Impact Tension Behaviors And Tensile Damage Mechanisms Of Co-woven-knitted Composites

Co-woven-knitted (CWK) fabric combines woven and knitted structure together, where warp and weft yarns in the woven structure insert into the weft-knitted structure. CWK fabric shows both the structure stability of woven and the extensibility and formability of knitted structure. The volume fraction of fibers can be increaseded and the tensile strength, stiffness and energy absorption of fabric reinforced composite can be increased in this novel structure. The CWK fabric is suitable to reinforce the composite in engneering design. This paper reports the tensile impact behaviours of CWK composite and its damage mechanisms due to the special structure of CWK reinforcement. In this study, the tensile impact behaviours of CWK composite have been studied via experimentation, the tensile damage processing of CWK composite has been investigated through Finite Element Analysis (FEA) simulation. The frequency features of damage mechanisms of CWK composite have also been researched by a seires of siginal processing methods.The tensile behaviours of CWK composite are studied at quasi-static (0.001/s) and high strain rates (up to2586/s) with the MTS803material tester and the split Hopkinson tension bar (SHTB), respectively. The stress-strain curves manifest the significant sensitivity with the strain rate. The tensile strength, failure strain, absorbed energy and resilient energy are all rate sensitive. The tensile strength, absorbed energy and resilient energy of the composite increase significantly with increasing strain rate. The fractograph shows the differences in damage modes at various strain rates. The damage modes are fibers breakage and matrix shear failure at quasi-static rate, and fiber pull-out at high strain rates.The tensile impact properties of CWK composite at different strain rates along various directions are modeled at the unit-cell level and compared with the FEM results and experimental results. The unit cell model is employed to define the CWK composite’s behaviour under tensile impact. A FORTRAN vectorized user-defined material (VUMAT) subroutine is developed and incorporated with a commercially available FEM code, ABAQUS/Explicit, to calculate the tensile impact. From the comparisons of the stress-strain curves at different strain rates and along various directions and CWK composite damages between FEM and experimental, it is indicated that there are reasonable agreements. The results also indicate that the unit-cell model can be used to simulate the tensile impact of extremely complex textile composites under high strain rates and along various directions.The tensile behaviours of the CWK composite are analyzed in frequency domain using fast Fourier transform (FFT) method. The features of amplitudes, phases and the energy absorption of tensile behaviours have been characterized. The FFT analysis illustrates that the CWK composite can absorb higher energy at the high frequency region. The FFT analysis illustrates that the CWK composite is strain rate sensitive under tensile impact. the amplitudes and energy absorption distribute at lower area close to zero, and the amplitudes and energy absorption distribute at higher range. It is also shown that the CWK composite can be used in high frequency loading and impact loading conditions.The Hilbert-Huang transform (HHT) method is first used to investigate the tension impact behaviours of CWK composite at various strain rates and along different directions. Using the HHT method, the stress-time history is decomposed into IMFs, then the considered IMFs are Hilbert transformed and the frequency-time spectra can be obtained. The tension impact processing can be observed both in time and frequency domain. The frequency distributions can be divided into several bands by the marginal frequency spectra. Each frequency band corresponding to a certain damage model is presumed.The tensile behaviors of the CWK composite are further analyzed in frequency domain by using Laplace-transform theory and Z-transform theory. The CWK composite is assumed as a continuous and a discrete system. The strain vs. time history is the system input and the stress vs. time history is the output. The transfer function, amplitude spectrum, and phase spectrum and pole distribution diagram of both the continuous and discrete systems are obtained. From the transformed results, it is found that the stress-strain curves of the CWK composite specimen under the different strain rates’tension have similar stability behaviours from the Laplace transform and Z-transform. For the continuous system, almost all of the pole plots are not distributed in the left side of the imaginary axis, which means the system is unstable. While for the pole plots before the post-critical deformation, it is more stable. For the discrete system, most of the poles are located inside of the unit circle, especially for the pole plots of the stress strain curves before the post-critical deformation. It indicates the system is stable.