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• Proposing an efficient DEM-building method for accurate beam vibration simulation. • DEM is built from continuum beam theory and the dispersion curve of a periodic chain. • The DEM can be expanded to cover a broad frequency range without limitation. • A graded cylinder chain having a broad frequency bandgap is designed. • Superimposing local resonances in beams efficiently forms a wide frequency bandgap. We propose an efficient method to build a simple discrete element model (DEM) that accurately simulates the oscillation of a continuum beam. The DEM is based on the Timoshenko beam theory of slender cylindrical members and their corresponding wave dynamics in assembly. This physics-informed DEM accounts for multiple vibration modes of the constituting beam elements in wide frequency ranges. We construct various DEMs mimicking cylinder chains and compare their wave dynamics with those measured in experiments to validate the proposed method. Furthermore, we construct a graded woodpile chain of slender cylinders. We experimentally and numerically investigate the frequency bandgaps of the system and demonstrate the possibility of constructing a wide bandgap by consecutively superposing multiple stop bands generated from cylinders of various lengths. This system is highly efficient in blocking propagating waves by leveraging the vibration isolation effect stemming from the local resonance of the cylinders. The proposed DEM method can be useful for investigating and designing complex vibration systems in an efficient and accurate manner. Moreover, the design approach of manipulating the frequency bandgap can be exploited for developing vibration filters and impact mitigation devices. [ABSTRACT FROM AUTHOR]

We numerically and experimentally investigate impact/blast-induced shock mitigation using a quasi-1D woodpile mechanical metamaterial consisting of slender cylindrical rods. In this system, nonlinear stress waves generated by the impact or blast are coupled with local bending vibration modes of the constituent rods. This result in an order of magnitude reduction of transmitted shock without relying on material damping. Specifically, we identify that the arrangement of the local resonances from low to high frequencies enables efficient mitigation of impact/blast energy by splitting strong incident waves into weak ones. We also observe that optimized patterns of the woodpile configurations based on genetic algorithm help making the wave energy equipartition and radiate efficiently in both temporal and spatial domains. This wave attenuation mechanism based on local resonances shows a great potential for the development of nonlinear elastic metamaterials for efficient impact mitigation. [ABSTRACT FROM AUTHOR]

In the present work, we numerically and experimentally investigate the propagation of nonlinear waves in a hexagonally packed granular channel. Specifically, we assemble a simple 1-2-1 granular chain, which triggers not only normal but also tangential interactions of particles up on a striker impact. We experimentally measure the transmission of nonlinear waves propagating along the channel direction using an embedded piezoelectric sensor particle. For numerical simulations, we introduce a discrete element model that accounts for both elastic and damping effects in axial and rotational directions. As a result, we find that the wave propagation in this 1-2-1 hexagonal architecture is governed strongly by the rotational dynamics of particles. We also verify that the effect of rotational damping is crucial for the accurate description of the particle’s dynamics. The findings in this study hints the significance of particles’ rotational dynamics when stress waves propagate along hexagonally packed particle channels even in highly ordered 2D/3D granular architectures. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the reflection of highly nonlinear solitary waves in one-dimensional granular crystals interacting with large plates. We observe significant changes in the reflected waves’ properties in terms of wave amplitude and time of flight in association with the intrinsic inelasticity of large plates, which are governed by the plate thickness and the size of the granular constituents. We also study the effects of fixed plate boundaries in the formation of reflected waves, and find the existence of a critical distance, within which the interaction between the granular chain and plate is strongly modified. We explain the effects of intrinsic inelasticity and of boundaries in the large plates by using plate theory and the contact mechanics between a plate and a spherical striker. We find that experimental results are in excellent agreement with the analytical predictions and numerical simulations based on the combined discrete element and spectral element models. The findings in this study can be useful for the nondestructive evaluation of plate structures using granular crystals, which can improve the resolution of in-situ, portable measurement instruments leveraging high acoustic energy and sensitivity of solitary waves. [Copyright &y& Elsevier]

This study investigates the mode I intra-laminar fracture and size effect in Discontinuous Fiber Composites (DFCs). Towards this goal, the results of fracture tests on geometrically-scaled Single Edge Notch Tension (SENT) specimens are presented and critically discussed for three platelet sizes. The results clearly show a decrease in nominal strength as the specimen size increases. It is found that, when the specimen is sufficiently large, the structural strength scales according to Linear Elastic Fracture Mechanics (LEFM) and the failure occurs in a very brittle way. In contrast, small specimens exhibit a more pronounced pseudo-ductility with a limited scaling effect and a significant deviation from LEFM. To characterize the fracture energy and the effective length of the fracture process zone, an approach combining equivalent fracture mechanics and stochastic finite element modeling is proposed. The model accounts for the complex random mesostructure of the material by modeling the platelets explicitly. Thanks to this theoretical framework, the mode I fracture energy of DFCs is estimated for the first time and it is shown to depend significantly on the platelet size. In particular, the fracture energy is shown to increase linearly with the platelet size in the range investigated in this work. Another importance of this work is that, compared to traditional unidirectional composites, DFC structures exhibit higher pseudo-ductility and their strength is, by far, less sensitive to notches, defects and cracks. This aspect can be used advantageously in structural design only upon the condition that proper certification guidelines acknowledging the more pronounced quasibrittleness of DFCs is formulated. The size effect analysis presented in this work represents a first step in this direction as it allows the assessment of the severity of a defect or notch in DFCs. [ABSTRACT FROM AUTHOR]

