Extreme Mechanics Letters

Print ISSN: 2352-4316
Publications
We have designed a new experimental setup able to investigate fracture of soft materials at small scales. At high crack velocity, where energy is mostly dissipated through viscoelastic processes, we observe an increasingly large high strain domain in the crack tip vicinity. Taking advantage of our ability to determine where linear elasticity breaks down, we derive a simple prediction for the evolution of the energy release rate with the crack velocity.
 
We report a high-throughput method that enables us to automatically compute the vibrational spectra of more than 100,000 proteins available in the Protein Data Bank to date, in a consistent manner. Using this new algorithm we report a comprehensive database of the normal mode frequencies of all known protein structures, which has not been available before. We then use the resulting frequency spectra of the proteins to generate audible sound by overlaying the molecular vibrations and translating them to the audible frequency range using the music theoretic concept of transpositional equivalence. The method, implemented as a Max audio device for use in a digital audio workstation (DAW), provides unparalleled insights into the rich vibrational signatures of protein structures, and offers a new way for creative expression by using it as a new type of musical instrument. This musical instrument is fully defined by the vibrational feature of almost all known protein structures, making it fundamentally different from all the traditional instruments that are limited by the material properties of a few types of conventional engineering materials, such as wood, metals or polymers.
 
FREE PRE-PRINT: http://arxiv.org/pdf/1709.07714 __________ABSTRACT: Alloyed MCrAlY bond coats, where M is usually cobalt and/or nickel, are essential parts of modern turbine blades, imparting environmental resistance while mediating thermal expansivity differences. Nanoindentation allows the determination of their properties without the complexities of traditional mechanical tests, but was not previously possible near turbine operating temperatures. Here, we determine the hardness and modulus of CMSX-4 and an Amdry-386 bond coat by nanoindentation up to 1000 °C. Both materials exhibit a constant hardness until 400 °C followed by considerable softening, which in CMSX-4 is attributed to the multiple slip systems operating underneath a Berkovich indenter. The creep behaviour has been investigated via the nanoindentation hold segments. Above 700 °C, the observed creep exponents match the temperature-dependence of literature values in CMSX-4. In Amdry-386, nanoindentation produces creep exponents very close to literature data, implying high-temperature nanoindentation may be powerful in characterising these coatings and providing inputs for material, model and process optimisations.
 
Adiabatic shear band (ASB) is widely observed in metals and alloys under impact loading as a significant failure mechanism. Exhibiting severe localization over a short period of time, ASB imposes great challenges in characterizing the evolution of the local deformation field. Most existing theoretical models employed the assumption of laminar-flow-like shear deformation inside the band, while non-uniform “hot spots” have also been observed along a propagating shear band. In this study, the evolution of the local deformation field inside the ASB was investigated based on the digital image correlation (DIC) technique with micro-speckles. Destructive micro-speckles were created using laser etching within the shearing region to resolve localized deformation. Based on experimental results of 1018 steel specimen under impact loading, ASB evolution along the shearing plane was divided into two distinct stages: in the first stage, the deformation localized almost uniformly along the band, which is consistent with the laminar flow assumption; whereas in the second stage, the deformation was highly non-uniform and exhibited distributed strain-localizations, which could be closely associated with hot-spot formation and should be taken into account especially during the post-localization stage. Our study revealed the existence of “hot spots” based on a direct measurement of the deformation field, and shed more light on the understanding of the entire evolution process of ASB formation.
 
Carbon fiber reinforced silicon carbide (C/SiC) composites usually serve in ultra-high-temperature extreme environments and are subjected to thermal-mechanical-oxidation coupled loads. However, the mechanical properties of C/SiC composites reported in the literature are mainly limited to room and moderate temperatures. In this work, the tensile properties of a two-dimensional plain-weave C/SiC composite are studied in air up to 1800 °C for the first time. C/SiC composite shows linear deformation characteristic initially and then strong nonlinear deformation behavior under the tensile load at room temperature. This is because that the initial defects and cracks propagate and result in the interfacial debonding and sliding. The nonlinear deformation behavior decreases as temperature increases because of the reduced thermal residual stresses. The Young’s modulus increases up to 1000 °C and then decreases as temperature increases. The tensile strength shows weaker temperature dependence than Young’s modulus because that oxidation reduces the strength at elevated temperatures. The failure mechanisms being responsible for the mechanical behavior are gained through macro and micro analysis. The results are useful for the applications of C/SiC composites in the thermal structure engineering.
 
Due to the lack of therapeutics and vaccines, diagnostics of COVID-19 emerges as one of the primary tools for controlling the spread of SARS-COV-2. Here we aim to develop a theoretical model to study the detection process of SARS-COV-2 in lateral flow device (LFD), which can achieve rapid antigen diagnostic tests. The LFD is modeled as the adhesion of a spherical nanoparticle (NP) coated with ligands on the surface, mimicking the SARS-COV-2, on an infinite substrate distributed with receptors under a simple shear flow. The adhesive behaviors of NPs in the LFD are governed by the ligand receptor binding (LRB) and local hydrodynamics. Through energy balance analysis, three types of motion are predicted: (i) firm-adhesion (FA); (ii) adhesive-rolling (AR); and (iii) free-rolling (FR), which correspond to LRB-dominated, LRB-hydrodynamics-competed, and hydrodynamics-dominated regimes, respectively. The transitions of FA-to-AR and AR-to-FR are found to be triggered by overcoming LRB barrier and saturation of LRB torque, respectively. Most importantly, in the AR regime, the smaller NPs can move faster than their larger counterparts, induced by the LRB effect that depends on the radius R of NPs. In addition, a scaling law is found in the AR regime that v∝γ̇Rα (rolling velocity v and shear rate γ̇), with an approximate scaling factor α∼−0.2±0.05 identified through fitting both theoretical and numerical results. The scaling factor emerges from the energy-based stochastic LRB model, and is confirmed to be universal by examining selections of different LRB model parameters. This size-dependent rolling behavior under the control of flow strength may provide the theoretical guidance for designing efficient LFD in detecting infectious disease.
 
