Compliant Layer Research Articles

Benefit of Backing‐Layer Compliance in Fibrillar Adhesive Patches—Resistance to Peel Propagation in the Presence of Interfacial Misalignment

AbstractInvestigations of backing‐layer effects in bioinspired fibrillar adhesives have shown that increased compliance is detrimental to the strength of fibril arrays under normal loading due to an increase in severity of a circumferential load concentration. In this work, the impact of misalignment on the performance of fibrillar adhesive patches contacting smooth flat surfaces is examined, demonstrating that the conditions for circumferential detachment are extremely limited. For an array of fibrils on a backing layer of varying thickness, normal adhesion tests are performed against a flat surface that maintains a fixed angle of misalignment with respect to the adhesive surface. In the aligned state the detachment is circumferential and the detachment force is highest for the thinnest, least compliant backing layer. However, for misalignment angles on the order of just 0.1°, peel‐like detachments are observed. The thickest backing layer, being 210% more compliant than the thinnest, yields a 43% increase in the adhesive strength at a misalignment angle of 0.4°. This suggests that out‐with conditions of precise alignment, backing‐layer compliance is beneficial to strength under normal loading. A mechanical model is presented, revealing the mechanism behind enhanced resistance to peel propagation is deformation of the backing layer at the detachment front which reduces differential stretching of fibrils.

A tribo-oxidation abrasive wear model to quantify the wear rate of a cobalt-based alloy subjected to fretting in low-to-medium temperature conditions

Abstract Wear mechanisms of cobalt-based alloys are commonly known to be dependent on temperature: above a glaze layer transition temperature, wear rates are very low and a compacted oxidized compliant debris layer is spontaneously created in the contact. The present study focuses on the high wear rate mechanism occurring below this transition temperature of a HS25/alumina contact subjected to gross-slip fretting. The investigation shows that the wear volume is proportional to the product of the Archard's work multiplied by the oxide thickness generated between each fretting sliding pass. A simple tribo-oxidation wear model was developed and a very good correlation was observed with experiments as long as the compaction of debris layer leading to a glaze layer structure was not activated.

Rock heterogeneity at the centimeter scale, proxies for interfacial weakness, and rock strength-stress interplay from downhole ultrasonic measurements

Analysis of data acquired with an ultrasonic rotating device lowered inside a vertical well of a highly laminated, kerogen-rich, carbonate source formation reveals centimeter-scale rock heterogeneities. The high-frequency (50–600 kHz) measurement used consists of a pulse-echo modality that yields a high-resolution (millimeter-scale) borehole shape and a pitch-catch modality with one transmitting and two receiving transducers that provide compressional (P) and shear (S) slownesses (depth-versus-azimuth) images estimated from signals propagating in the near-wellbore region as compressional head waves and pseudo-Rayleigh surface waves. The slownesses are compared with their counterparts estimated from a lower-frequency (1–15 kHz) sonic measurement logged in the same well interval. The sonic P and S logs are seen to average the centimeter-scale slowness spatial variation between compliant and stiff laminates at a scale larger than 30 cm (1 ft). This is also accompanied by a reduction in resolution of the slowness contrast, which markedly reduces the spectrum of rock-mechanical property variations that are estimated from the ultrasonic data. Further, ultrasonic images of the P and S slownesses and borehole acoustic caliper reveal a host of features associated with rock geomechanics in the near-wellbore and that inform on a first-order basis the interplay between rock strength and local stress regime, as affected by the lamination. The features include breakouts present in the stiffer limestone layers and their arrest at the intersection with the compliant siltstone layers, as well as azimuthal P and S slowness variations indicative of azimuthal stress concentrations, but without the appearance of breakouts. The ultrasonic borehole shape data also identify compliant thin layers that retract into the formation by amounts that are commensurate with their Young’s modulus, suggesting a proxy for detecting and characterizing thin-layer weakness in situ.

