Jakob Ilsted Bech

Entrapment and escape of liquid lubricant in metal forming

Using a transparent tool entrapment, compression and eventual escape of liquid lubricant in surface pockets is observed in plane strip drawing. The two mechanisms of lubricant escape, Micro Plasto HydroDynamic and Hydrostatic Lubrication (MPHDL and MPHSL), are observed and quantified experimentally with varying viscosity, speed, reduction, workpiece material, back tension and friction. The mechanisms are influenced by all these parameters in an explicable way. Theoretical models of the escape mechanisms are established combining continuum mechanic analyses of the die pressure distribution with a fluid mechanic analysis of the lubricant escape. Oscillations in the drawing force are caused by the local escape of lubricant.

A Study of Mechanisms of Liquid Lubrication in Metal Forming

Abstract Applying a transparent tool technique the lubrication in plane strip drawing of aluminium sheet is studied providing the strip with surface pockets for entrapment of lubricant. The compression and eventual escape of trapped lubricant by Micro Plasto HydroDynamic Lubrication (MPHDL) as well as Micro Plasto Hydrostatic Lubrication (MPHSL) is observed and quantified experimentally with varying lubricant viscosity, drawing speed, reduction, die angle, back tension, workpiece material and friction conditions All these parameters are shown to influence the mechanisms of lubricant escape in an explicable way. Theoretical models for the two observed mechanisms of lubricant escape, i.e. backward escape by MPHDL and forward escape by MPHSL are established combining a continuum mechanic analysis of the pressure distribution in the tool/workpiece interface with fluid mechanic analyses of the lubricant escape. Experimental observations of oscillations in the drawing force are quantitatively shown to be caused by the two mechanisms of lubricant escape.

A Basic Study of the Influence of Surface Topography on Mechanisms of Liquid Lubrication in Metal Forming

Abstract Applying a transparent tool technique the tool/workpiece interface in plane strip drawing of aluminium is studied. The strips are provided with macroscopic lubricant pockets, and the compression and eventual escape of trapped lubricant by the mechanisms Micro Plasto Hydro Dynamic Lubrication (MPHDL) and Micro Plasto Hydrostatic Lubrication (MPHSL) is observed and quantified experimentally with respect to the lubricant pocket parameters, shape, volume, and angle to the edge. The two mechanisms have proved to depend very differently upon these parameters. The level at which the hydrostatic pressure is stabilised is shown to be independent of the volume of a pocket and the MPHDL mechanism is therefore solely dependent on the angle to the edge, which is shown both experimentally and theoretically by a fluid mechanic analysis. For the MPHSL mechanism a dependency of the volume is observed.

Influence of Workpiece Surface Topography on the Mechanisms of Liquid Lubrication in Strip Drawing

The workpiece surface topography is an important factor controlling the mechanisms of lubrication in metal forming processes. In the present work, the microscopic lubrication mechanisms induced by lubricant trapped in pockets of the surface in strip drawing are studied. The experiments are performed with macroscopic model pockets in the surface studying the influence of the shape of the pockets on the lubrication mechanisms. A large radius of curvature on the rear edge and a small angle to the edge of the lubricant pocket induce a large area of backward escape of lubrication caused by microplasto hydrodynamic lubrication (MPHDL). On the other hand, when the radius on the edge is small MPHDL is impeded and microplasto hydrostatic lubrication appears instead implying forward escape of the lubricant. The occurrence of these mechanisms are quantitatively explained by a mathematical model combining a slab method of analysis of the strip drawing process and an analysis of lubricant escape by Reynolds equation.

Strength and Reliability of Wood for the Components of Low-Cost Wind Turbines: Computational and Experimental Analysis and Applications

This paper reports the latest results of the comprehensive program of experimental and computational analysis of strength and reliability of wooden parts of low cost wind turbines. The possibilities of prediction of strength and reliability of different types of wood are studied in the series of experiments and computational investigations. Low cost testing machines have been designed, and employed for the systematic analysis of different sorts of Nepali wood, to be used for the wind turbine construction. At the same time, computational micromechanical models of deformation and strength of wood are developed, which should provide the basis for microstructure-based correlating of observable and service properties of wood. Some correlations between microstructure, strength and service properties of wood have been established.

Diagnostic accuracy of heel pad palpation - a phantom study.

