The theory of extrusion—Advances and challenges

Plane-strain slip-line field theory, upper bound analysis and the visioplasticity technique form the basis for the mechanics of extrusion. Bishop [Metall. Rev. 2, 361 (1957)] pointed out in 1957 that to make the theory useful in practice, one had to use the mechanics of extrusion to quantify phenomena that were primarily metallurgical, such as speed-limit phenomena, flow resistance, the evolution of microstructure and properties of the extrudate.

Time has shown that the gap between the metallurgy of extrusion and the mechanics of extrusion was of such a character that only in recent years, by the full impact of computer simulation with thermomechanical coupling and realistic constitutive formulation of thermoviscoplastic solids, has one seen that the mechanics and metallurgy of extrusion have begun to emerge into a unified theory of extrusion.

The main challenge is now to develop the theory of plasticity and constitutive models that can handle metallurgical reactions and the development of microstructure in the sharp rates and gradients of strain rate and temperature observed in extrusion.

Another important challenge to the theory of extrusion is to transform basic knowledge about extrusion and the subsequent manufacturing processes into a form that is useful in product design engineering.

The practical part of this review focusses on unlubricated extrusion of aluminium alloys and on product and process development related to aluminium extrusions.     

Magnus效应下的前缘旋转圆柱参数设计

通过数值仿真方法研究了一种前缘附加旋转圆柱的NACA0015翼型气动性能,分析了圆柱与主体间间隙大小及圆心距离前缘位置两个主要参数对翼型升阻比的影响,获得了一组具有最佳气动特性的旋转圆柱参数,并利用此翼型建立了二维基于Magnus效应的H型垂直轴风力机模型。基于滑移网格技术,选用Spalart-Allmaras方程湍流模型和基于压力隐式的SIMPLE算法对风轮流场进行非定常数值模拟研究。制作了风机模型并进行测试,验证了前缘旋转圆柱对风力机性能有一定的提升作用。     

Monolayer freeze-fracture—a modified procedure

Monolayer freeze-fracture of biological membranes is a valuable tool for integrating membrane morphology with biochemical analysis of membrane components. This correlation has been restricted by the purity of the biochemical sample. In this article, the method is reviewed, and an improved method is described. The essential modification was the use of a polysaccharide-coated microscope slide, instead of a copper plate, to cover cells attached to a polylysine-coated coverslip. It was found that proper freeze-fracture will not occur unless there is a distinct temperature gradient, with its accompanying stresses, across the cell monolayer during the freezing process. This gradient is achieved by using glass slides of different thickness to cover each side of the monolayer. Comparison of the results with those obtained when using a copper-glass system demonstrated a consistently purer sample for the glass-glass system, with whole-cell contamination of the external membrane leaflet being reduced to 0.4%. Problems associated with obtaining pure samples for biochemical analysis are discussed, and the results of freeze-fracture with the glass-glass and glass-copper systems are compared. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of polypeptides associated with the separate halves of the erythrocyte membrane demonstrated that band 3, the anion transport protein, separates with the cytoplasmic face, whereas only sialoglycoproteins and their fragments are retained in the external face. This finding, obtained with the glass-glass system, is consistent with results of our earlier freeze-fracture study that used a copper-glass system which showed that covalent bonds may be broken during this procedure.     

Scuffing — a review: Part 2: The mechanism of scuffing

Surface films and experimental evidence of scuffing in discs and gears are discussed, and the importance of hydrodynamic and boundary lubrication mechanisms have been analysed. The breakdown conditions of these mechanisms and thermal instability work are summarised.     

The tribology of plastics — an overview

With the widening use of plastics, it has become an urgent task to study their behaviour in friction systems. The present paper reviews the applications of plastics during the past decades and their function in friction systems. The paper analyses the basic differences between metals and plastics in structure and properties, and points out the limitations, or inapplicability, of using metal tribology to solve problems in plastic friction systems when even different kinds of high-molecular materials have different shapes and applications. The paper also looks at the variety of plastic surfaces, and briefly presents the mechanism, theory, and calculation of the fiction, wear, and lubrication of plastics; theories of plastic wear are summarised. Also presented are the technical functions, simple application techniques, and economic benefits of plastic friction systems.     

