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Comments on "A Statistical Approach for Determining the Technological Value of Cotton Using HVI Fiber Properties"
Abstract
The fibre quality index of the original article (90W/05774) is modified to give more specific parameters of effective length from the 'bysectrice' method applied to the fibrogram, the Pressley strength, and the uniformity ratio divided by the Micronaire index. The Spanish workers have checked the validity of their index on 127 samples from 285 cotton lots.
This paper describes an experimental study of the frictional behaviour of diamond, graphite and of carbon which have been outgassed in vacuo. The removal of surface films which are normally present causes a large increase in the friction. The admission of a small amount of oxygen, water vapour or other contaminant will reduce the friction. Both physical adsorption and chemical adsorption are important. There is evidence that with clean graphite surfaces there is strong adhesion at the interface, so that when sliding takes place slip and shearing occurs beneath the surface. Carbon and graphite have a negative temperature coefficient of friction. The low friction normally observed with diamond is due to the presence of adsorbed oxygen and other gases. The friction of clean diamond on diamond is high, and the shear strength at the interface is comparable with the shear strength of diamond. Large-scale seizure does not occur because the deformation of the diamond in the region of contact is elastic and the real area of contact necessarily remains small.
A statistical approach for determining the technological value of a variety of cotton is presented. The approach suggests that the market value of cotton should correspond to its technological value in a particular manufacturing system; that is, the value of a bale of cotton should be determined based on its expected performance in the textile mill and the yarn quality obtained from it. A model relating fiber to yarn properties is a basic requirement for implementing the approach. The procedures used in this approach include developing a multiple regression model relating HVI fiber properties to the desired quality parameter of yarn (skein break factor), determining the percent relative contribution of a fiber property with respect to skein break factor, selecting a reference set of HVI fiber properties, determining a difference factor of the difference in value between fiber properties of a particular variety and the reference set, and finally, developing a premium/discount formula. The main feature of the approach is its flexibility in accommodating different fiber properties and yarns of different counts produced on different spinning systems.
The relation F = aRn between the frictional force and normal reaction is found to hold for fibers, and it is proposed to replace Amontons' law by this form. The friction index, n, is found to lie between 0.80 and unity. The significance of this relation is discussed in terms of the cohesion theory of friction. A satisfactory interpretation can be given if it is assumed that the areas of contact are determined by the visco-elastic properties of the surfaces. A compari son of this relation with that of Gralén's is made. Two important consequences arise from the adoption of this friction law: the frictional force is no longer independent of the apparent area of contact; and, in the case of friction of a string or fiber wrapped around a cylinder, the so- called "coefficient of friction" depends upon the initial tension and also upon the cylinder radius.
A study in terms of elastic asperity theory is made of the empirical frictional relation F=αRn where F and R are frictional force and normal reaction respectively, α is a coefficient of friction and n a friction index. Three different contact cases are treated: (i) plane surfaces, (ii) cylindrical surfaces, (iii) a string round a cylinder. Expressions are derived for F (or the tension in case (iii)) in terms of R and the size, number and elastic constants of the asperities. This enables the frictional `constants' to be related to these properties.
The relationship of the elastic properties of solids to their frictional properties is discussed. By applying Hertz's equation for deformation of solids in contact, it is shown that while the asperities on metallic surfaces are deformed plastically even at the smallest loads, this is not necessarily true of materials having lower elastic modulus. This is confirmed by measurement of the area of contact between a nylon hemisphere and a glass surface, and by frictional measurements on various nylon specimens. It is shown further that the observed decrease in friction of some high polymers on rough surfaces may be explained in terms of elastic deformation of the asperities.