Deflection-Force Measurements and Observations on Kevlar 29® Parachute Fabrics

Abstract

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...

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