Local and global measurements show that damage initiation in articular cartilage is inhibited by the surface layer and has significant rate dependence

Cracks in articular cartilage are a common sign of joint damage, but failure properties of cartilage are poorly understood, especially for damage initiation. Cartilage failure may be further complicated by rate-dependent and depth-dependent properties, including the compliant surface layer. Existing blunt impact methods do not resolve local cartilage inhomogeneities and traditional fracture mechanics tests induce crack blunting and may violate underlying assumptions of linear elasticity. To address this knowledge gap, we developed and applied a method to indent cartilage explants with a sharp blade and initiate damage across a range of loading rates (strain rates 0.5%/s–500%/s), while recording local sample deformation and strain energy fields using confocal elastography. To investigate the importance of cartilage’s compliant surface, we repeated the experiment for samples with the surface removed. Bulk data suggest a critical force at which the tissue cuts, but local strains reveals that the deformation the sample can sustain before reaching this force is significantly higher in the surface layer. Bulk and local results also showed significant rate dependence, such that samples were easier to cut at faster speeds. This result highlights the importance of rate for understanding cracks in cartilage and parallels recent studies of rate-dependent failure in hydrogels. Notably, local sample deformation fields were well fit by classical Hookean elasticity. Overall, this study illustrates how local and global measurements surrounding the initiation of damage in articular cartilage can be combined to reveal the importance of cartilage’s zonal structure in protecting against failure across physiologically relevant loading rates.

Interfacial debonding propagation in orthotropic layered plates with a compliant internal layer

The failure of layered plates with internal compliant layers is associated with interfacial debonding mechanisms, whose geometrically irregular propagation is governed by a three-dimensional stress state. This paper faces this challenge by developing a multi-layered, high-order plate theory and a triangular finite element that incorporate cohesive interfaces. The formulation tracks the evolution of two-dimensional debonding areas at the layered plate's interfaces. The model is used to study debonding mechanisms in a narrow plate with initial delaminations. Attention is given to the unstable response along equilibrium paths, the evolving irregular debonding front contours, and their impact on the structural resilience.

Ultrasonic tactile sensor integrated with TFT array for force feedback and shape recognition

In this study, we propose an ultrasonic tactile sensor for real time contact force measurements and high-resolution shape recognition to enable safe and reliable robotic grasping of objects that may vary in compliance or texture. The sensing mechanism utilizes piezoelectric transduction where pulsed alternating voltage signals are applied to a polyvinylidene fluoride (PVDF) thin film, which generates pulses of ultrasound waves that travel upwards through the sensor components to the object contact interface. These waves are reflected back onto a receiver PVDF thin film that produces a localized voltage output, which is detected by the TFT (Thin-Film Transistor) array layer and converted into a two-dimensional grayscale image after signal processing. The ability of the tactile sensor to detect contact forces can be attributed to the sensor surface having a thin compliant polymer layer with a microstructure array. When the sensor contacts objects, the microstructures act as force concentrators, resulting in the localized deformation of the polymer layer that can be observed by the proposed ultrasonic imaging technique with an observed linear response to normal static forces in the range of 1–6 N. Furthermore, the shape sensing resolution and force detection range of the tactile sensor can be tuned by varying the number of microstructures in the array and the utilization of polymers with varying hardness, respectively.

Development of a novel compressive h-BN based seal for planar intermediate temperature SOFC

A novel compressive boron nitride (h-BN) based seal has been developed for planar intermediate temperature solid oxide fuel cell (SOFC). It exhibited extremely low leakage rates and thermal cycling stability under simulated stack conditions. The h-BN based seal showed leakage rates of 0.01 sccm/cm under gas pressure of 6.8kPa in the temperature range of 650–800°C, and maintained similar leakage rates during 10 thermal cycles. The excellent sealing performance can be explained by oxidation of h-BN surface to form liquid B2O3 layers beyond the critical temperature of 700°C. Two of compliant B2O3 glass layers were observed on the surface of the h-BN to adjoin well cell and metallic interconnect. The sandwich structure with liquid B2O3 layer on two sides of the h-BN seal could not only enhance interfacial adherence but also eliminate the gas leakage paths at SOFC operation temperature. The applicability of the h-BN based seal has also been verified by single cell test.

