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Nanotube can be used as a mass sensor. To design a mass sensor for evaluating a high-speed nanoparticle, in this study, we investigated the impact vibration of a cantilever nanobeam being transversally collided by a high-speed C60 at the beam's free end with an incident velocity of vIn. The capped beam contains alternately two boron nitride zones and two carbon zones on its cross section. Hence, the relaxed beam has elliptic cross section. The vibration properties were demonstrated by molecular dynamics simulation results. Beat vibration of a slim beam can be found easily. The 1st and the 2nd order natural frequencies (f1 and f2) of the beam illustrate the vibration of beam along the short and the long axes of its elliptic cross section, respectively. f2 decreases with increasing temperature. A minimal value of vIn leads to the local buckling of the beam, and a different minimal vIn leading to damage of the beam. For the same system at a specified temperature, f2 varies with vIn. When the beam bends almost uniformly, f2 decreases linearly with vIn. If vIn becomes higher, the beam has a cross section which buckles locally, and the buckling position varies during vibration. If vIn approaches the damage velocity, a fixed contraflexture point may appear on the beam due to its strong buckling. Above the damage velocity, f2 decreases sharply. These results have a potential application in design of a mass sensor.

Elastic instability has been increasingly used to design buckling-induced auxetic metamaterials. However, only limited patterns of existing three dimensional (3D) buckling-induced auxetic metamaterials exhibit a reliable 3D auxetic behaviour, i.e., the material will contract along two normal lateral directions under compression along the third direction. In this paper, we study a simple geometrical shape for achieving 3D auxetic behaviour. The unit cell of the proposed 3D design is composed of a solid sphere and three cuboids. Two representative models, one with slender connecting bars and the other with thick connecting bars, are investigated both numerically and experimentally. The results indicate that the designed material with thick connecting bars did not exhibit auxetic behaviour while the one with slender connecting bars did. However, the anticipated 3D auxetic behaviour degraded to a two dimensional (2D) auxetic behaviour, i.e., the material would contract only in one lateral direction and maintain nearly the same dimension in the other lateral direction. This finding was further confirmed by experiments. This research has demonstrated challenges in designing and manufacturing of buckling-induced auxetic metamaterials with 3D auxetic behaviour and highlighted the importance of optimising and fine-tuning the auxetic unit cell using the pattern scale factor. Keywords: Auxetic, Metamaterial, Buckling, 3D printing, Pattern scale factor

Under uniaxial compression (tension), auxetic materials would shrink (expand) laterally. It has been speculated that the auxetic property could be used to design superior nails for easier push-in and harder pull-out. In this study, the first auxetic nails are designed, fabricated and experimentally investigated. Pine timber and medium-density fibreboard are selected as testing materials. The push-in and pull-out performance of auxetic and non-auxetic nails is compared by using two key parameters of the maximum compressive force and the maximum tensile force. It is found that the auxetic nails do not always exhibit superior mechanical performance to non-auxetic ones. Also, the small auxetic deformation of one typical designed auxetic nail is revealed by the experimentally validated finite element model. The experimental and numerical results illustrate the limitations of exploiting the auxetic property in the nail application. Some suggestions are provided for more effective designs of future auxetic nails.

The phenomenon of negative linear compressibility has attracted much interest because of its unusual deformation features with many potential applications. However, the design and fabrication of materials and structures with negative linear compressibility are limited. In this paper, we proposed two approaches to designing and fabricating new composite structures with negative linear compressibility. The effectiveness of the proposed design approaches was validated experimentally by applying uniformly distributed pressure to all surfaces of bulk specimens. The deformation features, strain history, and the effective area reduction of the specimens were analyzed from the experimental data. The results clearly demonstrated the feasibility of the proposed designing and manufacturing approaches for realizing composites with negative linear compressibility. Keywords: Negative linear compressibility, Composite material, Composite structure, Mechanical metamaterial

In this study a novel type of tubular structure was proposed, in which predesigned ellipsoidal dimples were introduced into conventional circular tubes. The influence of various design parameters of the dimples on the mechanical properties was extensively investigated by finite element modelling and was experimentally validated by quasi-static tests on 3D printed brass tubes. The results showed that properly designed dimpled tubes had substantially lower initial peak force and remarkably less fluctuation in the crushing force than circular tubes, without significantly sacrificing the mean crushing force.

