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Use of architected materials for the design of seismic isolation devices

Conference Proceedings
N. Bonessio, L. Valdevit, G. Lomiento
8th World Congress on Joints, Bearings and Seismic Systems for Concrete Structures, Atlanta, GA, September 25-29, 2016
Publication year: 2016

Abstract

The development of a novel class of materials that can be used for the seismic isolation protection systems is presented. The material is an architected cellular material obtained as periodic reproduction of a unit cell in all spatial directions. The mechanical properties of the constitutive unit-cell are tailored through an optimal design of the spatial configuration of voids and solids (cellular architecture) for a given solid constituent. The presented material can be designed at different scales in order to obtain unprecedented combinations of mechanical properties, such as high stiffness and strength in the vertical direction combined with high flexibility and dissipative capability in the lateral directions. Compact-shape, light-weight isolators overcoming traditional isolators’ limitations may be obtained through optimization of the unit cell properties. A mechanistic model of the isolator is provided after calibration on randomized parametric nonlinear Finite Element analysis of the unit cell. Keywords: Seismic isolation; periodic architected material; mechanistic model.

Multistable Shape-Reconfigurable Architected Materials

Journal paper
B. Haghpanah, L. Salari-Sharif, P. Pourrajab, J. Hopkins, L. Valdevit
Advanced Materials 28 (2016) 7915-7920
Publication year: 2016

Abstract

Multistable shape-reconfigurable architected materials encompassing living hinges and enabling combinations of high strength, high volumetric change, and complex shape-morphing patterns are introduced. Analytical and numerical investigations, validated by experiments, are performed to characterize the mechanical behavior of the proposed materials. The proposed architected materials can be constructed from virtually any base material, at any length scale and dimensionality.

This article was featured in a Nature Research Highlight, Nature 535 (07/2016) 32

Fabrication of 3D Micro-Architected/Nano-Architected Materials

Book Chapter
L. Valdevit, J. Bauer
In 'Three-dimensional Microfabrication using Two-photon Polymerization', T. Baldacchini, Editor. 2016, Elsevier.
Publication year: 2016

Abstract

Designing materials with exceptional combinations of properties at low weight is a continuous goal in many industries. Cellular (i.e., porous) materials with one or more phases topologically organized in a precisely designed configuration (often denoted as architected materials, or metamaterials) are excellent candidates to reach combinations of properties that are unattainable by existing monolithic materials. Additive manufacturing techniques are perfectly suited to implement the topological complexity that is often required for optimal performance. As beneficial size effects often arise in mechanical and functional properties as dimensions are shrunk to the nanoscale, 2PP becomes an ideal platform to investigate and ultimately fabricate topologically micro-architected and nano-architected materials with truly unique properties. The chapter reviews some notable features of architected materials, surveys commonly available manufacturing approaches, and presents challenges and opportunities for 2PP fabrication.

A Tri-Leaflet Nitinol Mesh Scaffold for Engineering Heart Valves

Journal paper
SH Alavi, MS Baliarda, N. Bonessio, L. Valdevit, A. Kheradvar
Annals of Biomedical Engineering 45 (2016) 413-426
Publication year: 2016

Abstract

The epidemiology of valvular heart disease has significantly changed in the past few decades with aging as one of the main contributing factors. The available options for replacement of diseased valves are currently limited to mechanical and bioprosthetic valves, while the tissue engineered ones that are under study are currently far from clinical approval. The main problem with the tissue engineered heart valves is their progressive deterioration that leads to regurgitation and/or leaflet thickening a few months after implantation. The use of bioresorbable scaffolds is speculated to be one factor affecting these valves’ failure. We have previously developed a non-degradable superelastic nitinol mesh scaffold concept that can be used for heart valve tissue engineering applications. It is hypothesized that the use of a non-degradable superelastic nitinol mesh may increase the durability of tissue engineered heart valves, avoid their shrinkage, and accordingly prevent regurgitation. The current work aims to study the effects of the design features on mechanical characteristics of this valve scaffold to attain proper function prior to in vivo implantation.

3D Manufacturing of Micro and Nano-Architected Materials

Conference Proceedings
L. Valdevit
SPIE Photonics West Conference, San Francisco, Feb 16-18, 2016, Paper # 9738-14, Invited.
Publication year: 2016

