Ballistic Penetration Property Of Full-Width Weft Insertion Knitted Fabric/Surlyn Resin Flexible Composite

Body armor can prevent bullets and debris from invading human bones and organs,causing penetrating and fatal injuries.Currently,body armor still has problems with poor comfort and flexibility,and the protective area needs to be improved.The method to balance the weight,flexibility,comfort and ballistic performance has become an important topic in the development of bulletproof materials.With the innovative development and commercialization of high-performance fibers,developing and optimizing high-performance fiber reinforced bulletproof composites has become development trends of future.High performance fiber reinforced composites are lightweight and flexible,with strong structural design,good fitness,and excellent impact resistance,providing more possibilities for the development of flexible bulletproof materials.The full-width weft insertion knitted fabric is to introduce high-performance and non-buckling insertion yarns into weft-knitted fabrics.The weft insertion yarns are evenly arranged and maintain straight states.When subjected to impact,they can quickly transmit the load and dissipate impact kinetic energy.Surlyn resin has excellent impact toughness and melting strength.In this paper,full-width weft insertion knitted fabrics were used as reinforcement structure,and flexible Surlyn resin was used as matrix material.The weft-knitted fabrics were stacked in an orthogonal manner to manufacture composite materials.Firstly,braided yarns with aramid filament as core yarns were prepared in order to alleviate the phenomenon of fuzzing and fiber slippage when aramid filaments are stressed.Three different specifications of full-width weft insertion knitted fabrics named AF-Ⅰ,AF-Ⅱand AB were prepared on the computerized flat-knitting machine.The fineness of ground yarn and weft insertion yarn material were controlled as single variables.AB was a kind of weft-knitted fabric with high breaking elongation braided yarns as insertion yarns.The bending stiffness was used to evaluate the flexibility of fabrics,the multi-directional damage resistance of fabrics was evaluated by bursting performance.The effects of ground yarn fineness and insertion yarn materials were explored.It was suggested that when the ground yarn is thin,the binding force on the weft insertion yarn is lower,and it is prone to fracture when subjected to bursting load.The weft insertion yarn is prone to slip,leading to "window opening effect",resulting in a lower utilization rate of the strength of weft insertion yarns.During the bursting process of AB,the braided yarn as the weft insertion yarn bears larger load,and the transverse tensile deformation generated by the ground coils will restrict the displacement of the weft insertion yarns,presenting "self-locking effect".After the fabric is laterally inserted with high stiffness yarns,the weft bending stiffness is mainly determined by the fabric area density,while the warp bending stiffness is mainly affected by the fineness of ground yarn.Thirdly,high velocity impact test was carried out on weft-knitted insertion fabric/Surlyn composite laminates.By comparing the ballistic limit velocities,impact deformations,and specific energy absorption of different specifications of laminates,the effects of coil structure and weft insertion yarn on the ballistic performance were investigated.The energy absorption mechanism of flexible composites under different impact velocities,the damage and failure mechanisms of the front and back layers,and the failure modes of composite laminates were deeply explored through analyzing the montage of impact event by high-speed photography,the macroscopic damage morphology of composite laminates,and X-ray computed tomography(XCT)scan images.It suggested that the ballistic limit velocities of AF-10/Surlyn,AF-6/Surlyn,and AB/Surlyn composites are 365.31m/s,339.76m/s,and338.29m/s,respectively.The main damage modes on the front layers of composite laminates are shear failure,while the damage modes on the back layers are tensile failure and delamination debonding.Therefore,using lightweight,tensile resistant fabrics as the back layers has greater energy absorption potential.Moreover,the impact velocity of the projectile can affect the failure mode and damage mechanism of the composites.The specific energy absorption of fabric reinforced flexible composite laminates first increases and then decreases with the increase of bullet impact velocity.The decrease in specific energy absorption is due to the influence of the dynamic response of the composite material and the material strain rate.Finally,the finite element model of weft-knitted insertion fabric/Surlyn resin composites under ballistic impact was established.Based on the impact response and damage evolution of laminates,damage mechanisms and failure laws were analyzed and verified deeply.It shows that at the center of the bullet hole,the main damage mode is impact compression;At the outer ring position of the bullet hole region,the specimen is subjected to greater shear stress at the edge of the bullet.Due to the cross configuration of the insertion yarns in the laminates at 0 ° and 90 °,the transverse waves of stress mainly propagate in the weft and warp directions of the fabric,and the damage area is in a cross shape,extending along both the weft and warp directions of the weft-knitted insertion fabric.