The sliding friction coefficient — its evolution and usefulness

Braithwaite (1978) expressed the opinion that the term coefficient of friction has served its usefulness and will eventually be of historical interest only.

After recalling the historical period (16th–18th centuries), this survey covers the period of more modern (simple and mixed) theories (18th–20th centuries); the energy problem is discussed separately. Then, the use of the friction coefficient is examined in thermal processes, vacuum, radiation, thin layers, solid lubricants, plastic and ceramic materials, and the friction-wear dependence, stick-slip motion and various lubrication regimes (from boundary to elastohydrodynamic lubrication) are considered.

The conclusion that is reached is that, instead of becoming a notion of historical interest only, the friction coefficient has been increasingly improved, adapted and employed, becoming at least a “tool” even in the new theories. It can hardly be avoided in the evaluation of friction effects.     

Design and performance of a small—scale diamond turning machine

Machining with single-point diamond tools is an area of increasing interest in the manufacture of optics, lens moulds, hard discs and other products Although many materials have been machined with impressive results, questions remain regarding the science of diamond turning. These questions concern the suitability of certain materials for turning, the properties of the tool and machine structure which limit the quality of the final product, and the modelling of the cutting process. To study these phenomena, a laboratory-scale diamond turning machine has been designed and built at the Precision Engineering Center. This machine, PAUL, although simple and compact, has produced excellent results on a variety of static and dynamic cutting experiments. The key points of its design, as well as an evaluation of PAUL's performance, are given in this paper.     

Analytical dynamics of mechanisms—a computer oriented overview

General-purpose computer programs such as IMP, DRAM, MEDUSA and DYMAC, for the solution of inertia-variant (linkage-type) mechanisms are beginning to emerge. These programs, which automatically generate, and numerically integrate, the equations of motions, are based upon different but related analytical and numerical principles. There has also been much recent discussion of “influence coefficient” methods, energy methods and related matters. This paper reviews the various principles and techniques available for formulating the equations of motion, for integrating them numerically, and for solving the associated kinetostatic problem for the determination of bearing reactions. The relative advantages of vector methods, d'Alembert's Principle, Lagrange's Equations with and without multipliers, Hamilton's Equations, Virtual work, and energy methods are discussed. Particular emphasis is placed on how well-suited the various methods are to the automatic generation of the equations of motion, and to the form and order of the systems of differential equations they lead to. It is shown how velocity ratios, influence coefficients, centripetal coefficients, generalized inertia coefficients, and Christoffel symbols interrelate the various methods, and tie them to classical results of Analytical Dynamics such as the “explicit” equations of motion and the power-balance principle. Methods for solving both the general dynamics problem, and the kinetostatic problem, are reviewed, and the particular methods of implementation used in the recently developed general-purpose computer programs, and in other recent literature, are described.     

Timber—a review of the structure-mechanical property relationship

Following a brief introduction concerned with present consumption and future prospects of timber and timber products the principal structure-property relationships of this low density, cellular, polymeric composite are reviewed and discussed. Structure is examined at four levels of magnitude—macroscopic, microscopic, sub-microscopic and chemical—and the various models used to interpret its composite nature are described. The dimensional instability of timber and loss of strength on wetting are discussed in terms of its fine structure. At low levels of stressing, and for short periods of time, timber can be treated as an elastic material, but at higher stresses and prolonged periods, especially with alternating humidity, timber behaves as a linear orthotropic viscoelastic material; the various factors influencing the elastic constants and the relationship of creep to fine structure are discussed. The anisotropy of wood is related to cell arrangement and microfibrillar orientation: strength and its variability are discussed in terms of structure at all four levels. Comparison of the strength of timber with that of other constructional materials especially on a weight basis shows timber in a very good light; the combination of high stiffness and high toughness is unique. Recent models using three-dimensional anisotropic elastic analysis to understand strength and deformation of timber are described. The morphology of fracture under different forms of stressing is illustrated.     

