Jongmin Shim

3D Soft Metamaterials with Negative Poisson's Ratio

Buckling is exploited to design a new class of three-dimensional metamaterials with negative Poissons ratio. A library of auxetic building blocks is identified and procedures are defined to guide their selection and assembly. The auxetic properties of these materials are demonstrated both through experiments and finite element simulations and exhibit excellent qualitative and quantitative agreement.

Buckling-induced encapsulation of structured elastic shells under pressure

We introduce a class of continuum shell structures, the Buckliball, which undergoes a structural transformation induced by buckling under pressure loading. The geometry of the Buckliball comprises a spherical shell patterned with a regular array of circular voids. In order for the pattern transformation to be induced by buckling, the possible number and arrangement of these voids are found to be restricted to five specific configurations. Below a critical internal pressure, the narrow ligaments between the voids buckle, leading to a cooperative buckling cascade of the skeleton of the ball. This ligament buckling leads to closure of the voids and a reduction of the total volume of the shell by up to 54%, while remaining spherical, thereby opening the possibility of encapsulation. We use a combination of precision desktop-scale experiments, finite element simulations, and scaling analyses to explore the underlying mechanics of these foldable structures, finding excellent qualitative and quantitative agreement. Given that this folding mechanism is induced by a mechanical instability, our Buckliball opens the possibility for reversible encapsulation, over a wide range of length scales.

Harnessing instabilities for design of soft reconfigurable auxetic/chiral materials

Most materials have a unique form optimized for a specific property and function. However, the ability to reconfigure material structures depending on stimuli opens exciting opportunities. Although mechanical instabilities have been traditionally viewed as a failure mode, here we exploit them to design a class of 2D soft materials whose architecture can be dramatically changed in response to an external stimulus. By considering geometric constraints on the tessellations of the 2D Euclidean plane, we have identified four possible periodic distributions of uniform circular holes where mechanical instability can be exploited to reversibly switch between expanded (i.e. with circular holes) and compact (i.e. with elongated, almost closed elliptical holes) periodic configurations. Interestingly, in all these structures buckling is found to induce large negative values of incremental Poissons ratio and in two of them also the formation of chiral patterns. Using a combination of finite element simulations and experiments at the centimeter scale we demonstrate a proof-of-concept of the proposed materials. Since the proposed mechanism for reconfigurable materials is induced by elastic instability, it is reversible, repeatable and scale-independent.

Highly Sensitive, Flexible, and Wearable Pressure Sensor Based on a Giant Piezocapacitive Effect of Three-Dimensional Microporous Elastomeric Dielectric Layer.

We report a flexible and wearable pressure sensor based on the giant piezocapacitive effect of a three-dimensional (3-D) microporous dielectric elastomer, which is capable of highly sensitive and stable pressure sensing over a large tactile pressure range. Due to the presence of micropores within the elastomeric dielectric layer, our piezocapacitive pressure sensor is highly deformable by even very small amounts of pressure, leading to a dramatic increase in its sensitivity. Moreover, the gradual closure of micropores under compression increases the effective dielectric constant, thereby further enhancing the sensitivity of the sensor. The 3-D microporous dielectric layer with serially stacked springs of elastomer bridges can cover a much wider pressure range than those of previously reported micro-/nanostructured sensing materials. We also investigate the applicability of our sensor to wearable pressure-sensing devices as an electronic pressure-sensing skin in robotic fingers as well as a bandage-type pressure-sensing device for pulse monitoring at the human wrist. Finally, we demonstrate a pressure sensor array pad for the recognition of spatially distributed pressure information on a plane. Our sensor, with its excellent pressure-sensing performance, marks the realization of a true tactile pressure sensor presenting highly sensitive responses to the entire tactile pressure range, from ultralow-force detection to high weights generated by human activity.

Modeling of cardiac muscle thin films: Pre-stretch, passive and active behavior

Recent progress in tissue engineering has made it possible to build contractile bio-hybrid materials that undergo conformational changes by growing a layer of cardiac muscle on elastic polymeric membranes. Further development of such muscular thin films for building actuators and powering devices requires exploring several design parameters, which include the alignment of the cardiac myocytes and the thickness/Youngs modulus of elastomeric film. To more efficiently explore these design parameters, we propose a 3-D phenomenological constitutive model, which accounts for both the passive deformation including pre-stretch and the active behavior of the cardiomyocytes. The proposed 3-D constitutive model is implemented within a finite element framework, and can be used to improve the current design of bio-hybrid thin films and help developing bio-hybrid constructs capable of complex conformational changes.