Abstract

Reducing mass without sacrificing mechanical integrity and performance is a critical goal in a vast range of applications. Introducing a controlled amount of porosity in a strong and dense material (hence fabricating a cellular solid) is an obvious avenue to weight reduction. The mechanical effectiveness of this strategy, though, depends strongly on the architecture of the resulting cellular material (i.e., the topology of the introduced porosity). Recent progress in additive manufacturing enables fabrication of macro-scale cellular materials (both single-phase and hybrid) with unprecedented dimensional control on the unit-cell and sub-unit-cell features, potentially producing architectures with structural hierarchy from the nano to the macro-scale. As mechanical properties of materials often exhibit beneficial size effects at the nano-scale (e.g., strengthening of metals and toughening of ceramics), these novel manufacturing approaches provide a unique opportunity to translate these beneficial effects to the macro-scale, further improving the mechanical performance of architected materials. The enormous design space for architected materials, and the strong relationship between the topological features of the architecture and the effective physical and mechanical properties of the material at the macro-scale, present both a huge opportunity and an urgent need for the development of suitable optimal design strategies. Here we present a number of strategies for the advanced manufacturing, characterization and optimal design of a variety of lightweight architected materials with unique combinations of mechanical properties (stiffness, strength, damping coefficient…). The urgent need to form strong synergies among the fields of additive manufacturing, topology optimization and architectureproperties relations is emphasized throughout.

Topology optimization of lightweight periodic lattices under simultaneous compressive and shear stiffness constraints

Journal paper
A. Asadpoure, L. Valdevit
International Journal of Solids and Structures 60-61 (2015) 1-16
Publication year: 2015

Abstract

This paper investigates the optimal architecture of planar micro lattice materials for minimum weight under simultaneous axial and shear stiffness constraints. A well-established structural topology opti- mization approach is used, where the unit cell is composed of a network of beam elements (Timoshenko beams are used instead of truss elements to allow modeling of bending-dominated architectures); start- ing from a dense unit cell initial mesh, the algorithm progressively eliminates inefficient elements and resizes the essential load-bearing elements, finally converging to an optimal unit cell architecture. This architecture is repeated in both directions to generate the infinite lattice. Hollow circular cross-sections are assumed for all elements, although the shape of the cross-section has minimal effect on most optimal topologies under the linear elasticity assumption made throughout this work. As optimal designs identi- fied by structural topology optimization algorithms are strongly dependent on initial conditions, a careful analysis of the effect of mesh connectivity, unit cell aspect ratio and mesh density is conducted. This study identifies hierarchical lattices that are significantly more efficient than any isotropic lattice (includ- ing the widely studied triangular, hexagonal and Kagomé lattices) for a wide range of axial and shear stiff- ness combinations. As isotropy is not always a design requirement (particularly in the context of sandwich core design, where shear stiffness is generally more important than compressive stiffness), the- se optimal architectures can outperform any established topology. Extension to 3D lattices is straightforward.

Push-to-pull tensile testing of ultra-strong nanoscale ceramic-polymer composites made by additive manufacturing

Journal paper
J. Bauer, A. Schroer, R. Schwaiger, I. Tesari, C. Lange, L. Valdevit, O. Kraft
Extreme Mechanics Letters 3 (2015) 105-112
Publication year: 2015

Abstract

The search for light yet strong materials recently benefited from novel high resolution 3D-printing technologies, which allow for fabricating lightweight porous materials with optimally designed micro-topologies. Architectural design improves mechanical properties significantly compared to stochastic porosity, as in foams. Miniaturization of the architectures offers to exploit material strengthening size-effects occurring at the nanoscale. However, these effects and their interaction with structural behavior are not yet well understood. We present tensile experiments of nanoscale alumina–polymer composite bars and cellular microarchitectures, applying 3D-printed push-to-pull mechanisms. The strength of alumina is found to strongly increase as the material thickness decreases. Below 50 nm thickness a plateau at about 5.5 GPa is reached, which is in the range of the theoretical strength. The characteristic low tensile strength of ceramics and its high variability seem not to hold at the nanoscale. Thus, when designed and fabricated appropriately, microarchitectures will facilitate carrying these size-effects beyond scales in future, allowing the use of ceramic materials far beyond what is possible to date.

Novel insights from 3D models: the pivotal role of physical symmetry in epithelial organization

Journal paper
A. Kurup, T. Tran, M. Keating, P. Gascard, L. Valdevit, T. Tlsty, E. Botvinick
Scientific Reports 5 (2015) 15153
Publication year: 2015

Abstract

3D tissue culture models are utilized to study breast cancer and other pathologies because they better capture the complexity of in vivo tissue architecture compared to 2D models. However, to mimic the in vivoenvironment, the mechanics and geometry of the ECM must also be considered. Here, we studied the mechanical environment created in two 3D models, the overlay protocol (OP) and embedded protocol (EP). Mammary epithelial acini features were compared using OP or EP under conditions known to alter acinus organization, i.e. collagen crosslinking and/or ErbB2 receptor activation. Finite element analysis and active microrheology demonstrated that OP creates a physically asymmetric environment with non-uniform mechanical stresses in radial and circumferential directions. Further contrasting with EP, acini in OP displayed cooperation between ErbB2 signalling and matrix crosslinking. These differences in acini phenotype observed between OP and EP highlight the functional impact of physical symmetry in 3D tissue culture models.

