Strain Rate Effect On Mechanical Behaviors Of Fibers And Ballistic Impact Damage Mechanisms Of Knitted Composites

The biaxial multi-layered warp knitted (BMWK) fabric structure consists of the inserted warp and weft yarns, and knitted yarn loops. The warp (0°) and weft (90°) yarns are held together with tricot yarns to form a stable fabric structure. This kind of fabric has both the good formability and high tension modulus and strength along the biaxial directions. As the axial fiber tows were not interwoven in knitted fabric, the axial fiber tows have the highest strength ratio. In theoretical, the ratio could be 100%. Under impact loading, the stress waves can spread to the largest area. This leads the BMWK composite has the highest energy absorption capacity. Such kind of composite material has great potential applications to the ballistic impact protection areas. This paper will report the strain rate effect of the fiber tows'mechanical behavior and investigate the ballistic impact damage of the BMWK composite at the unit-cell scale.The tensile properties of basalt filament tows were tested at quasi-static (0.001/s) and high strain rates (up to 3000/s) with MTS materials tester (MTS 810.23) and split Hopkinson tension bar (SHTB) respectively, and obtained the stress-strain curve under different strain rate. The results showed that the mechanical properties of the basalt filament tows were rather sensitive to strain rate. Specifically, the stiffness and failure stress of the basalt filament tows increased distinctly as the strain rate increased, while the failure strain decreased. It has analysis the fracture morphology with scanning electronic microscope (SEM) photographs of the fracture surface, it is indicated that the basalt filament tows failed in a more brittle mode and the fracture surface got more regular as the strain rate increases. In addition, based on the filament tows model and the statistical theory of fiber strength, a single Weibull statistical model was employed to describe the strength distribution of the basalt filament tows, and obtained the constitutive equation of parallel filament fiber tows, and characterized the strain rate effect of fiber tows, and simulate the experimental result with the constitutive equation.Ballistic impact tests were conducted to investigate the ballistic impact damage of the BMWK composite. Residual velocities of the projectile were obtained under the different strike velocities. The impact damage morphologies were photographed. From the testing results, the ballistic performance of the composite was calculated and the impact damage mechanisms were analyzed.From the ballistic tests, it was found that the BMWK composite has the higher energy absorption capacity than that of 3-D woven composite at the same area density. The strain rate effect of the mechanical behaviors of the basalt fiber tows will influence the interaction between the fiber tows and the projectile. As the increase of the strike velocity, the strain rate will also increase. This will lead the increase of the strength, modulus, and failure strain and rupture work of the basalt fiber tows.The straighten warp and weft yarns, tricot yarns in the BMWK structure will impart the high inter-laminar shear strength. Under the ballistic impacts, the damage area of the striking side is less than that in the rear side. The failure mode at the striking side is mainly the fiber compressive failure, resin shear failure and crack. While at the rear side, the failure mode is mainly the tensile failure of fiber tows and resins. Owing to the differences of the strain rate effects and strain wave propagation, the damage mode and failure extent will also differ under the different striking velocity.In experimental, we found that for the BMWK composite, the higher the striking velocity, the greater of the damage area.Based on the microstructure of the BMWK composite, the ballistic impact damage was investigated at the unit-cell level with the FE model. A unit-cell model of the BMWK composite has been established to characterize the stiffness matrix. The elastic-plastic constitutive equation of the composite, the maximal stress failure theory and the critical damage area (CDA) failure theory have been implemented as a user defined material law (VUMAT) using FORTRAN languages and connected with the commercial FEM software ABAQUS to define the composite constitutive equation and failure criterion under impact.Combined the commercial finite element code ABAQUS/Explicit with VUMAT, the damage process of BMWK composite is numerically simulated with explicit algorithm. The residual velocity, accelerated velocity of projectile, target deformation, geometry of bullet hole and propagate of stress wave, etc. were compared with experimental data and FEM. The damage mode of BMWK composite and energy absorbing mechanism difference between fiber and resin matrix are simulated accurately. From the comparisons of the impact damage and energy absorption, it was found there are good agreement between experimental and FEM calculation. The model and VUMAT for the BMWK composite could be extended to be optimization of the microstructure and the design of the high impact damage tolerance structures.