Pre-folded tubes can exhibit superior energy absorption characteristics under axial crushing compared to conventional straight-walled tubes. However, their construction can be difficult to automate. In this investigation, we present a novel filament winding approach to efficiently manufacture pre-folded carbon fiber reinforced plastic (CFRP) tubes featuring the Kresling fold pattern. We experimentally study the quasi-static axial crushing behavior of three Kresling geometries and compare them to conventional tubes with a circular and square cross-section. The data from experiments is used to develop a finite element model for estimating axial crush indicators. Despite the layup sequence not orienting the fibers in the direction of loading, we obtain superior structural performance in terms of the mean force divided by the initial peak crushing force, which represents the crushing force efficiency of the structure. The tested tubes exhibit further potential to improve their energy absorption performances, while preserving enhanced manufacturability. [ABSTRACT FROM AUTHOR]

We study the formation and propagation of nonlinear stress waves in 3D woodpile elastic metamaterials consisting of vertically stacked slender cylindrical rods. We find the nonlinear waveforms transmitted through the woodpile architectures under impact are highly sensitive to their design parameters, particularly stacking distances and angles in 3D assemblies. We numerically and experimentally demonstrate that the woodpile system can localize, modulate, and ultimately attenuate propagating nonlinear waves in an efficient manner without relying on material damping. We also show the feasibility of constructing metamaterials with a simultaneous characteristic of high damping and high stiffness by using the controllable wave dispersion in the woodpile structures. These woodpile metamaterials offer enhanced degrees of freedom in manipulating stress waves, thereby offering a new way to design efficient impact protectors with high stiffness. [ABSTRACT FROM AUTHOR]

Abstract: In this paper we present a nondestructive evaluation technique based on the propagation of highly nonlinear solitary waves (HNSWs) to monitor the hydration of cement. HNSWs are mechanical waves that can form and travel in highly nonlinear systems, such as one-dimensional chains of contacting spherical particles (i.e., granular crystals). In the present study, we use a granular crystal-based actuator/sensor to observe the solitary waves propagating to and from the mechanical interface between the transducer and a fresh gypsum cement sample. We hypothesize that the reflected HNSWs traveling along the crystal-based transducer are affected by the hydration process of the cement, and we assess the elastic modulus of the specimen in the localized region of the granular crystal contact. To verify the experimental results, we perform numerical simulations based on a simplified finite element model. The elastic properties of the cement specimen measured by the granular crystal transducer are compared with the compressive strength and the elastic modulus measurements obtained from destructive tests, conducted according to the ASTM C109. We observe good agreement between experiments and numerical simulations. [Copyright &y& Elsevier]

Metamaterials are a class of engineered materials that often violate the routine assumptions that apply to ordinary materials. While metamaterials are typically assembled from carefully designed mesoscale units, the intended bulk-scale functionality can be obscured by unintended emergent effects driven by non-additive unit-unit interactions. These interactions are often sensitive to the number of units and their overall arrangement, making them extrinsic to the unit-scale design. As such, the emergence of extrinsic effects adds a significant hurdle for the development of general-purpose metamaterial technologies. Here, we reconceptualize bulk-scale extrinsic properties as a design opportunity and develop an approach that repurposes them in a new class of exponentially reconfigurable origami-inspired mechanical metamaterials. We illustrate the use of extrinsic properties to design a single general-purpose structure that can be transformed into a variety of passive mechanical devices including a waveguide, a wave lens, and a wave cloak. Bench-top experiments validate the core concepts of our approach and show how unintentional extrinsic effects become useful for applications of reconfigurable metamaterials. Unlabelled Image • Modular metamaterials are proposed with an exponentially large number of configurations. • Each unit cell can switch into 4 patterns with multiple acoustic properties. • Cellular metamaterial can be in situ set as wave filter, guide, lens and cloak. • Extrinsic effect is reconceptualized as a design opportunity to form reconfigurable metamaterials. [ABSTRACT FROM AUTHOR]

We investigate the reconfigurability and tunability of the tessellation of Tachi-Miura Polyhedron (TMP), an origami-based cellular structure composed of bellows-like unit cells. Lattice-based three-dimensional mechanical metamaterials have recently received significant scientific interest due to their superior and unique mechanical performance compared to conventional materials. However, it is often challenging to achieve tunability and reconfigurability from these metamaterials, since their geometry and functionality tend to be pre-determined in the design and fabrication stage. Here, we utilize TMP's highly versatile phase-transforming and tessellating capabilities to design reconfigurable metamaterial architecture with tunable mechanical properties. The theoretical analyses and experiments with heat processing discover the wide range of the in-situ tunability of the metamaterial – specifically orders of magnitude change in effective density, Young's modulus, and Poisson's ratio – after its fabrication within the elastic deformation regime. We also witness a rather unique behavior of the inverse correlation between effective density and stiffness. This mechanical platform paves the way to design the metamaterial that can actively adapt to various external environments. • A post-fabrication tuning method for origami-based mechanical metamaterials is demonstrated. • A tessellation of Tachi-Miura Polyhedron is manufactured with an efficient method inspired by honeycomb structures. • Heat-processing method is leveraged to achieve the post-fabrication tuning, accompanied by theoretical analyses. • The result shows the wide range of in-situ tunability in effective density, Young's modulus, and Poisson's ratio. [ABSTRACT FROM AUTHOR]

