•experimental testing of chiral auxetic structures at low and high strain rates (up to 5000 s−1).•development and validation of the computational models.•analytical calculation and computational ...evaluation of critical velocities.•analysis of deformation modes and mechanisms based on experimental testing and subsequent parametric computational simulations.•the development of empirical polynomial approximation, which enables estimation of the plateau stress of auxetic cellular structures at arbitrary loading velocity.
The quasi-static and high strain rate response of chiral auxetic cellular structures were evaluated in this work. Samples of the chiral auxetic structures were fabricated with the Selective Electron Beam Melting (SEBM) technique from copper powder. Uniaxial quasi-static and low-speed dynamic compression tests were performed on a universal testing machine, while high strain rate tests up to 5000 s−1 were performed using the one-stage powder gun. The experimental measurements together with infrared thermography and high-speed camera images were used to study the deformation mechanism of chiral auxetic structures. A significant effect of the shock enhancement observed in experiments at higher loading velocities was characterised by evaluating the specific energy absorption and specific strength. This phenomenon was further analysed in more detail by using parametric computational simulations, which offered more detailed analysis of mechanical behaviour at different strain rates. The computational results indicate that the plateau stress of chiral auxetic structure increases exponentially with increasing loading velocity. An empirical polynomial approximation was extracted from the computational results, which enables estimation of the plateau stress of auxetic cellular structures at arbitrary loading velocity in between the analyses velocity limits.
The advanced composites – laminates, composed of auxetic and non-auxetic layers, were developed and subjected to tensile loading tests to characterize their deformation behaviour. The two-layer ...laminates were composed of the polymer foam with auxetic cellular structure and the conventional knitted fabric bonded together by the rubber-based adhesive, based on polymer binder, resins, additives and flammable organic solvents. All base materials, structured foams and laminates were tested. The influence of the adhesive on mechanical properties of the knitted fabric was also evaluated. The results of the study show that the cheaply fabricated laminates of knitted fabric reinforced by structured Ethylene-Vinyl Acetate (EVA) foam offer stiffness enhancement and decrease of the Poisson's ratio of the conventional fabric. The laminates also limit fabric warping during the deformation, thus increasing its stiffness. This behaviour can be beneficial in many applications in the textile industry by enhancing properties in durability, comfort and support.
•Auxetic cellular structures build from inverted tetrapods were experimentally tested at high strain rate compression loading for the first time.•The strain rates up to 10,000 s−1 were achieved with ...gas powder gun, where the shock deformation mode is predominant.•Shock deformation mode results in stiffness increases in comparison to the quasi-static response.•Validated computational models were used for critical strain rate analysis, determination of critical loading velocities and analysis of deformation modes together with analytical constitutive crushing models of cellular structures.
Auxetic cellular structures build from inverted tetrapods were experimentally tested at high strain rate compression loading for the first time. The strain rates up to 10,000 s−1 were achieved with gas powder gun, where the shock deformation mode is predominant. The deformation localizes in the deformation front between the impacting specimen and the fixed plate due to the inertia effects. This deformation mode results in stiffness increases in comparison to the quasi-static response. The results from experimental testing were used for validation of developed computational models in finite element explicit code LS-DYNA. Furthermore, the validated computational models were used for critical strain rate analysis, determination of critical loading velocities and analysis of deformation modes together with analytical constitutive crushing models of cellular structures.
This study presents the computational analysis for determining the fatigue life of two conventional 2D auxetic cellular structures: (i) A chiral auxetic structure, (ii) A re-entrant auxetic ...structure. The low-cycle fatigue analysis using a damage initiation and damage evolution law was performed, where the needed material parameters were obtained previously from the experimentally determined stabilised hysteresis loop of inelastic strain energy. The direct cyclic algorithm implemented in the Abaqus/Standard software was used for the computational analyses to obtain the stabilised response of a model subjected to the cyclic loading. In order to examine the damage evolution paths, finite elements with severe damage were detected, and then removed from the finite element model in the subsequent numerical simulations.
The computational fatigue analyses have shown that the chiral auxetic structure demonstrates a higher fatigue strength than the re-entrant auxetic structure which is probably due to the fact that, in the chiral structure, the stresses are distributed more uniformly along the individual cells. To confirm the computational results, the cyclic experimental testing has been performed for both auxetic structures. The comparison between computational and experimental results shows a reasonable agreement.
•Low-cycle fatigue behaviour of re-entrant auxetic cellular structures is presented.•The number of cycles required for crack initiation decreases from cell to cell.•The number of cycles required for ...crack propagation decreases from cell to cell.•The orientation of the base cells does not affect on the fatigue life.•The orientation of the base cells has influence on the crack path.
The investigation of the fatigue behaviour of an auxetic cellular structure made of Al-alloy 7075-T651 is presented in this study. The complete fatigue process of the analysed cellular structure is divided into the crack initiation (Ni) and crack propagation (Np) period, where the total fatigue life (Nt) is defined as the sum of Ni and Np. The crack initiation period, Ni, is determined using the strain life approach in the framework of the Fe-Safe computational code, where elastic-plastic numerical analysis is performed to obtain the total strain amplitude in the critical cross-section of the analysed cellular structure. The number of stress cycles, Np, required for the fatigue crack propagation from the initial to the critical crack length is also determined numerically using a finite element model in the framework of the Abaqus computation FEM code. The Maximum Tensile Stress (MTS) criterion is considered when analysing the crack path inside the cellular structure. Finally, experimental fatigue testing has also been performed to validate the computational results.
