Behavior of Yarns Subject to Impact Loading

Abstract

Stone, Schiefer, and Fox [6] have recently described some ingenious experiments relating to the behavior of yarns under impact loading. Among other results, they show, as others had shown earlier [5], that when nylon filaments fail under these conditions the new ends have been fused. They suggest that the cause of fusion may be &dquo;... the high, localized energy released by bond rupture.&dquo; We would like to suggest that fusion or pyrolysis is rather a normal concomitant of impact failure and perhaps other types of failure as well.We have discussed the problem of the energyabsorption capacity of materials in terms of the density of strain energy and the strained volume [3]. If a material &dquo;work hardens&dquo; in response to strain, when a strain is initiated at a point it will tend to become general so that the strained volume will be large. On the other hand, if &dquo;work softening&dquo; occurs, the strained volume will remain small and the net energy absorption (the product of the energy density and the volume) will be small. We think that this point of view provides some new insight into the nature of brittleness and toughness as well as providing an explanation of the fusion that is observed in the case at hand.In the present connection, the impact test is by its nature adiabatic. A work-softening process which must be considered, therefore, is the reduction in modulus that accompanies the work-induced temperature rise. In a qualitative way the importance of this work-softening process is shown by a consideration of the temperature-modulus relationships for various materials given by Schmieder and Wolf [4].For example, polystyrene, which has a very sharp drop in modulus at the glass temperature, is a brittle material. Nylon, on the other hand, has an attenuated temperature-modulus curve that depends on the existence of crystalline regions which function as crosslinking points in the rubbery material above the glass temperature. Nylon is, of course, an excellent example of a tough substance. Other similar examples can be given, and no exceptions to this general relationship between the temperature-modulus dependence and the toughness of uncrosslinked homopolymers have been found.The implication of this argument is that failure is a local catastrophe which may involve only a small fraction of the gross sample volume. This concentration of the strain energy at the point of failure can easily lead to fusion or pyrolysis. For nylon the strained volume is sufficiently large so that the fusion is apparent on inspection. The more brittle materials may fuse over so small a volume element that special methods would have to be devised to detect it, or, indeed, local pyrolysis may be involved.The development of high local temperatures during the mechanical working of materials has been clearly shown in experiments which involve less violent processes than actual failure. For example, Brauer and Mfller [2] have found temperature rises of about 70° C occurring during the drawing of polyvinyl chloride fibers. Bowden...

Microscopical Study of a Multilayer Nylon Body Armor Panel After Impact

“…It is obvious that the heat generated in the panel is very much concentrated [8] and cannot be readily dissipated. This causes the softening, fusion, and decomposition of nylon in localized regions.…”