Cell Wall Bending Research Articles

Fabrication and in-plane compressive collapse of CFRP honeycomb metamaterials

A pin-slot positioning method was proposed to fabricate carbon fiber reinforced polymer (CFRP) metamaterials of hexagonal and zero Poisson’s ratio semi-re-entrant honeycombs with the same mass of monolayer continuous twill-woven carbon fiber/epoxy prepregs. The in-plane compressive collapse of CFRP honeycombs was explored through experiments and finite element (FE) simulations. Furthermore, analytical models were developed to predict the modulus and initial collapse stress of the hexagonal and semi-re-entrant honeycombs. Good agreement is achieved between analytical predictions, FE simulations and experimental results. It is shown that the initial collapse of CFRP honeycombs is by the bending of cell walls. Once initial collapse has been attained, both hexagonal and semi-re-entrant honeycombs have a stress softening. The collapse and failure modes of CFRP honeycombs strongly depend on cellular configurations and loading directions.

Stiffness tailoring in sinusoidal lattice structures through passive topology morphing using contact connections

Structures with adaptive stiffness characteristics present an opportunity to meet competing design requirements, thus achieving greater efficiency by the reconfiguration of their topology. Here, the potential of using changes in the topology of planar lattice structures is explored to achieve this desired adaptivity and observe that lattice structures with rectangle-like unit-cells may undergo elastic buckling or bending of cell walls when subject to longitudinal compression. Under sufficient load intensity, cell walls can deform and contact neighbouring cells. This self-contact is harnessed to change the topology of the structure to that of a kagome-like lattice, thereby establishing new load paths, thus enabling enhancement, in a tailored manner, of the effective compressive and shear stiffness of the lattice. Whilst this phenomenon is independent of characteristic length scale, we focus on macroscopic behaviour (lattices of scale ≈ 200 mm). Experimentally observed responses of 3D-printed lattices correlate excellently with finite element analysis and analytical stiffness predictions for pre- and post-contact topologies. The role of key geometric and stiffness parameters in critical regions of the design space is explored through a parametric study. The non-linear responses demonstrated by this topology morphing lattice structure may offer designers a new route to tailor elastic characteristics.

Load-rate effects on the in-plane compressive behaviour of additively manufactured steel 316L honeycomb structures

Additive manufacturing (AM) offers the opportunity to enhance the mechanical performance of metallic honeycomb structures. However, there is limited research on the load-rate effects affecting in-plane compression. For the first time, this study investigates the in-plane compressive performance of Steel 316L honeycomb structures produced via material extrusion (ME) and laser powder bed fusion (LPBF). ME honeycombs of three cell sizes and single-cell size LPBF honeycombs were tested under uniaxial compression at varying quasi-static loading rates. Digital image correlation (DIC) was also employed to capture the deformation modes of the honeycombs and to map the local strain fields during compression. Empirical models were used to predict the in-plane compressive performance of the honeycomb structures. Particularly for the plastic collapse stress, an empirical formula incorporating viscoplastic dependency on the cell wall material within the quasi-static loading regime has been derived, producing accurate predictions. Both types of honeycomb structures are load-rate sensitive, as their compressive mechanical properties improved with the load rate increase. Moreover, the variation of the cell size was found to optimise the in-plane compressive performance. The deformation mode for both honeycomb structures was confirmed to be cell wall bending, plastic buckling, cell wall collapse, and folding.

Functionalized Wood Veneers as Vibration Sensors: Exploring Wood Piezoelectricity and Hierarchical Structure Effects.

Functional wood materials often rely on active additives due to the weak piezoelectric response of wood itself. Here, we chemically modify wood to form functionalized, eco-friendly wood veneer for self-powered vibration sensors. Only the piezoelectricity of the cellulose microfibrils is used, where the drastic improvement comes only from molecular and nanoscale wood structure tuning. Sequential wood modifications (delignification, oxidation, and model fluorination) are performed, and effects on vibration sensing abilities are investigated. Wood veneer piezoelectricity is characterized by the piezoresponse force microscopy mode in atomic force microscopy. Delignification, oxidation, and model fluorination of wood-based sensors provide output voltages of 11.4, 23.2, and 60 mV by facilitating cellulose microfibril deformation. The vibration sensing ability correlates with improved piezoelectricity and increased cellulose deformation, most likely by large, local cell wall bending. This shows that nanostructural wood materials design can tailor the functional properties of wood devices with potential in sustainable nanotechnology.