Nanotube can be used as a mass sensor. To design a mass sensor for evaluating a high-speed nanoparticle, in this study, we investigated the impact vibration of a cantilever nanobeam being transversally collided by a high-speed C60 at the beam's free end with an incident velocity of vIn. The capped beam contains alternately two boron nitride zones and two carbon zones on its cross section. Hence, the relaxed beam has elliptic cross section. The vibration properties were demonstrated by molecular dynamics simulation results. Beat vibration of a slim beam can be found easily. The 1st and the 2nd order natural frequencies (f1 and f2) of the beam illustrate the vibration of beam along the short and the long axes of its elliptic cross section, respectively. f2 decreases with increasing temperature. A minimal value of vIn leads to the local buckling of the beam, and a different minimal vIn leading to damage of the beam. For the same system at a specified temperature, f2 varies with vIn. When the beam bends almost uniformly, f2 decreases linearly with vIn. If vIn becomes higher, the beam has a cross section which buckles locally, and the buckling position varies during vibration. If vIn approaches the damage velocity, a fixed contraflexture point may appear on the beam due to its strong buckling. Above the damage velocity, f2 decreases sharply. These results have a potential application in design of a mass sensor.

It was discovered that a sudden jump of the output torque moment from a rotation transmission nanosystem made from carbon nanotubes (CNTs) occurred when decreasing the system temperature. In the nanosystem from coaxial-layout CNTs, the motor with specified rotational frequency (&omega, M) can drive the inner tube (rotor) to rotate in the outer tubes. When the axial gap between the motor and the rotor was fixed, the friction between their neighbor edges was stronger at a lower temperature. Especially at temperatures below 100 K, the friction-induced driving torque increases with &omega, M. When the rotor was subjected to an external resistant torque moment (Mr), it could not rotate opposite to the motor even if it deformed heavily. Combining molecular dynamics simulations with the bi-sectioning algorithm, the critical value of Mr was obtained. Under the critical torque moment, the rotor stopped rotating. Accordingly, a transmission nanosystem can be designed to provide a strong torque moment via interface friction at low temperature.

A simplified topology optimization was developed to prevent fatigue failure in this paper. It is based on the bi-directional evolutionary structural optimization (BESO) approach where the modified Goodman fatigue failure criterion was applied directly in the sensitivity analysis. The high-cycle fatigue under proportional loadings with constant amplitude was considered in fatigue analysis. To reduce the computational cost, the modified p-norm approach of the fatigue constraint was applied by assembling the contribution of the local fatigue failure criterion in one function. For the finite element analysis, the sensitivity number of the elements was calculated based on the results from the equivalent linear static analysis while the sensitivity number is based on the Goodman fatigue criterion. The optimization problem was defined as maximizing the stiffness of a structure with a volume constraint and a fatigue failure constrain to prevent fatigue within the prescribed life cycles. The effectiveness of the proposed optimization approach was confirmed through comparison between the new approach results and those from the traditional compliance minimization problem using several traditional examples with obvious stress concentration. Keywords: Fatigue failure, p-norm, Modified Goodman theory, Sensitivity analysis, BESO

This paper develops a method for dealing with the stress minimization of continuum structures based on the bidirectional evolutionary structural optimization (BESO) method. The modified p-n...

Auxetic materials exhibit uncommon behaviour, i.e. they will shrink (expand) laterally under compression (tension). This novel feature has attracted intense research interest. However, most of previous works focus on auxetic behaviour in either compression or tension. Most of the auxetic materials are not symmetric in tension and compression under large deformation. Studies on the auxetic performance of metamaterials both in compression and tension are important but rare. As an extension of our previous research on compressive auxetic performance of 3D metallic auxetic metamaterials, numerical simulations were carried out to investigate the auxetic and other mechanical properties of the 3D metallic auxetic metamaterials in tension. The preliminary results indicated that the designed 3D metallic auxetic metamaterials exhibited better auxetic performance in compression than in tension. By increasing a pattern scale factor, auxetic performance of the 3D metallic auxetic metamaterials under tension can be improved. With proper adjustment of the pattern scale factor, an approximately symmetric auxetic performance could be achieved in compression and tension.

The performance of a metamaterial is dominated by the geometric features and deformation mechanisms of its microstructure. For a certain mechanism, the geometric features have bounds in which the performance of a metamaterial such as negative Poisson’s ratio (NPR) can be designed. Previous investigation on buckling-induced auxetic metamaterial revealed that there is a geometric limit for its microstructure to exhibit auxetic behaviour in infinitesimal deformation. However, the limit for auxetic metamaterials undergoing large deformation is different from that under small deformation and has not been reported yet. In this paper, the geometric limit was investigated in an elastic and infinitesimal deformation range using linear buckling analysis. Furthermore, experimentally validated finite element models were used to identify the geometric limits for auxetic metamaterials undergoing large deformation. Depending on the control parameters of the topology, the bounds were represented by a line strip for one control parameter, an area for two control parameters and spatial domain surrounded by a 3D surface for three parameters. The limit was determined by the shape and size of the void of the metamaterials and it was identified through the large deformation analysis as well as the linear buckling analysis. We found that there was a significant difference in the geometric bounds obtained through those two methods. The results from this study can be used to design an auxetic metamaterial for different applications and to control the auxetic performance.