Coronavirus Disease 2019 (COVID-19) may spread through respiratory droplets released by infected individuals during coughing, sneezing, or speaking. Given the limited supply of professional respirators and face masks, the U.S. Centers for Disease Control and Prevention (CDC) has recommended home-made cloth face coverings for use by the general public. While there have been several studies on aerosol filtration performance of household fabrics, their effectiveness at blocking larger droplets has not been investigated. Here, we ascertained the performance of 11 common household fabrics at blocking large, high-velocity droplets, using a commercial medical mask as a benchmark. We also assessed the breathability (air permeability), texture, fiber composition, and water absorption properties of the fabrics. We found that most fabrics have substantial blocking efficiency (median values >70%). In particular, two layers of highly permeable fabric, such as T-shirt cloth, blocks droplets with an efficiency (>94%) similar to that of medical masks, while being approximately twice as breathable. The first layer allows about 17% of the droplet volume to transmit, but it significantly reduces their velocity. This allows the second layer to trap the transmitted droplets resulting in high blocking efficacy. Overall, our study suggests that cloth face coverings, especially with multiple layers, may help reduce droplet transmission of respiratory infections. Furthermore, face coverings made from materials such as cotton fabrics allow washing and reusing, and can help reduce the adverse environmental effects of widespread use of commercial disposable and non-biodegradable facemasks.
 
Understanding the outbreak dynamics of COVID-19 through the lens of mathematical models is an elusive but significant goal. Within only half a year, the COVID-19 pandemic has resulted in more than 19 million reported cases across 188 countries with more than 700,000 deaths worldwide. Unlike any other disease in history, COVID-19 has generated an unprecedented volume of data, well documented, continuously updated, and broadly available to the general public. Yet, the precise role of mathematical modeling in providing quantitative insight into the COVID-19 pandemic remains a topic of ongoing debate. Here we discuss the lessons learned from six month of modeling COVID-19. We highlight the early success of classical models for infectious diseases and show why these models fail to predict the current outbreak dynamics of COVID-19. We illustrate how data-driven modeling can integrate classical epidemiology modeling and machine learning to infer critical disease parameters—in real time—from reported case data to make informed predictions and guide political decision making. We critically discuss questions that these models can and cannot answer and showcase controversial decisions around the early outbreak dynamics, outbreak control, and exit strategies. We anticipate that this summary will stimulate discussion within the modeling community and help provide guidelines for robust mathematical models to understand and manage the COVID-19 pandemic. EML webinar speakers, videos, and overviews are updated at https://imechanica.org/node/24098.
 
One-dimensional (1D) granular crystals are demonstrated to be outstanding nonlinear systems for supporting various types of novel stress waves. This letter reports on the investigation of solitary wave propagation within 1D granular crystals based on composite cylinders. We design two types of composite particles by creating a material mismatch within the granular, i.e., core–shell and sandwich types. We discover that such 1D composite granular chains support the formation of strongly nonlinear solitary waves. A finite element model is constructed to describe wave propagation behaviors fully validated by experiments. We conduct a theoretical analysis that agrees with numerical results and uncovers the physical mechanisms of solitary wave propagation in 1D composite granular chains through the parametric study. Finally, the fundamental understanding of dynamic responses is extended to more generalized composite granular chains with sandwich configurations. Results provide in-depth physical understanding and engineering design guidance to quantitatively tailor wave properties through simple 1D granular structures.
 
Transition metal dichalcogenides (TMD) are currently among the most interesting two-dimensional (2D) materials due to their outstanding properties. MoTe2 involves attractive polymorphic TMD crystals which can exist in three different 2D atomic lattices of 2H, 1T and 1T', with diverse properties, like semiconducting and metallic electronic characters. Using the polymorphic heteroepitaxy, most recently coplanar semiconductor/metal (2H/1T') few-layer MoTe2 heterostructures were experimentally synthesized, highly promising to build circuit components for next generation nanoelectronics. Motivated by the recent experimental advances, we conducted first-principles calculations to explore the mechanical properties of single-layer MoTe2 structures. We first studied the mechanical responses of pristine and single-layer 2H-, 1T- and 1T'-MoTe2. In these cases we particularly analyzed the possibility of engineering of the electronic properties of these attractive 2D structures using the biaxial or uniaxial tensile loadings. Finally, the mechanical-failure responses of 1T'/2H-MoTe2 heterostructure were explored, which confirms the remarkable strength of this novel 2D system.
 
In the present paper, three kinds of novel two-dimensional enhanced star-shaped honeycombs (2D-ESSH) are proposed, which are consisted of the two-dimensional star-shaped honeycomb (2D-SSH) and reinforcing rods. Relative densities of the 2D-SSH and three kinds of 2D-ESSH are deduced. Mechanical properties of the proposed novel 2D-ESSH are investigated via finite-element simulations and compression experiments. The effects of the effective length, the angle of the outer cell walls, the thickness of the outer cell walls, the angle of the inner cell walls, and the thickness of the inner cell walls on the effective Poisson’s ratio and effective Young’s modulus of the structures are explored in detail. The results show that the novel 2D-ESSH can effectively regulate the effect of negative Poisson’s ratio and two kinds of the 2D-ESSH also improve the effective Young’s modulus with the appropriate parameters while ensuring the lightweight.
 