Study on the Cutting Damage Mechanism of Aramid Honeycomb Based on the Progressive Damage Model.

A progressive damage model for aramid honeycomb cutting was proposed to reveal its cutting damage mechanism. It established the relationship between the mesoscale failure modes and the macroscale cutting damage types of the aramid honeycomb. The proposed model addressed the material assignment problem of impregnated honeycomb by developing a material calculation method that simulates the real manufacturing process of the aramid honeycomb. Cutting experiment of aramid honeycomb specimen was conducted concerning on the cutting forces response and cutting damages, which validated that the proposed method was effective for investigating the cutting process and mechanism for the aramid honeycomb. Predicted cutting mechanism results show that: (a) cutting process of the aramid honeycomb can be divided into three stages with four characteristic states—initial state, cut-in state, cut-out state and final state; (b) cell wall bending in the cutting direction relieves the cutting force, and strong plasticity of the aramid fiber makes it hard to break, which lead to uncut fiber and burr damages; (c) using sharp tip cutting tool to reduce cutting force and bonding both top and bottom of the honeycomb to make it stiffer are beneficial to obtain good cutting quality with less damages.

Atomistic and continuum modeling of 3D graphene honeycombs under uniaxial in-plane compression

Using large-scale molecular dynamics (MD) simulations in conjunction with continuum modeling, the deformation behaviors of three-dimensional (3D) graphene honeycomb structures under uniaxial in-plane compression have been systematically investigated. The stress-strain responses of graphene honeycombs were found to be dependent on the loading direction, prism size and lattice orientation, but little affected by the junction type. Two critical deformation events, i.e., elastic buckling and structural collapse, were identified, with the associated local and global structural changes associated at these critical events clarified. Continuum models accounting for the effect of lattice orientation and size-dependent yielding have been developed to quantitatively predict the threshold stresses for those critical deformation events. In addition, it has been demonstrated that the overall stress-strain curve of graphene honeycomb can also be reasonably well predicted via continuum modeling, albeit deviation at large strains due to effect of junction on cell wall bending. The present study provides critical mechanistic understanding and predictive tools for optimizing and designing 3D graphene honeycombs in small-scale applications.

3D printed self-similar AlSi10Mg alloy hierarchical honeycomb architectures under in-plane large deformation

Self-similar hierarchical honeycombs with complicated cellular topologies are promising cores for lightweight sandwich structures ascribed to superior structural efficiency in terms of mitigating damage. In this paper, the in-plane compressive characteristics of AlSi10Mg alloy hierarchical honeycombs, fabricated using Selective Laser Melting (SLM) methodology, under large deformation are reported. The results suggest that higher hierarchies enhance the elastic modulus of honeycombs significantly, whereas the compressive strength is improved negligibly. There is a failure mechanism transition from cell wall bending dominated to cell wall fracture dominated with the increase of relative density and hierarchical order, and the failure mechanisms are related to the wall thickness with critical value of 0.345 mm. It is concluded that the in-plane failure of hierarchical honeycombs is dominated by the bending, axial compression and shear deformation of original unit cell walls. Besides, the parent material without heat treatment contributes to more excellent mechanical properties of hierarchical honeycombs owing to higher tensile strength and elastic modulus. This work enables to facilitate the structural design and optimization of high-performance hierarchical honeycomb metamaterials with desired properties and functions.

Interpretation of the Thickness Resonances in Ferroelectret Films Based on a Layered Sandwich Mesostructure and a Cellular Microstructure.

Transmission coefficient spectra of two ferroelectret films (showing several thickness resonances) measured with air-coupled ultrasound (0.2-3.5 MHz) are presented, and an explanation for the observed behavior is provided by proposing a film layered sandwich mesostructure (skin/core/skin) and by solving the inverse problem, using a simulated annealing algorithm. This permits us to extract the value of the ultrasonic parameters of the different layers in the film, as well as overall film parameters. It is shown that skin layers are thinner, denser, and softer than core layers and also present lower acoustic impedance. Similarly, it is also obtained that the denser film also presents lower overall acoustic impedance. Scanning electron microscopy was employed to analyze the films' cross section, revealing that both denser films and film layers present more flattened cells and that close to the surface cells tend to be more flattened (supporting the proposed sandwich model). The fact that more flattened cells contribute to a lower elastic modulus and acoustic impedance can be explained, as it has been made previously by several authors, by the fact that the macroscopic film elastic response is furnished by cell micromechanics, is governed, mainly, by cell wall bending. The consistency of extracted parameters with trends shown by a simple model based on a honeycomb microstructure is discussed, as well as the possibilities that this sandwich mesostructure and the associated impedance gradient could offer to improve the performance of FE films in ultrasonic transducers.

