The effect of repeating load on the crack growth initiation and crack propagation in delayed failure

The delayed failure test under repeating load was carried out with pre-cracked specimen. The incubation time and the crack propagation rate were correlated with the stress intensity factor K.

The incubation time is decreased by the superposition of repeating load, as the range of stress intensity factor ΔK or the repeating frequency f increase. The reason can be explained by the promotion of corrosion reaction due to, e.g. the destruction of oxide film on the crack tip, which facilitates the invasion of hydrogen atoms into the material.

The crack propagation rate da/dt is decreased by the superposition of repeating load, and there exist two valleys of crack propagation rate minima on the da/dt vs f and da/dt vs ΔK curves. One valley corresponds to the interaction between the cyclic movement of the region with tri-axial tensile stress and the hydrogen atoms diffused from crack tip, which disturbs the concentration of hydrogen atoms. Another seems te correspond to the generation of retained compressive stress which reduces the effective stress intensity at crack tip and supresses the invasion and diffusion of hydrogen atoms.     

Dislocations at ductile/plastic crack tips: in-situ TEM observations

 In-situ observations of dislocation structures ahead of crack tips in TEM metal foils are reviewed. Two cases are compared in particular: Structure development during in-situ straining to failure of (i) electron-transparent foils ahead of the tip of a growing crack that spreads from the thinnest regions or perforations and (ii) initially non-transparent thick foils. In the latter case cracks formed only after substantial in-situ straining, and they propagated along dislocation cell walls via repeated stimulated crack nucleation ahead of the tip. This behavior was shown to adequately simulate bulk behavior and such cracks do not exhibit dislocation-free zones at their tips. By contrast, dislocation-free regions along ligaments formed by crack propagation and observed in thin (e.g. about 100 nm or less) TEM foils are found to be artifacts due to strong dislocation image forces. These image forces at the same time limit mutual dislocation interactions to the thickness of the foil, and rotate the dislocations to be normal to the foil plane, meanwhile straightening them. This behavior has no correspondence to conditions at real cracks in bulk materials. Theoretical expressions are derived for the dislocation densities ahead of crack tips that give rise to long-range and shorter range stress fields in mode I crack tip configurations, respectively. Received:19 December 1997 / Accepted: 22 December 1997     

High-temperature X-ray structural,thermal and dielectric characteristics of ferroelectric Bi2VO5.5

The X-ray powder diffraction, dielectric and thermal studies of bismuth vanadate (Bi₂VO_5.5) ceramic have been carried out as a function of temperature (300–900 K). The hightemperature X-ray studies, supported by differential scanning calorimetry, clearly demonstrate that Bi₂VO_5.5 undergoes two major phase transitions at 730 and 835 K. It was found that the one at 730 K is associated with both the ferroelectric and the crystallographic transition, while at 835 K, Bi₂VO_5.5 undergoes only the crystallographic transition. Anomalies in both the dielectric constant and specific heat curves have been observed at 730 and 835 K. The total heat, Q, and entropy, S, associated with the transition at 730 K were found to be higher than those at 835 K.     

Workbench: an AI approach to materials modelling

The Workbench is an adaptable and efficient tool for performing thermodynamic and kinetic calculations. This computer program uses various artificial intelligence techniques to provide a more versatile modelling system than has been possible with conventional programs. The structure and operation of the program is described and a simplified example is worked through illustrating the Workbench environment. Practical applications are given of the use of the Workbench to predict magnetic microstructures in Nd-Fe-B permanent magnets and to study the long term structural stability of Co-Pd multilayers.     