Thermal fluctuation at finite temperature is a common feature for bio-membranes and two-dimensional (2D) layered crystals due to their flexibility of bending deformation. Previous works regarding thermal fluctuations of 2D materials mainly focused on monolayer systems, the finite temperature mechanics of multilayer 2D materials was rarely explored, especially the effect interlayer shear on the thermal fluctuation of multilayer 2D materials. In this work, we introduced a multi-beam shear model based on the statistical mechanical description of multilayer graphene (MLG) to explore the effects of thermal fluctuations. Various factors are considered, such as temperature T, size L, layer number n, interlayer shear modulus G and biaxial pre-strain ɛ0 etc. Within the harmonic approximation, the average root-mean-square height due to thermal fluctuation is found to be scaled as h∝lnL, which is distinctly different from that of monolayer graphene with a linear scaling (h∝L) within the harmonic approximation. In addition, h decreases with the increase of layer number n and interlayer shear modulus G. Overall, the thermal fluctuation of MLG is bounded by two theoretical limits, i.e., the case of perfect bonding without interlayer sliding and smooth interlayer sliding without interlayer shear stress. Furthermore, the thermal fluctuations also reduce the tangent biaxial Young’s modulus of multilayer 2D materials. The systematical MD simulations for MLG are performed and validate the effectiveness of the harmonic analysis.
 
2D materials are fascinating for numerous reasons. Their geometrical and mechanical characteristics along with other associated physical properties have opened up fascinating new application avenues ranging from electronics, energy harvesting, biological systems among others. Due to the 2D nature of these materials, they are known for their unusual flexibility and the ability to sustain large curvature deformations. Further, they undergo noticeable thermal fluctuations at room temperature. In this perspective, we highlight both the characteristics and implications of thermal fluctuations in 2D materials and discuss current challenges in the context of statistical mechanics of fluid and solid membranes.
 
To characterize the effects of free surfaces on dislocation mobility, the edge dislocation glide process in thin silicon films is modeled using an interatomic potential and first principles calculations. The influence of film thickness is determined, starting with the simulation of a silicene monolayer and then increasing the number of layers. The energy barrier for dislocation glide in silicene was calculated to be 1.5 eV, indicating a relatively high mobility of dislocation defects. In thin Si films a glide mechanism via consecutive bond rotations was identified, with kink nucleation being observed at the free surfaces of the film with subsequent migration. The influence of the free edge in finite size films is shown to be negligible in relation to glide for dislocations at distances from the free edge larger than three Burgers vectors. Molecular dynamics simulations reveal the possibility of more complex but lower energy barrier atomic reconstructions triggered at the free surfaces of the film near the dislocation core that may increase dislocation mobility at higher temperatures.
 
Layered 2D materials consist of a stack of 2D materials interacting weakly via van der Walls (vdW) forces. The vdW interactions are modeled using long-range interlayer potentials, such as Lennard-Jones (LJ) and Kolmogorov–Crespi (KC), that are expensive to compute in atomistic and multiscale simulations. We propose a discrete–continuum (DC) effective potential for LJ and KC that includes full atomistic resolution in the short range and a continuum integral approximation outside. This reduces the computational cost by two orders of magnitude while retaining excellent accuracy. With this method, we examine the relation between stacking states and interlayer energy of a graphene bilayer under relative in-plane translations and rotations. We compare the performance of LJ and KC and characterize the moiré patterns under rotation. We find that the energy of an infinite graphene bilayer is constant with rotation (with discontinuities at special angles) and explain this in terms of the moiré pattern supercell. We also find that the saddle point (SP) stacking configuration becomes the stable ground state (as opposed to AB stacking) when the bilayer is compressed.
 
Since the first successful synthesis of graphene just over a decade ago, a variety of two-dimensional (2D) materials (e.g., transition metal-dichalcogenides, hexagonal boron-nitride, etc.) have been discovered. Among the many unique and attractive properties of 2D materials, mechanical properties play important roles in manufacturing, integration and performance for their potential applications. Mechanics is indispensable in the study of mechanical properties, both experimentally and theoretically. The coupling between the mechanical and other physical properties (thermal, electronic, optical) is also of great interest in exploring novel applications, where mechanics has to be combined with condensed matter physics to establish a scalable theoretical framework. Moreover, mechanical interactions between 2D materials and various substrate materials are essential for integrated device applications of 2D materials, for which the mechanics of interfaces (adhesion and friction) has to be developed for the 2D materials. Here we review recent theoretical and experimental works related to mechanics and mechanical properties of 2D materials. While graphene is the most studied 2D material to date, we expect continual growth of interest in the mechanics of other 2D materials beyond graphene.
 
During charge and discharge of the lithium-ion batteries, deformation in the graphite-based electrode can induce high local strains and severe mechanical degradation. However, little quantitative method are available to help understand the mechanisms of deformation evolution at the microscopic scales. This work reports a combined method via in-situ scanning electron microscopy and digital image correlation technology to characterize the two-dimensional displacement and strain fields of the electrode throughout operation. It is found that 50% irreversible swelling in the initial cycle and reversible anisotropic deformation in the following cycles. The average strain in the vertical direction is 4.94%, which is about 18 times higher than that in the horizontal direction of electrode. This new combined method based on in-situ scanning electron microscopy and digital image correlation can be used to quantify the evolution of the displacement and stain fields in the electrode.
 