The partially wetted bearing—extended Reynolds equation

The artificial no-slip boundary conditions on the liquid/solid interfaces are traditionally used widely. Due to the advances on the measurement technique and interface sciences, the applications of no-slip boundary conditions on micro-systems are challenged continuously. The ‘slip effects’ are observed in small clearance measurement or by treating the surfaces hydrophobic. The non-Newtonian power-law fluid as well as the Navier-slip boundary conditions is considered in the partially wetted bearings. A perturbation technique is utilized to derive the extended Reynolds equations. The analysis applied either to Couette–dominated highly non-Newtonian fluids, or to Newtonian fluids with arbitrary Couette-Poiseuille components. Finally, the effects of slip parameters on the bearing performances are discussed.     

SP88 — The painkiller in validation

The process of establishing a pharmaceutical plant requires a well controlled engineering process, where the requirements to be met are clearly spelt out — and the design and the actual plant is tested against these requirements. In the context of batch processes — with the added complexity of time variance and some degree of flexibility — this becomes anything but trivial.

The approach described in this paper is an attempt to combine a consistent engineering approach for pharmaceutical plants with the models presented by SP88. The SP88 batch control standard provides an excellent platform for structuring this problem - not only from a control perspective, but from the perspective of the multi-disciplinary, complete specification and test of a plant. The following life-cycle models and terminology are therefore in no way control-specific, but cover the complete installation - including mechanical equipment and installations and manual functions.     

Ferrography—then and now

Since 1971, when ferrography was first introduced, there have been many developments in the techniques used to process samples and also the way in which debris is analysed and interpreted in relation to the underlying causes that produced them. The science and application of ferrography embraces both the engineering and medical fields and this has led to new and exciting ways of understanding the associated phenomena and how to extract the best information for the advancement of the subject matter.The purpose of this special issue is to provide an overview of some of the principal developments that have taken place over the past three decades, thereby placing on record what is widely acknowledged to be one of the most significant developments in the field of wear debris technology.     

Design of a six-axis micro-scale nanopositioner—μHexFlex

This paper presents the design of a small-scale nanopositioner, the μHexFlex, which is comprised of a six-axis compliant mechanism and three pairs of two-axis thermo-mechanical micro-actuators. In this paper, we cover the modeling, design and fabrication of the μHexFlex. Specific attention is given to: (1) the use of constraint-based design in generating the compliant mechanism design, (2) the modeling of the actuators, and (3) the system model which links the actuator input and mechanism response. The measured, quasi-static performance of a 3 mm diameter prototype shows a maximum range of 8.4 μm × 12.8 μm × 8.8 μm and 19.2 mrad × 17.5 mrad × 33.2 mrad (1.1° × 1.0° × 1.9°). Experimental results indicate that a constant mechanical/electrical material property system model may be used to predict the position and orientation over a range of 3.0 μm × 4.4 μm × 3.0 μm and 6.3 mrad × 6.3 mrad × 8.7 mrad (0.36° × 0.36° × 0.5°). The dynamic characteristics of the device were investigated experimentally. Experimental results show a lowest natural frequency of 4 kHz. The resolution characteristics of the device have been measured at 1 Å/mV. The device was created using deep reactive ion etching (DRIE). Bulk fabrication costs are estimated at less than $ 2 per device.     

Surface investigations with a combined scanning electron–scanning tunneling microscope

We have combined a scanning tunneling microscope (STM) with a scanning electron microscope (SEM) for surface investigations of atomically flat surfaces, ultrathin adsorbate films, and material surfaces. The mechanical stability of the hybrid instrument allows high-resolution SEM of samples mounted on the STM stage and atomic resolution with the STM. Experimental results of combined SEM/STM investigations on textured material surfaces, submicron structures, and atomically flat conducting surfaces are presented. An example is given for surface machining with the STM under SEM control.