Study Of Cut Resistance Of STF/UHMWPE Weft-knitted Fabric Flexible Protective Materials

STF/high performance fiber fabric flexible composites are commonly used in the field of individual mechanical protective equipment in recent years.This kind of composite material has good flexibility which can adapt to human movement,except for excellent mechanical properties.It first appeared in the field of bullet-proof research,and now extends to impact,stab and cut resistance.So,it is named as flexible mechanical protective material.Shear thickening characteristics and mechanism of STF by rheological properties,ballistic,impact and stab properties of high performance woven fabrics and STF/high performance woven fabric composites have achieved considerable accomplishment.However,study on cut resistance of high performance fibre fabrics and STF/fabric composites are just beginning.The protective mechanism of STF in the mechanical protective process is still unclear.The reasons are summarized as follows:First,the traditional rheological properties test can not characterize thickening characteristics of STF under normal mechanical behavior in the mechanical protective process.Secondly,thickening response of STF in the mechanical protective process and the mechanism of mechanical protection of STF/fabric composites before and after thickening are not clear.Therefore,this paper chooses UHMWPE weft knitted fabrics and STF to prepare flexible composites.Taking the thickening characteristics of mechanical properties of STF under normal load as the breakthrough point and cut resistance as the main line,through theoretical analysis and experimental research methods,combined with cut fracture failure mode of high performance fiber fabrics,and synergistic effect of thickening properties of STF under normal load and cut resistance of STF-fabric flexible composites is further studied.The main research contents and conclusions are as follows:Using self-made accessories and INSTRON universal mechanical instrument.Mechanical properties of STF were tested by quasi-static extrusion and pull-out experiments.The effects of experimental parameters such as speed,contact area,liquid level height of STF and roughness of contact interface on the normal mechanical properties of STF were discussed,and thickening characteristics of STF under normal mechanical behavior were revealed.The results show that there is a critical rate of STF in the extrusion mode and extrusion rate can be defined as the ratio of compression speed to the distance between compression rod and bottom boundary of container.At a certain extrusion speed,when metal bar reaches critical extrusion displacement,force of STF increases sharply,and STF becomes thickening.When extrusion speed increases,critical displacement decreases and STF thickens in advance;when liquid level height of STF increases,critical displacement increases,and STF delays thickening effect;the contact area between the rod and STF also affects thickening characteristics in the extrusion process.The larger contact surface,the smaller critical displacement.The roughness of contact surface has little effect on thickening characteristics of STF in the extrusion process.The results of STF pull-out experiments show that thickening phenomenon occurs in the first stage of STF extraction,that is,pull-out force at the initial pull-out rate is defined as pull-out modulus.Pull-out force of STF without thickening phenomenon is a constant,about 1.5N.With increase of pull-out speed,the maximum pull-out force reaches about 140 N.Pull-out force of STF which produces thickening increases by nearly 150%.There is thickening phenomenon of STF during pull-out process.There is a starting point of thickening of STF in the extraction process,i.e.the critical velocity and the maximum thickening degree,i.e.the maximum pull-out force.Jamming phenomenon in condensed-matter physics and Jamming dynamic forward stretch principle are used to analyze thickening characteristics of STF under normal quasi-static compression and pull-out modes,and a velocity-dependent Jamming dynamic forward stretch model is established.The anti-cutting performance of yarn was tested on the self-modified INSTRON universal material experimental machine.The experimental parameters were yarn structure,blade shape,cutting speed and cutting angle.The results show that the maximum specific cutting force of weak-twist multifilament yarn is the largest,but cut breakage elongation and cut specific work are the smallest,and yarns are rigid and strong.Cut breakage elongation of wrapped yarn is in the middle,the maximum specific cutting force is the smallest,and the cutting breaking specific work is also in the middle.The breaking elongation of staple yarn is the largest,the maximum specific cut force is in the middle,and the specific cut work is the largest.The samples show flexible and strong mechanical properties.Cutting angle is divided into orthogonal perpendicular and inclined angles.When yarn is cut orthogonally,yarn on the edge of knife has good bundling property,and yarn has greater cutting resistance.Because of different shape blade,contact form between the blade and the yarn is different during the cutting,so,cut resistance is affected by blade shape.The influence of cutting speed on cut resistance of yarn shows that with the increase of cutting speed,cut resistance property first increases and then decreases,which mainly reflects mechanical response characteristics of organic high performance fibers.Based on analysis of force and cut failure process of UHMWPE yarn,energy distribution of yarn in the cutting is proposed by using principle of energy conservation:compression and tension elastic deformation energy,cut fracture energy,viscous-flow energy of polymer material and plastic deformation energy in the cutting,and theoretical model of energy distribution is established.Finally,cutting failure mechanism of contact form and mechanical response of UHMWPE yarn and cutter is proposed.Based on the horizontal and vertical cutting experiments of UHMWPE weft knitted fabrics,force involved,cutting failure form,friction effect and deformation characteristics of weft knitted fabrics during cut process are analyzed comprehensively.The results show that cutting force can be divided into two categories:the normal force and the horizontal force,which are manifested in cutting force and friction force.The failure modes of fabric in the cutting include:fabric deformation,wear of the interface between fabric and tool,generation of fabric cut cracks,extension of fabric cut cracks to final complete failure of cutting.Friction effects in the cutting process of fabrics include friction between cutters and fabrics and slip friction between yarns.Deformation characteristics of fabric during vertical cutting:fabric back bulge and coil expansion.Based on the theory of fracture mechanics,it is proposed that the cutting fracture of weft knitted fabrics includes the generation of cracks,i.e.breaking energy of fabric and cracks expansion,i.e.the cutting impedance of fabrics.Calculation method of cutting impedance is given.According to the theory of energy balance,the relationship of energy conversion in each stage of cutting is analyzed,and a simple mechanical model of fabric in each stage is established.Cut experiment of STF/UHMWPE weft knitted fabric composite was carried out,and cut resistance of STF/UHMWPE weft knitted fabric was discussed.The results show that the anti-cutting performance of STF-weft knitted fabrics in normal quasi-static vertical cutting is improved due to the addition of STF.The synergistic effect of thickening characteristics and cutting resistance of STF/fabric composites in the cutting was analyzed from three aspects of speed,friction and fracture morphology.The contribution of thickening characteristics under normal mechanical behavior of STF to cut resistance of fiber fabrics was revealed.The results show:1)Cut speed has little effect on cut performance of UHMWPE weft knitted fabrics,but has great influence on STF/UHMWPE weft knitted fabrics composites.When cutting speed is small,STF does not produce thickening.The anti-cutting performance of STF/UHMWPE weft knitted fabrics composites has not been improved.Only by increasing cutting speed,when STF appears thickening,cut performance of composites can be significantly improved.2)There are two reasons for increase of friction of STF/weft knitted fabrics in the cutting:The one is that solid hard SiO₂ particles in STF change roughness of material surface and friction force was increases;The other is that thickening of STF occurs in the cutting,which changes from liquid to solid,hinders movement between fabric and blade,inter-yarns.Friction force was imprived;The greatest contribution of increased friction is attributed to thickening characteristic of STF.There is synergistic effect between the thickening property of STF and the sharp increase of friction property of STF/fabric composite during cutting process.3)STF fills gap between fibre bundles.Cut broken-end of fibre bundles is covered with a layer of STF protective"film".Fracture surface is smooth.Fructure of a single fibe have a large number of STF,and agglomeration phenomenona of lots of SiO₂ nanoparticles can be observed.It indicates that thickening characteristic of STF in the cutting makes SiO₂particles agglomerate rapidly at stress concentration to resist the invasion of cutter.