Si nanospring films for compliant interfaces

Films comprised of dense arrays of Si nanosprings were studied for their potential as compliant interface layers. The films, fabricated via glancing angle deposition (GLAD), were comprised of 10-µm high Si nanosprings with 4 or 10 coil turns and seed spacings of 900 nm or 1500 nm. Unseeded films with the same height and 4 or 10 coil turns were co-fabricated as the control material. The film mechanical behavior was a combination of the mechanical response of the individual springs and their interactions inside the film, resulting in increasing compressive stiffness with applied stress. The shear stiffness of all types of Si spring films was in the narrow range of 17–27 MPa. On the contrary, depending on coil geometry, the low-stress compressive stiffness could be varied in the broad range of 13–150 MPa. Thus, this class of nanostructured films allows for relatively decoupled normal and shear properties with values comparable to those of soft polymers. Despite that unseeded films have the thinnest coil wire, it was shown that seeding can produce equal or lower film stiffness, while also increasing the resistance to permanent compression at high stresses. Similarly, a capping layer increased the coherency of the films and their resistance to permanent compression without affecting significantly the compressive film stiffness. Capped films with 10-turn coils, with 900-nm seed spacing, provided the most resistance to permanent compression for stresses at least as high as 15 MPa, while maintaining compressive stiffness values that were equivalent to unseeded 10-turn springs. Finally, capped 4-turn springs with 900-nm seed spacing were shown to be the most compliant of all film types, including unseeded films, while also exhibiting the best agreement between film-level stiffness and estimates based on individual spring properties. Therefore, seeding of GLAD springs can produce films with increased resistance to permanent deformation while maintaining the low stiffness of unseeded films.

Interrelated chemical-microstructural-nanomechanical variations in the structural units of the cuttlebone of Sepia officinalis

“Cuttlebone,” the internalized shell found in all members of the cephalopod family Sepiidae, is a sophisticated buoyancy device combining high porosity with considerable strength. Using a complementary suite of characterization tools, we identified significant structural, chemical, and mechanical variations across the different structural units of the cuttlebone: the dorsal shield consists of two stiff and hard layers with prismatic mineral organization which encapsulate a more ductile and compliant layer with a lamellar structure, enriched with organic matter. A similar organization is found in the chambers, which are separated by septa, and supported by meandering plates (“pillars”). Like the dorsal shield, septa contain two layers with lamellar and prismatic organization, respectively, which differ significantly in their mechanical properties: layers with prismatic organization are a factor of three stiffer and up to a factor of ten harder than those with lamellar organization. The combination of stiff and hard, and compliant and ductile components may serve to reduce the risk of catastrophic failure, and reflect the role of organic matter for the growth process of the cuttlebone. Mechanically “weaker” units may function as sacrificial structures, ensuring a stepwise failure of the individual chambers in cases of overloading, allowing the animals to retain near-neutral buoyancy even with partially damaged cuttlebones. Our findings have implications for our understanding of the structure-property-function relationship of cuttlebone, and may help to identify novel bioinspired design strategies for light-weight yet high-strength foams.

Investigating ultraflexible freestanding graphene by scanning tunneling microscopy and spectroscopy

A strictly two-dimensional (2D) material such as freestanding graphene (FSG) is rarely investigated at the atomic scale by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). A basic difficulty in probing FSG by STM and STS is the mechanical instability when a highly compliant 2D atomic layer interacts with a proximal tip. Here we report a detailed method to conduct reliable STM and STS on FSG with atomic precision. We found that FSG is intrinsically rippled and exhibits a nonlinear strain-stress relation under applied normal forces; it shows a very soft region of bending strain and stiffer regions of in-plane tensile strain once the nanoscale ripples of FSG are eliminated. The elimination of the nanoripples can be controlled by tip-induced pulling or pushing force through the so-called closed-loop Z-V STS mode which can monitor the FSG deformation. A key factor for controllable STM and STS measurements is to select tunneling set points to place FSG in metastable configurations, as determined from stress-strain (i.e., Z-V) response. Atomic imaging and electronic states thus measured must be interpreted by considering the dynamical deformation of FSG as tunneling parameters, and therefore tip-FSG forces, are varied.