Macroscopic strain controlled ion current in an elastomeric microchannel

Journal paper
C-C. Kuo, Y. Li, D. Nguyen, S. Buchsbaum, L. Innes, A. P. Esser-Kahn, L. Valdevit, L. Sun, Z. Siwy, M. Dennin
Journal of Applied Physics 117 (2015) 174904
Publication year: 2015

Abstract

We report on the fabrication of an ultra-high aspect ratio ionically conductivesingle microchannel with tunable diameter from ≈ 20 μm to fully closed. The 4 mm-long channel is fabricated in a Polydimethylsiloxane (PDMS) mold and its cross-sectional area is controlled by applying macroscopic compressive strain to the mold in a direction perpendicular to the channel length. We investigated the ionic conduction properties of the channel. For a wide range of compressive strain up to ≈ 0.27, the strain dependence of the resistance is monotonic and fully reversible. For strain > 0.27, ionic conduction suddenly shuts off and the system becomes hysteretic (whereby a finite strain reduction is required to reopen the channel). Upon unloading, the original behavior is retrieved. This reversible behavior is observed over 200 compression cycles. The cross-sectional area of the channel can be inferred from the ion current measurement,as confirmed by a Nano-Computed Tomography investigation. We show that the cross-sectional area decreases monotonically with the applied compressive strain in the reversible range, in qualitative agreement with linear elasticity theory. We find that the shut-off strain is affected by the spatial extent of the applied strain, which provides additional tunability. Our tunable channel is well-suited for multiple applications in micro/nano-fluidic devices.

Incorporating Fabrication Cost into Topology Optimization of Discrete Structures and Lattices

Journal paper
A. Asadpoure, J. Guest, L. Valdevit
Structural and Multidisciplinary Optimization 51 (2015) 385-396
Publication year: 2015

Abstract

In this article, we propose a method to incorporate fabrication cost in the topology optimization of light and stiff truss structures and periodic lattices. The fabrication cost of a design is estimated by assigning a unit cost to each truss element, meant to approximate the cost of element placement and associated connections. A regularized Heaviside step function is utilized to estimate the number of elements existing in the design domain. This makes the cost function smooth and differentiable, thus enabling the application of gradient-based optimization schemes. We demonstrate the proposed method with classic examples in structural engineering and in the design of a material lattice, illustrating the effect of the fabrication unit cost on the optimal topologies. We also show that the proposed method can be efficiently used to impose an upper bound on the allowed number of elements in the optimal design of a truss system. Importantly, compared to traditional approaches in structural topology optimization, the proposed algorithm reduces the computational time and reduces the dependency on the threshold used for element removal.

Glass-Blown Pyrex Resonator with Compensating Ti Coating for Reduction of TCF

Conference Proceedings
J. Giner de Haro, L. Valdevit, A. Shkel
International Symposium of inertial Sensors and Systems, Laguna Beach, CA, Feb 25, 2014
Publication year: 2014

Abstract

This paper demonstrates that the Thermal Coefficient of resonant Frequency (TCF) of a micro glass-blown Pyrex spherical resonator can be substantially reduced by the application of a titanium (Ti) coating. Finite Elements Analysis (FEA) is used to demonstrate that the temperature dependence of the Young’s modulus of the shell material is the dominant parameter affecting the TCF of the resonator, clearly suggesting the use of a metallic compensating layer. Experimental characterization demonstrates that the TCF of a Pyrex glass-blown resonator is reduced by 70% (from 73 ppm/°C to 24 ppm/°C) by the application of a 1.33 μm thick layer of Ti. It is predicted by FEM that for a Ti layer thickness on the order of 2.5 μm the TCF will fall below 10 ppm an acceptable value for high performance resonators. This investigation is a step forward in the quest to employ the desirable properties of micro-blown resonators, such as high symmetry, manufacturing tolerances and environmental robustness.

Fabrication and Deformation of Metallic Glass Micro-Lattices

Journal paper
J. Rys, L. Valdevit, T.A. Schaedler, A.J. Jacobsen, W.B. Carter, J.R. Greer
Advanced Engineering Materials 16 (2014) 889-896
Publication year: 2014

Abstract

Recent progress in micro- and nanofabrication techniques enables the creation of hierarchically architected microlattices with dimensional control over six orders of magnitude, from centimeters down to nanometers. This hierarchical control facilitates the exploration of opportunities to exploit nano-sized material effects in structural materials. In this work, we present the fabrication, characterization, and properties of hollow metallic glass NiP microlattices. The wall thicknesses, deposited by electroless plating, were varied from %60 nm up to 600 nm, resulting in relative densities spanning from 0.02 to 0.2%. Uniaxial quasi-static compression tests revealed two different regimes in deformation: (i) Structures with a wall thickness above 150 nm failed by catastrophic failure at the nodes and fracture events at the struts, with significant micro- cracking and (ii) Lattices whose wall thickness was below 150 nm failed initially via buckling followed by significant plastic deformation rather than by post-elastic catastrophic fracture. This departure in deformation mechanism from brittle to deformable exhibited by the thin-walled structures is discussed in the framework of brittle-to-ductile transition emergent in nano-sized metallic glasses.

