Impact Responses And Energy Absorptions Of 3-D Cellular Woven Composites

3-D cellular woven composites have widely applications to aircrafts, vehicles, ships and sports facilities because of the high ratio of strength/weight and high energy absorption under impact loading. The objectives of this investigation are to characterize the microstructures, impact responses and failure modes of the 3-D cellular composite under transverse impact.The 3-D cellular woven fabrics and composites were manufactured based on the 3-D angle-interlock woven fabric. The impact responses and energy absorption of the composite were tested with a modified split Hopkinson bar (SHPB) apparatus. A unit-cell model of the composite was established from the microstructure features, i.e., same fiber volume fraction and mechanical behaviors. A user subroutine VUMAT (FORTRAN Vectorized User-Material) was developed and connected with a commercial available FEM software package ABAQUS/Explicit for calculating the impact responses of the composite. This subroutine describes the elasto-plastic constitutive equations of the unit-cell, maximum stress failure criteria and critical damage area failure criteria. It was found that there is a good agreement of the impact load-displacement, failure modes between FEM calculation and experimental. This proves the validity of the unit-cell model, failure criteria and user-defined subroutine VUMAT. The VUMAT can also be extended to characterize the impact responses of other engineering structures manufactured from the 3-D cellular woven composites.The main contents are as follows:1 Analysis and discussion of principles and structures of the modified SHPB apparatus2 Design and manufacture of the 3-D cellular woven preform and composite The cellular structure was formed from the connection of different layers of fabric. The vacuum aided resin transfer molding (VARTM) technique was employed to manufacture the composite. The cellular composites have a hollow structure, low area density and high bending rigidity.3 The three-point bending properties of the 3-D cellular composite under quasi-static and impact loading were tested with MTS 810.23 universal tester and the modified SHPB apparatus. The energy absorption and failure modes were also investigated. At the initial stage of the quasi-static testing, the composite coupon manifested the whole structure deformation because of the very slow deformation. As the load reached the maximum value, the coupons had structure failure. The different bending properties along warp and weft directions depend on the different structure features along the two directions. The maximum load along warp direction is lower owing to the stress concentration at the local parts. While the failure load is higher along weft direction because of this direction is the main loading direction. The failure mode of the 3-D cellular composite has compression failure at the inner surface and tension failure at the rear surface, i.e., the 3-D cellular composite manifests a feature of typical bending failure. Furthermore, there was not delamination in the post-mortem of the composite coupons.4 As for the impact tests, the failure loads and energy absorptions of the composite increase with the increase of impact velocity. There were fluctuations of the load-displacement curves under impact test for the inliomogeneous microstructure of the composite. The failure load and energy absorption along weft direction were much higher than those in the warp direction. As the same phenomenon found in the quasi-static test, the weft direction is also the main loading direction.5 A unit-cell model which based on the microstructure of the 3-D cellular composite was established to characterize the impact responses of the composite. The unit-cell model composes of three parts: warp tows, weft tows and resin. The 3-D elasto-plastic constitutive equation and compliance matrix could be deduced from a 3-D plastic potential function for orthotropic materials. The failures of the resin and fiber tows were determined with the maximum stress criteria and critical damage area criteria respectively.6 A user subroutine VUMAT was compiled with FORTRAN to define the elasto-plastic constitutive equation of the composite and maximum stress criteria, critical damage area criteria. In the FEM calculation, the resin failure was described with the Octahedral shear stress and maximum stress criteria, the damage of fiber tows was described with the critical damage area criteria. The impact damage and energy absorption of the composite were calculated with the FEM software package of ABAQUS/Explicit and the VUMAT. There are good agreements of the load-displacement curves, energy absorption and failure modes between FEM results and experimental. This proves the validity of the unit-cell model and the subroutine VUMAT. The details of the impact responses and impact damage evolution could be revealed from the FEM calculation.It has not escaped our notice that because the subroutine VUMAT developed is based on the 3-D cellular woven composite, the subroutine is not only limited in composite panel, but also can be extended to impact damage simulation of other engineering structures which manufactured with the 3-D cellular woven composite. From the changes of the geometrical parameters of the unit-cell, the structure of the 3-D cellular woven composite could be optimized to obtain higher energy absorption.