Bohuslav Neckář

Modelling of fibre orientation in fibrous materials

In this article, mathematical models of fibre orientation in fibrous materials are derived and demonstrated with the help of case studies of real fibrous materials. The theoretical results on fibre orientation in plane are found to be in good agreement with the experimental results obtained from real fibrous structures.

Tensile behavior of staple fiber yarns part I: theoretical models

Abstract In this work, an attempt is made to create theoretical models on tensile behavior of staple fiber yarns and verify them with experimental results. While the first part of this work deals with model development, the second part reports on model validation. This part starts with a description of stress–strain diagrams of fiber and yarn. It then discusses the tensile behavior of twisted yarns according to helical model. Afterward, it proceeds to partial generalization of helical model by taking into account of the influence of fiber orientation distribution on fiber stress utilization in yarn. At the end, it provides a theoretical example for better illustration of the models. In the second part of this work, we report on comparison of the models with experimental results.

A mathematical model of fiber orientation in slivers

The directional arrangement of fibers significantly determines the internal structure and the mechanical properties of the slivers and the ultimate yarns produced from such slivers. In order to evaluate the directional arrangement of fibers in slivers, the experimental method according to Lindsleys’s idea, based on the weighing of suitable combed-out and cut-out fringes, is most frequently used in practice. But, the traditional Lindsley’s evaluation takes on some empirical ratios of fringe weights only. In this work, the experimental method according to Lindsley’s idea is analyzed and a set of relevant equations are derived for the theoretical determination of the measured fringe weights. The new characteristic of fiber orientation represents the harmonic mean of cosines of angles among the short fiber segments and the longitudinal direction of the sliver. The probability density function of the directional arrangement of fibers in a sliver is also estimated. The experimental data obtained on polyester-drawn sliver is compared with the theoretical results and a satisfactory correspondence between them is observed. The fiber orientation function reported in this article can be used in practice as a new quality parameter for the slivers and also to judge the effectiveness of the fiber preparation processes.

Modelling of yarn shrinkage due to washing

Abstract Yarn shrinkage due to washing is modelled using the idea of helical model of yarn. The derived theoretical relation between yarn shrinkage and yarn twist intensity is experimentally verified. It is proved that yarn shrinkage increases non-linearly with the increase in yarn twist intensity.

Tensile behavior of staple fiber yarns part II: model validation

Abstract The theoretical models of tensile behavior of staple fiber yarns derived in Part I of this work are compared and validated with experimental results. It is observed that the stress–strain curve of yarn always lies under the stress–strain curve of fiber. The well-known Gégauff’s theory is found to overestimate fiber stress utilization in yarn. The partial generalization of helical model by taking fiber orientation into account results in satisfactory agreement with the experimental results. Clearly, fiber orientation plays an important role in deciding the fiber stress utilization in yarns. The lower is the variability of fiber direction in relation to the corresponding helical direction of fibers, the higher is the fiber stress utilization in yarn.

Prediction of spun yarn strength at different gage lengths

Abstract In this article, the influence of tensile tester gage length on ring, rotor and air-jet spun yarns tenacity and its coefficient of variation have been investigated. A statistical model has been presented that correlates yarn tenacity and coefficient of variation of yarn tenacity to gage length. The model is based on Peirce model and assuming the three-parameter Weibull distribution of yarn strength values. A reasonable agreement has been shown between the experimental and predicted values. The model successfully captured the change in yarn strength and its coefficient of variation at different gage lengths. Results showed that at longer gage lengths, yarn strength decreases and its coefficient of variation decreases as well.

Influence of Twist on Selected Properties of Multifilament Yarn

Abstract Owing to twisting of filament fiber bundle, the structure and consequently various parameters and properties of a fiber bundle are changed. The aim of the work is to verify the effect of multifilament yarn twist (or twist coefficient) on selected mechanical properties such as multifilament tenacity, breaking elongation, and coefficient of fiber stress utilization in the yarn. Furthermore, the influence of twist on structural parameters such as the angle of peripheral fibers, the packing density, and the substance cross-sectional area of fiber bundle is observed. Two multifilament yarns with different filament cross-section shape and material were used for the experiment. Experimentally obtained data was compared with the known model dependencies derived decades ago based on the helical model. It can be stated that multifilament yarn retraction can be predicted based on the angle of peripheral fibers using the Braschler’s model. The coefficient of fiber stress utilization in the multifilament yarn determined experimentally corresponds with a theoretical curve, constructed according to Gégauff and Neckář, in the area of Koechlin’s twist coefficient α > 54 ktex1/2 m−1. Results as well as possible causes of deviations of experimental data from the theoretical one are discussed in this work.

Predicting Specific Stress of Cotton Staple Ring Spun Yarns: Experimental and Theoretical Results

The aim of this research is to predict the yarn specific stress from fiber specific stress and fiber stress utilisation. In this paper a new approach is introduced to predict the specific stress-strain curves of cotton carded and combed yarns. The force on single fiber is worked out and these fiber forces are combined together to obtain forces acting on yarn. The theoretical model introduces the utilisation of fiber stress on the basis of the fiber specific stress-strain curve, twist angle, fiber directional distribution parameter C and contraction ratio. A comparison of experimental results suggests that the specific stress-strain curves predicted have reasonable agreement with the experimental yarn specific stress-strain curves for all types of yarns. Thus this model is valid to predict the specific stress-strain curves for carded and combed cotton ring spun yarns.

Hairiness of staple fiber yarns part I: mathematical modeling

In this work, an attempt is made to create probabilistic models of yarn hairiness and verify them with experimental results. The first part of this work deals with model development, while the second part focuses on model validation. In this part, a set of generalized relationships on yarn hairiness are developed. Assuming only one type of hairs present in the yarns, a single exponential model of yarn hairiness is created. The number of protruding fibers is found to decrease exponentially with the increase in yarn radius. Under the consideration of two mutually independent types of hairs in the yarns, a double exponential model of yarn hairiness is developed. Further, the evaluation of yarn hairiness according to both the models is discussed. In the second part of this work, we make an attempt to validate the models with the experimental results and develop better understanding of the characteristics of yarn hairiness.

Hairiness of staple fiber yarns Part II: model validation

An attempt is made here to verify and validate the probabilistic models of yarn hairiness reported in Part I of this work. It is shown that the double exponential model of yarn hairiness corresponds more close to the experimental results than the single exponential one. This suggests that two types of hairs of very different nature are present in yarns. The first type of hairs falls with the increase in yarn radius quickly to (practically) zero. These hairs create something like “moss” on the yarn surface. They are considered to be good as they bring pleasant handle, fullness of the fabric, etc. On the other hand, the second type of hairs falls with the increase in yarn radius very slowly. These hairs are created mostly from “long flying fibers,” which often bring difficulty in the subsequent technological processes, namely weaving as well as make the fabric look bad.