Influence of porosity on cavitation instability predictions for elastic–plastic solids

Cavitation instabilities have been found for a single void in a ductile metal stressed under high triaxiality conditions. Here, the possibility of unstable cavity growth is studied for a metal containing many voids. The central cavity is discretely represented, while the surrounding voids are represented by a porous ductile material model in terms of a field quantity that specifies the variation of the void volume fraction in the surrounding metal. As the central void grows, the surrounding void volume fractions increase in nonuniform fields, where the strains grow very large near the void surface, while the high stress levels are reached at some distance from the void, and the interaction of these stress and strain fields determines the porosity evolution. In some cases analysed, the porosity is present initially in the metal matrix, while in other cases voids nucleate gradually during the deformation process. It is found that interaction with the neighbouring voids reduces the critical stress for unstable cavity growth.     

Elastic–plastic void expansion in near-self-similar shapes

For void growth in an elastic–plastic strain hardening material the preferred shape of the void is calculated, dependent on the macroscopic stress state. Axisymmetric cell model analyses are carried out with a very small initial void size relative to the cell dimensions. Large deformations of the material around the void are modeled until the void volume is four orders of magnitude larger than the initial volume. An iterative procedure is used until the final void shape and the initial void shape are identical. Even when this convergence has been obtained, the void shape does not stay constant during the growth. Thus, the shapes found give only approximately self-similar growth. The results are compared with self-similar shapes determined previously for nonlinear viscous solids, subject to power law creep. For the time independent elastic–plastic material considered here the effect of the strain hardening level and of the initial yield strain are studied.     

Analysis of creep rupture in a notched tensile bar

For circumferentially notched, round tensile bars the creep rupture behaviour is analysed, based on constitutive relations that account for the nucleation and growth of grain boundary cavities in polycrystalline metals at high temperatures. Both diffusive cavity growth and growth by dislocation creep of the surrounding grains is incorporated in the model, and in some cases free grain boundary sliding is assumed. Failure by cavity coalescence is predicted at small overall strains in the range where cavity growth is constrained by the rate of dislocation creep of the grains, whereas outside this range large occur prior to failure.In the analyses for notched specimens, where the stress fields are strongly non-uniform, first failure occurs at the notch tip, and subsequently a macroscopic crack grows into the material. Various combinations of material parameters are considered, and in most cases the crack is found to grow in the plane of the notch. The results are related to earlier experimental and computational investigations of creep rupture in notched bars.     

Building a Vision - A Practical Guide

Vision building is as old and natural as man himself. Vision building has become an effective means for individuals and organizations to set and achieve goals. A practical step by step guide to building a vision is presented, which was originally designed for building visions in groups and organizations, although with some modifications may also be used for generating individual visions.     

Fibre debonding and breakage in a whisker-reinforced metal

A numerical cell-model analysis of the development of damage in aluminium reinforced by aligned, short SiC fibres, is given for materials with arrays of transversely aligned fibres, to be compared with the results obtained previously for transversely staggered fibres. The stress distributions in the matrix and fibres are quite different for the different fibre arrangements, and this has a significant effect on failure by fibre breakage or decohesion. Fibre fracture is represented by a critical value of the average tensile stress on a cross-section, while interfacial failure is modelled in terms of a cohesive zone formulation that accounts for decohesion by normal separation as well as by tangential separation. It is shown that highly constrained plastic flow associated with transversely aligned fibres gives rise to early transverse void growth towards coalescence, but the onset of failure requires higher stress levels.     

On the buckling of elastic-plastic columns with asymmetric cross-sections

The post-bifurcation behaviour of elastic-plastic perfect columns compressed into the plastic range is studied analytically by application of an asymptotic theory, and the behaviour of columns with initial imperfections is computed numerically using an incremental method. For columns with asymmetric cross-sections both the plastic post-bifurcation behaviour and the sensitivity to imperfections are found to be asymmetric even though the same columns behave symmetrically when made of a material that stays linearly elastic. Numerical results are given for a column of U-shaped cross-section and for a column of solid triangular cross-section.     

A creep rupture model accounting for cavitation at sliding grain boundaries

An axisymmetric cell model analysis is used to study creep failure by grain boundary cavitation at facets normal to the maximum principal tensile stress, taking into account the influence of cavitation and sliding at adjacent inclined grain boundaries. It is found that the interaction between the failure processes on these two types of adjacent facets reduces the failure time significantly when cavitation is creep constrained. In all cases the time to cavity coalescence on transverse facets appears to be a useful lower bound measure of the material life-time. Sliding at the boundaries of the central grain of the cell model is accurately represented; but in some computations a stress enhancement factor is used to incorporate also the effect of sliding between surrounding grains. The influence of grain boundary viscosity is included in the model and it is found that even in the absence of sliding, cavitation on inclined boundaries may significantly reduce the failure time.     

Tracking airborne Legionella and Legionella pneumophila at a biological treatment plant

Biological treatment plants are frequently used to degrade organic substances in wastewater from wood refinement processes. Aeration ponds in such plants provide an optimal growth environment for many microorganisms, including Legionella species. To investigate whether legionellae could be dispersed as aerosols from the ponds and transported by the wind, the wetted-wall cyclone SASS 2000(PLUS) and the impactors MAS-100 and STA-204 were used to collect air samples directly above, upwind, and downwind of aeration ponds during a 4-month period. Computational fluid dynamics was used a priori to estimate the aerosol paths and to determine suitable air-sampling locations. Several Legionella species, including Legionella pneumophila, were identified in air samples at the biological treatment plant using microbiological and molecular methods. L. pneumophila was identified up to distances of 200 m downwind from the ponds, but, in general, not upwind nor outside the predicted aerosol paths. The highest concentration level of viable legionellae was identified directly above the aeration ponds (3300 CFU/m3). This level decreased as the distance from the aeration ponds increased. Molecular typing indicated that a single clone of L. pneumophila was dispersed from the ponds during the period of the study. Thus, our study demonstrated that aerosols generated at aeration ponds of biological treatment facilities may contain L. pneumophila, which then can be transported by the wind to the surroundings. The methods used in this study may be generically applied to trace biological aerosols that may pose a challenge to environmental occupational health.