Tarras Wanheim

Real area of contact and friction stress at high pressure sliding contact

Abstract An experimental investigation of the plastic deformation of the surface asperities of a soft rough specimen surface in sliding contact with a hard smooth tool surface is described. The experimental equipment allows independent variation of the normal pressure and sliding length. The normal pressure was varied from 0.5 to 2.5 times the yield stress of the specimen material and the sliding length was varied from 0 to 20 mm. Experiments with soap-coated phosphated specimen surfaces showed that the real area of contact and the friction stress were related to normal pressure and sliding length as expected from theoretical considerations. Measurements of the change of the asperity slopes were also in agreement with theory. Experiments with dry friction showed a steadily increasing contact area with increasing sliding length; this explains the fact that the coefficient of friction can rise to a very high value, increasing with decreasing normal pressure. On the basis of the experimental results a new slip-line field was developed taking the slope of the asperities into consideration.

A theoretically determined model for friction in metal working processes

Abstract A general theory for friction in metal working processes is developed based upon the slip-line theory as a model of analysis. The real area of contact α and the nominal friction stress τn are determined as functions of the nominal normal pressure q/2k and the friction factor m. The results show how the real area of contact increases and approaches the apparent, as the normal pressure increases. Furthermore it is found that Amontons law is valid only until q/2k = 1.3 irrespective of the m- m value . Curves of τn/k as functions of q/2k and m show that τn/k approaches the friction factor m as q/2k tends to infinity. This means that Amontons law in the case of small m-values (m

Friction at high normal pressures

Abstract Friction conditions between tool and workpiece in metal working are of the greatest importance to a number of factors such as force and mode of deformation, properties of the finished specimen and resulting surface roughness. It is shown, theoretically and experimentally, that the Amonton friction law expressed by τ = μq does not apply when normal pressure is higher than approximately the yield stress of the specimen; in this case it is necessary to consider the frictional stress as a function of normal pressure, surface topography, length of sliding, viscosity, and compressibility of the lubricant. The theoretical work was carried out by means of upper bound and slipline field analysis based on experiments with model surfaces in wax and metal. The theoretical model applied is one of multihole extrusion, the material beneath the valleys of the workpiece surface being extruded up towards the tool when the real area of contact exceeds a certain value. The effect of the trapped lubricant is to build up a back-pressure on the extrusion process. The experimental work was carried out with newly developed equipment enabling direct determination of the abovementioned function; construction and calibration of the equipment are described. The equipment allows determination of frictional stress on a surface with well-defined values of normal pressure, sliding length, and sliding velocity. The normal pressure can attain about 8 times the yield stress for commercially pure aluminium. The results obtained show reasonably good agreement between theory and experiment, and a dependence of the frictional stress on the sliding length, this dependence being a function of normal pressure.

Real area of contact and friction stress — The role of trapped lubricant

Abstract A theory is developed which shows the influence of the normal pressure, asperity slope, friction factor and pressure-sensitive lubricant bulk modulus on the real area of contact and the friction stress between a rough specimen and a smooth tool surface. Theory as well as experiments show that the normal pressure and bulk modulus have great influence on the real area of contact, whereas the asperity slope and friction factor are of minor importance.

Real area of contact between a rough tool and a smooth workpiece at high normal pressures

Abstract A theory is developed determining the real area of contact between a hard rough tool and a soft smooth specimen surface. The accordance with experiment is good. Both theory and experiment show the slope of the asperities to be of minor importance.

Ra and the average effective strain of surface asperities deformed in metal-working processes

Abstract Based upon a slip-line analysis of the plastic deformation of surface asperities, a theory is developed determining the R a -value (c.l.a.) and the average effective strain in the surface layer when deforming asperities in metal-working processes. The ratio between R a and R a 0 , the R a -value after and before deformation, is a function of the nominal normal pressure and the initial slope γ 0 of the surface asperities. The last parameter does not influence R a significantly. The average effective strain \ ge in the deformed surface layer is a function of the nominal normal pressure and γ 0 . \ ge is highly dependent on γ 0 , \ ge increasing with increasing γ 0 . It is shown that the R a -value and the strain are hardly affected by the normal pressure until interacting deformation of the asperities begins, that is until the limit of Amontons law is reached. After that R a decreases and the strain increases rapidly with the normal pressure, R a approaching zero whilst \ ge approaches a limiting value depending on the initial slope of the asperities.

The basis for a design support system to prevent defects in forging

Abstract The present paper describes the basis for a system to reduce the number of defects in a forged component. For this purpose two classifications are created as originally suggested by Dodd. The first divides the possible defects into six groups and the second classifies the probable causes of the defects. By use of these classifications, a catalogue showing the relation between the defects and their causes is established. This catalogue, called the Defect-Matrix, can be created for all types of forging processes and workpiece geometries as a valuable aid for the designer.

The use of a square grid as an alternative to a circular grid in the determination of strains

Abstract The circle grid strain analysis method has hitherto been the most commonly used method to determine strains in sheet metal parts. This method has proved very successful in a number of applications. However, the method has drawbacks, some of which can be overcome by employing a grid strain analysis — the coefficient method — based on the measuring of a grid consisting of squares. The present paper compares the circle grid method and the coefficient method, the conclusions of which comparison are that the coefficient method (i) can yield the strain distribution much faster and more accurately, (ii) is very flexible with respect to choice of optimum reference area, (iii) can be used to analyse the influence of the strain path on the formability of sheet metal, and (iv) is very easy to computerize.

Physical and mathematical modelling of extrusion processes

Abstract The main objective of the work is to study the extrusion process using physical modelling and to compare the findings of the study with finite element predictions. The possibilities and advantages of the simultaneous application of both of these methods for the analysis of metal forming processes are shown.

Asperities on asperities

Abstract The effect of load on the flattening of asperities on top of others is discussed. It is shown that the real area of contact on top of an asperity that is not interfering with others lies between 85 and 90% when only second generation asperities are involved. When third and fourth generations are taken into account, the real area of contact on the parent asperity might be as low as 75%. A simple hypothesis for the survival of an asperity on the flank of another is suggested.