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The effect of filling and warp yarn variables on the strength of a nominally designated 60 kN, 2.9 cm Kevlar® 29 parachute webbing was investigated. The goal was to increase webbing and jointed webbing strength without in creasing the weight of the structure. Webbing of varying pick spacing, filling yarn strength, and amount of warp yarn twist was fabricated and tested at quasi-static and high strain rates. Improved strength was obtained when: a the filling was reduced from two to single-ply yarn, b) pick spacing was increased to 6.3 picks per cm, and c twist was removed from the warp. Kevlar® webbing strength was not reduced at high strain rates of 3000% s compared with quasi-static values. The strength of jointed samples at high strain rate loading was higher than that obtained in quasi static tests. ,

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Deflection-force relations for plain weave Kevlara fabrics have been determined under conditions of uniaxial loading. In these experiments, the loading is stopped at a given level and a portion of the fabric is encapsulated. The fabric is then unloaded, sectioned, and photographed. Measurements on the photographs reveal the changes in weave geometry and yam cross section with loading. The initial geometrical data are used in a large deformation mechanical model, which couples yam bcnding and stretching effects to predict theoretical displacement-force relations for the fabric. Experimental and theoretical deflection-force curves arc in good agreement; they show that during initial loading the response is dominated by yarn bending, while for large loads the response is dominated by yarn stretching.The high tenacity of yam spun from Kcvlar 2g9 fibers make it an attractive material for use in parachute decelerator applications where reduced weight and bulk are particularly important. When Kevlar was first considercd for parachute applications, Coskrcn and Abbott [2] pointed out that replacing nylon with Kevlar on an equal-strength basis results in a 67% reduction in weight and a 75% reduction in volume. Extensive use of Kevlar in high performance ribbon parachutes has generally supported these predictions [8, 91. Nylon is a relatively efficient parachute material, so the degree to which the potential of Kevlar can be achieved is contingent on the effective translation of beneficial fiber mechanical properties into the mechanical properties of the textile structure.The design of parachute fabrics has been primarily a trial-and-error process in the absence of any ihcoretical models that effectively relate the overall fabric mechanical properties to specific aspects of the fabric microstructure and morphology. A number of experimental investigations to determine the effects of specific fabric microstructure on the ultimate strength of woven Kevlar fabrics have been reported. These have included studies of the effects of both warp and fill yam spacing, denier, and strcngth [ 1,3,4] on the overall fabric strength when loaded uniaxially in the warp direction. These studies have also included investigations into the strength ofjointssewn into these fabrics. ' This work was done at Sandia National Lnboratories and s u p ported by the U.S. Dtpanment of Energy undcrcontna # DE-AW-76DPW789. The effects of moisture on the strength of Kevlar 29 parachute fabrics have been investigated by Ericksen and Orear [ 5 ] . An excellent summary of the analysis of mechanical properties of general woven fabrics prior to 1969 is included in the monograph by Hearle, Grosberg, and Backer 171.In this paper, we provide deflection-force measurements obtained on three different plain weave fabrics woven from Kevlar 29 yarns. Each of the three fabrics was tested with an Instron testing machine under conditions of uniaxial loading in both warp and fill dircctions to providc a total of six deflection-force curves. In each of these six cases, four sep...

The effect of moisture on the strength of Kevlar 29 ribbon parachute fabrics was investigated. Individual yarn samples and various fabrics were soaked in water for periods of 15-60 min. Yarn strength did not change; however, fabric strength was reduced. The reductions ranged 3-13% depending upon the fabric construction. Additional experiments eliminated factors such as the weaving process, yarn swelling, and increased abrasion in the presence of moisture as possible mechanisms. Other tests demonstrated that increased interyarn friction occurs when moisture is present. These results suggest that the moisture-enhanced friction restrains highly loaded filaments from adjusting their position and relieving stress concentrations. Specific construction parameters governing the amount of strength reduction were not defined in this study. However, it is evident that wet strength should be considered in parachute material selection and design.

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