Thin-walled tubes are widely used as energy absorption components. In this study, two different origami patterns were introduced to circular tubes. The influence of the origami patterns on the energy absorption capacity and the deformation mechanism of tubes under uniaxial loading were investigated both numerically and experimentally. The results showed that the initial peak force of origami tubes would be significantly reduced, while the energy absorption capacity could be improved or maintained. Brass tubes with and without origami patterns were fabricated using 3D printing and were tested to validate the finite element models.

The development of porous metals to alleviate the effects of stress shielding in bone will help improve the function of metallic biomaterials in orthopaedic applications. A critical step in advancing this technology is to design metallic structures with low rigidity that is comparable with bone tissue, but with good mechanical strength. In this study, porous titanium (Ti) structures with periodic cell topologies were designed to achieve tunable mechanical properties. The versatility of the design scheme was demonstrated by examining lattice designs with different stiffness properties achieved by using the Selective Laser Melting (SLM) technology. The fabricated porous Ti exhibited a low modulus of 1.05 GPa but a high compressive strength of 55 MPa. Large deformation analysis using digital image correlation (DIC) technique indicated uniform strain patterns at micro-trusses, suggesting the overall high quality of the structure with absence of local flaws. A functionally-graded stiffness design was further investigated by varying the diameters of micro-trusses within the structure. A stiffness graded material may be favourable for anatomical site that has strong depthdependent variations, such as in trabecular bone microstructures.

Recent advances in additive manufacturing make it possible to fabricate periodic lattice structures with complex configurations. However, a proper design strategy to achieve lattice structures with controlled anisotropy is still unavailable. There is an urgent need to fill this knowledge gap in order to develop mechanical metamaterials with prescribed properties. Here we propose two different methodologies to design lattice structures with controlled anisotropy. As examples, we created two new families of lattice structures with isotropic elasticity and cubic symmetric geometry. The findings of this work provide simple and effective strategies for exploring lightweight metamaterials with desired mechanical properties. Keywords: Metamaterials, Lattice structures, Anisotropy, Homogenization

It was discovered that large-amplitude axial oscillation can occur on a rotor with an internally hydrogenated deformable part (HDP) in a rotation&ndash, translation nanoconverter. The dynamic outputs of the system were investigated using molecular dynamics simulations. When an input rotational frequency (100 GHz &gt, &omega, &gt, 20 GHz) was applied at one end of the rotor, the HDP deformed under the centrifugal and van der Waals forces, which simultaneously led to the axial translation of the other end of the rotor. Except at too high an input rotational frequency (e.g., &gt, 100 GHz), which led to eccentric rotation and even collapse of the system, the present system could generate a periodic axial oscillation with an amplitude above 0.5 nm at a temperature below 50 K. In other ranges of temperature and amplitude, the oscillation dampened quickly due to the drastic thermal vibrations of the atoms. Furthermore, the effects of the hydrogenation scheme and the length of HDP on the equilibrium position, amplitude, and frequency of oscillation were investigated. The conclusions can be applied to the design of an ideal nano-oscillator based on the present rotation&ndash, translation converter model.

Using molecular dynamics simulation, the dynamic response of a nanogear drive system with intersecting axes was investigated. The nanogear drive system made from carbon nanotubes (CNT) contains two parts, i.e., a motor and a nanobearing. Their tube axes have an intersecting angle of θ. Due to van der Waals (vdW) interaction at their adjacent edges, the rotor in the bearing can be driven to rotate by the constant-speed motor. By increasing the value of θ, the vdW interaction decreases and the difference between the rotational frequencies of the motor and the rotor becomes larger. Especially, at temperature below 100 K, the system with θ = 60° has very stable and asynchronous transmission effect, which will benefit potential application of an asynchronous rotation transmission nanosystem in a nanomachine.

The mechanical properties of bonded graphene ribbon were evaluated via uniaxial tensile tests by molecular dynamics simulations. For an ideal/pristine few-layered graphene ribbon, the interlayer superlubrication leads to easily relative sliding between neighboring layers. When the layers in the ribbon are sewn by collision with high speed fullerenes from both sides, a new two-dimensional material is fabricated. A part of bonds in the original graphene layers are broken, and new bonds are generated to connect the neighboring layers at the impacted areas. The in-plane mechanical properties, e.g., Young’s modulus, maximal engineering strain (which demonstrates deformability), deformation energy (which illustrates structural stiffness) and the maximal stress (which describes strength), of the newly fabricated defective ribbon are different from the original ideal ribbon. By comparison with the ideal ribbon, the changes of the mechanical properties were demonstrated. Meanwhile, temperature effect on the changes of properties was also discussed for potential application of the new two-dimensional material. In brief, the maximal strain slightly depends on temperature and loading speed. The defective ribbon breaks in tension failure mode when stretched along x/zigzag direction, but in shear failure mode when stretched along y/armchair direction. The Young’s modulus reduces 10%–12%, but, the maximal strain reduces more than 50% when compared with the ideal ribbon.

