Effects of geometric and material nonlinearities on tunable band gaps and low-frequency directionality of phononic crystals

We investigate the effects of geometric and material nonlinearities introduced by deformation on the linear dynamic response of two-dimensional phononic crystals. Our analysis not only shows that deformation can be effectively used to tune the band gaps and the directionality of the propagating waves, but also reveals how geometric and material nonlinearities contribute to the tunable response of phononic crystals. Our numerical study provides a better understanding of the tunable response of phononic crystals and opens avenues for the design of systems with optimized properties and enhanced tunability.Engineering and Applied Science

Finite strain behavior of polyurea for a wide range of strain rates

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, February 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 101-112).Polyurea is a special type of elastomer that features fast setting time as well as good chemical and fire resistance. It has also good mechanical properties such as its high toughness-to-density ratio and high strain rate-sensitivity, so its application is recently extended to structural purpose to form sandwich-type or multi-layered plates. Those structures can be used for retrofitting of military vehicles and historic buildings, absorbing energy during structural crash. In order to investigate its behavior of hysteresis as well as rate-sensitivity, three different testing systems are used to cover a wide range of strain rates up to strain of 100%. In view of impact and blast events, the virgin state of polyurea is considered throughout the experiments. First, a hydraulic universal testing machine is used to perform uniaxial compressive loading/unloading tests in order to investigate its hysteresis behavior at low strain rates (0.001/s to 10/s). Second, two distinct gas-gun split Hopkinson pressure bar [SHPB] systems are employed to cover high strain rates: a nylon bar system (700/s to 1200/s) and an aluminum bar system (2300/s to 3700/s). Lastly, the rate-sensitivity for intermediate strain rates (10/s to 1000/s) is characterized using a modified SHPB system.(cont.) The device is composed of a hydraulic piston along with nylon input and output bars. A finite strain constitutive model of polyurea is presented in order to predict the hysteresis and rate-sensitivity behavior. The 1-D rheological concept of two Maxwell elements in parallel is employed within the framework of the multiplicative decomposition of the deformation gradient. Model parameters are calibrated based on the uniaxial compressive tests at various rates. The corresponding algorithms is implemented as a user-defined material subroutine VUMAT for ABAQUS/Explicit, and used to predict the response of polyurea. The proposed constitutive model reasonably captures the experimentally observed asymmetric rate-sensitivity and stress-relaxation behavior: strong rate-sensitivity and large amount of stress relaxation during loading phase, but weak rate-sensitivity and smaller amount of stress relaxation during unloading phase. In order to validate the proposed model, various dynamic punching tests are performed, and their results are well compared with the model predictions during loading although the prediction of unloading behavior can be further improved.by Jongmin Shim.Ph.D

Prediction of early-age cracking of UHPC materials and structures : a thremo-chemo-mechanics approach

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2005.Includes bibliographical references (p. 152-154).Ultra-High Performance Concrete [UHPC] has remarkable performance in mechanical properties, ductility, economical benefit, etc., but early-age cracking of UHPC can become an issue during the manufacturing process due to the high cement content and the highly exothermic hydration reaction. Because of the risk of early-age UHPC cracking, there is a need to develop a material model that captures the behavior of UHPC at early-ages. The objective of this research is to develop a new material model for early-age UHPC through a thermodynamics approach. The new model is a two-phase thermo-chemo-mechanical model, which is based on two pillars: the first is a hardened two-phase UHPC material model, and the second is a hydration kinetics model for ordinary concrete. The coupling of these two models is achieved by considering the evolution of the strength and stiffness properties in the two-phase UHPC material model in function of the hydration degree. The efficiency of the model and finite element implementation is validated with experimental data obtained during the casting of a DuctalTM optimized bridge girder. Based on some decoupling hypothesis, the application of the early-age UHPC model can be carried out in a two-step manner: the thermo-chemical problem is solved first, before solving the two-phase thermochemomechanical problem. It is shown that the newly developed model is able to accurately predict temperature history and deformation behavior of the bridge girder. Furthermore, with this versatile engineering model, it is possible to predict the risk of cracking, and eventually to reduce it.by JongMin Shim.S.M