[Display omitted] • A graph-based method is proposed to assess reconfigurability in origami metamaterial. • Tachi-Miura Polyhedron tessellations have an abundance of reconfigurability. • The heterogeneity and connectivity of unit cells contribute to reconfigurability. • This method contributes to designing origam metamaterial with tailored properties. Origami-based mechanical metamaterials have recently received significant scientific interest due to their versatile and reconfigurable architectures. However, it is often challenging to account for all possible geometrical configurations of the origami assembly when each origami cell can take multiple phases. Here, we investigate the reconfigurability of a tessellation of origami-based cellular structures composed of bellows-like unit cells, specifically Tachi-Miura Polyhedron (TMP). One of the unique features of the TMP is that a single cell can take four different phases in a rigid foldable manner. Therefore, the TMP tessellation can achieve various shapes out of one original assembly. To assess the geometrical validity of the astronomical number of origami phase combinations, we build a graph-theoretic framework to describe the connectivity of unit cells and to analyze the reconfigurability of the tessellations. Our approach can pave the way to develop a systematic computational tool to design origami-based mechanical metamaterials with tailored properties. [ABSTRACT FROM AUTHOR]

Silicon carbide (SiC) fibers can act not only as a load-bearing component, but also as a piezoresistive and microwave lossy material when embedded in fiber-reinforced polymer composites. By leveraging these multifunctional characteristics, we propose simultaneous self-sensing and microwave-absorbing composite structures based on embedded SiC fiber networks. A SiC fiber network consists of SiC fiber yarns embedded in each interlaminar of the host composite at regular intervals. The SiC fiber yarns, which do not come into contact with other SiC fiber yarns, act as independent sensing elements using their piezoresistivity. By positioning the SiC fiber yarns at regular intervals within the wavelength of the targeted microwave-absorption bandwidth, the SiC fiber network functions as a microwave lossy material. In this study, specific focus has been placed on designing a multi-functional composite that senses external impacts and also absorbs microwaves in the X-band (8.2–12.4 GHz). The simultaneous self-sensing and microwave-absorbing performance of the fabricated SiC-embedded composite was experimentally verified through microwave return loss measurements and dynamic signal acquisition tests. The proposed SiC fiber network-embedded composite structure has great potential for use in future aerospace structures. [ABSTRACT FROM AUTHOR]

Pyrotechnic separation devices have been widely used in various missions of spacecraft in order to separate the structural parts with high reliability. Although they afford advantages of cost-effectiveness and high reliability, pyrotechnic separation devices generate an intensive dynamic response called pyroshock, which can lead to fatal damage to the mounted electronic equipment. Previous studies have attempted to resolve these issues with a shock isolator. However, these studies have limitations such a possible dynamic instability in low frequency and complexity in design. In the present study, a novel design of the sandwich insert is proposed to achieve enhanced shock attenuation without a shock isolator. Static-load tests were conducted to validate the structural performance of the shock-absorbing insert. In order to observe the attenuation performance, shock propagation and response experiments were carried out with consideration that the inserts were utilized to connect parts and to mount devices on the panel. In addition, swept sine vibration tests were carried out to investigate the dynamic response in low frequency. The shock-absorbing insert reduced propagating shock for the joint structure, and the shock response of the mounted equipment was significantly attenuated. The novel shock-absorbing insert saves space and is dynamically stable, and furthermore has the ability to protect the small electronic equipment, which is mounted on the honeycomb sandwich panel, from severe shock. These advantages make it a suitable insert for space structure application for connecting parts or mounting equipment. [ABSTRACT FROM AUTHOR]

Deployable structures are typically made of thin membranes and slender elements, which often require foldable – yet stiff – mechanical properties. The use of carbon fiber reinforced polymer (CFRP) composites for such deployable structures has been limited due to the their rigid and unfoldable nature in general. Here, we design, fabricate, and demonstrate foldable – yet stiff – structures made of CFRP composites. To achieve this, we leverage origami design principles based on the Tachi-Miura-Polyhedron (TMP) architecture. To manufacture TMP structures, we devise a unique vacuum-bag-only composite fabrication method by using compliant urethane epoxies impregnated into woven glass fiber layers on which pre-made CFRP tiles are positioned. We show the resulting structures feature self-deployability, high compactness, and deterministic force–displacement characterization. Potential applications of the proposed composite origami are abundant, including deployable habitats for space exploration and disaster relief, deployable solar arrays and antennas, actively-controlled aerodynamic surfaces, and impact mitigation structures. [ABSTRACT FROM AUTHOR]