Dynamic Response Of 3D Biaxial Spacer Weft-knitted Composite T-beam Under Transverse Impact

T-beams are widely used in aerospace, airplane and navigation fields as one common engineering structure. These T-beams are liable to be impacted by the birds, hails, floating stuff and dock. Therefore, designing and manufacturing the composite T-beams with high flexural stiffness, strength and excellent anti-impact properties, investigating the dynamic response and characterizing the mechanical properties under the impact load, are meaningful for the practical use.The quasi-static flexural and dynamic impact properties of 3D biaxial spacer weft knitted composite T-beams with different rib heights were investigated in this paper. The impact response and damage modes under the quasi-static and impact loadings were compared, and rib height influence on the dynamic response was also investigated. Using unit-cell model of the microstructure of the 3D biaxial weft-knitted structure, stiffness/compliance matrices of each component in the unit-cell were deduced through the coordinate transformation. The VUMAT (Vectorized User-defined Material Subroutine) was firstly written basing on the maximum stress failure theory and the critical damage area (CDA) failure theory, and then it was cooperated with commercial finite element analysis software ABAQUS, to calculate the dynamic impact response of 3D biaxial spacer weft-knitted composite T-beam. FEM results and experimental ones were compared to check the validity of unit-cell model and user-defined subroutine VUMAT for the calculation of 3D biaxial spacer weft-knitted composite T-beam.The main investigations include the following parts: 1. Fabrication of 3D biaxial spacer weft-knitted fabric and testing its mechanical behaviorAs one of novel textile reinforcement structures,3D biaxial spacer weft-knitted fabric was not only strengthened in axes due to the insertion of warp and weft yarns, but also improved concerning the anti-delamination ability of reinforced T-beam composite duo to the linking yarns. The modified special knitting machine was employed to produce the required fabric. The reinforcement was tested and analyzed on tensile, tear, peeling properties. The results indicated that:(a) the tensile properties are closely related to the warp yarn and weft yarn densities; (b) the anti-delamination ability is greatly improved by linking yarns; (c) the peeling behavior differences between the weft direction and warp direction are resulted from the difference of linking yarn densities in these two directions.2. Fabrication of T-beamManufacturing 3D biaxial spacer weft knitted composite T-beam required rib to be vertical with panel surface, to have uniform thickness and smooth surface. Self-designed glass mould and vacuum assisted resin transferring molding (VARTM) technique were employed to produce the 3D biaxial spacer weft knitted composite T-beam with three different rib heights (Ocm,3cm and 5cm).3. Quasi-static and impact dynamic responsesThe quasi-static and dynamic impact loading tests on the composite T-beam with different rib heights were conducted on MTS and modified split Hopkinson bar respectively. The results showed that for the quasi-static loading test, the composite T-beams with 3cm and 5cm rib heights were indicated with higher maximum load and energy absorption ability, because rib improved the flexural stiffness of T-beam effectively. For the dynamic impact loading, it was shown that maximum load and energy absorption ability increased with the increasing of impact velocity for the T-beam with the same rib height, and also increased with increasing of rib height of the T-beam under the same impact velocity.The comparison between quasi-static and dynamic impact loadings also showed that the dynamic impact damage mode includes shearing damage besides matrix crack, fiber breakage, tension, compression. Compared with Ocm T-beam(namely panel composite), the T-beam with 3cm and 5cm rib heights were observed with the relatively less damage where compressive failure appeared on the front side and tensile failure on the rib. The reason is that the rib improved the stiffness and anti-impact resistance of T-beam.4. FEM simulation calculation of T-beam under quasi-static and dynamic impact loadingsBased on the microstructure of unit-cell of 3D biaxial spacer weft-knitted fabric, stiffness/compliance matrices of five components (warp yarn, weft yarn, loop yarn, linking yarns and matrix) in unit cell were obtained through the coordination transformation, and 3D plastic potential function was introduced to deduce elasto-plastic constitutive equations of the 3D biaxial spacer weft-knitted composite T-beam. VUMAT was written based on the elasto-plastic constitutive equations, maximum stress failure theory and critical damage area (CDA) failure theory in FORTRAN. The Octahedral shear failure theory and the maximum stress failure theory were used to control matrix failure in unit-cell models. The maximum stress failure theory and the critical damage area (CDA) failure theory were used to control yarns failure in unit-cell models. In cooperating with finite element software ABAQUS, VUMAT was implemented to stimulate the load-displacement curves, energy-displacement curves, damage mode and damage process of 3D biaxial spacer weft-knitted composite T-beam under the quasi-static flexural and dynamic impact loadings. The good agreement between FEM results and experimental ones showed that the unit-cell and VUMAT used not only suitable for the 3D biaxial spacer weft-knitted composite panel, but also appropriate for 3D biaxial spacer weft-knitted composite T-beam.The paper has characterized the dynamic impact response of 3D biaxial spacer weft-knitted composite T-beam. The obtained results are beneficial for the further optimization of 3D biaxial spacer knitted fabric, and can also be used as one reference for other kinds of 3D biaxial spacer weft-knitted composite structures.