable applications enables a 2.9 times faster read-and-write transaction time over EEPROM-based tag ICs. The resulting FeRAM-based tag has a nominally identical communication range for both read and write operations, which is indispensable for data write applications. The evaluated tag communication range with a folded dipole antenna is from 0 m to 4.3 m, at the 953-MHz carrier frequency with 4-W transmitting Effective Isotropic Radiated Power (EIRP) from a reader/writer. The developed tag IC features two circuit blocks to maximize the communication range in 0.35-mum CMOS/FeRAM technology. First is a CMOS-only full-wave rectifier, which can improve the measured efficiency by up to 36.6% by reducing the input parasitic capacitances and optimization of multiplier structure. This efficiency is more than twice that of previously-published results. Second is a low-voltage current-mode ASK demodulator to accommodate a low-breakdown voltage of FeRAM, which converts the ASK power modulation into a linearly modulated current over an incoming power range of 27 dB, corresponding to the entire communication range. The developed demodulator can thus resolve the primary design tradeoff issue between device protection and detection sensitivity in the conventional voltage-mode demodulator     

Improving sinterability of ceramics using hybrid microwave heating

Microwave processing, as a new method for sintering ceramics, has key advantages such as increased heating rate, uniform heating and reduced cost compared to conventional methods. It is generally accepted that microwave sintering can improve the macroscopic mechanical performances of ceramics, however, the performances of microwave-sintered ceramics on the microscopic scale are rarely investigated. In the present study, the ceramics are sintered by hybrid microwave sintering (HMS), which combines the characteristics of microwave heating and conventional heating. To evaluate the homogeneous performance of the sintered ceramics, the behaviors of thermal residual stress distribution in the microwave-sintered and conventionally sintered ceramics were investigated by X-ray diffraction technique. The thermal residual stress investigation shows microwaves can sinter ceramics in entire volume while offering improved mechanical properties. Subsequently, the distribution behaviors of pore ratio and hardness in the ceramics were investigated, respectively. The experiment results confirm that the sinterability of ceramics is homogenously improved by hybrid microwave sintering.