Filters
Availability
Publication Year
Resource Type
Faculty
School/Institute
Publication Search Results
Sort
Results per page
1 - 10 of 13
-
(1999)Journal ArticleThis paper examines the history of selenium pollution in Lake Macquarie, NSW, Australia, and three factors that may affect the redistribution and remobilisation of particle bound selenium: changes in redox state, bioturbation, and bioaccumulation by macrobenthos and bacteria. Sediment cores were taken from Nords Wharf, a relatively unpolluted area, and from Mannering Bay near the Vales Point coal-fired power station. The age profile at the unpolluted site seems to indicate that mild selenium pollution has been occurring for over 100 years, however, some mixing of the sediments has occurred. At the polluted site, the age profile indicated that major contamination has occurred in the last 30 years, due to an ash dam associated with nearby electric power generation facilities. The contamination chronology suggests that remobilisation and reduction processes have affected the selenium profile. Changing the redox state of Lake Macquarie sediment results in a release of selenium under oxidising conditions and immobilisation under reducing conditions. The sediment-bound selenium was associated with the operationally defined `organic/sulfide' fraction under reducing conditions, and as the redox potential increases this moves into the `exchangeable' and `iron/manganese oxyhydroxide' phases to a limited extent. Bioturbation by the animals Marphysa sanguinea and Spisula trigonella caused increases in the redox potential and pore water selenium concentrations in surfcial sediments relative to unbioturbated controls. Both animals accumulated significantly more selenium when exposed to contaminated sediment than when exposed to uncontaminated control sediments. Selenium concentrations in molluscs from Mannering Bay were all significantly higher than those collected from Nords Wharf. Most of the selenium in the mollusc tissues was found to be associated with the protein fraction. Selenium isolated from hydrolysed muscle tissue was not present as selenate or selenite but as selenomethionine and an unidentified compound. Seven types of bacteria were isolated from Lake Macquarie sediment. All seven isolates were able to transform selenite quantitatively to elemental selenium as evidenced by a red precipitate and identified by X-ray diffraction. Six isolates grew on media containing selenate but no elemental selenium was formed. Mass balances showed that for three isolates total selenium was conserved, selenate decreased and selenium (0; II-) increased indicating the production of non-volatile organic selenium compounds. For two isolates both total selenium and selenate decreased with no increase in selenium (0; II-), therefore, loss of selenium occurred from the media. Selenium is immobile in anoxic reduced sediments but may become available to benthos and fish as a consequence of sediment oxidation associated with bioturbation leading to bioaccumulation and transformation by macrobenthos and bacteria. These mechanisms can be invoked as possible transport pathways to explain the presence of selenium above background concentrations in preindustrial sediments, but further work dating the sediments in which elevated concentrations of selenium are found is needed to confirm this hypothesis.
-
(1999)Journal ArticleMeasurements of selenium in sediments and benthic infauna of Lake Macquarie, an estuary on the east coast of Australia, indicate that sediments are a significant source of selenium in the lake's food web. Analysis of surficial sediment samples indicated higher selenium con- concentrations near what are believed to be the main industrial sources of selenium to the lake: a smelter and a power station. Sediment cores taken from sediments in Mannering Bay, near a power station at Vales Point, contained an average of 12 times more selenium in surficial sections than sediment cores from Nord's Wharf, a part of the lake remote from direct inputs of selenium. The highest selenium concentration found in Mannering Bay sediments (17.2 mg/g) was 69 times the apparent background concentration at Nord's Wharf (0.25 mg/g). Pore water concentrations in Mannering Bay were also high, up to 5 mg/l compared to those at Nord's Wharf which were below detection limits (0.2 mg/l). Selenium concentrations in muscle tissues of three benthic-feeding fish species (Mugil cephalus, Platycephalus fuscus, Acanthopagrus australis) were significantly correlated ( p < 0:05) with surficial sediment selenium concentra- tion. Selenium concentrations in polychaetes and molluscs of Mannering Bay were up to 58 times higher than those from Nord's Wharf. Two benthic organisms, the eunicid polychaete Marphysa sanguinea and the bivalve mollusc Spisula trigonella, were maintained at different densities in selenium-spiked sediments. Both animals accumulated selenium from the spiked sediment, confirming that bioaccumulation from contaminated sediments occurs. Collectively, these data suggest that benthic food webs are important sources of selenium to the fish of Lake Macquarie.
-
(1998)Journal ArticleA recent publication says that the primary spelling of the word for the depth of water required to float a ship is draught, with draft as the secondary spelling. A survey of thirty practising naval architects was made to check the claim, and found that a large majority of Australian-educated naval architects spell it draft. The overall results are interesting and are given here.