Sagittal plane waves in periodic multilayered composites composed of alternating viscoelastic and elastic solids

In order to design phononic crystals whose band-gaps are located in low-frequency ranges, researchers commonly adopt low stiffness polymeric materials as key constituents and exploit the high impedance mismatch between metals and polymers. However, there has been very little research on wave propagation at arbitrary angles in the sagittal plane of viscoelastic-elastic multilayered composites because there exist the intricate wave attenuation characteristics at the layer interfaces. The objective of our investigation is to obtain analytical dispersion relation for oblique wave motion in the sagittal plane of infinitely periodic multilayered composite composed of alternating viscoelastic and elastic solids, where the attenuation of harmonic plane waves is found to occur only in the direction perpendicular to the layers. By using this wave propagation characteristic, we directly apply the semi-analytical approach employed in elastic multilayered composites to calculate the dispersion relation of sagittal plane waves in alternating viscoelasticelastic multilayered composites. Specifically, we consider a bilayered composite composed of alternating aluminum and polyurethane elastomer, whose complex-valued viscoelastic moduli are experimentally determined by performing dynamic mechanical analysis (DMA). The analysis shows that the alternating viscoelastic-elastic layered composite does not possess a phononic band-gap regardless of incident angles. In addition, wave motions at oblique angles are found to travel with a wide range of frequency contents compared to wave motions perpendicular to the layers. The presented analysis demonstrates that wave dispersion relation in viscoelastic-elastic layered composites is distinctly different from the corresponding elastic counterpart, and highlights the importance of the viscoelastic modeling of polymeric materials in wave dispersion analysis. [DOI: 10.1115/1.4039039]

Generalized Spatial Aliasing Solution for the Dispersion Analysis of Infinitely Periodic Multilayered Composites Using the Finite Element Method

Qatar National Research Fund through Grant No. NPRP8-1568-2-666. Shim acknowledges start-up funds from the University at Buffalo (UB), and he is grateful to the support of UB Center for Computational Research.

Experimental determination of pressure-dependent stiffness of a nonlinear acoustic metamaterial

The field of acoustic metamaterials (AMM) involves the design of subwavelength structures to create material properties exceeding those of traditional composite materials. One such structure developed recently, known as a “Buckliball,” displays strong pressure-dependent stiffness [Proc. Natl. Acad. Sci. USA 109, 5978 (2012)]. The unique response of these structures results from element geometry: spherical elastomeric shells with thin, patterned circular membranes whose skeleton buckles when subjected to a differential pressure. We present measurements of the properties of a nonlinear effective medium consisting of Buckliballs immersed in water. The experimental apparatus consists of four elements: (1) a pressure vessel, which contains (2) a resonance tube filled with Buckliballs suspended in water, (3) an air-coupled ultrasonic system to monitor water height in the resonance tube, and (4) acoustic excitation and measurement instrumentation to obtain the frequency dependent acoustic response of the Bucklib...

Failure of an Impulsively-Loaded Composite Steel/Polymer Plate

The concept of spraying thick layer of polymer material onto metal plate has recently received considerable interest in many civilian and military applications. There are numerous analytical and numerical solutions for single thin plates (membrane) made of either a steel or an elastomer. However, solutions for composite plate made of both of the above constituents are lacking. The objective of the present paper is to formulate a model for composite steel/elastomer plate, derive an analytical sol ution of the impulsive loading problem and compare it with a more exact numerical solution. It is assumed that the circular plate is fully clamped around its peripheral and it is loaded by uniformly distributed transverse pressure of high intensity and short du ration. The pressure is imparting initial impulse which is proportional to initial transverse velocity of the plate. As an example, DH-36 is used for steel backing plate while polyurea is chosen to represent a typical polymer coating. In the analytical model, an iterative method is developed in which steel layer treated as a rigid perfectly-plastic material with ma gnitude of flow stress adjusted according to calculated magnitude of average strain. A linear elastic material is assumed with elas tic modulus in the tensile range calculated from the Arruda-Boyce model for an specific type of polyurea. It was found that the magnitude of the average strain rate is relatively low, abou t 100 sec −1 . Therefore, the effect of strain rate is not considered in this paper. A comprehensive parametric study was performed by varying various material and structural parameters of the model. A closed form solution was compared with the results of de