Mechanical peeling is a well-known route to transfer a single piece of two-dimensional (2D) materials from one substrate to another one, yet heavily relies on trial and error methods. In this work, we propose a liquid-assisted, etching-free, mechanical peeling technique of 2D materials and systemically conduct a theoretical study of the peeling mechanics for various 2D materials and substrates in a liquid environment. The surface wettability of 2D materials and substrates and surface tension of liquids have been incorporated into the peeling theory to predict the peel-off force. The theoretical model shows that the peel-off force can be significantly affected by liquid solvents in comparison with that in dry conditions. Moreover, our analysis reveals that the mechanical peeling-induced selective interface delamination in multilayered 2D materials can be achieved by employing a liquid environment. These theoretical results and demonstrations have been extensively confirmed by comprehensive molecular dynamics simulations and good agreement is obtained between them. The present work in theory provides a new approach of peeling 2D materials from substrates and can also be extended for peeling thin films and membranes.
 
It is known that metamaterials with negative Poisson’s ratio (NPR) can be designed to possess negative thermal expansion (NTE) property. However, it is uncommon to use NTE structures to obtain NPR behavior. In this paper, based on a typical NTE unit structure, two novel meta-structures with auxetic behavior are proposed, which are demonstrated by the micro-scale mechanical experiments and numerical simulations. By changing the angle of the structure, the Poisson’s ratio of the structures could be positive, zero and negative. The extreme negative Poisson’s ratio of the structures is about -0.94. Moreover, the thermal expansion of the novel 2D structures can also be designed and modulated to be positive, zero and negative. This concept of building 2D metamaterials with NPR in this study would open up new avenues to understand and design metamaterials with NPR and NTE for potential engineering applications, such as smart sensors and electronic components.
 
2-dimensional (2D) random fiber networks are extremely thin plate-like structures constructed by randomly distributed fibers, and have been widely used in many fields from nano to macro scales due to their excellent properties. To make a comprehensive understanding on their in-plane and especially out-of-plane elastic properties, a mechanics model is established and analytical expressions for the in-plane and out-of-plane stiffnesses are derived based on micromechanics analysis. It is revealed that the fiber networks carry in-plane loads by axial stretching/compression of the fibers in the networks and resist out-of-plane loading by bending and torsion of the fibers. Due to the discreteness and randomness of the networks, the in-plane and out-of-plane stiffnesses present a non-classical relation, which can be described by a newly proposed formula. This work gives an insight into the understanding and designing of the mechanical properties of discrete materials/structures ranging from nano scale to macro scale.
 
Two-dimensional (2D)materials have been attracting numerous research attention due to their distinctive physical properties and boundless application potential in various fields. Among diverse physical properties, the mechanical property is the most basic one and plays a crucial role in ensuring the high reliability of 2D material-based devices and products. However, characterizing the mechanical properties of 2D materials is always a challenge due to their atomic thickness. Here, we propose a facile method to decipher the mechanical property of 2D materials from the statistical distribution of the size of the fragments acquired via mechanical exfoliation. This method is essentially based on a probabilistic mechanics model correlating the distribution pattern of fragment size and the intrinsic mechanical properties of 2D materials. The ensuing experimental verifications on both graphene and 2H-MoS 2 show good agreement between our measurements and the results reported in literature. This work not only provides a facile method for characterizing the mechanical properties of 2D materials, but also implies approaches to attaining 2D material fragments with controllable size via mechanical exfoliation.
 
The plasticity and ductile fracture behavior of stainless steel 304L fabricated by laser powder bed fusion additive manufacturing was investigated under both uniaxial and multiaxial loading conditions through the use of specialized geometry mechanical test specimens. Material anisotropy was probed through the extraction of samples in two orthogonal material directions. The experimentally measured plasticity behavior was found to be anisotropic and stress state dependent. An anisotropic Hill48 plasticity model, calibrated using experimental data, was able to accurately capture this behavior. A combined experimental-computational approach was used to quantify the ductile fracture behavior, considering both damage initiation and final fracture. An anisotropic Hosford-Coulomb model was used to capture the anisotropic and stress state dependent fracture behavior.
 
Honeycomb structures have significant advantages in load-bearing and energy absorption. In recent years, some researches have been carried out on the in-plane mechanical properties of honeycombs with variable-thickness cell edges, but the out-of-plane compressive properties of this type of honeycombs have not been well studied. In this paper, the out-of-plane compressive properties of honeycombs with variable-thickness cell edges are investigated through experimental analysis. Here square and hexagonal honeycombs with variable-thickness cells are described with one geometric parameter. The experimental samples of these honeycombs were fabricated using 3D printing technology, and the quasi-static compression tests of these honeycombs were conducted. Experimental results illustrate that the honeycombs with variable-thickness cell edges show enhanced compressive mechanical properties compared to the conventional honeycombs. The highest increase rates of compressive strength of the presented square and hexagonal honeycombs are around 57% and 19%, respectively. In addition, the highest increase rate of specific energy absorption of these square honeycombs reaches up to 172%. The deformation and failure modes of square and hexagonal honeycombs with different geometric parameter values are also discussed and compared. Experimental results show that the square honeycombs with appropriate geometric parameter can achieve more desirable damage tolerance than the conventional square honeycombs.
 
Wrinkling of thin films in micro- and nanoscale has found wide applications in stretchable electronics, optical gratings, material characterization, and biology. However, most wrinkling work is in planar fashion and only limited work on 3D wrinkling has been reported. In this paper, we present experimental demonstration of 3D Cr film wrinkling on PDMS micro-ridges induced by strain mismatch. We first fabricated PDMS micro-ridges using replication from a silicon mold, prepared by conventional microfabrication processes. While pre-stretching the PDMS specimen along the ridge direction, thin Cr film was deposited through physical sputtering. After releasing the prestrain, two wrinkling waves formed on the two inclined surfaces of the PDMS ridges with the same wavelength but opposite phases. These two wrinkling waves twist the top of the ridge into a wavy shape, forming wrinkling waves oscillating in the direction parallel to the substrate plane. In contrast, 2D wrinkling waves oscillate in the direction perpendicular to the substrate plane. Using finite element simulations, we demonstrate that the 180° phase shift between the two sides is favored by the dominant buckling modes. While the 3D wrinkling wavelength is consistent with 2D theoretical prediction, wrinkles on the ridge feature non-uniform wrinkling wave amplitude along the width of the inclined surface. This 3D wrinkling work opens new opportunities in manufacturing curved-shape mold for contact printing and increases surface to volume ratio for wetting and battery applications.
 