Molecularly imprinted polymer beads: suspension polymerization using a liquid perfluorocarbon as the dispersing phase

A suspension polymerization technique suitable for molecular imprinting is described, based on the use of a liquid perfluorocarbon as the dispersing phase. This dispersant does not interfere with the interactions between functional monomers and print molecules required for the recognition process during molecular imprinting. The method produces polymer beads, with almost quantitative yield, which can be used after only a simple washing step. An acrylate polymer with perfluorocarbon and poly(oxyethylene) ester groups was used to stabilize an emulsion of functional monomer, cross-linker, print molecule, initiator, and porogenic solvent in perfluoro(methylcyclohexane). Initiation of polymerization by UV irradiation resulted in polymer beads. The average bead size could be controlled between about 50 and 5 μm by varying the amount of stabilizing polymer. SEM of the beads indicated spherical particles with morphology typical of beads made by suspension polymerization. The technique was applicable to a range of conditions typically used for molecular imprinting. A detailed chromatographic study of the polymer beads confirmed that α values and resolution factors were similar to those achieved with traditional ground and sieved imprinted polymers. Small (5 μm) beaded packings gave low back pressure and rapid diffusion, giving good separation even at high flow rates.     

Fracture toughness—microstructure investigation of hyperbarically welded joints in offshore application

The metallurgical changes have been studied in a typical offshore structure material, BS4360 grade 50E steel at various simulated depths (0–185 m) in the dry underwater welding conditions and the weld metal and heat affected zone (HAZ) fracture toughness have been related with the microstructure and micromechanisms of fracture. The results indicated substantial decrease in weld metal CTOD toughness at 185 m welding depth whereas HAZ toughness remained generally unaffected. High toughness of weld metal was found to be associated with greater proportion of the polygonal and the acicular ferrites and negligible proportion of aligned cerbide. High HAZ toughness was due to fine ferrite (6–8 μm) and absence of martensite in the structure.     

Preparation of YBa2Cu3O7–x high-Tcceramic superconductors from melt

Fabrication of high-T _c ceramic superconductor in the system Y₂O₃-BaO-CuO by melting a mixture of component oxides has been investigated. The compositions of the resulting specimens and the effects of heat treatment have been investigated. It was determined that molten material was composed of phases including BaCuO₂, CuO, Y₂O₃, and Y₂BaCuO₅. A subsequent heat treatment in air produced a nominal amount of the high-T _c phase, while heat treatment in an O₂ atmosphere resulted in a significantly large percentage of the superconducting phase.     

On the feasibility of high spatial resolution superconducting X-ray detectors

We discuss a class of superconducting particle/radiation detectors in which the readout is by magnetic means. Even a small energy deposition can change the state of a superconductor, leading to a drastic change of its electromagnetic properties. To increase the detector sensitivity, a highly granulated superconducting medium is used. Millions of physically separated sensors are read out in parallel by only a few channels of very sensitive D.C.-SQUID based electronics. We present the mathematical formulation of the problem and discuss how the excellent signal-to-noise ratio obtained using SQUIDs permits us to determine particle/photon localization.     

Splitting of the phonon spectrum in3He-4He solutions

The dynamic structure function for ⁴ He- ⁴ He density correlationsS ⁴⁴(k, ) is calculated as a function of the wave vectork and frequency for a simple model of a ³ He- ⁴ He solution at various temperatures and small ³ He molar concentrationx. The inputs to the model include the measured ⁴ He phonon spectrum and the zero-concentration ³ He quasiparticle energy (above thek=0 value), which is taken to be quadratic ink and to intersect the phonon spectrum atk _c near the roton minimum. Taking into account the decay of a phonon into a quasiparticle-hole pair, which is effected in the model by a quantum hydrodynamic interaction, we find that the phonon spectrum is split into two distinct branches. Atk _c the splitting of the phonon branches depends on a coupling constant and the crossover parameter (k _c )/k _c . If in ³ He- ⁴ He solutions is large enough and (k _c )/k _c is small enough, then the phonon spectrum is split into two branches.Work performed in part under the auspices of the U.S. Atomic Energy Commission.Resident Student Associate, Summer 1972, at Argonne National Laboratory, Argonne, Illinois.Supported in part by a Faculty Research Grant and Fellowship from the Horace H. Rackham School of Graduate Studies at the University of Michigan.     

Low-temperature specific heat of high-purity calcium

Low-temperature specific heat of high-purity calcium has been measured with an adiabatic calorimeter with a mechanical heat switch to avoid helium exchange gas errors, over the temperature range 1.1–4.2 K. The values obtained for the electronic coefficient of specific heat and the Debye temperature _D are =1.99±0.05 mJ/moleK ² and _D=250±4K.