Elastic properties of particle-reinforced composites containing nonspherical particles of high packing density and interphase: DEM–FEM simulation and micromechanical theory

The physical and morphological properties of particles and interfaces can seriously impact the whole mechanical behavior of particle-reinforced composites. In this work, we devise a robust coupling model of the discrete element method (DEM) and the finite element method (FEM) to numerically investigate the effective elastic moduli of particle-reinforced composites consisting of elliptical particles of high packing density, compliant interfaces and homogeneous matrix. In the numerical model, a novel parametric equation for the topological geometry of an interface that is treated as an interphase model is formulated to realize a compliant (penetrable) layer with a constant finite thickness coated surrounding each elliptical particle. Additionally, a convenient strategy is implemented to cope with periodic boundary conditions containing numerous particles. On the other hand, we also propose a micromechanical theoretical framework to derive the effective elastic properties of such three-phase composites by incorporating the fraction of compliant interfaces that is theoretically computed by using the statistical geometry of composites. It is shown that our numerical and theoretical models lead to the prediction of elastic moduli of particle-reinforced composites with nonspherical particles of high packing density to a reasonable accuracy by comparing with available experimental data. Moreover, utilizing the proposed models, we systematically investigate the influence of the characteristics of particles and interphase such as the high packing density and geometries of elliptical particles, and interphase fraction, thickness and stiffness on the elastic moduli of particle-reinforced composites. We find that these physical characteristics play significant roles in determining the mechanical behavior of composites, suggesting that the properties of such materials can be tailored via proper composites engineering and design.

Elasto-plastic contact of materials containing double-layered inhomogeneities

Double-layered inhomogeneities consist of enclosed, or layered, inhomogeneities of different material properties, embedded in a matrix material. They are found in engineering materials used for components under contact and relative motion. Currently, most theoretical investigations on the double-layered inhomogeneities are limited to elastic or plane-strain problems. This work proposes a novel model, based on the numerical equivalent inclusion method, for the elasto-plastic contact of materials with double-layered inhomogeneities. The current analysis is focused on such inhomogeneities as a stiff core enclosed by a compliant outer layer. A group of in-depth parametric studies is performed to reveal the effects of this type of double-layered inhomogeneities on the contact plasticity of the matrix material. The results indicate that the plastic strain distribution in the matrix material is related to the Young's modulus and geometric eccentricity of the inner and outer inhomogeneities, as well as the location and shape of the inhomogeneities. For the cases of individual double-layered inhomogeneity embedded at different locations, the maximum equivalent plastic strain in the matrix appears in the vicinity of the double-layered inhomogeneity, wherever is the closest to the location of the theoretical maximum elastic stress from the homogeneous solution. For the cases of multiple double-layered inhomogeneities, the overlap of the plastic strain concentration regions amplifies the disturbance caused by these inhomogeneities, and the amplification effect is related to the inhomogeneity layout. If the stiff core is completely encircled in the outer inhomogeneity layer, plastic strains would initiate from the outer layer and then permeate to the matrix material.