Energy Dissipation Mechanisms in Hollow Metallic Microlattices

Journal paper
L. Salari-Sharif, T. A. Schaedler, L. Valdevit
Journal of Materials Research 29 (2014) 1755-1770
Publication year: 2014

Abstract

When properly designed at ultra-low density, hollow metallic microlattices can fully recover from compressive strains in excess of 50%, while dissipating a considerable portion of the elastic strain energy. This article investigates the physical mechanisms responsible for energy loss upon compressive cycling, and attributes the most significant contribution to a unique form of structural damping, whereby elastic local buckling of individual bars releases energy upon loading. Subsequently, a simple mechanical model is presented to capture the relationship between lattice geometry and structural damping. The model is used to optimize the microlattice geometry for maximum damping performance. The conclusions show that hollow metallic microlattices exhibit exceptionally large values of the damping figure of merit, (Young’s modulus)^(1/3) (loss coefficient)/(density), but this performance requires very low relative densities (<1%), thus limiting the amount of energy that can be dissipated.

Accurate Stiffness Measurement of Ultralight Hollow Metallic Microlattices by Laser Vibrometry

Journal paper
L. Salari-Sharif, L. Valdevit
Experimental Mechanics, 54 (2014) 1491-1495
Publication year: 2014

Abstract

Recent progress in advanced manufacturing enables fabrication of macro-scale hollow metallic lattices with unit cells in the millimeter range and sub-unit cell features at the submicron scale. If designed to minimize mass, these metallic microlattices can be manufactured with densities lower than 1 mg/cm3, making them the lightest metallic materials ever demonstrated. Measuring the compressive stiffness of these ultralight lattices with conventional contact techniques presents a major challenge, as the lattices buckle or locally fracture immediately after contact with the loading platens is established, with associated reduction in stiffness. Non-contact resonant approaches have been successfully used in the past for modulus measurements in solid materials, at both small and large scales. In this work we demonstrate that Laser Doppler Vibrometry coupled with Finite Elements Analysis is a suitable technique for the reliable extraction of the Young’s modulus in ultralight microlattices.

The Effects of Tine Coupling and Geometrical Imperfections on the Response of DETF Resonators

Journal paper
K. Azgin, L.Valdevit
Journal of Micromechanics and Microengineering, 23 (2013) 125011 (12p)
Publication year: 2013

Abstract

This paper presents a two-degree-of-freedom analytical model for the electromechanical response of double ended tuning fork (DETF) force sensors. The model describes the mechanical interaction between the tines and allows investigation of the effect of a number of asymmetries, in tine stiffness, mass, electromechanical parameters and load sharing between the tines. These asymmetries are introduced during fabrication (e.g., as a result of undercut) and are impossible to completely eliminate in a practical design. The mechanical coupling between the tines induces a frequency separation between the in-phase and the out-of-phase resonant modes. The magnitude of this separation and the relative intensity of the two modes are affected by all the asymmetries mentioned above. Two key conclusions emerge: (i) as the external axial compressive load is increased, the in-phase mode reaches zero frequency (buckling) much faster than the out-of-phase (i.e., operational) mode, resulting in a device with a decreased load range. (ii) During the operation, balanced excitation is essential to guarantee that the out-of-phase mode remain significantly stronger than the in-phase mode, thus allowing sharp phase locked loop locking and hence robust performance. The proposed model can be used to assess the magnitude of asymmetries introduced by a given manufacturing process and accurately predict the performance of DETF force sensors. For the specific sensor characterized in this study, the proposed model can capture the full dynamic response of the DETF and accurately predict its maximum axial compressive load; by contrast, the conventional single-DOF model does not capture peak splitting and overpredicts the maximum load by ~18%. The proposed model fits the measured frequency response of the electromechanical system and its load-frequency data with coefficient of determination (R2) of 95.4% (0.954) and 99.2% (0.992), respectively.

Microlattices as Architected Thin Films: Analysis of Mechanical Properties and High Strain Elastic Recovery

Journal paper
K. J. Maloney, C. S. Roper, A. J. Jacobsen, L. Valdevit, W. B. Carter, T. A. Schaedler
APL Materials, 1 (2013) 022106
Publication year: 2013

Abstract

Ordered periodic microlattices with densities from 0.5 mg/cm3 to 500 mg/cm3 are fabricated by depositing various thin film materials (Au, Cu, Ni, SiO2, poly(C8H4F4)) onto sacrificial polymer lattice templates. Young’s modulus and strength are measured in compression and the density scaling is determined. At low relative densities, recov- ery from compressive strains of 50% and higher is observed, independent of lattice material. An analytical model is shown to accurately predict the transition between recoverable “pseudo-superelastic” and irrecoverable plastic deformation for all con- stituent materials. These materials are of interest for energy storage applications, de- ployable structures, and for acoustic, shock, and vibration damping.