UNLABELLED Falanga torture involves repetitive blunt trauma to the soles of the feet and typically leaves few detectable changes. Reduced elasticity in the heel pads has been reported as characteristic sequelae and palpatory testing of heel pad elasticity is therefore part of medicolegal assessment of alleged torture victims. The goal was to test the accuracy of two experienced investigators in determining whether a heel pad model was soft, medium or hard. The skin-to-bone distance in the models varied within the human range. METHOD Two blinded investigators independently palpated nine different heel pad models with three different elasticities combined with three different skin-to-bone distances in five consecutive trials and categorized the models as soft, medium or hard. RESULTS Two experienced investigators were able to identify three known elasticities correctly in approximately two thirds of the cases. The skin-to-bone distance affected the accuracy. CONCLUSION The use of clinical examination in documenting alleged exposure to torture warrants a high diagnostic accuracy of the applied tests. The study implies that palpatory testing of the human heel pad may not meet this demand. It is therefore recommended that a device able to perform an accurate measurement of the viscous-elastic properties of the heel pad be developed.

Influence of process parameters in drawing of superconducting wire

Superconducting wire is often produced by the powder-in-tube method involving a number of different mechanical processes, among these is multi-step drawing operations of single filament wire. A silver tube is filled with ceramic powder (BiSrCaCuO). This composite is drawn in up to 100 steps, from an initial diameter of 20 mm to the final one of 1 mm. The present paper shows how two of the main parameters, the reduction and the semi-die angle, influences the drawing stress and the powder compaction of the single filament wire during drawing. The optimum die angle and the largest acceptable reduction per drawing step are determined. In powder compaction tests a linear relationship between microhardness and powder density is determined. This relationship is applied for determination of the density distribution in the cross section of drawn single filament wire. It is found that the drawing process causes an increase of the powder compaction near the interface between the silver and powder, whereas the density increase near the centre axis of the tube is smaller. The average density determined in this way corresponds well with the one determined by geometrical measurements.

Influences of lubricant pocket geometry and working conditions upon micro-lubrication mechanisms in upsetting and strip drawing

Micro-lubricant pockets located in the surface of plastically deforming workpieces are recognised to improve the performance of fluid lubrication in a metal-forming process. This work investigates the joint influence of pocket geometry and process working conditions on micro-lubrication mechanisms, during upsetting and strip drawing, by means of a rigid-viscoplastic finite-element formulation. Special emphasis is placed on the effect of pocket geometry on the build-up of hydrostatic pressure, which is responsible for the onset of micro-lubrication mechanisms. A good agreement is found between the numerically predicted and the experimentally measured distributions of hydrostatic stress.

Analysis of flat rolling of superconducting silver/ceramic composites

The flat rolling process from wire to tape is presumably the most crucial link in the chain of mechanical processes leading from loose powder and silver tubes to the final superconducting Ag/BSCCO tape. In order to improve the critical current density of the superconducting filaments, one must process these to the highest possible density without at the same time introducing failures as large cracks and macroscopic shear bands. In order to analyse and optimise the process, the interaction between the involved materials and their very different mechanical properties must be taken into account. The silver is considered an incompressible von Mises material, hardening as a function of equivalent strain. The powder on the other hand is a compressible friction material implying pressure dependent “flow stress”. In this paper particular attention is paid to stresses and strains and defects in the deformation zone. The evolution of density in the powder cores is analysed by the finite element method.

Manufacturing of Superconducting Silver/Ceramic Composites

Abstract Manufacturing of superconducting metal/ceramic composites is a rather new discipline within materials forming processes. High Temperature Superconductors, HTSC, are manufactured applying the Oxide-Powder-In-Tube process, OPIT. A ceramic powder containing lead, calcium, bismuth, strontium, and copper oxides is inserted into a silver tube and reduced by multi-step drawing. These single-filaments are packed in a new silver tube thus forming a multi-filament containing e.g. 37 single-filaments, which is subsequently reduced by drawing and rolling to tapes approximately 0.2 mm thick by 3 mm wide. After the mechanical deformation the tapes are heat treated at approximately 835°C whereby the powder-cores by phase diffusion and grain growth are converted into superconducting, ceramic fibres. The geometry, density and texture of the powder cores before heat treatment is essential for the quality and current leading properties of the final superconducting fibres. The present work describes studies on alternative packing geometries and process parameters in the flat rolling operations. The aim is to obtain homogenous filaments with advantageous geometry and good texture while avoiding potential defects such as cracks, shear band formation and sausaging.