Buckled surfaces induced by mismatched swelling and elastic properties of materials are commonly observed in nature, such as on cacti and euphorbias. The rational design and mimicry of such buckling surfaces could lead to the development of smart, adaptive, and stimuli responsive devices. We designed a 3D printed tubular structure, composed of soft swellable poly N-isopropylacrylamide (pNIPAM) segments and stiff non-swellable polyacrylamide (pAAM) segments. Similar to the shape change of Saguaro stems after rainfall, the tubes show tunable periodic buckling modes in water at the room temperature. The buckling behavior was harnessed through the development of compressive stresses in the soft swellable segments induced by the constraint of the stiff non-swellable segments. We developed a finite element model to explore the design space of this periodic buckling behavior for the tube, and used a chemo-mechanically coupled constitutive model to describe the swellable hydrogel. Inspired by the classic bar buckling problem, we constructed a phase diagram and discovered a universal design parameter that combines the effects of geometric and material properties to guide the design of periodic buckling tubes for bioinspired functional gel structures.
 
Mechanical metamaterials/meta-structures with bistability/multi-stability are of great potential use in engineering applications ranging from shape reconfiguration to impact mitigation. Current designs are mainly based on typical 2D bistable components with two stable states, thus having limited deformation amplitude and stiffness tunability. Here in this paper we propose an innovative 3D modular bistable meta-structure based on two different types of mechanisms with partially compatible motion ranges, which have two distinct ranges of low-stiffness mechanism motion where the two based-mechanisms are kinematically compatible, separated by high-stiffness structural deformation due to the incompatibility between the motion of base-mechanisms. By setting up the mechanism kinematics model and structure mechanics model, we are capable of programming the detailed ranges of mechanism motion and structural deformation, even to obtain a stable state for the meta-structure, while the stiffness transition can also be fine-tuned by adjusting the geometric parameters and hinge stiffness. This work is expected to open a new revenue for the design of 3D modular metamaterials with customized multi-stability.
 
Loosening of the glenoid component is the most common cause of failure of total shoulder arthroplasty. While the underlying mechanisms are not fully understood, mechanical factors are widely reported to play a key role in glenoid component loosening. In this study, mechanical testing coupled with micro X-ray computed tomography (micro-CT) is performed to apply various physiologically realistic loads on a native and implanted glenoid. Digital volume correlation of micro-CT images is used to compute the 3D full-field deformation and strain inside the glenoid. The measured strain distributions are in good agreement with the analytical solutions of beam bending models, especially for anteriorly and posteriorly eccentric loadings. The effective moduli of the overall native and implanted glenoid were similar. However, under the same eccentric loading conditions, implanted glenoid exhibited a wider range of strain, because the placement of glenoid component increases the bending moment inside the glenoid. This proof-of-concept study provides a feasible and powerful method for the study of 3D full-field biomechanics in native and implanted glenoids.
 
Mechanical impact protection is an important consideration in many applications, ranging from product transportation to sports. Cellular materials are typically used due to their desirable energy absorption properties and light weight. However, their large deformation and rate dependent responses (especially of polymer foams) are challenging to consider in design. Additionally, the use of foams with uniform properties, such as uniform density and uniform stiffness, often restricts the designed foams to only be suitable for a narrow range of mechanical impact conditions whereas real applications commonly face unpredictable situations. 3D printing offers fabrication flexibility and thus opens the door to create foams with tailored properties. In this work, we investigate the feasibility of using 3D printing for functionally graded foams (FGFs) that are optimal over a broad range of mechanical environments. The foams are fabricated by the recently developed grayscale digital light processing (g-DLP) method which can print parts with locally designed properties. These foams are tested under both drop test conditions and with slower displacement control. We also model the large deformation behavior of FGFs using finite element analysis in which we account for the different viscoelastic behaviors of the distinct grayscale regions. We then use the model to examine the impact mitigation capabilities of FGFs in different loading scenarios. Finally, we show how FGFs can be used to satisfy real-world design goals using the case study of a motorcycle helmet. In contrast to prior work, we investigate continuous, functionally graded foams of a single density that differ in their viscoelastic responses. This work provides further insight into the benefits of viscoelastic properties and modulus graded foams and presents a manufacturing approach that can be used to produce the next generation of flexible lattice foams as mechanical absorbers.
 
Our daily experiences suggest that friction would increase when a sliding interface is pressed harder. At the nanoscale, however, recent experiments showed that friction on chemically modified graphite can decrease with increasing normal load due to atomic interlayer delamination. In this paper, we report that, through rational design of surface components, the friction of a macroscopic surface can also counterintuitively decrease with increasing normal load, resulting in a so-called macroscopic negative friction coefficient. This unusual feature is enabled by the coupling of contact pressure and deformation of the microstructured surface, which is achieved via two distinct microscale architectures, referred to as ‘lollipop’ and inverted ‘Y’ structures respectively. This work offers a novel strategy for designing meta-surfaces that possess unusual tribological properties, which may eventually lead to revolutionary engineering applications.
 
