Mary Lynn Realff
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Research Abstracts - Mary Lynn Realff |
Last Updated: May 11, 1998 |
The mechanics of the failure process and ultimate strength of a twisted yarn structure are studied using a newly proposed stochastic model of the failure process. The importance of twist reinforcing mechanism to the strength of a twisted structure with continuous components, the interaction patterns between different types of components during yarn extension, and the significance of multiple breaks along a component are demonstrated. Based on such concepts as fragmentation and chain-of-subbundles, the changing lateral constraint between components due to twist and its effect on component strength, the load sharing between broken and still surviving members during yarn breakage, a new mechanistic approach is proposed and a stochastic computer model is developed in a manner similar to that developed earlier by Boyce et al [3] to study the failure process in woven fabrics. The model acts to predict the strength and fracture behavior of a blended yarn with continuous components. The predicted results are illustrated in comparison with the experiments of Montego et al [17,18,19]. By means of this new model, issues like the strength reinforcing mechanism of twist in a blended yarn, the yarn break propagation pattern, and twist effect on yarn fracture behavior as well as the shape effect of component stress-strain curves are simulated and elucidated. Moreover the relationship between strength of a structure and of its components is investigated in this study. Knitted fabric is produced in the form of interlaced loops. Under different deformations of the fabric, a loop changes shape and length. Empirical, geometric and mechanical models have been developed in the past to study the structural and mechanical properties of knitted fabrics. This research extends previous research by measuring and describing "real" loops within single knit jersey fabrics. An image processing technique is used to obtain the coordinates of the loops within the fabric during deformation and the relationship between fabric strain and yarn strain is studied. In the past, many attempts have been made to quantify the characteristics of knitted fabrics. The defining element of a knitted structure is the single knitted loop. It has been shown that the length of yarn knitted into a single loop is a key factor in such overall fabric qualities as hand, wearer comfort, weight, extensibility, finished size, and cover factor. Therefore, many models have been proposed which describe a loop and provide a means by which to calculate the loop length. This study uses image processing to find loop lengths and then compares the results to past models to provide a critical assessment of which equations and methods provide accurate values of a knitted loop length.
This work develops an automated creeling system for the carpet and textile industries. Its goal is to increase the competitiveness of the industries by addressing the critical need for creeling automation through: (1) improving working conditions and reducing work-related injuries, (2) reducing operational costs, (3) increasing process efficiency, (4) improving product quality, and (5) reducing yarn waste. The project focuses on the creeling process for carpet tufting operations. A low-cost prototype of an automated creeler has been developed to demonstrate feasibility. Together with an on-line package weight matching scheme, this system is advanced as a tool to increase productivity in the textile industry. Realff, M.L., Boyce, M.C., and Backer, S., "A Micromechanical Modeling of the Tensile Behavior of Woven Fabrics", submitted to Text. Res. J., Jan. 1996. In this paper a micromechanical approach to fabric tensile modelling has been taken. The entire uniaxial tensile stress-strain behavior of the fabric is modelled from the constituent yarn properties (tensile, bending, flattening, and consolidation behavior) and the original fabric geometry. In most cases, good agreement between the theoretical and experimental results on several fabrics of differing weave and yarn construction is observed. Two possible approaches are suggested for those cases where the prediction of the fabric stress-strain behavior deviates from the experimental data. Goddard, T., Realff, M.L., and Realff, M.J., "Modeling and Analysis of Weaving Preparation Systems with Respect to Small Lot Manufacturing, Part I: Problem Definition", to appear J. Text. Inst., 1996. The implementation of quick response manufacturing strategies in the textile industry has led to the realization of smaller lot sizes for woven fabric producers. In this first of a two part work the effects of this shift are explored, and an approach for producing within a short run environment presented. Also, a definition for small lot sizes is developed for weaving preparation that holds for a wide range of product mixes. Basic models of the weaving preparation processes as multi-stage flow shops are given for the purpose of design and analysis. Finally, a framework for the design of appropriate small lot weaving preparation systems is defined. In Part II of this work, the analysis component of this framework will be used to characterize the performance of weaving preparation systems with respect to small lot manufacturing. Goddard, T., Realff, M.L., and Realff, M.J., "Modeling and Analysis of Weaving Preparation Systems with Respect to Small Lot Manufacturing, Part II: Analysis of Candidate Systems", to appear J. Text. Inst., 1996. In Part I of this work a framework for the design of efficient small lot weaving preparation systems was presented. The design objective was to specify the lowest cost system that will meet the desired production goals for a given product mix. A key element of this framework is the Analysis Engine, which evaluates the performance of candidate systems in light of production targets and generates information about the direction in which to evolve the design. In this work it is shown that simulation methodologies and sensitivity analysis can be used to perform the Analysis Engine function. Two candidate systems representing possible points in the overall design space are evaluated to determine if one has an advantage in producing small lots. In addition, information