Emergence of film thickness and grain size dependent elastic properties in nanocrystalline thin films

Journal paper
J. Lian, S-W. Lee, L. Valdevit, M. I. Baskes, J. R. Greer
Scripta Materialia, 68 (2013) 261–64
Publication year: 2013

Abstract

Molecular dynamics simulations of nanocrystalline Ni revealed that the in-plane Young’s modulus of 2.2 nm grained Ni film with 10 grains across its thickness was only 0.64% smaller than that of bulk, while it dropped to 24.1% below bulk value for ~1 grain across film. This size dependence arises from the increased number of more compliant grains adjacent to the free surface. Simulations of nanocrystalline diamond revealed that the anharmonicity of the potential curve determined the sensitivity of the Young’s modulus to variations in the sample size.

Compressive Strength of Hollow Microlattices: Experimental Characterization, Modeling and Optimal Design

Journal paper
L. Valdevit, S. W. Godfrey, T. A. Schaedler, A. J. Jacobsen, W. B. Carter
Journal of Materials Research, Special Issue on Porous Metals, 28 (2013) 2461-2473
Publication year: 2013

Abstract

Recent advances in multiscale manufacturing enable fabrication of hollow-truss based lattices with dimensional control spanning seven orders of magnitude in length scale (from ;50 nm to ;10 cm), thus enabling the exploitation of nano-scale strengthening mechanisms in a macroscale cellular material. This article develops mechanical models for the compressive strength of hollow microlattices and validates them with a selection of experimental measurements on nickel microlattices over a wide relative density range (0.01–10%). The limitations of beam-theory-based analytical approaches for ultralight designs are emphasized, and suitable numerical (finite elements) models are presented. Subsequently, a novel computational platform is utilized to efficiently scan the entire design space and produce maps for optimally strong designs. The results indicate that a strong compressive response can be obtained by stubby lattice designs at relatively high densities (~10%) or by selectively thickening the nodes at ultra-low densities.

Ceramic and hybrid micro-architected materials for high temperature applications

Conference Proceedings
A. L. R. Moodie, J. P. Angle, E. C. Tackett, T. J. Rupert, M. L. Mecartney, L. Valdevit
SAMPE 2013, Long Beach, CA, May 6-9, 2013
Publication year: 2013

Abstract

This article presents an innovative additive manufacturing approach for fully optimized ceramic and hybrid (ceramic/polymer) hierarchical micro-architected materials for extreme environments. The developed materials combine high-temperature capabilities, extremely low thermal conductivity, high stiffness and strength per unit weight, sufficient toughness and great resistance to oxidation. Processing involves 3D printing of a ceramic architecture, followed by bisque firing and sintering. When sufficient porosity remains after sintering (~50%), infiltration by a polymeric matrix is possible, resulting in a cellular architecture where the constituent material is a fully dense ceramic/polymer hybrid with exceptional ductility. After characterizing the microstructure and mechanical properties of the constituent material (ceramic and hybrid), we demonstrate the fabrication of a truss-core sandwich panel. Both the internal architecture and the external shape can be controlled at will in the manufacturing process. Multifunctional Thermal Protection Systems (TPS) for the next generation of high-speed aircraft (particularly hypersonics) are the prototypical, albeit not the only, application.

Ultra-high dynamic range resonant MEMS load cells for micromechanical test frames

Journal paper
K. Azgin, T. Akin, L. Valdevit
Journal of Microelectromechanical Systems 21 (2012) 1519-1529
Publication year: 2012

Abstract

This paper presents a resonant double-ended tuning fork (DETF) force sensor with an experimentally demonstrated resolution of 7 nN and a compressive load range of 0.08 N, exceeding a dynamic range of 140 dB (100 parts per billion). The resonator has a scale factor of 216 kHz/N, a Q-factor exceeding 60 000 at 3-mtorr ambient pressure, and a zero-load resonant frequency of 47.6 kHz. The resonator is kept at resonance via a phase-locked loop composed of discrete elements. The sensor is implemented with a silicon-on-glass process with a 100-μm-thick 111 silicon structural layer. The sensor and the complete readout circuit are fully embedded in a compact 65 mm × 52 mm printed circuit board (PCB). The outof-plane parasitic modes of the DETF are also investigated with finite-element simulations and laser Doppler vibrometry experiments, and are verified to be outside of the device working range. The PCB is mounted on a microstage and coupled with an off-the-shelf displacement actuator to realize an economical, versatile, and robust micromechanical test frame with unprecedented combination of force and displacement resolution and range.