This paper develops a method for considering the critical fatigue stress as a constraint in the Bi-directional Evolutionary Structural Optimization (BESO) method. Our aim is to reach an optimal design in which high cycle fatigue failure does not occur for a specific life time. The critical fatigue stress is calculated based on modified Goodman criteria and used as a stress constraint in our topology optimization problem. Since fatigue generally does not occur for compressive stresses, we use the p-norm approach of the stress measurement that considers the highest tensile principal stress in each point as stress measure to calculate the sensitivity numbers. The BESO method has been extended to minimize volume an object subjected to the critical fatigue stress constraint. The optimization results are compared with the results from the compliance minimization problem which shows clearly the merits of our newly developed approach., {"references":["Bendsøe, M. P. and N. Kikuchi, Generating optimal topologies in structural design using a homogenization method. Computer methods in applied mechanics and engineering, 1988. 71(2): pp. 197-224.","Svanberg, K., The method of moving asymptotes—a new method for structural optimization. International journal for numerical methods in engineering, 1987. 24(2): pp. 359-373.","Bruggi, M. and P. Duysinx, Topology optimization for minimum weight with compliance and stress constraints. Struct Multidisc Optim, 2012. 46(3): pp. 369-384.","Holmberg, E., B. Torstenfelt, and A. Klarbring, Stress constrained topology optimization. Struct Multidisc Optim, 2013. 48(1): pp. 33-47.","Jeong, S. H., D.-H. Choi, and G. H. Yoon, Separable stress interpolation scheme for stress-based topology optimization with multiple homogenous materials. Finite Elements in Analysis & Design, 2014. 82: pp. 16-31.","Jeong, S. H., et al., Topology optimization considering static failure theories for ductile and brittle materials. Computers and Structures, 2012. 110-111: pp. 116-132.","Le, C., et al., Stress- based topology optimization for continua. Struct Multidisc Optim, 2010. 41(4): pp. 605-620.","París, J., et al., Stress constraints sensitivity analysis in structural topology optimization. Computer Methods in Applied Mechanics and Engineering, 2010. 199(33): pp. 2110-2122.","Huang, X. and Y. M. Xie, Convergent and mesh-independent solutions for the bi-directional evolutionary structural optimization method. Finite Elements in Analysis & Design, 2007. 43(14): pp. 1039-1049.\n[10]\tRozvany, G. I. N., A critical review of established methods of structural topology optimization. Structural and Multidisciplinary Optimization, 2009. 37(3): pp. 217-237.\n[11]\tHolmberg, E., B. Torstenfelt, and A. Klarbring, Fatigue constrained topology optimization. Structural and Multidisciplinary Optimization, 2014. 50(2): pp. 207-219.\n[12]\tBendsøe, M. P., Topology Optimization: Theory, Methods, and Applications, ed. O. Sigmund. 2004, Berlin/Heidelberg: Berlin/Heidelberg : Springer Berlin Heidelberg.\n[13]\tCheng, G. and Z. Jiang, Study On Topology Optimization With Stress Constraints. Engineering Optimization, 1992. 20(2): pp. 129-148.\n[14]\tCheng, G. and X. Guo, ε-relaxed approach in structural topology optimization. Structural Optimization, 1997. 13(4): pp. 258-266.\n[15]\tRozvany, G. I. N., On design-dependent constraints and singular topologies. Struct Multidisc Optim, 2001. 21(2): pp. 164-172.\n[16]\tKirsch, U., On singular topologies in optimum structural design. Structural Optimization, 1990. 2(3): pp. 133-142.\n[17]\tCook, R. D., Concepts and applications of finite element analysis. 4th ed. ed, ed. R. D. Cook. 2001, New York: Wiley.\n[18]\tHuang, X. and Y. Xie, Convergent and mesh-independent solutions for the bi-directional evolutionary structural optimization method. 2007.\n[19]\tHuang, X. and Y. Xie, Evolutionary topology optimization of continuum structures with an additional displacement constraint. Struct Multidisc Optim, 2010. 40(1): pp. 409-416.\n[20]\tHuang, X. and Y. Xie, Evolutionary topology optimization of geometrically and materially nonlinear structures under prescribed design load. Structural Engineering and Mechanics 2010. 34(5): pp. 581-595.\n[21]\tYi, J., et al., A topology optimization method for multiple load cases and constraints based on element independent nodal density. Structural Engineering and Mechanics, 2013. 6(45): pp. 759-777.\n[22]\tJensen, K. E., Solving stress and compliance constrained volume minimization using anisotropic mesh adaptation, the method of moving asymptotes and a global p-norm. Structural and Multidisciplinary Optimization, 2016. 54(4): pp. 831-841."]}