Preparation and properties of open-cell zinc foams as human bone substitute material

Foamed zinc was prepared by infiltration casting process. The mechanical properties and corrosion resistance of the samples were studied, and the feasibility of the foamed zinc as a bone implant material was discussed. All the compression stress-strain curves of open-cell zinc foams with various cell size (1–4 mm) and porosity (55%–67%) show three stages: elastic stage, plastic stage, and densification stage. The compression strength increases with decreasing density. The smooth stress-strain response indicates a progressively deformation of open-cell zinc foam. In addition, the cell wall or edge bending and fracture are the dominated mechanisms for failure of open cell zinc foam. The immersion test for determining the corrosion rate of open cell zinc foam was conducted in simulated body fluid. It was found that zinc foam with a small cell size and high porosity showed a higher corrosion rate. In addition, open-cell zinc foams can effectively induce Ca-P deposition in immersion tests, showing good bioactivity. Therefore, the open cell zinc foam prepared in this experiment has a good potential application as a human bone substitute material.

Three dimensional modelling of closed-cell aluminium foams with predictive macroscopic behaviour

This study presents an efficient approach to generate a realistic micromechanical model for closed-cell foams using irregular Voronoi models. Starting with a reference microstructure consisting of regular Kelvin cells, the nucleating points underlying each reference unit cell is subjected to a random displacement, the magnitude of which depends on the pore size and a misalignment parameter. For convenience, the misalignment parameter is calibrated from available statistical data on unit cell geometries for the commercially available closed-cell aluminium foam, and a common value is utilised for all cases considered. Numerical analyses with the irregular 3D micromechanical models are done using ABAQUS/Explicit. Considering both uniaxial and hydrostatic compressive loading cases, the numerical responses compare well with the experimental data in literature, over a wide range of relative densities. The deformation mechanism is next elaborated. In uniaxial compression, the deformation initiates at the weaker regions induced by the geometrical irregularities, before evolving into a localized deformation band through the coalescence between neighbouring regions of local weakness. This localized deformation band forms an irregular 3D relief in space, a phenomenon which cannot be captured by regular periodic microstructures. In hydrostatic compression, the irregular geometry also induces deformation through cell wall bending. In tension, a stretching mode induces a shift in the yield surface towards the positive mean stress quadrant, which is consistent with experimental observations. The proposed approach thus generates the irregular 3D microstructures efficiently through a single calibrated misalignment parameter, leading to realistic deformation mechanisms, with good predictions on the macroscopic behaviour, over a wide range of relative densities and different loading conditions.

Dynamic deformation and failure process of quasi-closed-cell aluminum foam manufactured by direct foaming technique

The application of cellular structure for energy dissipation requires the investigation on the deformation and failure process of the foam under dynamic loading. In this study, the split-Hopkinson pressure bar technique with high speed video camera was used to directly observe the dynamic deformation in quasi-closed-cell aluminum foam fabricated by an in-house direct foaming methodology. The characterization of cellular structure was first performed on the developed aluminum foam, indicating that the cell size follows a log-normal distribution. The measurements revealed that the deformation mode varies with the strain rate. After that an experimentally validated X-ray micro-computed tomography based 3D finite element model of the as-fabricated specimen was established to predict the stress distribution and deformation history under impact loading, and correlated to the experimental observations to explore the deformation mechanisms. It was found that shear band is formed after peak stress. The dynamic deformation and failure process of the quasi-closed-cell aluminum foam result from the coexistence of cell wall bending and buckling, cell collapse and shear traction induced tearing breakage.

Microstructure and compressive deformation behavior of SS foam made through evaporation of urea as space holder

Abstract Open cell 316L stainless steel foam (SSF) of varying porosities have been developed through powder metallurgy route using evaporative spherical urea particles (UP) as a space holder. Stainless steel powders (SSP) were cold compacted under 500 MPa pressure and sintered at 1200 °C for 1 h in a high vacuum atmosphere (10−4 mbar). Detailed Energy dispersive X-ray spectroscopy (EDS) analysis and X-ray diffraction pattern (XRD) conformed that no residue of space holder (urea) in sintered samples. The compressive deformation behavior of sintered foam samples with varying relative densities (ρrd) was conducted at 0.01s−1 strain rate. The yield strength, elastic modulus (Ef), plastic modulus, average plateau stress (σpl) and energy absorption (Eab) of the foam increases with increases in the relative density and these follows power law relationship with relative density. On the other hand, densification strain (ɛd) decreases with increases relative density. This has been discussed with the deformation mechanism of the stainless steel foam (SSF). Deformation of SSF is associated with cell wall (CW) bending, CW collapse due to bulking followed by shearing and facture of cell wall layer by layer.