Adaptive Scalable Video Coding: An HEVC-Based Framework Combining the Predictive and Distributed Paradigms

The emerging scalable High Efficiency Video Coding (SHVC) video coding standard provides an efficient solution for transmission of video over heterogeneous and time dynamic networks, terminals, and usage environments. The encoding complexity and the error sensitivity associated with the efficient HEVC coding tools adopted in SHVC make this scalable codec less attractive to some emerging applications such as video surveillance, visual sensor network, and remote space transmission where these requirements are critical. To address the requirements of these application scenarios including scalability, this paper proposes a novel HEVC-based framework offering quality scalability on top of an HEVC compliant base layer while appropriately combining the predictive and distributed coding paradigms. To achieve the best enhancement layer compression efficiency, two novel coding tools are proposed, notably a machine learning-based side information creation mechanism and an adaptive correlation modeling process. The experimental results reveal that the rate-distortion performance of the proposed distributed scalable video coding-HEVC solution outperforms the relevant alternative coding solutions, notably by up to 52.9% and 23.7% BD-rate gains regarding the HEVC-Simulcast and SHVC standard solutions, respectively, for an equivalent prediction configuration, while achieving a lower encoding complexity.

Diffusion-induced bending of viscoelastic beams

The contribution of local volumetric change due to the diffusion/migration of solute atoms to viscoelastic deformation is incorporated in the theory of linear viscoelasticity, following the elastic theory of diffusion-induced stress. Three-dimensional constitutive relationship in differential form for diffusion-induced stress in linear viscoelastic materials is proposed. Using the correspondence principle between linear viscoelasticity and linear elasticity and the results from the diffusion-induced bending of elastic beams, the radii of curvature of the centroidal plane of viscoelastic beams of single layer and bilayer with top layer being viscoelastic in the transform domain are obtained. For viscoelastic beams of single layer, closed-form solution of the radius of curvature of the centroidal plane is derived, and the radius of curvature is inversely proportional to the diffusion moment created by non-uniform distribution of solute atoms. For the condition of constant concentration on free surface, there is overshoot behavior; for the condition of constant flux on free surface, there is no overshoot behavior. For viscoelastic beams of bilayer with top layer being the Maxwell-type standard material, the numerical results show the presence of the overshoot behavior for a very compliant elastic layer under the condition of constant concentration on free surface, and there is no overshoot behavior under the condition of constant flux on free surface.

Modeling and Controlling an Active Constrained Layer (ACL) Beam Actuated by Two Voltage Sources With/Without Magnetic Effects

A fully dynamic three-layer active constrained layer (ACL) beam is modeled for cantilevered boundary conditions by using a thorough variational approach. The Rao-Nakra thin compliant layer assumptions are adopted to model the sandwich structure, and all magnetic effects for the piezoelectric layers are retained. The piezoelectric layers are activated by two different voltage sources. When there are no "mechanical" boundary forces acting in the longitudinal direction, it is shown that the system with certain parameter combinations is not uniformly strongly stabilizable by the $B^*-$type feedback controller, which is the total current accumulated at the electrodes for the piezoelectric layers. However, as the magnetic effects are ignored (electrostatic assumption), the closed-loop system with all mechanical feedback controllers is shown to be uniformly exponentially stable.

Computational optical palpation: a finite-element approach to micro-scale tactile imaging using a compliant sensor.

High-resolution tactile imaging, superior to the sense of touch, has potential for future biomedical applications such as robotic surgery. In this paper, we propose a tactile imaging method, termed computational optical palpation, based on measuring the change in thickness of a thin, compliant layer with optical coherence tomography and calculating tactile stress using finite-element analysis. We demonstrate our method on test targets and on freshly excised human breast fibroadenoma, demonstrating a resolution of up to 15-25 µm and a field of view of up to 7 mm. Our method is open source and readily adaptable to other imaging modalities, such as ultrasonography and confocal microscopy.