Auxetic metamaterials have enhanced indentation and penetration resistance due to their high shear strength and modulus. Its auxetic performance under dynamic loading cases is an important property for shields and armour applications. In the present study, compressive tests at different impact velocities on buckling-induced auxetic metamaterials were conducted for two different initial geometries. A photographic technique was applied to measure the Poisson's ratio. When the dynamic data were compared with those of quasi-static experiments, it was found that the negative Poisson's ratio for the buckling-induced metamaterial is sensitive to the rate of loading, while the negative Poisson's ratio for the metamaterial with initial auxetic behaviour is insensitive to the loading rate. It was also found that the deformation pattern is similar to that in the quasi-static loading condition when the impact force measured by the test machine is dominated by the inertia force of the metamaterials.

The behaviour of luffa and luffa-filled tubes under uniaxial compression was investigated numerically using finite element analysis (FEA) and analytically by theoretical models. The FEA models were validated against experimental data. Parametric study was carried out using the validated FEA models to examine the effects of the density of luffa, the thickness to diameter ratio of tube and the cross-sectional topology of luffa core. It was found that the optimal density of the luffa as filler for the luffa-filled tubes was closely related to the optimal density of the luffa sponge. It increased with the increase of the thickness to diameter ratio of the tube. The cross-sectional topology of the filler material had a negligible effect on the specific energy absorption per unit mass even when the deformation pattern of the luffa-filled tube was changed from the diamond mode to the concertina one.

This paper is concerned with the lateral crushing behaviour of short sandwich tubes, which consist of two concentric aluminium tubes of different diameters filled with aluminium foam. Experimental results and corresponding analytical models are presented. Aluminium sandwich tubes 50 mm long and of different values of diameter to thickness ratios were laterally crushed using an MTS machine at a displacement rate of 2 mm/min. Two bonding cases between the foam and monolithic component tubes, i.e. fully bonded and bonding free, were employed for all the configurations. Load–displacement curves were obtained. Deformation mechanisms were observed and three different crushing patterns have been identified. The experiments reveal that the bonding between the tubes and core for sandwich tubes played a different role in the three crushing patterns. Analytical models using rigid, perfectly plastic theory have been developed, which agree well with the corresponding experimental results.

In this paper we propose a general method for creating a new type of hierarchical structures at any level in both 2D and 3D. A simple rule based on a rotate-and-mirror procedure is introduced to achieve multi-level hierarchies. These new hierarchical structures have remarkably few degrees of freedom compared to existing designs by other methods. More importantly, these structures exhibit synchronized motions during opening or closure, resulting in uniform and easily-controllable deformations. Furthermore, a simple analytical formula is found which can be used to avoid collision of units of the structure during the closing process. The novel design concept is verified by mathematical analyses, computational simulations and physical experiments.

Elastic instability of soft cellular solids plays an increasingly important role in the creation of metamaterials with smart properties. Inspiration for much of this research comes from a planar metamaterial with negative Poisson's ratio behavior induced by elastic instability. Here we extend the concept of buckling induced pattern switch further to the design of a new series of three-dimensional metamaterials with negative Poisson's ratio over a large strain range. The highlight of this work is that our designs are based on very simple initial geometric shapes.

Insight into the stimuli-responsive behaviour of biological materials with hierarchical microstructures is essential for designing new sustainable materials and structures. Shape memory, self-healing and self-repairing will become valuable characteristics of advanced materials. Here we report the water-triggered shape recovery of a natural biological material, the luffa sponge. The longitudinally crushed luffa sponge column can recover up to 98% of its original shape after it is immersed in water. The mechanical properties of the luffa sponge can also be recovered, to a large extent, after a subsequent drying process. The effects of strain rate, crushing strains, loading cycles, and temperature/duration of water treatment of the drying process on the shape recovery ratio and the energy dissipation recovery ratio have been investigated. The results from this study have demonstrated that the luffa sponge material possesses remarkable shape memory effects and mechanical recovery features which could be exploited or biomimicked for the design of water-responsive smart materials undergoing large deformations.

