Mary Lynn Realff

Processing, structure, and properties of fibers from polyester/carbon nanofiber composites

Poly(ethylene terephthalate) (PET) resin has been compounded with carbon nanofibers. The amount of carbon nanofibers utilized in each case was 5 wt.%. Compounding methods included ball-milling, high shear mixing in the melt, as well as extrusion using a twin-screw extruder. PET/CNF composite resins were melt-spun into fibers using the conventional PET fiber spinning conditions. Morphology and mechanical properties of these composite fibers have been studied. The results show that CNFs can be incorporated into PET matrix with good dispersion. Compressive strength and torsional moduli of PET/CNF composite fibers were considerably higher than that for the control PET fiber.

Mechanical Properties of Fabrics Woven from Yarns Produced by Different Spinning Technologies: Yarn Failure as a Function of Gauge Length

In a study of yam strength translation into woven fabric behavior, experiments were conducted to establish the effect of test gauge length on yarn properties. Yams produced on each of the three major spinning systems were tensile tested at varying gauge lengths. Yam 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 yam 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 yams produced on the different spinning systems. The resultant fabric tenacities approximated corresponding yarn tenacities only for the shortest gauge lengths.

A Micromechanical Model of the Tensile Behavior of Woven Fabric

This work takes a micromechanical approach to fabric tensile modeling. The entire uniaxial tensile stress-strain behavior of the fabric is modeled from the constitutive yarn properties (tensile, bending, flattening, and consolidation behavior) and the original fabric geometry. Techniques for measuring these yarn properties are described. In most cases, there is good agreement between the theoretical and experimental results for several fabrics of differing weave and yarn construction. Modified approaches are suggested for those cases where prediction of fabric stress-strain behavior deviates from the experimental data.

Mechanical Properties of Fabric Woven from Yarns Produced by Different Spinning Technologies: Yarn Failure in Woven Fabric

A study has been conducted on the mechanisms of in-situ tensile failure of staple yams 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 yam 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 yams could straighten without cross yam 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 yams. The interaction between yarn properties and weave geometry had a strong influence on the local disturbance of cloth structure resulting from isolated yam failure during fabric tensioning. The extent of such dis turbance permitted estimates of the stress recovery length of the failed yam and showed its dependence on cloth tightness and yarn type.

Identifying Local Deformation Phenomena During Woven Fabric Uniaxial Tensile Loading

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.

A Stochastic Simulation of the Failure Process and Ultimate Strength of Blended Continuous Yarns

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 impor tance of the twist reinforcing mechanism to the strength of a twisted structure with continuous components, the interaction patterns between different component types dur ing yam extension, and the significance of multiple breaks along a component are demonstrated. Building on the three basic concepts of fragmentation and chain-of-sub- bundles, changing lateral constraint between components due to twist and its effect on component strength, and load sharing between broken and still surviving members during yam breakage, a new mechanistic approach is proposed and a stochastic computer model is developed to predict the behavior of blended yams. The approach is 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 Monego et al. [20, 21, 22]. By means of this new model, fundamental features of blended yam behavior are simulated and elucidated, including the strength reinforcing mechanism of twist in a blended yam, the yarn break propagation pattern, and the effect of twist on yam fracture behavior as well as the shape effect of component stress-strain curves. Moreover, the relationship between the strength of a structure and that of its components is also investigated.

The mechanical behavior of poly(lactic acid) unreinforced and nanocomposite films subjected to monotonic and fatigue loading conditions

The mechanical and fatigue behavior of neat poly(lactic acid) (PLA) films and PLA films reinforced with 5 wt% nanoclay particles has been examined using various analytical procedures. The results showed that for the films tested in this study, PLA-5 wt% samples were more susceptible to crazing at the same maximum fatigue stress as the neat PLA samples, as evidenced by results from light transmission experiments. Optical microscopy results confirmed this observation. In addition, under fatigue loading conditions, the neat PLA samples displayed almost the same fatigue resistance (number of cycles to failure) at 3 and 30 Hz, while the PLA-5 wt% samples showed a shift in the number of cycles to failure to higher values at a frequency of 30 Hz. Using the linear regression curves from the S–N data (stress vs. number of cycles to failure), time-to-failure curves were generated to show the difference between the neat PLA and PLA-5 wt% samples when tested at frequencies of 3 and 30 Hz. Based on these results, it is known that the nanoclay particles served to increase the fatigue resistance at the higher frequency of 30 Hz, when compared to the neat PLA sample.

Fatigue response and constitutive behavior modeling of poly(ethylene terephthalate) unreinforced and nanocomposite fibers using genetic neural networks

The constitutive behavior of poly(ethylene terephthalate) (PET) unreinforced (control) and PET fibers reinforced with 5 wt% vapor-grown carbon nanofibers (VGCNFs) under uniaxial tension and subsequent to fatigue loading has been evaluated utilizing various analytical models. Two types of fatigue tests were performed: (1) Long cycle fatigue at 50 Hz (glassy fatigue) to evaluate fatigue resistance and (2) fatigue at 5 Hz (rubbery fatigue) to evaluate residual strength performance. The long cycle fatigue results at 50 Hz indicated that the PET-VGCNF sample exhibited an increased fatigue resistance of almost two orders of magnitude when compared to the PET unreinforced filament. The results of the fatigue tests at 5 Hz indicated that the constitutive response of both the PET control and PET-VGCNF samples changed subsequent to fatigue loading. The large deformation uniaxial constitutive response of the PET and PET-VGCNF fibers was modeled utilizing genetic-algorithm (GA) based training neural networks. The results showed that the large deformation uniaxial tension constitutive behavior of both PET unreinforced and PET-VGCNF samples with and without prior fatigue can be represented with good accuracy utilizing neural networks trained via genetic-based backpropagation algorithms, once the appropriate post-fatigue constitutive behavior is utilized. Experimental data of uniaxial tensile tests and experimental postfatigue constitutive data have been implemented into the networks for adequate training. The fatigue tests were conducted under tension-tension fatigue conditions with variations in the stress ratio (R), maximum stress (σmax), number of cycles (N), and the residual creep strain (εR).

Understanding thermomechanical failure of athletic textiles via the pendulum skid method

Fiber textiles worn by some athletes and basketball and volleyball players experience higher than usual thermomechanical stresses compared to everyday garments because these athletes slide and dive on hardwood courts. Common textile testing procedures, such as the Martindale abrasion tester, effectively test textiles under modest loads and thousands of cycles, but this methodology does not suffice for athletic textiles. In addition, there is not a robust model nor a repeatable test that mimics high thermomechanical stress on fabrics and provides insights on fabric abrasion resistance. We present a model to calculate the temperatures and strain rates that are seen by fabrics undergoing thermomechanical deformation. To enable validation of the model, a fabric pendulum abrasion tester, an adaptation of the Cooper pendulum skid tester, was developed. The tester characterizes high-strain fabric abrasion deformation. This adaptation is statistically reliable and induces repeatable and realistic fabric failure within tens to hundreds of cycles, proving to be analogous to the loads athletes place on their textiles. Analog electronics on the pendulum abrasion tester generate real-time temperature and velocity profiles. A series of 11 unique athletic fabrics were abrasion tested, and it was found that fabrics with macroporosity experience the largest abrasion degradation. Significant degradation sites were further explored using scanning electron microscopy and X-ray diffraction analysis, and it was shown that thermomechanical loading’s effect on fiber microstructure is a function of the fabric construction. This novel abrasion tester and quantitative relationships between fabric structure and degradation mechanisms will enable more data-driven decisions when designing textiles for thermomechanical loads.