Plasticity Theory at Small Scales

Book Chapter
L. Valdevit, J. W. Hutchinson
In Encyclopedia of Nanotechnology, B. Bhushan, Editor. 2012, Springer
Publication year: 2012

Abstract

Plasticity theory is the mathematical formalism that describes the constitutive model of a material undergoing permanent deformation upon loading. For polycrystalline metals at low temperature and strain rate, the J2 theory is the simplest adequate model. Classic plasticity theory does not include any explicit length scale, and as a result, the constitutive behavior is independent of the sample dimensions. As the characteristic length of a sample is reduced to the micro (and nano) scale, careful experimental observations clearly reveal the presence of a size effect that is not accounted for by the classical theory. Strain gradient plasticity is a formalism devised to extend plasticity theory to these smaller scales. For most metals, strain gradient plasticity is intended to apply to objects in the range from roughly 100 nm to 100 μm. Above 100 μm, the theory converges with the classical theory and below 100 nm surface and grain boundary effects not accounted for in the theory begin to dominate the behavior. By assuming that the plastic work (or in some theories, the yield strength) depends not only on strain but also on strain gradients (a hypothesis physically grounded in dislocation theory and, in particular, in the notion of geometrically necessary dislocations (GND) associated with incompatibility due to strain gradients), an intrinsic length scale is naturally introduced, allowing the theory to capture size effects. According to most theories, the intrinsic length scale is of the order of the distance between dislocation-clipping obstacles or cellular dislocation structures (typically, submicron to tens of microns). This continuum theory is appropriate for length scales that remain large relative to the distance between dislocations. As the sample length scale is dropped below this level, dislocations must be modeled individually, and discrete dislocations simulations (DSS) are the preferred approach. At even smaller scales, molecular dynamics (MD) becomes the applicable tool. This article presents a brief overview of one of the simplest continuum strain gradient plasticity theories that reduces to the classical J2 theory when the scale of the deformation becomes large compared to the material length scale. This simple theory captures the essence of the experimental trends observed to date regarding size effects in submicron to micron scale plasticity.

Characterization of nickel-based microlattice materials with structural hierarchy from the nanometer to the millimeter scale

Journal paper
A. Torrents, T. A. Schaedler, A. J. Jacobsen, W. B. Carter, L. Valdevit
Acta Materialia, 60 (2012) 3511-3523
Publication year: 2012

Abstract

Novel nickel-based microlattice materials with structural hierarchy spanning three different length scales (nm, μm, mm) are characterized microstructurally and mechanically. These materials are produced by plating a sacrificial template obtained by self-propagating photopolymer waveguide prototyping. Ni–P films with a thickness of 120 nm to 3 μm are deposited by electroless plating, whereas thicker films (5–26 μm) are obtained by subsequent electrodeposition of a pure Ni layer. This results in cellular materials spanning three orders of magnitude in relative density, from 0.01% to 8.5%. The thin electroless Ni–P films have ultra-fine grain size (7 nm) and a yield strength of ∼2.5 GPa, whereas the thicker electrodeposited Ni films exhibit a much broader distribution with average grain size of 116 nm and strong (1 0 0) texture in the plating direction, resulting in a yield strength of ∼1 GPa. Uniaxial compression experiments reveal two distinct mechanical responses. At ultra-low densities (<0.1%), these lattices exhibit nearly full recovery after strains up to more than 50%, and damping coefficients an order of magnitude larger than for conventional Ni foams. At higher densities (0.1–10%), the compression behavior is fully plastic, similar to traditional cellular metals. A simple mechanical analysis reveals that the transition occurs when the thickness-to-diameter ratio of the truss elements is of the order of the yield strain of the material, in agreement with experimental observations. Optical and electron imaging of deformed lattices show that the deformation largely localizes around the nodes. In the ultra-light regime, the microlattice materials are stiffer and stronger than any existing alternative.

A novel modeling platform for characterization and optimal design of micro-architected materials

Conference Proceedings
S.W. Godfrey, L. Valdevit
2012 AIAA Structural Dynamics and Materials Conference, Honolulu, HI, Apr 2012. AIAA Paper # 2012-2003
Publication year: 2012

Abstract

Recent advances in multi-scale manufacturing enable fabrication of hollow-truss based lattices with dimensional control spanning seven orders of magnitude in length scale (from ~50nm to ~10cm), offering tremendous potential for multifunctionality. Topology optimization is essential to realize the full potential of these micro-architected materials. This paper presents a novel optimal design and modeling platform, consisting of four interconnected tools: (i) a geometric modeling algorithm; (ii) a meshing algorithm; (iii) an optimal design code; and (iv) a communication interface with a commercial Finite Elements program (Abaqus). The powerfulness of the proposed platform is demonstrated for the optimization of specific stiffness in pyramidal hollow micro-lattices.