The response of sandwich tubes under internal explosive loading was investigated experimentally, numerically and analytically in this paper. Experiments were conducted first to capture the fundamental deformation and failure patterns and they served as a basis of validation for both the FE and analytical models. Further detailed deformation and blast loading history were revealed by the FE model. An explicit analytical solution for the deformation of sandwich tubes under blast loading has been worked out and used to obtain the optimum sandwich configurations, which would outperform their corresponding monolithic tubes.

Luffa sponge is a light-weight natural material which has the potential to be used as an alternative sustainable material for various engineering applications such as packaging, acoustic and vibration isolation, and impact energy absorption. The strain rate effect is an important material property for such applications. In the present study, compressive tests at different strain rates on luffa sponge material were conducted over a wide density range from 24 to 64 kg m−3. A photographic technique was applied to measure the sectional area which has an irregular shape. The stress–strain curves of luffa sponge material at various strain rates were calculated based on this measurement. When the dynamic data are compared with those of quasi-static experiments, it is found that the compressive strength, plateau stress and specific energy absorption of the luffa sponge material are all sensitive to the rate of loading. It is also found that the dynamic enhancement for the compressive strength was more prominent than that for the plateau stress. The underlying mechanism was discussed and clarified. Empirical formulae were proposed for the macroscopic strength, densification strain and specific energy absorption at various strain rates. A comparison study shows that the luffa sponge has better energy absorption capacity per unit mass than other cellular materials with similar plateau stress at various strain rates.

This paper concerns on the aluminum foam material modeling and identification of constitutive parameters at high strain rates. Lured by its excellent energy absorption capacity under the impact conditions, aluminum foam has been widely used in automotive and aerospace industry as a lightweight filler material. Nevertheless, aluminum foam shows different mechanical properties at low and high plastic deformation rates, moreover, in the engineering practice, the occurrence of high strain rate appears more often than low strain rate. Generally speaking, to obtain the constitutive model parameters of aluminum foam material, a large number of expensive experiments need to be conducted. In addition, the plastic deformation behavior of aluminum foam at high strain rate follows a highly non-linear dynamic process, and its parameter identification requires much more complex numerical procedure. For these reasons, this paper proposes a new procedure to predict Deshpande and Fleck (DC) model parameters for aluminum foam based on successive artificial neural network (SANN) technique and particle swarm optimization (PSO) algorithm. Finite element analyses are performed by using Design of Experiment (DoE) method to establish the SANN model for each of the five constitutive parameters of the DC model. The constitutive model parameters with the minimum discrepancy between experimental and SANN curves are obtained by using the surrogate modeling and PSO methods. Finally, this approach is validated by comparing the FE modeling results against the experimental results. This study demonstrates the effectiveness of such a three phase identification procedure comprising experimentation, optimization, and verification. It provides a useful means for parametric identification of other similar lightweight foam or porous materials.

The strain rate effect of luffa sponge material is an indispensable property for it to be used for acoustic, vibration, and impact energy absorption. Compressive tests at different strain rates on cylindrical column specimens of luffa sponge material were conducted over a wide density ranging from 24 to 64 kg/m3. A photographic technique was applied to measure the section area of the specimen with irregular shape. The mechanical properties of luffa sponge material at various strain rates were obtained based on this measurement. The dynamic data were compared to those of quasi-static experiments. It was found that compressive strength, plateau stress and specific energy absorption of luffa sponge material were sensitive to the rate of loading. Empirical formulae were developed for strength, densification strain and specific energy absorption at various strain rates in the macroscopic level by considering the luffa fiber as base material.

Metallic auxetic metamaterials are of great potential to be used in many applications because of their superior mechanical performance to elastomer-based auxetic materials. Due to the limited knowledge on this new type of materials under large plastic deformation, the implementation of such materials in practical applications remains elusive. In contrast to the elastomer-based metamaterials, metallic ones possess new features as a result of the nonlinear deformation of their metallic microstructures under large deformation. The loss of auxetic behavior in metallic metamaterials led us to carry out a numerical and experimental study to investigate the mechanism of the observed phenomenon. A general approach was proposed to tune the performance of auxetic metallic metamaterials undergoing large plastic deformation using buckling behavior and the plasticity of base material. Both experiments and finite element simulations were used to verify the effectiveness of the developed approach. By employing this approach, a 2D auxetic metamaterial was derived from a regular square lattice. Then, by altering the initial geometry of microstructure with the desired buckling pattern, the metallic metamaterials exhibit auxetic behavior with tuneable mechanical properties. A systematic parametric study using the validated finite element models was conducted to reveal the novel features of metallic auxetic metamaterials undergoing large plastic deformation. The results of this study provide a useful guideline for the design of 2D metallic auxetic metamaterials for various applications.

