Analysis of semi-solid response under rapid compression tests using multi-scale modelling and experiments

Simulating semi-solid metal forming requires modelling semi-solid behaviour. However, such modelling is difficult because semi-solid behavior is thixotropic and depends on the liquid-solid spatial distribution within the material. In order to better understand and model relationships between microstructure and behavior, a model based on micromechanical approaches and homogenisation techniques is presented. This model is an extension of a previous model established in a pure viscoplastic framework to account for elasticity. Indeed, experimental load-displacement signals reveal the presence of an elastic-type response in the earlier stages of deformation when semi-solids are loaded under rapid compression. This elastic feature of the behaviour is attributed to the response of the porous solid skeleton saturated by incompressible liquid. A good quantitative agreement is found between the elastic-viscoplastic predicted response and the experimental data. More precisely, the strong initial rising part of the load-displacement curve, the peak load and the subsequent fall in load are well captured. The effect of solid fraction on mechanical response is in qualitative agreement with experiments.     

A laboratory investigation of a physical rnechanisim for the extended infrared absorption (‘red shift‘) in wheat

Laboratory spectral measurements, on the components of both greenhouse and field grown winter wheat, were performed to identify the component and its appropriate response which gave rise to the extended infrared absorption or ‘red shift’ reported by Collins. Results of this study indicated that inherent intraplant adaxial (upper) leaf reflectances were of sufficient variability to suggest that an admixture of mechanisms may have utility on identifying the booting and head emergence stages in the life cycle of wheat. The physical mechanism for the shift was found to be relatively independent of the inherent variability in leaf spectra, and to be dependent upon the difference in the mode of deposition of cuticle upon the abaxial (lower) surface relative to that of the adaxial (upper) surface, ihe position of the flag leaf, and thus the surface exposed to the incident light during heading and after emergence of the head.     

Surface Analytical Study of Synroc Subjected to Hydrothermal Attack

Specimens of the ceramic Synroc, both with (Synroc C) and without the addition of a simulated nuclear waste (Synroc B), were subjected to hydrothermal attack. The near-surface regions, both before and after attack, were characterized using X-ray photoelectron spectroscopy, Auger electron spectroscopy, and scanning electron microscopy. Ion-beam erosion was used in conjunction with these surface analysis techniques to obtain information from below the original surface. It was found that hydrothermal attack led to the formation of a modified layer extending to a maximum depth of ∼200 nm after 25 days at 350°C. Within this modified layer, the abundances and chemical states of the elements were different from those in the unattacked Synroc. The changes which were observed can be explained qualitatively by postulating that selective dissolution (e.g., for Ca and Ba), hydration (e.g., for Al and Fe), recrystallization (e.g., for Ti and Zr), and reaction with Co₂ dissolved in the liquid phase (e.g., to form CaCO₃) are all taking place.     

Exploiting Persistence in Build Management

A challenging issue in the construction and maintenance of large application systems is how to determine which components need to be rebuilt after change, when and in which order. Rebuilding is typically recompilation and linking, but may also include update of derivable components such as cross-reference databases and re-creation of library indexes. Type definitions or schema, and data values in a file store, database or persistent store may also need to be rebuilt. The main purpose of this paper is to describe how persistent language technology can be exploited to enhance build management. In particular, the paper describes a method for transactional, incremental linking and the implementation of its support. To help implement this method, and to make it safer and more efficient to carry out rebuild activities in general, we have defined a set of automatically checkable constraints on the software. The build management tool we have implemented, the Builder, derives rebuild dependencies automatically and offers partitioning of dependency graphs—a means to defer or avoid unnecessary rebuilding. The Builder is implemented in a persistent programming language and provides build management for applications written in the language. It exploits features such as strong typing, runtime linguistic reflection, and referential integrity provided by the language processing technology. The Builder operates over both programs and (complex) data, which is in contrast to conventional language-centred tools. © 1997 by John Wiley & Sons, Ltd.     

