Experimental And Numerical Analyses Of The Strain Rate Effect Of Tensile Behavior Of3D Woven Fabrics&Mechanical Testing And Numerical Analyses Of Novel Honeycomb Sandwich Structures

Three-dimensional (3D) textile fabrics can be used as soft bulletproof vest or as reinforcement of advanced composite materials due to the higher fracture toughness, interlaminar shear strength and impact damage tolerance. The impact response of3D textile fabrics under high strain rate loading indicates obvious difference from those subjected to quasi-static loading. It is therefore critical to characterize the mechanical behavior of3D textile fabrics under high strain rate loading. Such an investigation can provide accurate and reliable reference for structural components design.This study is dedicated to conduct quasi-static and impact tensile tests on different3D textile fabrics using MTS810material system and split Hopkinson tension bar system (SHTB), including3D angle-interlock woven fabrics (3DAWFs) and3D orthogonal woven fabrics (3DOWFs). The mechanical responses of3D woven fabrics under different strain rates are discussed. The sensitivity of tensile properties, including tensile stiffness, strength, failure strain and failure modes, to strain rate is revealed. An obvious difference in the failure mechanism is revealed between the quasi-static and high strain rate loading cases. The loading rate is constant and relatively small under quasi-static loading, so there will be enough time for the whole specimen to be in a state of uniform stress and minimum energy. The stress wave propagation effect and the inertia of the specimen can be neglected. The effect of temperature on the tensile behavior of3D woven fabrics are also not be considered since the tensile process under quasi-static loading can be regarded as isothermal. It can be observed from the final fracture morphologies that the weft yarn system is the primary-load-carrying component when stretched along the weft direction. The warp yarn or Z yarn system only adjust the position to be in the minimum energy state according to the deformation or break of the weft yarns. The deformation in the warp or Z yarn system is quite small and sustain hardly no tensile loading. While for3D woven fabrics under high strain rate loading, the tensile stress wave propagates so quickly that the stress wave effect in the whole structure cannot be neglected. The stress and strain state of different positions in the specimen will vary depending on the propagation history of stress wave. The stress wave is so strong that it will reflect and transmit in the interlacing area and contacting region of different yarn systems. It eventually enables the secondary-load-carrying yarn systems (i.e. warp and Z) to deform measurably and absorb part of the impact energy. The whole impact tensile process can be regarded as adiabatic since the impact fracture is achieved instantaneously and there is no time for the specimen system to release energy to the outside.The experimental tensile stress strain curves obtained by SHTB system indicate that the three kind of3D textile fabrics show various dependence on the strain rate. It is found that the tensile behaviors of the3DAWF are sensitive to the strain rate. Both the failure stress and failure strain increase with the strain rate. But for the3DOWF, it is found that both the tensile strength and the failure strain increased with the increase of the strain rate. Furthermore, two-phase tensile stiffness phenomenon is observed in3DAWF and3DOWF, which can be attributed to the unique architecture of3D textile fabrics.Full size geometric models of3D textile fabrics are established on finite element analysis (FEA) software package ABAQUS/Explicit. Finite element models for impact tensile simulation of3D textile fabrics are developed after material behaviors definition, analysis step setup, interaction and boundary condition defining, loading and meshing. The tension load and displacement curves for each kind of3D textile fabric can be extracted and the according fracture mophorlogy can be obtained after faiure element deletion. There are good agreements between the FEM and experimetal in impact tension load, tension stiffness and frcture morphology. The tensile failure mechanism is revealed through the stress wave propagation, reflection and transmission along each yarn system and in the interlacing points and contacting area.In the second part of this paper, the research work in the Advanced Composites Centre for Innovation and Science (ACCIS) in Bristol University is summarized and reported. The main topic there involves the mechanical testing of honeycomb sandwich panels made by novel honeycomb structures, which derived from the concept of negative Poisson’s ratio (NPR or auxetics) structures. For one thing, two angle gradient honeycombs are designed and manufactured with ABSplus by Fuse Deposite Modeling (FDM) rapid prototyping (RP). The according sandwich panels are prepared though bonding the honeycomb core and two composite skins together. The flexure properties of the sandwich panels with gradient honeycomb core are investigated by3-point bending tests. A full size finite element model (FEM) of the sandwich panel with angle gradient honeycomb is established to predict the bending behavior. The validated FEM is used to optimize the geometric parameters of honeycomb core. For another, honeycombs with traditional hexagonal and re-entrant shape are manufactured by Kirigami/Origami techniques. Hence, stepwise honeycombs can be obtained by gluing the two kinds of honeycombs together. Similarly, sandwich panels with stepwise honeycomb cores are produced by attaching two composite facings. The mechanical properties of the stepwise honeycomb sandwich panels are studied through series mechanical tests, including flatwise compression and edgewise compression and impact. Finally, it is found that functional gradient honeycombs with variation in mechanical properties (Poisson’s ratio, transverse shear modulus) or bending curvature can be developed through changing the geometric and physical parameters of the unit cell. Also the mismatch of the wave impedance of a structural component can be realized through the alternative arrangement of honeycombs with positive Poisson’s ratio (PPR) and negative Poisson’s ratio (NPR). As a result, this kind of periodic structures can behave as directional mechanical filters.