Novel three-dimensional (3D) axisymmetric chiral structures with negative and zero Poisson's ratios are presented based on the existing 3D conventional chiral unit cell. The conventional tetra-chiral ...unit cell is mapped to the axisymmetric space to form the new 3D axisymmetric chiral structure. Two different structure designs are characterised depending on the period delay of the sine curve representing the horizontal struts of the structure. The structures are fabricated using additive manufacturing technology and experimentally tested under compression loading conditions. The digital image correlation methodology is used to determine the Poisson's ratio dependence on the axial strain. The computational model of axisymmetric chiral structures is developed and validated using the experimental data. The computational model is then used to evaluate the new virtual axisymmetric chiral structures with graded cell structures. The newly developed axisymmetric structures show enhanced mechanical properties when compared to the existing 3D chiral structures.
•Fracture behavior of auxetic cellular structures is investigated.•A ductile damage computational modeling technique is presented.•Identification of ductile damage material parameters is given.•The ...influence is evaluated of the shape of unit cells on fracture behavior.•Experimental testing is carried out to validate the numerical results.
Auxetic cellular structures are advanced materials with negative Poisson’s ratios, which exhibit some unique features which are useful for various applications. The objective of this paper is the numerical simulation and experimental analysis of evolution propagation under quasi-static loading conditions in selected auxetic cellular structures with different geometries and orientations of unit cells that is: (i) honeycomb structure, (ii) re-entrant structure, and (iii) rotated re-entrant structure. The failure modeling capability of Simulia Abaqus code for ductile materials is used for the numerical simulation. The paper presents in detail the calculation methodology for determination of ductile damage material parameters that, are used for further numerical simulations of the damage initiation and evolution. Standard Compact Tension (CT) specimens are selected for the numerical simulation and experimental testing. Here, the specimens are made from 7075-T651 aluminum plate, cut using water jet cutting technology. Using a numerical approach, the fracture behavior of the selected auxetic structures is estimated first. Experimental testing is also carried out to validate the numerical results. The comparison between computational and experimental results regarding crack propagation path and force-displacement diagrams showed a reasonable agreement.
This paper proposes an innovative multi-material approach for introducing auxetic behaviour to syntactic foams (SFs). By carefully designing the size, shape, and orientation of the SFs, auxetic ...deformation behaviour was induced. Re-entrant hexagon-shaped SF elements were fabricated using expanded perlite (EP) particles and a plaster of Paris slurry first. Then, an auxetic pattern of these SF elements was arranged within a stainless-steel casting box. The empty spaces between the SF elements were filled with molten aluminium alloy (A356) using the counter-gravity infiltration casting technique. The cast auxetic composite had a bulk density of 1.52 g/cm3. The cast composite was then compressed under quasi-static loading to characterise its deformation behaviour and to determine the mechanical properties, especially the Poisson’s ratio. The cast composite deformation was auxetic with a Poisson’s ratio of −1.04. Finite Element (FE) simulations were conducted to understand the deformation mechanism better and provide means for further optimisation of the geometry.
Chiral-type cells consisting of rigid nodes and rotatable ligaments provide an opportunity to develop lightweight engineering structures with unique mechanical performances. For a 2D or 3D ...chiral-type cellular structure system with a periodic arrangement of chiral cells, a distinct rotational response will present because of the asymmetrical and geometrical configuration. In this article, a tetra-chiral cylindrical shell is proposed on the basis of natural plant architecture. This shell exhibits a reversible bi-directional twisting deformation in the axial compression and tension processes. A theoretical model is proposed via the geometrical parameters of cells to describe the relationship between twist angle and axial displacement. Two categories of tetra-chiral cylindrical shell specimens are fabricated utilizing additive manufacturing technique involving the application of nylon and AlSi10Mg materials. Uniaxial compressive tests and finite element simulation are conducted to reveal the twist deformation mechanism. Results verify that the twist characteristics of the chiral-type shell, which are adjustable in terms of the rotational direction and angle, are only related to the distribution and geometrical sizes of ligaments. The innovative chiral-type cylindrical shell provide a new design strategy which can be used in engineering applications as compress- or stretch-twist coupled smart actuators, biomechanical devices, and micro sensors.
Hybrid metamaterial with auxetic cellular structure and silicon filler was manufactured and mechanically tested for the first time with aim to study its enhanced mechanical properties. The uniform ...and graded base specimens with auxetic chiral cellular structure were fabricated from copper alloy using the Selective Electron Beam Melting (SEBM) method. The fabricated specimens were then fully infiltrated with silicone filler under vacuum conditions to avoid any voids (air gaps) and to achieve a high degree of homogeneity in the composite structure. The mechanical behaviour of hybrid specimens under quasi-static and dynamic compressive loading conditions was investigated experimentally. The results show that hybrid specimens with auxetic cellular structure and silicon filler exhibit much better mechanical response with increased stiffness and smooth response in comparison to specimens with conventional (non-hybrid) auxetic cellular structure. They also have a higher plateau stress but lower densification strain and possess much higher energy absorption capacity in comparison to the base specimens with chiral auxetic structure. This is attributed to the interaction between the filler and the structure, which results in the improvement of the macroscopic mechanical properties.
PDF