Antenna, which is the essential electric component for wireless signal communication, requires one fixed electrical size for the stable working frequency. However, the frequency of the existing flexible antenna varies with its changing deformation, which leads to the loss of data packet or even transmission failure. Hence the fixed size of the existing flexible antenna is severely inconsistent with the deformation of flexible substrate. Here we propose a strategy to design and fabricate stretchable antennas with robust frequency based on three dimensional (3D) Kirigami, which is lightweight and deformation insensitive. With Kirigami design and assembling in 3D style, the radiation element of the antenna is mostly detached and strain-isolated from the soft substrate. Thus, not only bending, stretching and, compressing, but also the random dynamic disturbance deviates the resonant frequency little from the designed frequency in the wind disturbing experiments, as shown in the wind experiments It exhibits a superior strain insensitivity compared with the stretchable antennas ever reported. With mechanics guided design, the antenna’s property remains stable after 5000 cycles bending deformation in the fatigue tests. Besides, an omnidirectionally stretchable and fully integrated device featured Bluetooth wireless communication enabled by 3D Kirigami antenna (2.45 GHz) has been designed and fabricated. Results show that the stretchable antenna strategy based on 3D Kirigami antenna is robust for wearable electronics’ wireless transmission.
 
Three-dimensional (3D) mesostructures that can reversibly change their geometries and thereby their functionalities are promising for a wide range of applications. Despite intensive studies, the lack of fundamental understanding of the highly nonlinear multistable states existing in these structures has significantly hindered the development of reconfigurable systems that can realize rapid, well-controlled shape changes. Herein we exploit systematic energy landscape analysis of deformable 3D mesostructures to tailor their multistable states and least energy reconfiguration paths. We employ a discrete shell model and minimum energy pathway methods to establish design phase diagrams for a controlled number of stable states and their energy-efficient reconfiguration paths by varying essential geometry and material parameters. Concurrently, our experiments show that 3D mesostructures assembled from ferromagnetic composite thin films of diverse geometries can be rapidly reconfigured among their multistable states in a remote, on-demand fashion by using a portable magnet, with the configuration of each stable state well maintained after the removal of the external magnetic field. The number of stable states and reconfigurable paths observed in experiments are in excellent agreement with computational predictions. In addition, we demonstrate a wide breadth of applications including reconfigurable 3D light emitting systems, remotely-controlled release of particles from a multistable structure, and 3D structure arrays that can form desired patterns following the written path of a magnetic “pen”. Our results represent a critical step towards the rational design and development of reconfigurable structures for applications including soft robotics, multifunctional deployable devices, and many others.
 
Metamaterials with compression-induced-twisting (CIT)features shows great potential applications in sensors and actuators. In this paper, a series of 3D metamaterials are developed with the inspiration of the shear–compression coupling effect of the 2D materials, which exhibits twisting behavior when subjected to uniaxial loading. Analytical solutions and numerical simulations are both carried out to demonstrate the possible CIT performance of the proposed 3D metamaterial models, with good agreement obtained. A linear relationship between the 2D shear–compression coupling effect and the twist angle per axial strain of the 3D structures is also deduced. In addition, the twist angle per axial strain can be designed to be infinite. This concept of CIT metamaterials in this paper might shed light on the design and optimization of 3D architectures with multifunctional applications.
 
Inspired by the hierarchical/fractal topological interlocking innature, Koch fractal interlocking with different numbers of iteration N are designed. To better understand the mechanics of fractal interlocking, the designs are also fabricated via a multi-material 3D printer. Mechanical experiments and finite element (FE) simulations are performed to further explore the mechanical performance of the new designs. Analytical model is also developed to capture the deformation mechanisms of the fractal interlocking through contact. It is found that the load-bearing capacity of Koch fractal interlocking can be effectively increased via fractal design. However, the mechanical responses of fractal interlocks are also sensitive to imperfections, such as the gap between the interlocked pieces and the rounded tips. When fractal complexity increases, the mechanical properties will become more and more sensitive to the imperfection and eventually, the influences from imperfection can even become dominant. By considering the influences of imperfection, the theoretical model predicts the existence of an optimal level of fractal complexity for maximizing mechanical performance.
 
Use of nanoscale architecturing that exhibits superior strength-to-weight ratio has been of recent interest, and here we demonstrate a bulk fabrication of a fully recoverable, ultralight 3D porous composite with high strength composed of Ag nanowire/cellulose nanofiber using a freeze-casting method. A one-step process for highly efficient bulk formation of 3D porous structure via ice crystal formation followed by sublimation was demonstrated that can overcome the cost and scalability associated with lithography methods. 3D porous composite with controlled geometry was fabricated by controlling the nucleation and growth kinetics of ice formation that resulted in highly anisotropic compressive strength and resilience that depends on the wall orientations and composition of composite wall. Strength of the 3D porous composite increased with an increase in the Ag nanowire content as the deformation transitioned from bending dominant toward stretch dominant behavior for the case of vertically aligned walls, and an optimized concentration of Ag nanowires resulted in the compressive strength of 100 kPa at a relative density of 0.96%, which has 1.5 times higher strength when normalized by the material density (2.3 ± 0.2 MPa⋅cm³/g) in comparison to that of metal microlattice (1.7 MPa⋅cm³/g) fabricated by lithography methods. Horizontally oriented 3D porous structure interestingly showed a fully reversible deformation while also maintaining sufficient conductivity that makes this new material well-suited for a variety of flexible electronics applications.
 