Effect of magnesium alloying additions on infiltration threshold pressure and structure of SiC powder compacts infiltrated by aluminium-based melts

Infiltration of 53% dense preforms of 23 m size SiC particulate by Al–0 to 14 wt% Mg alloy melts has been carried out at different applied pressures at 750°C, together with microstructural characterization of the resulting composites. The threshold pressure P* for infiltration decreased with increasing Mg content in the melt at twice the rate by which its surface tension decreased, the residual effect being attributable to the expected effect of Mg on the contact angle between Al and SiC. Periodic bands of entrapped porosity at lower Mg contents and infiltration pressures just above P* are thought to have arisen from periodic arrest of the infiltration front pending ventilation of the gas accumulated at the front. The formation of monolithic Mg2Si and areas of lamellar Al–Mg2Si eutectic observed at higher Mg contents is associated with the accumulation of 5 wt% Si in the melt as a result of reaction between the melt and SiC.     

Coarsening rate of microstructure in semi-solid aluminium alloys

The rate of microstructural coarsening of recrystallisation and partial melting (RAP) and cooling slope (CS) processed materials in the semi-solid state is compared with rates found in the literature. The rate of coarsening depended on the liquid fraction but RAP route 2014 alloy with 37% liquid coarsened slightly more slowly than the CS route 2014 alloy with a lower liquid fraction of 17%, contrary to expectations. For the CS route, an increase in liquid fraction resulted in faster coarsening. A modified 2014 alloy with Fe, Mn and Zn stripped out of the composition gave a relatively high coarsening rate. The coarsening rate was also relatively high for CS 201 alloy in comparison with either RAP 2014 or CS 2014. Low coarsening rates are thought to be associated with the presence of particles which inhibit the migration of liquid film grain boundaries. This could be the result of pinning or of the liquid film impeding diffusion at the boundary.     

BIMATERIAL INTERFACE CRACK TIP STRESS ANALYSIS: ANISOTROPIC THEORY

Abstract— Near crack tip stress and displacement fields are obtained for anisotropic bimaterial interface cracks. A contact zone model is used in order to get rid of the unphysical oscillatory interpenetration between the edges of the crack. Semi-infinite and the finite crack problems have been studied. Using the near crack tip results of this model crack branching angles can be predicted. These results are illustrated by numerical results for various materials.     

Hot isostatic pressing of metal reinforced metal matrix composites

Metal reinforced Metal Matrix Composites (MMMCs) made by combining an aluminium alloy matrix with stainless steel reinforcing wires are potentially cheaper and tougher than continuous fibre ceramic reinforced Metal Matrix Composites (MMCs). Although they do not give as great enhancements in stiffness and strength, worthwhile gains are achieved. Such MMMCs can be produced by Hot Isostatic Pressing (HIPping), which reduces interfacial reactions in comparison with liquid metal routes. Here, stainless steel (316L) and commercial purity aluminium wires were used to make bundles which were inserted into mild steel cans for HIPping at 525 °C/120 min/100 MPa. Some stainless steel wires were pre-coated with A17Si, to examine the effect of coatings on mechanical properties. Specimens were evaluated in terms of their tensile and fatigue properties. During HIPping, cans collapsed anisotropically to give different cross-section shapes, and for larger diameter cans, there was also some longitudinal twisting. Wires tended to be better aligned after HIPping in the smaller diameter cans, which produced material having higher modulus and UTS. Higher volume fractions of reinforcement tend to give better fatigue properties. Composites with coated stainless steel wires gave higher composite elongation to failure than uncoated wires. Both uncoated and coated wires failed by fatigue during fatigue testing of the composite. This contrasts with ceramic reinforced MMCs where the fibres fracture at weak points and then pull out of the matrix.     

Adaptive non-linear Bayesian estimation for bounded-noise systems†

An adaptive non-linear filtering algorithm is developed for handling linear, time-invariant, discrete, dynamical systems with bounded input and measurement noise disturbances. The new result yields the conditional-mean state estimate by combining the Bayesian decision rule, a reachable set concept and a technique of approximating probability density functions by their moments. The adaptive feature is needed because only the input noise bounds, rather than the complete input density function, are assumed to be known. Numerical comparisons on a second-order system are presented to demonstrate that the non-linear filtering algorithm adapts itself to yield considerably more accurate results than those obtained by applying the best linear filter.