Study on Load Transfer of Aramid Fabric by Micro-Raman Spectroscopy
Author:Qin Fu Yong
Supervisor:lei zhen kun
Flexible fiber fabrics have the advantages of light weight and toughness,and have great application space and development potential in the field of outer protection of human body or equipment.The essence of optimum design of elastic performance of soft fabric structure is to study the interaction between bullet and fabric.From the mechanical point of view,it is the process of load transfer and energy absorption of yarn.It is of great significance to model the yarn deformation mechanism in the ballistic penetration experiment into the yarn drawing process,which can be used to study the yarn crimp and elongation,adhesion-slip,energy absorption mechanism and friction characteristics during bullet penetration.Because of the complexity and multi-level of the fabric’s micro-structure,the deformation of the fabric structure will lead to the physical phenomena of crimp exchange,adhesion and slip at the warp and weft interweaving points,and the fibers inside the yarn will also slip and destroy,resulting in the damage and breakage of the yarn eventually.At present,the experimental characterization of micro-deformation of fabric structure is the most difficult and key stage to establish load transfer model and energy absorption mechanism of yarn.In this study,the basic mechanical problems such as yarn stress transfer,crimp elongation,friction slip and energy absorption mechanism during impact of bullet-proof plain fabric(Kevlar 49)are taken as research contents,and micro-scale experimental characterization and modelling test methods are developed to reveal the load transfer mechanism of the fabric,which is helpful to further explore the penetration failure and energy absorption mechanisms of the bullet-proof fabric.On the basis of the experimental results,the stress transfer of fabrics in single yarn drawing and out-of-plane pushout test with circlely-fixed support is analyzed,and the stress transfer model in single yarn drawing test is proposed to reveal the relationship between yarn uncrimp and shear stress,and the stress transfer model in out-of-plane pushout test is established to reveal the stress transfer mechanism of fabrics.The penetration failure mechanism and energy absorption mechanism of bullet-proof fabrics are further discussed by interpreting the two transfer models.The main contents of this study are as follows:(1)Tensile mechanical properties of Kevlar 49 single yarn were measured by digital image correlation marker method.The experimental results show that the stress-strain curve of single yarn transits from linear to non-linear evolution,which is due to the successive failure of yarn.The elastic modulus of warp and weft yarn varies slightly due to the different initial crimp degree of single yarn.The average modulus of elasticity of single yarn is about 51GPa is lower than that of single fiber about 98.63 GPa,which is due to yarn failure mechanism,initial crimp,manufacturing defects and damage.(2)The theoretical formula between single yarn Raman shift and multi-fiber stress is deduced,and the Raman measurement principle of single-yarn equivalent stress is proposed.The Raman test of Kevlar 49 single yarn drawing was carried out.The experimental results show that the red shift or blue shift(Raman shift)of G’ peak position of single-yarn has the highest load sensitivity.The Raman shift-stress factor is obtained by fitting curve and a set of steps for analyzing Raman spectrum are given,about 1 82.44 MPa/cm-1.(3)A single-yarn drawing test device was built.The single-yarn drawing test was carried out with or without transverse preload.The stress distribution on the yarn was measured by Raman spectroscopy.The rule of friction and shear stress on the yarn uncrimp section and the crimp section was discussed,and the interfacial friction and shear stress model and the axial stress transfer model were established.The interface parameters of the yarn drawing model are reverse identified from the experimental data of micro-Raman spectroscopy.The experimental results show that the proposed single yarn drawing model conforms to the yarn stress distribution and effectively explains the load transfer mechanism in the single-yarn drawing process.The load transfers from the yarn entry to the inside and the adhesion and friction phenomena run through the whole process of yarn drawing test.Static friction exists in the yarn crimp section and The slip friction occurs in the yarn uncrimp section.At the transition point of crimp/uncrimp,the shear stress reaches the maximum.(4)Fabric out-of-plane pushout setup was built,and fabrics circularly fixing and center out-of-plane loading test were carried out.The stress distribution in the whole field was measured by Raman spectroscopy.The experimental results show that there exists a high stress cross distribution in the primary yarn area and a low stress distribution in the secondary yarn area.The primary yarn is the main path of load transfer,and the load is gradually transferred to the adjacent secondary yarn.So the secondary yarn is the branch of load transfer.A yarn drawing model with end fixing is established to characterize the stress transfer in fabric out-of-plane pushout test.The interface adhesion determines the load transfer from the yarn axial stress to the interface shear stress.Experiments show that the proposed model conforms to the yarn stress distribution and effectively explains the load transfer mechanism in fabric out-of-plane pushout test.The fabric modeling test method and its load transfer model studied in this thesis will be helpful in understanding failure and energy absorption mechanisms of flexible fabrics under impact,and provide experimental and theoretical supports for the optimization of bulletproof fabrics.The research work of this thesis was funded by the National Natural Science Foundation of China,"Experimental study on interfacial loading transfer and energy absorption behavior of bullet-proof fabric with stitching structure(11472070,2015-2018)".