gained from the analysis procedures can be used to point to further improvements in system performance for a relatively small increase in capital expenditure. Realff, M.L., "Identification of Local Deformation Phenomena During Woven Fabric Uniaxial Tensile Loading", Text. Res. J., Vol. 64 (3), pg. 135-141, 1994. Experimental methods are presented for observing local fabric deformation during uniaxial tensile loading of a woven fabric. These experiments include the traditional ravel strip test as well as the use of video taping, encapsulation, and photographing fabrics varying in weave texture, yarn type (ring and rotor spun), and yarn size. Through these techniques, changes in yarn curvature and cross-sectional area ( shape ) are monitored along with the global stress-strain response of the fabric. Fabric response depends on both fabric structure and constituent yarn properties. By choosing fabrics in which single parameters, such as picks per inch, vary systematically, trends in fabric behavior can be tracked. Seo, M.H., Realff, M.L., Boyce, M.C., Schwartz, P., and Backer, S., "Mechanical Properties of Fabrics Woven from Yarns Produced by Different Spinning Technologies: Yarn Failure in Fabric", Text. Res. J., Vol. 63 (3), pg. 123-134, 1993. A study has been conducted on the mechanisms of in-situ tensile failure of staple yarns during uniaxial tensioning, as in a conventional ravel strip test. The yarns were PET/cotton blends processed on ring, rotor, and airjet spinning systems, and then woven into plain or twill weave fabrics. Load-extension behaviors of the yarns were recorded for the in-fabric state as well as for the free state (out-of-fabric), and SEM comparisons were made of the fractured yarn ends obtained in the two states. When the tensioned yarns became jammed between cross yarns before straightening, the fracture ends were abrupt, similar to those observed in near zero gauge length tests of free-state yarns. However, when fabric structure was such that tensioned yarns could straighten without cross yarn jamming, the resulting failure zones were considerably longer, with a mixture of fiber fracture and slippage similar to that observed in long gauge length tests of free-state yarns. The interaction between yarn properties and weave geometry had a strong influence on the local disturbance of cloth structure resulting from isolated yarn failure during fabric tensioning. The extent of such disturbance permitted estimates of the stress recovery length of the failed yarn and showed its dependence on cloth tightness and yarn type. Realff, M.L., Boyce, M.C., and Backer, S., "A Micromechanical Approach to Modeling Tensile Behavior of Woven Fabrics", Proceedings of the 1993 ASME Conference, December 1993. In this paper a micromechanical approach to fabric tensile modelling has been taken. The entire uniaxial tensile stress-strain behavior of the fabric is modelled from the constituent yarn properties (tensile, bending, flattening, and consolidation behavior) and the original fabric geometry. In most cases, good agreement between the theoretical and experimental results on several fabrics of differing weave and yarn construction is observed. Two possible approaches are suggested for those cases where the prediction of the fabric stress-strain behavior deviates from the experimental data. Realff, M.L., Seo, M., Boyce, M.C., Schwartz, P., and Backer, S., "Mechanical Properties of Fabrics Woven from Yarns Produced by Different Spinning Technologies: Yarn Failure as a Function of Gauge Length", Text. Res. J., Vol. 61, pg. 517, 1991. In a study of yarn strength translation into woven fabric behavior, experiments were conducted to establish the effect of test gauge length on yarn properties. Yarns produced on each of the three major spinning systems were tensile tested at varying gauge lengths. Yarn strength data were fit to two-parameter Weibull distributions and corresponding shape and scale parameters were determined. Strength increased as gauge lengths decreased, a trend indicated by the weakest-link theory. At very short gauge lengths, however, the data deviated from prediction based on the weakest-link theory, thus suggesting a change in the yarn failure mechanism, as one would expect when the gauge length approximates the staple length. More direct evidence of such a change is provided in SEM photomicrographs of tensile failures of long versus short gauge test specimens. Combined fiber slippage/pullout and breakage prevailed at longer gauges, whereas a greater extent of fiber breakage with less slippage occurred at shorter gauge lengths. The balance between fiber slippage and fiber breakage varied with yarn structure as produced on different spinning systems. Finally, tensile tests were con- ducted on plain and twill weave fabrics woven from yarns produced on the different spinning systems. The resultant fabric tenacities approximated corresponding yarn tenacities only for the shortest gauge lengths. Boyce, M.C., Palmer (Realff), M.L., Seo, M., Schwartz, P. and Backer, S., "A Model of the Tensile Failure Process in Woven Fabrics", Journal of Applied Polymer Science: Applied Polymer Symposium, 47, pg. 383-402, 1991. In this study, a model for simulating the tensile failure process of woven fabric is constructed which accounts for several major factors responsible for the translation of yarn properties into fabric strength. These factors are strongly influenced by the nature of yarn-yarn interaction within the fabric as determined by yarn type, weave construction, and conditions of manufacture and are modeled as fabric assistance effects on ( I ) the strength distribution of the constituent yarns, (2 ) the recovery length of the yarn in fabric subsequent to a break, and ( 3 ) the local load redistribution- (load sharing rule) subsequent to a break. The model is capable of predicting the load at which the first yarn breaks; the number of isolated yarn breaks prior to catastrophic fabric failure; the critical crack length upon catastrophic failure; and the tensile strength of the fabric. A parametric study reveals a strong dependence of the tensile failure process on constituent yarn strength distribution and load sharing rule with comparatively small dependence on recovery length. Strong connection is made with the tensile failure process of a real fabric. |