Yehia E. El Mogahzy

Friction in Fibrous Materials Part I: Structural Model

The classical laws of friction adequately describe the behavior of materials that deform plastically but fail to do so in fibers that deform viscoelastically. This paper presents a structural model that characterizes friction in fibrous materials. The theory is general and can account for the behavior of a wide range of materials. The model provides a theoretical base for the empirical equation F = aNn, which has been fitted successfully to experimental data from previous investigations. It gives theoretical meaning to the indices a and n, which heretofore were empirical constants, and brings out the factors, structural as well as procedural, that affect their values. The factors affecting friction in fibers are shown to fall in two main groups, one governing the morphology of contact and the other the mechanical properties of the junctions. A detailed discussion of these factors is given in this paper. The results of an experimental investigation, where acrylic and polypropylene yarns varying systematically in structure were the materials, will be submitted for publication later. Friction is measured in dry and wet media using both line and point contact methods. The effects of structural factors are examined and interpreted in light of the model.

A Statistical Approach for Determining the Technological Value of Cotton Using HVI Fiber Properties

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.

Friction in Fibrous Materials: Part II: Experimental Study of the Effects of Structural and Morphological Factors

In this part of the series, the effects of a number of structural and morphological factors on fiber friction are examined and rationalized on the basis of the structural model discussed in Part I. The factors are fiber cross-sectional shape, molecular ori entation, annealing, and fiber type. Friction tests have been conducted using techniques that allow measurements in both the point contact and line contact modes. Selected tests have also been done with the contact regions immersed in water. The results show that the coefficient of friction μ is higher for annealed than unannealed structures and increases with increasing molecular orientation. Circular fibers exhibit higher values than noncircular fibers. The value of the friction index a varies in the same fashion as the value of the coefficient of friction μ in all cases, except when fiber orientation is the variable. The friction index n increases with orientation and is higher for circular fibers than for noncircular ones. Wetting of the contact region or annealing of the structure has no effect on the value of n. Results are explained in light of the structural model presented in Part I.

Mechanism of Yarn Failure

The importance of interfiber friction in determining yarn strength has been acknowledged by several authors. Studies of the effect of friction on yarn strength were often based on determining the influence of twist level, a structural factor, to change the level of friction. To our knowledge, no study is available in which varying fiber frictional characteristics are introduced into a constant yarn structure (i.e., the same twist, fiber type, fiber length, etc.). This effect has been accomplished through a surface treatment that changes the level of interfiber friction, and subsequent yarn testing provides useful and interesting information about how fiber interaction contributes to yarn strength. The results presented here show that interfiber friction can (under certain circumstances) be the dominant factor in determining the tensile properties of a ring spun staple yarn. Friction and yarn strength results show that moderate changes in the interfiber friction can produce large changes in yarn strength. We suggest that interfiber friction should receive more attention as a determinant of yarn properties, particularly strength.

Evaluating Staple Fiber Processing Propensity Part I: Processing Propensity of Cotton Fibers

In this study, we present a novel approach to simulate and characterize the behavior of fibers during processing using a modified version of the familiar rotor ring system. With this modification, we attempt to create an area approximately resembling the carding zone and measure the energy required to shear the fibers in this area. We then use the energy readings to provide an index of fiber processing propensity. Such an index is believed to be a function of combined fiber cohesion and fiber resiliency. In addition to the rotor ring, we present corresponding results from other independent techniques such as NIR wax analysis and sliver cohesion. The most important finding of this part of the study is that deterioration in the quality of yarns and fabrics should not be rationalized only on the basis of standard fiber properties (such as length, fine ness, and strength) and that measures of processing propensity, wax content, and sur face cohesion provide a complete picture of fiber processibility and its impact on end product quality.

Selecting Cotton Fiber Properties for Fitting Reliable Equations to HVI Data

An advanced statistical procedure for developing reliable prediction equations relating cotton fiber properties and the skein break factor of yarn is demonstrated. The procedure consists of a sequential analysis in which subsets of independent variables are obtained by linear regression to best predict a dependent variable. Selection of the most reliable equation is based on three main criteria: the R 2 statistic, the Cp statistic, and the MSE. The independent variables considered in the analysis are the cotton fiber properties measured by the HVI system (MCI3000). A reliable regression equation relating cotton fiber properties and yarn quality should simultaneously satisfy the two criteria: it has a high R 2 value and uses only the most important fiber properties. Written primarily for textile scientists and engineers, the details of the statistical analysis are minimized, and the analysis is presented in its proper context, as a useful tool for the nonstatistician.

Theory and Practice of Cotton Fiber Selection Part II: Sources of Cotton Mix Variability and Critical Factors Affecting It

This part of the study demonstrates the complex nature of variability in multi component blends and the effectiveness of bale picking schemes in handling this com plexity, using analysis of variance. Such analysis provides useful guides in the selecting fibers for uniform blending. Critical factors affecting mix uniformity are examined, including population variability, location of category break points, number of cate gories, and laydown size. Methods for optimizing blend uniformity in view of the effects of these factors are recommended.

Theory and Practice of Cotton Fiber Selection Part I: Fiber Selection Techniques and Bale Picking Algorithms

The cotton mixture as a multi-component blend of inherently variable natural fibers imposes several challenges with regard to the proper method of selecting fibers for a uniform blend and consistent output characteristics. Part I introduces a number of fiber selection techniques and based on these techniques, proposes three different bale picking schemes. The first scheme is random picking, which resembles the tra ditional massive blending and serves as the basis for more advanced schemes. The second scheme is proportional weight category picking, in which the distribution of a fiber characteristic is divided into a number of classes, and bales are picked from each class in quantities proportional to class relative frequency. The third scheme is optimum category picking, in which bales are selected on the basis of optimizing factors that contribute to blend uniformity. These factors include category variance, picking cost, and category inventory.

Theory of Soil / Geotextile Interaction

In this study, soil/geotextile interaction is characterized by the following general equation-τ = τa + tan δ*( σnor) n -where τ and σnor are shear and normal stresses on the failure plane at failure, τa is the soil/fabric adhesion component, and δ* and n are friction indexes. This equation provides a general relationship in which the classical Mohr-Coulomb law is considered as a special case. It holds for a wide range of normal stresses and describes soil/geotextile interaction in terms of material-related parameters. A theoretical interpretation of the effects of critical factors affecting soil / geotextile interaction is based on this equation. The four main factors examined are soil/geotextile adhesion, normal stress, deformation of junctions at the soil/fabric interface, and surface roughness. To evaluate interactive deformational modes of soil / geotextile systems, a pull-out test device was designed and constructed and used to test woven and nonwoven textiles. Identified interactive modes were divided into three main categories: pure slippage, complete rupture, and combined slippage and longi tudinal deformation. This categorization supported by the equation above can help rationalize the complex mechanism involved in pull-out conditions.

Diagnostic Procedures for Multicollinearity Between HVI Cotton Fiber Properties

An advanced statistical procedure for diagnosing multicollinearity between HVI fiber properties is demonstrated. The procedure consists of sequential analyses in which validation of a regression model is examined, diagnostic procedures for collinearity are demonstrated, and remedies for the problem of collinearity are proposed. This paper is presented to textile scientists in an effort to introduce new statistical techniques that should improve the performance of linear multiple regression analyses relating HVI fiber properties to yam strength.