Compressive behaviour of a nickel superalloy Superni 263 honeycomb sandwich panel

Metallic thermal protection systems comprising of sandwich panels consisting of hexagonal honeycomb sandwich structures are envisaged to be used in advanced transportation systems like hypersonic vehicles and reusable launch vehicles. The assessment of compressive mechanical behaviour is necessary to understand the response of sandwich structures to aerothermal loads. The fabrication methodology for realizing Ni based superalloy Superni 263 hexagonal honeycomb sandwich panels is established. This work is aimed at understanding the effect of sandwich panel geometry parameters like hexagonal cell size and core thickness on the out-of-plane flatwise compressive behaviour at room temperature. The ultimate compressive strength decreases with increasing core height irrespective of the cell sizes investigated. The dependence of specific compressive strength on the cell size is established by a power law relationship. The compressed sandwich panels subjected to understand the deformation behaviour indicated the dominance of cell wall bending and occasional fracture, however in the case of sandwich panels with higher core thickness cell wall buckling coupled with shearing at the face sheet vicinity is noticed.

Elastoplastic response and recoil of honeycomb lattices

Abstract This paper presents elasto-plastic response and recoil analysis of two-dimensional honeycombs. The architecture studied here possesses cell wall bending as the dominant mechanism of deformation. Elasto-plastic analysis of cell walls, in conjunction with the kinematics of the deformation of the lattice, enables us to obtain closed form plastic response as well as recoil upon unloading. Elastic-perfectly plastic cell wall material is considered. A smooth apparent structural response is observed although the material model is piece-wise linear. The apparent Poisson's ratio of the porous honeycomb material, in the plastic regime, is derived from the lateral response calculation. In the non-linear regime of deformation, the Poisson's ratio of honeycombs remains independent of the apparent strain of the lattice. A parametric study of the non-linear response involving systematic changes in the parameters is carried out. This suggests the existence of non-dimensional groups that could provide response relationship valid for all geometric and material parameters . Scaling arguments are developed and a scaling ansatz is proposed which leads to data collapse, based upon which, a family of honeycombs can be analytically characterised for elasto-plastic response. Data collapse thus obtained provides a master-curve for structure-property relationship for plasticity of honeycombs which separates the individual cell wall mechanics from the lattice kinematics.

3D printed structures for modeling the Young’s modulus of bamboo parenchyma

Bamboo is a sustainable, lightweight material that is widely used in structural applications. To fully develop micromechanical models for plants, such as bamboo, the mechanical properties of each individual type of tissue are needed. However, separating individual tissues and testing them mechanically is challenging. Here, we report an alternative approach in which micro X-ray computed tomography (µ-CT) is used to image moso bamboo (Phyllostachys pubescens). The acquired images, which correspond to the 3D structure of the parenchyma, are then transformed into physical, albeit larger scale, structures by 3D printing, and their mechanical properties are characterized. The normalized longitudinal Young’s moduli of the fabricated structures depend on relative density raised to a power between 2 and 3, suggesting that elastic deformation of the parenchyma cellular structure involves considerable cell wall bending. The mechanical behavior of other biological tissues may also be elucidated using this approach. Statement of SignificanceBamboo is a lightweight, sustainable engineering material widely used in structural applications. By combining micro X-ray computed tomography and 3D printing, we have produced bamboo parenchyma mimics and characterized their stiffness. Using this approach, we gained insight into bamboo parenchyma tissue mechanics, specifically the cellular geometry’s role in longitudinal elasticity.

Bending processing and mechanism of laser forming pure aluminum metal foam

Laser forming of pure aluminum closed-cell foam has been studied by means of an effective and cellular model, and the results are compared with experiments and numerical simulations. In order to reveal the forming process and bending mechanism during the process of laser forming closed-cell foam aluminum, a simulation was performed. It is the first time to study the laser forming pure aluminum closed-cell foam aluminum when compared to Al-Si closed-cell foam aluminum. It is found that the bending angle increases with increasing the laser power, and decreases with the increasing scan velocity in the experiment results. The stress/strain profile is similar to that in laser forming Al-Si foam aluminum in simulation results. It is confirmed that temperature gradient mechanism (TGM) is the dominant mechanism during the present laser forming of the closed-cell pure aluminum foam material. And the plastic compressive strain is caused by overall response of the cell wall bending and cell bucking, not by the material shortening.

