| Three-dimensional angle-interlock woven composite (3DAWC) owns higher interlaminar shear strength, fracture toughness and impact damage tolerance than the laminated composite, and has great potential applications to the impact protection. The mechanical performances of composite material are significantly affected by reinforcement structure under impact loading. And the micro-structure performance and dynamic response with accurate characterization can provide theoretical guidance for the impact-resistance design of composite material.The purposes of this research are to study the ballistic impact behaviors and structural characterization of3DAWC at microstructural scale, to analyze its dynamic responses by considering the strain rate effect and the visco-elastic mechanical constitutive relation of Twaron(?) filament bundle, and to reveal energy absorption mechanism of3DAWC during ballistic penetration.The research procedures and approaches are as follows:(1) Establishing a geometrical model of3DAWC: Basing on the microstructural theory of composite material and fundamental structure parameters of three-dimensional angle-interlock woven fabric (3DAWF), the geometrical model of3DAWC is created in computer-aided design software. And this model can be discretized into the finite element model (FEM) of3DAWC under a hemispherical-cylindrical steel projectile.(2) Utilizing two research schemes of yarn material for numerical calculation of FEM: Elastic plastic model with kinematic hardening (EPM):Calculate the FEM of3DAWC under a hemispherical-cylindrical steel projectile with standard material model in the commercial-available finite element software package LS-DYNA. User-defined material model:Characterize strain rate sensitivity of Twaron(?) filament bundle by the dynamic mechanical constitutive relation, and compile the user defined material subroutine (UMAT) of Twaron(?) filament bundle, which is invoked into calculation process of FEM in LS-DYNA. (3) Verifying the efficiency and accuracy of two simulation schemes according to the ballistic experimental data.(4) Predicting and revealing the ballistic performances, damage modes and penetration mechanism of3DAWC from theoretical model.(5) Comparing the theoretical predictions between micro structure model and unit-cell model of3DAWC under a hemispherical-cylindrical steel projectile, to demonstrate its ballistic penetration features in the view of calculation time, damage morphology and stress distribution and demonstrate its ballistic damage characteristics.The main conclusions are as follows:3DAWC has excellent ballistic performances and higher impact damage tolerance, in which the fabric reinforcement dominates the energy absorption of3DAWC during ballistic penetration. Along with the loading velocity variation, the strain rate sensitivity of Twaron(?) filament bundle significantly influences dynamic properties and ballistic performances of composite material. Therefore, a fully understand about mechanic constitutive relationship of Twaron(?) filament bundle will provide accurately theoretical demonstration to structural composite design. It can also assist structural design to increase the damage tolerance of3DAWC under the dynamic loading with high impact velocity.The velocity of projectile was controlled within the range between200m/s and600m/s in the ballistic experiment. The experimental results include the initial impact velocity and residual velocity of projectile, ballistic limit velocity, and damage morphologies of incident surface, rear surface and cross section in the through thickness direction of3DAWC. The fit curve of impact velocity vs. residual velocity has a nonlinear incensement which indicates that the3DAWC under ballistic penetration is strain rate sensitivity. The damage morphologies of3DAWC include matrix cracking, filaments pull-out and fracture, yarn bending, interactive deformation between matrix and filaments, impact debris and so on. Its main damage modes are compression failure in incident surface, shear failure in the through thickness direction and tensile failure in the rear surface of composite target.The micro structure model of3DAWC is in accordance with actual basic configurations and the fiber volume fraction of3DAWF obtaining from the yarn spatial distribution in the matrix of the composite. It is an accurate characterization to utilize this model to evaluate dynamic mechanics of3DAWC, because of building up the link between micro structure and basic material properties. The simulation results of penetrated3DAWC under a hemispherical-cylindrical steel projectile show good agreements with experimental. And the ballistic damage mechanism of3DAWC can be achieved by the FEM from views of ballistic penetration progress, damage modes and energy absorption. Basing on the PPTA molecular structure and mechanic characterization, the dynamic responses of Twaron(?) filament is represented by the standard linear solid model which contains Hookean springs and Newtonian dashpot. The shear modulus in transversely isotropic material model of Twaron(?) filament bundle is treated as strain rate sensitivity, and this constitutive relation is compiled into UMAT to calculate and update stress increments, strain increments and history variables in main program. The simulation results demonstrate that the constitutive relations with strain rate sensitivity can improve the accuracy of ballistic calculation and also can increase fitting degree between theoretical prediction and experimental results.The comparison between theoretical predications and experimental results shows that the micro structure model can accurately reflect ballistic behaviors of3DAWC. From the simulated ballistic penetration progress, the deform morphology of filament bundle, fracture of resin matrix, interaction between two components caused by high velocity impact are reappeared. Otherwise, some precise results in ballistic tests, such as velocity vs. time history curves, acceleration vs. time history curves, deform deflection curves of composite target, are rarely achieved for ballistic equipment restrictions. Yet those can be obtained from simulation results for further revealing the ballistic damage mechanism and the energy absorption difference between reinforcement and resin matrix of3DAWC.Comparing the two theoretical schemes results, UMAT results show better agreements with experimental than EPM indicating that the constitutive relationship in UMAT can efficiently improve accuracy of ballistic process simulation. The comparison also shows that the main energy absorption media in3DAWC is filament bundle. The filament bundles in UMAT scheme absorb more internal energy than that in EPM scheme, as well as absorbing more proportion of kinematic energy of projectile. All those illustrate that the dynamic mechanic properties of filament bundle are affected by strain rate effect when suffering from ballistic impact, which further affect the energy absorption and transformation in3DAWC. Otherwise, wefts absorb more energy than warps during ballistic penetration, demonstrating that the design of wefts in3DAWC is an important factor.Comparing results of the micro structure model and the unit-cell model, the damage morphologies of composite panel, filament yarn and resin matrix, and the interactions between projectile and different components of3DAWC were clearly recognized by the micro structure model. While only perforated hole and deformation of composite panel were manifested in the damage morphologies of the unit-cell model. Moreover, the micro structure model can show stress distributions in the filament yarn and resin matrix, providing further theoretical evidences to analyze the stress propagation and energy absorption mechanism. And the unit-cell model can show a schematically stress distribution. Also, it is helpful to discuss the calculation time of the two model for understanding the relationship between calculation efficiency and simulation accuracy. From those discussion, the best simulation model can be chosen for engineering evaluation or scientific research basing on different features of two theoretical model. |
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