A representative volume element based micromechanical analysis of a Bi-layered Ganoid Fish scale

The Mississippi Alligator gar (Atractosteus spatula) possesses a flexible exoskeleton armor consisting of overlapping ganoid scales used for predatory protection. Each scale is a two-phase biomineralized composite containing bio-modified hydroxyapatite (hard) minerals and collagen (soft) fibers. The protective layer consists of a stiff outer ganoine layer, a characteristic “sawtooth” pattern at the interface with the compliant bone inner layer. The garfish scale exhibits a decreasing elastic modulus from the external to the internal layers. Scanning electron microscopy (SEM) images of the cross-section revealed a two-layered structure. Elastic moduli, measured from nanoindentation experiments, were correlated to structural changes across each layer. The “material” symmetry of this materially and geometrically nonlinear biomineralized composite is unknown. Therefore, to be able to determine the stiffness tensor requires the use of finite element analysis (FEA). The gar fish scale was computationally modeled using the representative volume element (RVE) based approach. As a result, the unknown symmetry induced by the architecture and material layering require the use of complex FEA boundary conditions. The simulation was conducted in the pure uniaxial strain regimes of tension and shear, which necessitated the mathematical determination so appropriate surface loading conditions could be applied. This paper provides the results from a highly-resolved mesoscale RVE model based on iso-strain boundary conditions (ISBC) to determine the elastic stiffness tensor for the composite system. By assuming isotropic behavior in individual elements, the results for the RVE reveal the fish scale has an “orthotropic symmetry” with slight local strain variations occurring at the sawtooth interface.

Experimental Validation of Metaconcrete Blast Mitigation Properties

We provide experimental evidence of the mitigation properties of metaconcrete under blast loading. Mitigation is achieved through resonance of engineered aggregates consisting of a heavy and stiff core coated by a light and compliant outer layer. These engineered aggregates replace the standard gravel in conventional concrete. To assess experimentally the attenuation properties of metaconcrete, we have cast two batches of cylindrical specimens. The mortar matrix of the first batch consists of cement combined with a regular sand mix, while the mortar matrix of the second batch consists of cement combined with sand mix, fine gravel, and polymeric fibers. One of the specimens of each batch was cast with no aggregates, while the other two contained 40 and 60, respectively, randomly arranged 22 mm diameter commercially available computer mouse balls. We performed nondestructive dynamic tests by applying a 10 V amplitude periodic signal to one end of the specimens and measuring the amplitude of the transmitted signal received at the other end. We observed a remarkable 2 order of magnitude reduction in the amplitude of the transmitted signal in metaconcrete relative to conventional concrete.

Thermal wrinkling behavior of formable decorative film laminates

This paper focuses on wrinkle development in decorative film laminates during heating operations with the goal to understand their driving factors and develop strategies to overcome such defects. The study looked at temperature and heating rate effects on the wrinkling behavior of a commercial black-out film laminated onto a metal substrate. The 135℃ threshold temperature identified for our film under which no wrinkles formed, related to the stiffness of its different construction layers. Heating rate was also noted by this study to be an important parameter in wrinkling; values between 1℃ and 350℃/min were tested. It was possible to exceed the threshold temperature stated above without wrinkling when the heating rate was sufficiently low (closer to 1℃/min, though less than 50℃/min was often sufficient depending on the final temperature). The heating rate effect is believed to be related to the time-dependent viscoelastic response of the compliant layer in relation to building thermal stresses.

Standard-based IoT platforms interworking: implementation, experiences, and lessons learned

The Internet-of-Things (IoT) provides a great opportunity to many vertical industries because IoT interconnects various devices such as sensors and actuators and collects/processes data from them in order to improve services and reduce costs. As there exists many IoT technologies in the market, global standards and interworking mechanisms are critical to the success of the IoT. This article introduces standardized interworking interfaces and procedures based on oneM2M global standards, and tests them through use cases involving multiple IoT service platforms. The interworking involves smart city applications/services running on multiple IoT service layer platforms interoperating with each other. The main purpose of the interworking experiment is to show how machine-to-machine (M2M)/IoT service providers are using oneM2M compliant service layer platforms to deliver services more efficiently across multiple technology domains such as smart city. Because the deployment configurations of this interworking experiment span multiple domains, and the IoT devices and platforms are from different companies, we believe that this interworking experiment clearly proves that global IoT standards specifications can foster implementations of a service layer that enables services and interoperability between devices/device networks and cloudbased applications.