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/Young's 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.Engineering and Applied Science

Harnessing instability to control wave propagation in phononic crystals and acoustic metamaterials

Artificially structured composite materials have the ability to manipulate the propagation of elastic waves due to the existence of band gaps, i.e., frequency ranges of strong wave attenuation. However, most configurations proposed to date cannot be tuned after the manufacturing process. We propose new strategies using elastic buckling mechanisms to design novel devices with in-situ adaptive properties that can be reversibly tuned. Buckling and large deformations can be effectively exploited to reversibly tune not only the width and location of band gaps, but also the directional preferences of the wave propagation, even for low-frequency elastic waves. Our proof-of-concept demonstrations also indicate that the proposed mechanisms work robustly over a wide range of length scales, opening avenues for the design of smart systems for applications, such as vibration/noise reduction, wave guiding, frequency modulation, and acoustic imaging

Switching Periodic Membranes via Pattern Transformation and Shape Memory Effect

We exploited mechanical instability in shape memory polymer (SMP) membranes consisting of a hexagonal array of micron-sized circular holes and demonstrated dramatic color switching as a result of pattern transformation. When hot-pressed, the circular holes were deformed to an array of elliptical slits (with width of tens of nanometers), and further to a featureless surface with increasing applied strain, therefore, switching the membrane with diffraction color to a transparent film. The deformed pattern and the resulting color change can be fixed at room temperature, both of which could be recovered upon reheating. Using continuum mechanical analyses, we modeled the pattern transformation and recovery processes, including the deformation, the cooling step, and the complete recovery of the microstructure, which corroborated well with experimental observations. We find that the elastic energy is roughly two-orders of magnitude larger than the surface energy in our system, leading to autonomous recovery of the structural color upon reheating. Furthermore, we demonstrated two potential applications of the color switching in the SMP periodic membranes by (1) temporarily erasing a pre-fabricated "Penn" logo in the film via hot-pressing and (2) temporarily displaying a "Penn" logo by hot-pressing the film against a stamp. In both scenarios, the original color displays can be recovered.Engineering and Applied Science

Direct Laser Writing of Air-Stable p–n Junctions in Graphene

Photo-oxidation of spin-cast films of 6,13-bis(triisopropylsilylethynyl) pentacene has been exploited to develop a novel means of spatially modulating doping in graphene. The degree of n-doping of initially p-type graphene can be varied by laser irradiation time or intensity with carrier density change up to ∼7 × 10¹² cm^–2. This n-doping approach is demonstrated as an effective means of creating p–n junctions in graphene. The ability to direct-write arbitrary shapes and patterns of n-doped regions in graphene simply by scanning a laser source should facilitate the exploitation of p–n junctions for a variety of electronic and optoelectronic device applications

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.Harvard University. Materials Research Science and Engineering Center (National Science Foundation (U.S.) (Award DMR-0820484))MIT-France ProgramMassachusetts Institute of Technology. Department of Mechanical EngineeringMassachusetts Institute of Technology. Department of Civil and Environmental Engineerin

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/Young's 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

Generalized spatial aliasing solution for the dispersion analysis of infinitely periodic multilayered composites using the finite element method

The finite element (FE) method offers an efficient framework to investigate the evolution of phononic crystals which possess materials or geometric nonlinearity subject to external loading. Despite its superior efficiency, the FE method suffers from spectral distortions in the dispersion analysis of waves perpendicular to the layers in infinitely periodic multilayered composites. In this study, the analytical dispersion relation for sagittal elastic waves is reformulated in a substantially concise form, and it is employed to reproduce spatial aliasing-induced spectral distortions in FE dispersion relations. Furthermore, through an anti-aliasing condition and the effective elastic modulus theory, an FE modeling general guideline is provided to overcome the observed spectral distortions in FE dispersion relations of infinitely periodic multilayered composites, and its validity is also demonstrated.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