Ultralight Metallic Microlattices

Journal paper
T. A. Schaedler, A. J. Jacobsen, A. Torrents, A. E. Sorensen, J. Lian, J. R. Greer, L. Valdevit, W. B. Carter
Science, 334 (6058) 962-965 (2011)
Publication year: 2011

Abstract

Ultralight (<10 milligrams per cubic centimeter) cellular materials are desirable for thermal insulation; battery electrodes; catalyst supports; and acoustic, vibration, or shock energy damping. We present ultralight materials based on periodic hollow-tube microlattices. These materials are fabricated by starting with a template formed by self-propagating photopolymer waveguide prototyping, coating the template by electroless nickel plating, and subsequently etching away the template. The resulting metallic microlattices exhibit densities ρ ≥ 0.9 milligram per cubic centimeter, complete recovery after compression exceeding 50% strain, and energy absorption similar to elastomers. Young’s modulus E scales with density as E ~ ρ2, in contrast to the E ~ ρ3 scaling observed for ultralight aerogels and carbon nanotube foams with stochastic architecture. We attribute these properties to structural hierarchy at the nanometer, micrometer, and millimeter scales.

Protocol for the Optimal Design of Multifunctional Structures: From Hypersonics to Micro-Architected Materials

Journal paper
L. Valdevit, A. J. Jacobsen, J. R. Greer and W. B. Carter
Journal of the American Ceramic Society, Special Issue in Honor of Anthony G. Evans, 94 [S1] (2011), S15-S34
Publication year: 2011

Abstract

Cellular materials with periodic architectures have been extensively investigated over the past decade for their potential to provide multifunctional solutions for a variety of applications, including lightweight thermo-structural panels, blast resistant structures, and high-authority morphing components. Stiffer and stronger than stochastic foams, periodic cellular materials lend themselves well to geometry optimization, enabling a high degree of tailorability and superior performance benefits. This article reviews a commonly established optimal design protocol, extensively adopted at the macro-scale for both single and multifunctional structures. Two prototypical examples are discussed: the design of strong and lightweight sandwich beams subject to mechanical loads and the combined material/ geometry optimization of actively cooled combustors for hypersonic vehicles. With this body of literature in mind, we present a motivation for the development of micro-architected materials, namely periodic multiscale cellular materials with overall macroscopic dimensions yet with features (such as the unit cell or subunit cell constituents) at the micro- or nano-scale. We review a suite of viable manufacturing approaches and discuss the need for advanced experimental tools, numerical models, and optimization strategies. In analyzing challenges and opportunities, we conclude that the technology is approaching maturity for the development of micro-architected materials with unprecedented combinations of properties (e.g., specific stiffness and strength), with tremendous potential impact on a number of fields.

Mechanical Characterizations of Cast Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate)/Polyvinyl Alcohol thin films

Journal paper
C-H. Chen, A. Torrents, L. Kulinsky, R. D. Nelson, M. Madou, L. Valdevit, J.C. LaRue
Synthetic Metals, 161 (2011) 2259-2267
Publication year: 2011

Abstract

The polymer Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate), hereafter referred to as PEDOT:PSS, has electrical properties superior to those of most conducting polymers, but it is too brittle to be employed in many applications. Blending PEDOT:PSS with other polymers is a promising route to reach a good trade-off between electrical and mechanical properties. This paper describes the mechanical characterization of PEDOT:PSS/PVA (Polyvinyl Alcohol) blends. The PEDOT:PSS/PVA films used in this study are produced by casting, and uniaxial tensile tests are performed to characterize the Young’s modulus, fracture strain, tensile strength, and plastic deformation behavior of the blends as a function of the weight fraction of the components. For pure PVA, the Young’s modulus, fracture strain and tensile strength are found to be, respectively, 41.3 MPa, 111% and 41.3 MPa. The strength exhibits a nearly perfect bimodal behavior, suddenly increasing by a factor 2 at a PEDOT:PSS content of 30%. Importantly, the ductility remains extremely high (∼94%, only 20% lower than pure PVA) up to PEDOT:PSS fractions of ∼50%. The Young’s modulus monotonically increases with PEDOT:PSS content, reaching 1.63 GPa at 50%. SEM imaging and XRD analysis allows correlation of these evolutions to substantial morphological changes in the PEDOT:PSS/PVA microstructure. When combined with a previously published electrical characterization study, the current work suggests that a PEDOT:PSS/PVA polymer blend with 30–40 wt% of PEDOT:PSS provides the best trade-off of conductivity and ductility. For non free-standing films, higher PEDOT:PSS fractions (70%) might be preferable.

