The nature of the change in the geometrical restraints which occurs at the yarn crossover points during fabric bending is found from analysis of a simple waveform model of fabric crimp. This same model is also used to predict radial growth in bent, twisted multifilament yarns. The analysis shows that bending of idealized plain-woven fabrics will always result in increased amplitude of the fabric crimp and, for certain fabrics, a reduction in restraining forces on the crossing yarns. Similarly, bending of twisted multifilament yarns will always result in an increase in yarn diameter in the plane of bending and a decrease in the number of filament-to-filament contacts. Photographs of bent yarn and fabric structures illustrate these effects. The combined influence of both yarn radial growth and increased fabric free space on interyarn constraints is also discussed.

The variation of loss coefficient with stress and strain amplitude has been derived theoretically for idealized multicomponent fibrous assemblies with frictional interactions, and the results are compared with measurements made on a range of textile materials.

Expressions are derived for determining theoretically the number of filaments accommodatable in successive concentric rings in a multifilament yarn cross section as a function of yam twist and filament diameter. Increasing filament ellipticity with increasing twist is considered. Numerical values are tabulated for several typical filament diameters and a range of yam twists. The yarn packing factor is also given. Photomicrographs of model yarns illustrate that migration of filaments occurs with increasing twist; filaments are forced outward from one ring to the next as their ellipticity is increased.Photographs of bent, multifilament model yams show evidence of an increase in helix angle of the filaments on the inside of the bend and a decrease of those on the outside of the bend. A change of filament packing density upon bending is also evident. An increase in packing density on the inside of the bend and a decrease on the outside is observed for some degrees of bending; however, for severe bending the packing density appears to decrease throughout.Analytical results are presented graphically for the curvature and change in curvature of the filaments in a bent, twisted yam as a function of filament position in the yam cross section, yam twist, and radius of curvature. The most significant findings relate to the locations within the yam cross section of the maximum and minimum curvature and changes in curvature. A discussion of the classic geometrical model of a bent yarn as it relates to fiber mobility is included. ' The work was partially completed while this author was employed at Fabric Research Laboratories.

A new model has been developed that predicts the compression behavior of thick. nonwoven fiber assemblies at high stress levels. The model uses the Poisson distribution to describe how fibers are stacked on top of one another during web formation. The stress/thickness relationships developed are based solely on the compression properties of the stacked fiber mass. Effects predicted by the model include greater packing uni formity for increasing web weight, decreasing fiber diameter. and decreasing fiber transverse compression modulus. An expression for the fractional area of a compressed web that is actually under stress is also given. The stress/thickness behavior predicted by the model is compared to the measured compression characteristics of nylon. Spec tra®, Kevlar®, and fiberglass needled batts. The measured and predicted behavior agree well enough in most cases to validate both the Poisson distribution approach to the placement of fiber mass in randomly formed nonwoven materials and the simple com pressive stress relationships developed from it.

Factors affecting the thermal performance of protective clothing are discussed in a general way, and a laboratory method of achieving heat absorption rates typical of those occurring during exposure to a large fire is described. Using this method, the strength retention of fabrics during short-term exposure at high heat flux levels has been found to depend on the temperature achieved at a given instant during exposure and to be independent of the mechanism of heat absorption. A comparison of the duration of exposure to high heat flux levels that causes various polymeric fabrics to lose most of their original strength or to autoignite predicts that such high-temperature materials as polybenzimidazole (PBI) and Nomex/Kevlar can provide a few extra seconds of protection against the extreme heat of a large fire. Some difficulties associated with the use of an instrumented skin-simulant device for determining the rate of conductive heat transfer through fabrics of various kinds during standard gas flame impingement tests are also discussed. There is, at present, no accurate way to translate temperatures measured at a depth of 500 μm in a fabric-covered skin simulant to temperatures appropriate for a depth of 80 μm, the skin depth to which clinical data regarding tissue damage are related. Until the conductive heat flow equation can be revised, ranking of fabrics by the maximum temperature achieved in the skin simulant remains the only reliable method of using data from this device.