Seismic retrofit of structures using steel honeycomb dampers

The purpose of this paper is to investigate the seismic performance of a honeycomb shaped steel hysteretic damper applied to seismic retrofit or strengthening of a structure. The formulas for the initial stiffness and yield strength of a damper unit were derived based on the cell wall bending model, and the results were compared with those obtained from finite element analysis. Bilinear model of the honeycomb damper was developed based on the nonlinear force-displacement relationship obtained from finite element analysis. The honeycomb dampers were applied for seismic retrofit of a 15-story apartment building designed without considering seismic load and for seismic design of a 3-story moment frame designed with reduced seismic load. The analysis results showed that the honeycomb dampers were effective in the enhancement of seismic-load resisting capacity of the model structures.

Image-based correlation between the meso-scale structure and deformation of closed-cell foam

In the correlation between structural parameters and compressive behaviour of cellular materials, previous studies have mostly focused on averaged structural parameters and bulk material properties for different samples. This study focuses on the meso-scale correlation between structure and deformation in a 2D foam sample generated from a computed tomography slice of Alporas™ foam, for which quasi-static compression was simulated using 2D image-based finite element modelling. First, a comprehensive meso-scale structural characterisation of the 2D foam was carried out to determine the size, aspect ratio, orientation and anisotropy of individual cells, as well as the length, straightness, inclination and thickness of individual cell walls. Measurements were then conducted to obtain the axial distributions of local structural parameters averaged laterally to compression axis. Second, the meso-scale deformation was characterised by cell-wall strain, cell area ratio, digital image correlation strain and local compressive engineering strain. According to the results, the through-width sub-regions over an axial length between the average (lower bound) and the maximum (upper bound) of cell size should be used to characterise the meso-scale heterogeneity of the cell structure and deformation. It was found that the first crush band forms in a sub-region where the ratio of cell-wall thickness to cell-wall length is a minimum, in which the collapse deformation is dominated by the plastic bending and buckling of cell walls. Other morphological parameters have secondary effect on the initiation of crush band in the 2D foam. The finding of this study suggests that the measurement of local structural properties is crucial for the identification of the “weakest” region which determines the initiation of collapse and hence the corresponding collapse load of a heterogeneous cellular material.

Interfacial behavior and mechanical properties of aluminum foam joint fabricated by surface self-abrasion fluxless soldering

Fluxless soldering with surface self-abrasion has been developed for joining aluminum foams with metallic bonding. The effect of the self-abrasion on the wettability of molten solder alloy and mechanical properties is determined by microstructural observation, tension and compression tests. No apparent macroscopic deformation and collapse of foam structure are observed adjacent to the joint interface. The average tensile strength of the joints is about 14% higher than that of aluminum foam, and the compressive strength can reach 200% of that of aluminum foam. The deformation mechanisms and energy absorbing characteristics of aluminum foam and the joint are investigated. The aluminum foam joint fails primarily by bending, crushing, and compaction of cell walls and cracking of the solder seam. The interdiffusion process is explained based on thermodynamic equations.

Mechanical properties and pore structure deformation behaviour of biomedical porous titanium

Porous titanium has been shown to exhibit desirable properties as biomedical materials. In view of the load-bearing situation, the mechanical properties and pore structure deformation behaviour of porous titanium were studied. Porous titanium with porosities varying from 36%–66% and average pore size of 230 μm was fabricated by powder sintering. Microstructural features were characterized using scanning electron microscopy. Uniaxial compression tests were used to probe the mechanical response in terms of elastic modulus and compressive strength. The mechanical properties of porous titanium were found to be close to the those of human bone, with stiffness values ranging from 1.86 to 14.7 GPa and compressive strength values of 85.16–461.94 MPa. The relationships between mechanical properties and relative densities were established, and the increase in relative density showed significant effects on mechanical properties and deformations of porous titanium. In a lower relative density, the microscopic deformation mechanism of porous titanium was yielding, bending and buckling of cell walls, while the deformation of yielding and bending of cell walls was observed in the porous titanium with higher relative density.