The paper presents the first scientific study of the stiffness, strength and energy absorption characteristics of the luffa sponge with a view to using it as an alternative sustainable engineering material for various practical applications. A series of compression tests on luffa sponge columns have been carried out. The stress-strain curves show a near constant plateau stress over a long strain range, which is ideal for energy absorption applications. It is found that the luffa sponge material exhibits remarkable stiffness, strength and energy absorption capacity that are comparable to those of some commonly-used metallic cellular materials. These properties are due to its light-weight base material, and its structural hierarchy at several length scales. Empirical formulae have been developed for stiffness, strength, densification strain and specific energy absorption at the macroscopic level by considering the luffa fiber as the base material. A comparative study shows that the luffa sponge material outperforms a variety of traditional engineering materials.

The paper presents the first scientific study of the stiffness, strength and energy absorption characteristics of the luffa sponge with a view to using it as an alternative sustainable engineering material for various practical applications. A series of compression tests on luffa sponge columns have been carried out. The stress-strain curves show a near constant plateau stress over a long strain range, which is ideal for energy absorption applications. It is found that the luffa sponge material exhibits remarkable stiffness, strength and energy absorption capacities that are comparable to those of some metallic cellular materials in a similar density range. Empirical formulae have been developed for stiffness, strength, densification strain and specific energy absorption at the macroscopic level. A comparative study shows that the luffa sponge material outperforms a variety of traditional engineering materials.

Curved sandwich panels with two aluminium face sheets and an aluminium foam core under air blast loadings were investigated experimentally and numerically. Specimens with two values of radius of curvature and different core/face sheet configurations with the same projected area were tested for three blast intensities. All four edges of the panels were fully clamped. The experiments were carried out by a four-cable ballistic pendulum with corresponding sensors. The impulse acting on the front face of the assembly, the deflection history at the center of the back face sheet, and the strain history at some characteristic points on the back face were obtained. Then the deformation/failure modes of specimens were classified and analyzed systematically. The commercial software LS-DYNA was employed to simulate those physical processes. The finite-element (FE) model was validated by the data from experiments. Detailed deformation and energy dissipation mechanisms were further revealed by the FE models. The valuable experimental data and results from FE models show that the initial curvature of a curved sandwich panel changes the deformation/collapse mode with an extended range for bending-dominated deformation mode, which suggests that the performance of the sandwich shell structures slightly exceeds that of both their equivalent solid counterpart and a flat sandwich plate in certain blast intensity ranges.

Compressive behavior of closed-cell aluminum alloy foams at strain rates of 10 −3 –450 s −1 has been studied experimentally. The fully stress–strain curves of specimens at medium strain rates were obtained using the High Rate Instron Test System, which can maintain a constant loading rate. The experimental results show that plateau stress and energy absorption capacity are remarkable dependent on strain rate, while the densification strain has a negligible dependence.

In this paper curved sandwich panels with two aluminium face sheets and an aluminium foam core under air blast loadings were investigated experimentally. Specimens with two values of radius of curvature and different core/face sheet configurations were tested for three blast intensities. All the four edges of the panels were fully clamped. The experiments were carried out by a four-cable ballistic pendulum with corresponding sensors. Impulse acting on the front face of the assembly, deflection history at the centre of back face sheet, and strain history at some characteristic points on the back face were obtained. Then the deformation/failure modes of specimens were classified and analysed systematically. The experimental data show that the initial curvature of a curved sandwich panel may change the deformation/collapse mode with an extended range for bending dominated deformation, which suggests that the performance of the sandwich shell structures may exceed that of both their equivalent solid counterpart and a flat sandwich plate.

Failure of metal foams caused by dynamic indentation and penetration is very common in practice, such as light-weight structural sandwich panels, packing materials and energy absorbing devices. Rational application of these materials requires a sound understanding of deformation and energy absorption mechanisms of the aluminium foams as well as the effect of impact velocity. In this study, following experimental investigations into compression, tension, sharing and indentation of CYMAT aluminium foams of various densities, a finite element (FE) analysis using ABAQUS is conducted for dynamic indentation process of aluminium foams under a rigid, flat-headed indenter. Two methods of applying impact velocities are considered: the indenter is pushed into the foam at a constant velocity through the whole process or with an initial velocity which then decreases with indentation. Two energy dissipation mechanisms are considered: compression of the foam ahead of the indenter and fracture along the indenter edge. Effect of impact velocity is noted on the size of a localized deformation and the total energy absorbed. A plastic structural shock theory developed by previous researchers is applied to calculate the resistance force with indentation depth during indentation process and fair agreement is obtained between the analytical and numerical results.