-
(1998)Journal ArticleThe two types of marine hydrometer commonly used by naval architects and surveyors measure two distinct, but related, properties. The properties have different units and are used for different purposes. Some users may not be aware of the two types of hydrometer, the properties measured, or the errors which can significantly affect the end result. The relationships between the measured properties are discussed, together with the specific applications of the two types of hydrometer. The conversion of measurements made on one type of hydrometer to the other type is given, with examples of the conversions. Application of the principles presented here will prevent confusion and ensure the use, rather than abuse, of the two types of marine hydrometer.
-
(1999)Journal ArticleAbrasive Waterjet (AWJ) cutting is an emerging technology for material processing with the distinct advantages of no thermal distortion, high machining versatility, high flexibility and small cutting forces. In this paper, an experimental investigation of the machinability and kerf characteristics of polymer matrix composite sheets under abrasive waterjets is presented. It shows that this unique ‘cold’ cutting technology is a viable and effective alternative for polymer matrix composite processing with good productivity and kerf quality. Plausible trends of kerf quality with respect to the input parameters are discussed, from which recommendations are made for process control and optimization.
-
(1999)Journal ArticleAn investigation of the cutting performance and erosive process in abrasive waterjet (AWJ) machining of polymer matrix composites is presented. It shows that AWJ cutting can produce good quality kerf at high production rate if the cutting parameters are properly selected. Plausible trends of the cutting performance, as assessed by the various kerf geometry and quality measures, with respect to process parameters are discussed. The traverse speed, water pressure and abrasive flow rate are found to have profound effect on the total depth of cut and kerf taper angle, while the first two variables also affect heavily on the kerf width. The study shows that the optimum jet forward impact angle in the cutting plane is about 80 which increases only marginally the total depth of cut and has little effect on the other kerf characteristics. It is found that good quality kerf without delamination can be achieved if through cut is attained. A scanning electron microscopy (SEM) analysis of the cut surfaces reveals that the erosive process for the matrix material (resin) involves shearing and ploughing as well as intergranular cracking. Shearing or cutting is found to be a dominant process for cutting the fibres in the upper cutting region but the fibers are mostly pulled out in the lower region. Mathematical models for the total depth of cut are finally developed and verified, together with empirical models for the other kerf geometrical features.
-
(1999)Journal ArticleA study of Abrasive Waterjet (AWJ) cutting of metallic coated sheet steels is presented based on a statistically designed experiment. It shows that AWJ cutting is a viable technology for processing metallic coated sheet steels with good productivity and kerf quality. A scanning electron microscopy analysis indicates that micromachining and plastic deformation are the dominant cutting phenomena in sheet steel processing. Plausible trends and relationships between kerf characteristics and process parameters are discussed. It is found that an optimum water pressure together with small standoff distance between the nozzle and workpiece may be used, while the traverse speed should be selected as high as possible for through cuts in order to increase the cutting rate. Empirical models for kerf geometry and quality are finally established for the prediction and optimization of AWJ cutting performance.
-
(1998)Journal ArticleThe selection of economic cutting conditions in machining operations is becoming increasingly important in modern computer based-manufacturing. Using a generic deterministic approach, the optimization analyses, strategies and software for selecting the economic cutting conditions in drilling operations are presented based on the criteria typified by the maximum production rate (or minimum production time per hole) and incorporating a range of machine tool constraints. The deterministic optimization approach involving mathematical analyses and graphic presentation of economic characteristics provides clearly defined optimization strategies which guarantee the global optimum solution. Numerical simulation studies have verified the optimization strategies and demonstrated the economic benefits of using optimization in process planning as well as the suitability of the developed optimization strategies for on-line applications in Computer-Aided Manufacturing (CAM).
-
(1999)Journal ArticleSheet steels with zinc and aluminium coatings, such as GALVABOND, exhibit an anomalous behaviour when subjected to laser light and pose some severe machining limitations by virtue of the high light reflectivity and thermal conductivity of the coatings as well as the difference in physical properties between the coating and substrate materials. An investigation of the machinability of GALVABOND using CO2 laser cutting technology is presented. It shows that by proper control of the cutting parameters, good quality cuts are possible at high cutting rate. Visual examination indicates that when increasing the cutting rate to as high as 5000 mm/min, kerfs of better quality than those produced using the parameters in an early study can be achieved. Some kerf characteristics such as the width, heat affected zone and dross with respect to the process parameters are also discussed. A statistical analysis has arrived at the relationships between the cutting speed, laser power and workpiece thickness, from which a recommendation is made on the selection of cutting parameters for process control and optimization.
-
(1998)Journal ArticleOptimization analysis, strategy and CAM software for single pass end-milling on CNC machine tools are outlined and discussed based on criteria typified by the maximum production rate and allowing for a range of machine tool and component surface roughness constraints. It is shown that the deterministic optimization approach involving mathematical analyses of constrained economic trends and their graphical representation on the feed-speed domain provides a deeper understanding of the influences of constraints and a clearly defined strategy which guarantees the global optimum solutions. Numerical simulation studies have amply demonstrated the economic benefits of using this strategy over handbook recommendations as well as in assessing, selecting and improving machine tool design specifications.