J.A. McGeough

Age-related changes in the tensile properties of cortical bone. The relative importance of changes in porosity, mineralization, and microstructure.

Tensile testing to failure was done on 235 cortical specimens that had been machined from forty-seven femora from human cadavera. The donors had ranged in age from twenty to 102 years at the time of death. After mechanical testing, the porosity, mineralization, and microstructure were determined. Linear regression analysis showed that the mechanical properties deteriorated markedly with age. Ultimate stress, ultimate strain, and energy absorption decreased by 5, 9, and 12 per cent per decade, respectively. The porosity of bone increased significantly with age, while the mineral content was not affected. Microstructural analysis demonstrated that the amount of haversian bone increased with age. Both bivariate and multivariate analyses demonstrated the importance of age-related changes in porosity to the decline in mechanical properties. Changes in porosity accounted for 76 per cent of the reduction in strength. Microstructural changes were highly correlated with porosity and therefore had little independent effect. Mineral content did not play a major role. Thus, the quantitative changes in aging bone tissue, rather than the qualitative changes, influence the mechanical competence of the bone.

New Developments in Electro-Chemical Machining

Abstract Electrochemical machining (ECM) has traditionally been used in highly specialized fields such as those of the aerospace and defense industries. It is now increasingly being applied in other industries where parts with difficult-to-cut materials and complex geometry are required. In this paper the latest advances are discussed, and the principal issues in ECM development and related research are raised. Developments in tool design, pulse current, micro-shaping, finishing, numerically controlled, environmental concerns, hybrid processes, and recent industrial applications, are covered.

Age-related changes in the Compressive strength of cancellous bone : The relative importance of changes in density and trabecular architecture

Compressive testing to failure in the weight-bearing axis was done on 255 specimens of cancellous bone that had been machined from forty-four femora from human cadavera. The donors had ranged in age from twenty to 102 years at the time of death. After mechanical testing, the apparent density and trabecular architecture were determined. Linear regression analysis showed that the compressive strength decreased by 8.5 per cent each decade (p < 0.001). Apparent density and volume fraction also decreased significantly with age (p < 0.001). Histomorphometric analysis demonstrated that the surface-to-volume ratio and the mean separation of the trabecular plate increased with age, whereas the mean thickness and connectivity of the trabecular plate decreased. Both bivariate and multivariate analyses demonstrated that age-related changes in apparent density played an important role in the decrease in mechanical strength, accounting for a 92 per cent reduction. Microstructural changes were highly correlated with apparent density and therefore had little independent effect. Thus, similar to the situation with cortical bone, the quantitative changes in aging cancellous-bone tissue, rather than the qualitative changes, influenced the mechanical competence of the bone. CLINICAL RELEVANCE: This study provides information concerning the difference in the properties of human cancellous bone as a function of age. Because of the importance of changes in apparent density, non-invasive means can be used to estimate the mechanical properties of cancellous bone in vivo. Thus, it may be possible to predict the risk of fracture and to explain further some aspects of the mechanics of fracture in the elderly.

Electroforming process and application to micro/macro manufacturing

Abstract Electroforming is the highly specialised use of electrodeposition for the manufacture of metal parts. This paper describes the process principles and mechanisms of electroforming, outlining its advantages and limitations. A review of modelling and simulation of electroforming and experimental analysis work is also presented. The metals that can be electroformed successfully are copper, nickel, iron or silver, thickness up to 16 mm, dimensional tolerances up to 1 μm, and surface finishes of 0.05 μm R a . The ability to manufacture complex parts to close tolerances and cost effectively has meant that electroforming has applications both in traditional/macro manufacturing and new micromanufacturing fields. These include tooling; mould making; fabrication of microelectromechanical systems (MEMS) and the combination of lithography, electroforming and plastic moulding in the LIGA process. Applications in micro-optics and medicine are included.

Precision ECM by Process Characteristic Modelling

Abstract Electrochemical machining (ECM), which is not normally considered as a precision process, is used to achieve accuracy better than 5 urn and surface finish 0.03 μm R a by using pulsed power of relatively short durations (1–10 ms) and narrow inter-electrode gaps (10–50 μm). The narrow gaps, however, make the control and prediction of the process much more complex than normal ECM. An empirical model is developed based on the characteristic relationships of ECM to predict and optimise the process parameters such as dissolution efficiency, current density, electrolyte concentration and pulse duration, in narrow gaps. This model is then used to facilitate new applications of high precision ECM without recourse to the lengthy trial-and-error approach, by predicting selective dissolution of the workpiece and tool design.

Influence of Electrolyte Concentration on Copying Accuracy of Precision-ECM

Abstract A precision ECM process, dimensional accuracy ± 2 μm, surface finish 0.01 μm Ra has been developed using narrow inter-electrode gaps (

Process monitoring of electrochemical micromachining

Abstract Electrochemical micro-machining (ECμM), utilises very small inter-electrode gaps (

Computer applications in unconventional machining

Abstract The progress of unconventional machining through the application of computer-aided technology is presented. The processes of ECM, EDM, LBM, AWJM are discussed in relation to the influence of CAD, CAM, CAE, CNC, CIM and on their acceptance by manufacturing industry. EDM and LBM can readily be integrated onto the machine shop floor today. Other processes have lesser degrees of automation, mainly because their applications tend to be more specialised. All processes need to be able to be fully automated and integrated if they are to remain competitive.

An intelligent knowledge-based system for wire-electro-erosion dissolution in a concurrent engineering environment

Abstract An intelligent knowledge-based system for evaluating wire-electro-erosion dissolution (WEED) in a concurrent engineering (CE) environment and based on object-oriented techniques, is introduced. The design description is obtained through a feature-based approach. Nine different classes of design features are interactively acquired. The attributes of steel as a workpiece material, copper wire as a tool material, a single electrolyte solution, one type of WEED machine, and machining conditions such as current pulse on- and off-time, and nozzle distance, are stored in a database. For each design feature, information needed in manufacturing, such as the machining cycle time and cost, material removal rate, width of cut, maximum and working feed-rate, cutting area, and operation efficiency are estimated.

Process Energy Analysis for Aluminium Alloy and Stainless Steel in Laser-Assisted Jet Electrochemical Machining:

Abstract Laser-assisted jet electrochemical machining (LAJECM) is a hybrid process that combines a relatively low-power laser beam (typically 375 mW) with an electrolyte jet to facilitate metal removal. The main purpose of the laser is to enhance the localization effect of electrochemical dissolution from the workpiece, thus giving better precision and efficiency. The laser thermally activates the material surface where it impinges, thereby increasing the electrochemical current density in that localized zone. This paper reports recent investigations of LAJECM process energy distribution and explains the influence of laser assistance on dissolution removal rates using theoretical as well as experimental analysis. It was found that laser assistance increases energy in the LAJECM process by up to 45 per cent compared with jet electrochemical machining (JECM). The process energy has also been related to volumetric removal rate, and the specific energy required for machining with given variables has been calculated. It has been proved that LAJECM is more effective then JECM as the calculated specific energy was lower by up to 30 per cent.