| Kevlar which has high strength,high modulus,high strength to weight ratio,acid and alkali resistance and other properties is widely used in various fields of aerospace,military,engineering.It is vital to study the mechanical properties of Kevlar for playing the excellent performance.In this paper,we mainly consider the influence of friction and scale effect on Kevlar:Kevlar 49 and Kevlar 29 filament is tested by MTI micro tensile testing machine with different gauge lengths(12.5 mm,25.0 mm,40.0 mm).The results shows that in a certain range,tensile strength,toughness,ultimate strain of Kevlar 49(Kevlar 29)decreased with the gauge length increasing,the elastic modulus increased with the increase of gauge length.Kevlar 49 and Kevlar 29 have the same structure,so their mechanical properties are similar,but not quite the same,which is due to the different of their processing technology.At the same time,Carbon,Glass and Basalt filaments with the gauge length of 25.0 mm were tested and then analyzed.Tensile strength and elastic modulus of Kevlar filament is lower than that of Carbon filament,however,the ultimate strain of Carbon filament is smaller than that of Kevlar filament.The mechanical properties of Glass filament is similar to that of Basalt filament,which is caused by the similar structure.The treated(oiling)and untreated Kevlar 49 yarns with the gauge lengths(25.0 mm,50.0 mm,100.0 mm,200.0 mm)are tested by MTS universal testing machine for static tensile test.The treated and untreated yarns with the gauge length of 25.0 mm were held on dynamic tests by Instron system(Strain rate=40s-1,80s-1,120s-1,160s-1).Weibull distribution model statistical analysis was conducted on the test data to quantify the discreteness of fiber strength.The results showed that in a certain range,with the increasing of gauge length,tensile strength,toughness,ultimate strain of both the treated and untreated Kevlar 49 yarns decreased,while the Young's modulus increased gradually,Kevlar 49 yarns had obvious scale effect.The treated Kevlar 49 yarns compared with untreated Kevlar 49 yarns(in the same gauge length)the ratio of mechanical parameters of static tensile strength,Young's modulus,toughness decreased and ultimate strain increased with the gauge length increasing,however,the variation was very small,namely decreasing the friction between filaments of Kevlar 49 yarn(in the same gauge length)had little effect on the static tensile mechanical properties.With the increasing of strain rate,both the treated Kevlar 49 yarns and the untreated yarns whose tensile strength,Young's modulus increased,toughness and ultimate strain decreased.Compared with the untreated yarns,tensile strength of the treated yarns decreased gradually,and decreasing amplitude showed increasing trend.These results indicated that friction between filaments had a significant effect on the mechanical behaviors of Kevlar yarns under high strain rates.According to the analysis of Kevlar 49 fiber testing results to analysis the scale effect(different gauge length and different gauge structure).The results showed that in a certain range,tensile strength,toughness,ultimate strain of Kevlar 49 filament decreased with the gauge length increased,the Young's modulus increases with the gauge length increasing;In a certain range,tensile strength,toughness,ultimate strain of Kevlar 49(treated)yarns decreased while the Young's modulus increased with the gauge length increasing,and with the gauge length increasing,mechanical parameters of Kevlar 49 filaments and yarns had the same variation tendency.Kevlar fiber had obvious scale effect with the gauge length increasing.It is found that tensile strength of filaments is greater than that of yarns,then tensile strength of yarns is larger than that of fabrics.Specifically,tensile strength,Young's modulus of Kevlar 49 filaments,yarns,fabrics decreased in turn,toughness and ultimate strain was decreasing first then increasing,namely toughness,ultimate strain of yarns were the lowest.Fiber has obvious scale effect. |
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