Study On The Mechanical Property And Protective Application Of The Shear Thickening Fluid

Shear thickening fluid(STF)is a type of dense granular suspension.When the applied shear rate or shear stress exceeds a critical value,the STF will undergo shear thickening.In particular,its viscosity will rise sharply,and it can even result in the transition from continuous shear thickening(CST)to discontinuous shear thickening(DST)or shear jamming(SJ).The STF has great potential in practical applications of damper,personal protection,and etc,because of this special shear thickening property.In recent years,researchers have carried out a series of studies on the performance characterization,shear thickening mechanism,and practical applications of STF,and have achieved fruitful research results.However,there are still many problems in rheological property regulation,shear thickening mechanism explanation,and protection application that need to be explored and solved.To this end,this dissertation started from the characterization of mechanical properties of STF and studied its stress relaxation behavior.Based on the regulation of the rheological properties of STF,the basis for judging the occurrence of SJ was proposed by studying the evolution of the first normal stress difference(N1)of STF in the stress-controlled mode.The competition mechanism between the hydrodynamic lubrication and frictional contact during the thickening state transition process was revealed.Then,STF and Kevlar fabric were compounded to prepare a composite material with high resistance to knife and spike stab.Its mechanical properties at high strain rates were characterized by the split Hopkinson pressure bar(SHPB)system.Besides its low velocity anti-impact property,this composite was also applied to the field of ballistics,and its ballistic performance was optimized by adjusting parameters.Combined the experiments and finite element simulation together,the enhancement mechanism of STF on Kevlar’s ballistic resistance was presented.The specific work includes the following aspects:1.Preparation of STF and characterization of its stress relaxation behavior.The polystyrene ethyl acrylate(PSt-EA)particles were firstly prepared by emulsion polymerization,and then the STF with obvious shear thickening properties was developed.Then the relationship between the stress relaxation phenomenon of STF and its internal structure was studied by the rheological method.Based on the particle chain model,the viscoelastic model successfully predicted the stress relaxation,which was in good agreement with the experimental results.By studying the stress relaxation behavior,it was concluded that STF could achieve the transformation of thickened state not only by increasing the volume fraction but also the shear rate and shear time.2.Study on the mechanism of STF thickening state transition.Since the W-C model cannot accurately describe the flow curve of high volume fraction STF under the stress-controlled mode,the onset jamming stress obtained by the W-C model will no longer be accurate.For this reason,the basis for judging the occurrence of SJ in STF was given by the investigation of N1.In the DST state,N1 is negative.Although there is frictional contact inside the STF at this time,its influence is masked by hydrodynamic lubrication.With the increase of shear stress,the effect of frictional contact becomes more obvious,gradually surpasses the fluid lubrication effect,occupying a dominant position,N1 changes from negative to positive,and STF changes from DST to SJ.3.Preparation of STF/Kevlar and research on its impact resistance.The STF/Kevlar composite was prepared by immersing Kevlar fabric in STF.The drop tower test results showed that the presence of STF significantly improved the knife and spike stab resistance of Kevlar fabrics.Fabric failure in the spike stab test was mainly due to the slippage of the yarn,while the failure mode in knife stab test was mainly due to yarn breakage.The SHPB results showed that as the impact velocity increased,both the impact strain rate and the modulus of the composite were increasing.The modulus of composites increased with increasing the volume fraction of STF.The increase in particle volume fraction and the number of fabric layers decreased the energy transfer rate.Kevlar fabrics undergo four stages when impacted.STF plays a reinforcing role mainly in the slippage and deformation stages by increasing the friction between fibers and preventing yarn slippage.4.Research on the ballistic performance of STF/Kevlar.This kind of composite material was extended to the bulletproof field,and the bulletproof characteristics of the composite material were optimized by changing the parameters in the influencing factors.The results depicted that increasing the particle volume fraction of STF can enhance the ballistic performance of STF/Kevlar.The added amount of CNT had the optimal value,and the protective effect of adding CNT with a mass fraction of 1%was even better than 1.5%within the concentration range studied in this work.The trend of the puncture force in the quasi-static puncture was roughly the same as that of the bulletproof test result.The yarn pull-out results revealed that STF could increase the frictional factor between the yarns,the addition of CNT further increased the friction.The finite element analysis results of the ballistic impact corresponded well to the experimental results.The STF enhancement mechanism was revealed through simulation.STF increases the coefficient of friction between yarns,increases the load-bearing area of the fabric,and makes the stress distribution more uniform,thereby improves the ballistic performance of Kevlar.