Dynamic Response And Finite Element Analysis Of 3-D Textile Structural Composites Under Transverse Impact

3-D textile structural composites have higher inter-laminar shear strength, fracture toughness and impact damage resistance tolerance than laminates because the fiber tows run through the thickness direction. These features lead to the extensive application of 3-D textile structural composites in higher impact damage resistance tolerance fields. There are differences in mechanical properties and dynamic response of the composites between quasi-static and impact loading since the rate sensitivity of component materials. Therefore, it is necessary to accurately characterize the mechanical properties and dynamic response of the composites under dynamic loading. If the high precise design of engineering structures were needed, investigating the mechanical properties under impact has important values to design the impact damage resistance tolerance 3-D textile structure and composites.The goal of the thesis is to investigate the dynamic responses of 3-D orthogonal woven and biaxial spacer weft knitted composites under transverse impact using a modified split Hopkinson pressure bar (SHPB) and finite element method. A unit-cell model based on the microstructure of the 3-D textile structural composites was established to determine impact deformation and damage. Incorporated with the unit-cell models, elasto-plastic constitutive equations of the 3-D textile structural composites, the maximum stress failure theory and the critical damage area (CDA) failure theory have been implemented as a user defined material law (VUMAT) with FORTRAN for finite element code ABAQUS/Explicit. The results of load-displacement curves, maximal load-velocity curves, impact energy absorptions-displacement and impact damages calculated from FEM code ABAQUS were compared with those in experiment. The good agreements between FEM and experiment prove the validity of the unit-cell model and user-defined subroutine VUMAT. This unit-cell model can also be extended to analyze the impact responses of the complex engineering structures made from such kinds of composites.The main contents are as follows:(1)The quasi-static experiments of 3-D orthogonal woven and biaxial spacer weft knitted composites were tested on MTS machine, the load-displacement curves of two kinds of composites under quasi-static transverse load were compared. The failure modes of 3-D orthogonal woven and biaxial spacer weft knitted composites were the compression failure on the front side and tension failure at the rear side of the composite panels and no delamination was found. However, the damaged areas and degree were more severe for the biaxial spacer weft knitted composites under the same velocity because the fiber volume fraction of the composites was more little than that of 3-D orthogonal woven composites.(2)The split Hopkinson pressure bar (SHPB) was modified for the transverse impact tests. The results of the two kinds of composites in warp and weft directions indicated that the damages only occur in a local area under dynamic transverse impact. The maximal loads, damage areas, and energy absorptions increased with the increasing of impact velocity, i.e., the mechanical behaviors of the two kinds of composites are rate sensitive.(3)Unit-cell models based on the microstructure of the 3-D textile structural composites were established both for 3-D orthogonal woven composites and biaxial spacer knitted composites. The unit-cell of 3-D orthogonal woven composites was comprised of warp bundles, weft bundles, Z bundles and matrix. The unit-cell of biaxial spacer weft knitted composites was comprised of loop bundles, warp bundles, weft bundles, connecting bundles and matrix. The stiffness/compliance matrixes of each components in unit-cell are deduced from the coordinate transformation. Elasto-plastic constitutive equations of the 3-D textile structural composites can be obtained by introducing the 3-D plastic potential function.(4)Unit-cell models based on the microstructure of the 3-D textile structural composites are established to analyze impact deformation and damage. Incorporated with the unit-cell models, elasto-plastic constitutive equations of the 3-D textile structural composites, the maximum stress failure theory and the critical damage area (CDA) failure theory have been implemented as a user defined material law (VUMAT) with FORTRAN. The Octahedral shear failure theory and the maximum stress failure theory are used to control matrix failure in unit-cell models. The maximum stress failure theory and the critical damage area (CDA) failure theory are used to control yarns failure in unit-cell models. The load-displacement curves, maximal load-velocity curves, impact energy absorptions and impact damages calculated by FEM were compared with those in experiment for the 3-D orthogonal woven and biaxial spacer weft knitted composites. The results show a good agreement between the experimental and FEM results. This proves the validity of the unit-cell models and user-defined subroutine VUMAT.By adjusting the parameters of the unit-cell model and it is possible for the optimization of the composite microstructure. This is also benefit for the design of the engineering structures with high impact damage tolerance.