(a) Schematic of plate embedded with meta-slab for focusing incident flexural wave. (b) Enlarged view of the ellipse zone in (a). (c) Two-dimensional model of a shuttle-like subunit. (d) Enlarged view of the subunit in (c). The subunit is divided into discrete stepped beams for deriving the transmission coefficient and phase shifts.
(a) Transmission coefficient and (b) phase shift difference versus subunit cross-section angle θ and incident frequency. (c) Transmission coefficient difference between shuttle-like and uniform subunits versus phase shift and incident frequency. (d) Equivalent thickness versus θ and incident frequency.
Schematic of broadband meta-slab for focusing incident flexural waves.
(a) Normalized displacement field of transmitted region for incident frequency of 6 kHz, (b) that for incident frequency of 7 kHz and (c) that for incident frequency of 8 kHz. (d) Normalized displacement amplitude field of transmitted region for incident frequency of 6 kHz, (e) that for incident frequency of 7 kHz and (f) that for incident frequency of 8 kHz. (g) Displacement amplitudes along the line of y=0m and (h) that along the line of x=0.03m for incident frequencies of 6 kHz, 7 kHz and 8 kHz. All analytical solutions are calculated by Eq. (5).
(a) Transient displacement fields of incident and transmitted regions for incident frequency of 6 kHz, (b) those for incident frequency of 7 kHz and (c) those for incident frequency of 8 kHz. The preset focal points and white dotted lines are marked in the transmitted regions. (d) Energy intensities in white dotted lines for incident frequencies of 6 kHz, 7 kHz and 8 kHz. (e) Focal point positions obtained by multiple measurements plotted with error bars for different incident frequencies.
Focusing of elastic waves by meta-structures is of considerable significance in energy harvesting. However, meeting both high transmission and wide bandwidth for these meta-structures is a challenge. In this paper, we propose a 3D-printed meta-slab with shuttle-like subunits with gradually varying thickness to focus incident flexural waves in broadband. Shuttle-like subunits have high transmission performance and frequency-dependent phase shifts. Based on these subunits, the meta-slab is designed by the mechanism of phase compensation, which makes the focal position unchanged with the incident frequencies. The wave fields of the transmitted region are obtained by the phased array theory and measured by the experiments. Their results are consistent and demonstrate well the focus ability of the meta-slab. Our design effectively concentrates the energy of broadband flexural waves, providing a new approach for energy harvesting.
 
Recent advances in soft magnetic nanocomposites have enabled myriads of magnetic robots with programmable shape transformation, shedding light on various biomedical applications. However, the complex shape transformation of magnetic hydrogels remains a challenge because of their ultralow magnetization and simple geometry. Here we demonstrate an approach to meet this challenge by fabricating composite structures of magnetic hydrogels and elastomers with extrusion-based 3D printing. Under an alternating magnetic field, magneto-thermo-sensitive hydrogels — poly(N-isopropylacrylamide) (PNIPAm) embedding Fe3O4 nanoparticles, can undergo abrupt volume collapse due to magnetothermal effect. The mismatch in the responsiveness of magnetic hydrogels and elastomers enables the shape transformation of the composite structure. We have printed magnetic hydrogels with various geometries and achieved complex shape transformation of the composite structure. The shape transformative structure can simultaneously encase and kill cancer cells (human malignant melanoma cells) through magnetic hyperthermia. ∼50% of cancer cells can be killed by the heated magnetic hydrogel during deformation. This approach may open opportunities for applications in medicine and bioengineering.
 