Implications of Shakedown for Design of Actively-Cooled Thermostructural Panels

Journal paper
N. Vermaak, L. Valdevit, A. G. Evans, F. W. Zok and R. M. McMeeking
Journal of the Mechanics of Materials and Structures 6 (2011) 1313-1327
Publication year: 2011

Abstract

Propulsion systems in future hypersonic vehicles will require use of actively cooled structures that can withstand extreme thermomechanical loads. Candidate designs and materials for such structures have previously been identified through conventional yield-based design principles. The present article out- lines an approach that utilizes concepts of localized plasticity and shakedown under cyclic loading in the design process. For this purpose, an established computational technique is used to determine shakedown limits for prototypical cooled structures. The results are employed in a design sensitivity study. The study demonstrates that, by allowing for shakedown, structures with areal densities significantly lower than those obtained from yield-limited design can be obtained. The magnitude of the benefits depends on the specific geometry of interest, the thermomechanical boundary conditions and the constraints placed on the design.

Concentration independent modulation of local micromechanics in a fibrin clot

Journal paper
M. A. Kotlarchyk, S. G. Shreim, M. B. Alvarez-Elizondo, L. C. Estrada, R. Singh, L. Valdevit, E. Kniazeva, E. Gratton, A. J. Putnam and E. L. Botvinick
PLoS ONE, 6 (2011) e20201
Publication year: 2011

Abstract

Methods for tuning extracellular matrix (ECM) mechanics in 3D cell culture that rely on increasing the concentration of either protein or cross-linking molecules fail to control important parameters such as pore size, ligand density, and molecular diffusivity. Alternatively, ECM stiffness can be modulated independently from protein concentration by mechanically loading the ECM. We have developed a novel device for generating stiffness gradients in naturally derived ECMs, where stiffness is tuned by inducing strain, while local mechanical properties are directly determined by laser tweezers based active microrheology (AMR). Hydrogel substrates polymerized within 35 mm diameter Petri dishes are strained non-uniformly by the precise rotation of an embedded cylindrical post, and exhibit a position-dependent stiffness with little to no modulation of local mesh geometry. Here we present the device in the context of fibrin hydrogels. First AMR is used to directly measure local micromechanics in unstrained hydrogels of increasing fibrin concentration. Changes in stiffness are then mapped within our device, where fibrin concentration is held constant. Fluorescence confocal imaging and orbital particle tracking are used to quantify structural changes in fibrin on the micro and nano levels respectively. The micromechanical strain stiffening measured by microrheology is not accompanied by ECM microstructural changes under our applied loads, as measured by confocal microscopy. However, super-resolution orbital tracking reveals nanostructural straightening, lengthening, and reduced movement of fibrin fibers. Furthermore, we show that aortic smooth muscle cells cultured within our device are morphologically sensitive to the induced mechanical gradient. Our results demonstrate a powerful cell culture tool that can be used in the study of mechanical effects on cellular physiology in naturally derived 3D ECM tissues.

Catastrophic vs. gradual collapse of thin-walled nanocrystalline Ni cylinders as building blocks of micro-lattice structures

Journal paper
J. Lian, L. Valdevit, T. A. Schaedler, A. J. Jacobsen, W. B. Carter and J. R. Greer
Nano Letters, 11 (2011) 4118-4125
Publication year: 2011

Abstract

Lightweight yet stiff and strong lattice structures are attractive for various engineering applications, such as cores of sandwich shells and components designed for impact mitigation.
Recent breakthroughs in manufacturing enable efficient fabrication of hierarchically architected microlattices, with dimensional control spanning seven orders of magnitude in length scale. These materials have the potential to exploit desirable nanoscale-size effects in a macroscopic structure, as long as their mechanical behavior at each appropriate scale nano, micro, and macro levels is properly understood. In this letter, we report the nanomechanical response of individual microlattice members. We show that hollow nanocrystalline Ni cylinders differing only in wall thicknesses, 500 and 150 nm, exhibit strikingly different collapse modes: the 500 nm sample collapses in a brittle manner, via a single strain burst, while the 150 nm sample shows a gradual collapse, via a series of small and discrete strain bursts. Further, compressive strength in 150 nm sample is 99.2% lower than predicted by shell buckling theory, likely due to localized buckling and fracture events observed during in situ compression experiments. We attribute this difference to the size-induced transition in deformation behavior, unique to nanoscale, and discuss it in the framework of “size effects” in crystalline strength.

A resonant tuning fork sensor with unprecedented combination of resolution and range

Conference Proceedings
K. Azgin, C. Ro, A. Torrents, T. Akin and L. Valdevit
2001 IEEE MEMS Conference, Cancun, Mexico, Jan 2011
Publication year: 2011

Abstract

This paper presents a double-ended tuning fork (DETF) force sensor with a resolution of 7nN and a range of 0.12N. The resonator has a scale factor of 216 kHz/N, a Q-factor exceeding 60,000 at 3mTorr ambient pressure and a zero-load resonant frequency of 47.6 kHz. The sensor and the complete readout circuit are fully embedded in a compact 65 mm × 52 mm printed circuit board (PCB). The PCB is mounted on a micro-stage and coupled with an off-the-shelf displacement actuator to realize an economical, versatile and robust micro mechanical test frame with unprecedented combination of force and displacement resolutions and ranges.