Failure of aluminium foams due to dynamic indentation and penetration is very common in their application such as light-weight structural sandwich panels, packing materials and energy absorbing devices. This requires a sound understanding of deformation and energy absorption mechanisms of the aluminium foams as well as the effect of impact velocity. In this study, a finite element analysis using ABAQUS is conducted for the dynamic indentation/penetration process of aluminium foams under a rigid flat-headed indenter. The indenter is pushed into the foam either at a constant velocity or with an initial velocity. Two mechanisms exist: compression of the foam ahead of the indenter and fracture along the indenter edge. Effect of impact velocity is noted on the size of a localized deformation and the total energy absorbed.

Experiments were carried out to examine the behaviour of aluminium foams under quasistatic shear loading. Special fixtures were designed to clamp the both ends of a beam-like specimen while the load was applied via a punch, which led to failure by shearing along the clearance near the fixed end. Specimens were made of CYMAT foams with two nominal relative densities (12% and 17%) and several values of width. This study focuses on the maximum strength under shear and the essential energy in fracture. It has been found that the ultimate shear force increases linearly with the width of the beam, so does the total energy absorbed. Two empirical formulae have been obtained which relates to the relative density, respectively, the ultimate shear strength and energy absorbed in shearing.

We propose a technique capable of designing fishnet metamaterials that have a negative refractive index (NRI) over a broad range in the visible and infrared. The technique relies on optimizing the shape and scale of the fishnet apertures as well as the depth of different layers of the composite. A metamaterial is obtained that exhibits an unbroken 552 nm bandwidth of NRI, covering the entire red and infrared regions. Moreover, two fishnet structures perforated with star-like holes are found to render refractive index negative in the yellow and green spectra.

Auxetic materials and structures are increasingly used in various fields because of their unusual properties. Auxetic tubular structures have been fabricated and studied due to their potential to be adopted as oesophageal stents where only tensile auxetic performance is required. However, studies on compressive mechanical properties of auxetic tubular structures are limited in the current literature. In this paper, we developed a simple tubular structure which exhibits auxetic behaviour in both compression and tension. This was achieved by extending a design concept recently proposed by the authors for generating 3D metallic auxetic metamaterials. Both compressive and tensile mechanical properties of the auxetic tubular structure were investigated. It was found that the methodology for generating 3D auxetic metamaterials could be effectively used to create auxetic tubular structures as well. By properly adjusting certain parameters, the mechanical properties of the designed auxetic tubular structure could be easily tuned.

The energy absorption of sandwich tubes impacted between two rigid plates was investigated experimentally and numerically in the paper. Based on the collapse patterns observed in quasi-static experiment, several typical sandwich specimens were made and tested. The specimens were placed on the bottom platen of an Instron machine and gained a constant upwards velocity, followed by the impact with the top rigid platen. Their deformation history and load-compression curves were recorded. The energy absorption of sandwich tubes were then calculated and analyzed. Three different crushing patterns have been identified from previous experiments. The dynamic enhancement of energy absorption of the sandwich tubes were only observed in collapse pattern III under the tested velocity up to 10 m/s. Finite element (FE) models using ABAQUS were developed and validated against experimental results and the strain rate effect of metallic foam was considered. They were used to explore the detailed energy-absorption characteristics beyond the experimental range for impact velocities up to 100 m/s. The dynamic enhancement occurred for each configuration of sandwich tubes when the impact velocities were greater than 20 m/s. It was found that increasing the compression velocity leads to an increase in total plastic energy dissipation. Sandwich tubes with a thicker foam core are proved to be the optimum design.

The dynamic lateral crushing behaviour of short empty and sandwich circular tubes is examined in this paper. Unlike the conventional impact method, the specimens were placed on the bottom platen of an Instron machine with a constant upwards velocity and then the tube collided with the fixed upper rigid platen. Load-deflection curves of empty tubes were first obtained and analysed. From the viewpoint of deformation modes and plastic strain energy absorbed by the tube quadrants around the proximal surface and distal surface, a critical velocity of impact is determined which corresponds to a mode change. A relationship is found to exist between the critical velocity and thickness-to-diameter ratio as well as the yield stress and density of the material. To understand the dynamic crushing behaviour of short aluminium foam-filled sandwich tubes by two rigid platens, further tests and corresponding finite element analysis were performed, respectively. Similar to the observations in the quasi-static tests, the mode of dynamic collapse is bending, with the formation of plastic bending zones accompanied with core crushing. Corresponding FE models for ABAQUS/Explicit were developed and validated against the experimental observations. Detailed deformation features and energy absorption characteristics during the crushing process were identified. It was found that increasing the compression velocity leads to an increase in the total internal plastic energy dissipation for both empty and sandwich tube. The propagation of plastic bending in the form of double-moving-hinges is the main mechanism of energy dissipation, as opposed to the low velocity impact which involves stationary plastic deformation zones.