Schematic of aerosol jet printing with electrical interconnect design and process flow. (A) Schematic of aerosol jet printing where ink is atomized in the vial and carried towards the print head where ink is directly patterned on the substrate. The cross section of the meandering silver electrical interconnect embedded in PI is shown in the inset image. (B) Sintering and curing profiles for each fabrication step (1) spinning PDMS and release layer onto silicon wafer followed by (2) printing of the base layer of PI, (3) printing of silver, and (4) printing of the top layer of PI. (C) Representative SEM image of the PI-encapsulated silver interconnect printed on a PDMS platform. Bar: 50 µm. (D) Profilometer scans on a silicon substrate of a printed silver profile, and printed PI profiles with varied number of printing passes. The solid lines are the average of three scans at different locations along the trace, and the shaded envelope shows the minimum and maximum extent of the scans.
Delamination between PI layers of prototype stretchable interconnect. (A) SEM image of bonded sample with a single print pass of PI on the top and bottom layers encapsulating the silver. (B) SEM image of delaminated sample with three printing passes of PI comprising the top and bottom layers encapsulating the silver.
Thermoelastic and vapor driven delamination model. (A-B) The thermoelastic model has a multilayered stack up (A) matching that of the experimental design with the modeled delamination surface in the hashed area between the Ag and bottom PI layer. (B) Exploded free body diagrams for the layers. The interfacial forces are summed over the length from both neighboring layers to form the equivalent axial force from the assumption that the layers are thin. The interfacial forces arise from the thermal expansion differences of the materials and develop the elastic strain energy in each layer that is available to delaminate the surfaces. (C-D) The vapor driven model consists of the top and bottom layer of PI shown in (C) where the bottom layer of PI is saturated with water moisture. This water moisture is then able to diffuse to the boundary with silver and vaporize in the voids between the rough surface of the sintered silver and the PI as shown in the inset image. The area directly below the delaminated surface (purple region in C) is the volume of saturated PI available to deliver water to the crack void area. The crack is considered to be straight and long in the z direction as shown in (C). The corresponding free body diagram for the vapor driven model is shown in (D) for the deformed state of the top layer of PI. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) cross sectional area, E ′ i = E i /(1 − v i ) is the biaxial elastic modulus
Delamination Mechanism Maps (A) The surface plots show the mapping of the energy release rate varying both the top and bottom PI thickness for two different crack lengths. The blue surface on both plots is the critical energy release rate of 30 J/m 2 and the isolines are highlighted in black at the critical energy release rate. Any portion of the surface higher than the critical energy release rate will delaminate. These isolines are then plotted in (B) for each silver width or crack length (2a) to create a design map of regions with and without delamination for all three geometric parameters. The delamination region is highlighted in blue as an example for the 60 µm crack length. (C) Individual graphs hold the top layer of PI constant, while showing curves for three different thicknesses of the bottom layer of PI. (D) Individual graphs hold the bottom layer of PI thickness constant, while showing curves for three different thicknesses of top PI. Part (C) and (D) show the individual effects of the top and bottom PI thicknesses in the vapor starved and vapor rich region along with the critical energy release rate of 30 J/m 2 shown by the dotted line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Finite element model results and comparison with analytical model. (A-C) Compares the analytical model to the direct comparison FE model. (A) Comparison of the maximum displacement w max at the center of the blister. (B) Comparison of the u factor, which determines whether the deformation behavior in the blister is membrane or plate dominated. (C) Comparison of the energy release rate development between the two models also showing the critical energy release rate of 30 J/m 2 . (D-F) Results of the progressive crack growth model with meander geometry. Growth of the crack is shown in (D-F) for a PI thickness of 6 µm and silver width of 50 µm where (D) the crack is just opening due to a lower pressure, (E) the crack is just about to grow, and (F) the crack has preferentially grown towards the inside edge. The energy release rate just before growth in (E) is G i = 29.7 J/m 2 and G o = 27.9 J/m 2 . The plastic strain ε pl is displayed by the color map
Flexible electronic systems integrate heterogeneous materials such as ceramics, metals, and elastomers, which results in interfaces prone to delamination under stress. In this research, we show that delamination can be initiated in a polyimide-based flexible interconnect system directly printed on a polydimethylsiloxane substrate where the metallic interconnect acts as the crack initiation site. This problem is experimentally and analytically evaluated to identify the controlling parameters and propose pathways to prevent delamination. The driving force for delamination is shown to be the vapor pressure of the absorbed moisture in the polymer. Based on the dimensions of the cracks in our system (20–60 μm) and the thickness of the delaminated polymer films (2–6 μm), nonlinear von-Kármán plate theory is utilized to capture both membrane stretching and thin plate bending behavior in the polymer films. The model yields ‘delamination mechanism maps’ that relate the energy release rate to the geometric dimensions of the flexible interconnects/circuits. For thin polymer films, a ‘vapor starved’ regime is shown where insufficient moisture reduces the driving force for delamination. For thicker films, a higher resistance to fracture is observed due to an increased rigidity of the polymer layer which behaves as a ‘plate’ rather than a ‘membrane’. Under these conditions, however, the higher retained moisture in the thicker films sustains the driving force for fracture capable of reaching the critical energy release rate. The mechanism maps also reveal the width of the metallic conductor (i.e., the initial crack size) as an important factor controlling fracture. For example, it is shown that the energy release rate for fracture is reduced from 20.5 to 4.6 J/m² when the conductor width is reduced from 50 μm to 30 μm for a polymer film thickness of 6 μm. These predictions are shown to be in reasonable agreement with our experimental observations. A finite element model is also developed and used to further validate the analytical model. The work presented in this paper provides highly important and practical design guidelines for improved reliability of flexible electronic systems.
 
This work proposes a data-driven approach, G-MAP123, using discrete data directly for nonlinear elastic materials to solve boundary value problems, avoiding analytic-function based constitutive models. G-MAP123 is formulated in the current configuration in which the Cauchy stress and the left Cauchy–Green strain are adopted as the stress–strain measures of the data. Data generated under both uniaxial tension and equibiaxial tension experiments is used. A data search employing stress triaxiality as the index is here proposed for the stress update. Furthermore, including additional data from other loading paths is also rendered possible. Comparison with reference analytic-function based models such as Arruda-Boyce, Yeoh, Mooney–Rivlin and Van der Waals is carried out. Results show that the predictions from G-MAP123 are in agreement with all those of the reference models. Moreover, the classic experimental data of rubber from Treloar is here used to demonstrate the capability of the proposed G-MAP123 in the practical setting. This approach opens a new avenue to modeling soft materials accurately and conveniently at large deformation, directly from the data.
 
Combining high strength, hardness and high toughness remains a tremendous challenge in materials engineering. Interestingly nature overcomes this limitation, with materials such as bone which display unusual combinations of these properties in spite of their weak constituents. In these materials, highly mineralized “building-blocks” provide stiffness and strength, while weak interfaces between the blocks channel non-linear deformation and trigger powerful toughening mechanisms. This strategy is also exploited in multilayered ceramics, fiber-reinforced composites, and more recently in topologically-interlocked materials. In this work we apply these concepts to the toughening of glass panels by incorporating internal architectures carved within the material using three-dimensional laser engraving. Glass is relatively stiff and hard but it has no microstructure, no inelastic deformation mechanism, low toughness and poor resistance to impacts. We demonstrate how introducing controlled architectures in glass completely changes how this material deforms and fails. In particular, our new architectured glass panels can resist about two to four times more impact energy than plain glass. Our architectured glass also displays non-linear deformation, progressive damage and failure contained within a few building blocks. This work demonstrates how micro-architecture, bio-inspiration and top-down fabrication strategies provide new pathways to transform the mechanics and performance of materials and structures.
 
Top-cited authors
Huajian Gao
  • Brown University
Yong-Wei Zhang
  • Institute Of High Performance Computing
Nanshu Lu
  • University of Texas at Austin
Yong Zhu
  • North Carolina State University
Rui Huang
  • University of Texas at Austin