Publication Types:

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.

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.

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.

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.

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.

Pressure Induced Amorphization in Silicon Caused by the Impact of Electrosprayed Nanodroplets

Journal paper
M. Gamero-Castano, A. Torrents, L. Valdevit, J-G. Zheng
Physical Review Letters 105 (2010) 145701
Publication year: 2010

Abstract

This Letter describes the shock-induced amorphization of single-crystal Si bombarded by nanodroplets. At impact velocities of several kilometers per second, the projectiles trigger strong compression pulses lasting tens of picoseconds. The phase transition, confirmed via transmission electron microscopy and electron backscatter diffraction, takes place when the projectile’s stagnation pressure is approximately 15 GPa. We speculate that the amorphization results either from the decompression of the beta-Sn phase or during the compression of the diamond phase.

MEMS resonant load cells for micro-mechanical test frames: Feasibility study and optimal design

Journal paper
A. Torrents, K. Azgin, S. W. Godfrey, E. S. Topalli, T. Akin, L. Valdevit
Journal of Micromechanics and Microengineering, 20 (2010) 125004 (17pp)
Publication year: 2010

Abstract

This paper presents the design, optimization and manufacturing of a novel micro-fabricated load cell based on a double-ended tuning fork. The device geometry and operating voltages are optimized for maximum force resolution and range, subject to a number of manufacturing and electromechanical constraints. All optimizations are enabled by analytical modeling (verified by selected finite elements analyses) coupled with an efficient C++ code based on the particle swarm optimization algorithm. This assessment indicates that force resolutions of ∼0.5–10 nN are feasible in vacuum (∼1–50 mTorr), with force ranges as large as 1 N. Importantly, the optimal design for vacuum operation is independent of the desired range, ensuring versatility. Experimental verifications on a sub-optimal device fabricated using silicon-on-glass technology demonstrate a resolution of ∼23 nN at a vacuum level of ∼50 mTorr. The device demonstrated in this article will be integrated in a hybrid micro-mechanical test frame for unprecedented combinations of force resolution and range, displacement resolution and range, optical (or SEM) access to the sample, versatility and cost.

Influence of Configuration on Materials Selection for Actively-Cooled Combustors

Journal paper
N. Vermaak, L. Valdevit, A. G. Evans, F. W. Zok and R. M. McMeeking
AIAA Journal of Propulsion and Power 26 (2010) 295-302
Publication year: 2010

Abstract

The influence of combustor size and shape on material feasibility is explored using (structural and fuel) weight, as well as fuel economy as metrics. A materials selection methodology developed for actively cooled rectangular panels has been embellished to include cylindrical/annular configurations. The procedure incorporates an analytical model for temperature and stress distributions subject to thermomechanical loads representative of hypersonic flight conditions. The model has been numerically verified using finite element simulations. By combining the model with optimization routines, materials robustness maps have been produced, depicting the range of thermal loads and fuel flow rates that satisfy all design constraints. A wide selection of high-temperature materials has been investigated. Comparisons of cylindrical and rectangular combustors are made for the leading candidates. It is established that the cylindrical designs allow both lighter optimal structures as well as greater robustness and fuel economy.

Materials Property Profiles for Actively Cooled Panels: An Illustration for Scramjet Applications

Journal paper
N. Vermaak, L. Valdevit, A.G. Evans
Metallurgical and Materials Transactions A 40A (2009) 877-890
Publication year: 2009

Abstract

A scheme for identifying and visualizing the material properties that limit the performance of candidate materials for actively cooled aerospace propulsion components is presented and illustrated for combustor panels for Mach 7 hypersonic vehicles. The method provides a framework for exploring the nonlinear interactions between design and materials optimization. By probing the active constraints along the border of feasible design space, the limiting properties have been elucidated for a representative group of candidate materials. Property vectors that enhance design options have also been determined. For one of the promising candidate alloys (the Ni-based superalloy, INCONEL X-750), the possibilities of reclaiming design space and lowering optimal combustor panel weight by tailoring its strength properties are assessed.

Feasibility of metallic structural heat pipes as sharp leading edges for hypersonic vehicles

Journal paper
C. Steeves, M.Y. He, S.D. Kasen, L. Valdevit, H.N.G. Wadley and A.G. Evans
Journal of Applied Mechanics 76 (2009) 031014 (9p)
Publication year: 2009

Abstract

The influence of combustor size and shape on material feasibility is explored using (structural and fuel) weight, as well as fuel economy as metrics. A materials selection methodology developed for actively cooled rectangular panels has been embellished to include cylindrical/annular configurations. The procedure incorporates an analytical model for temperature and stress distributions subject to thermomechanical loads representative of hypersonic flight conditions. The model has been numerically verified using finite element simulations. By combining the model with optimization routines, materials robustness maps have been produced, depicting the range of thermal loads and fuel flow rates that satisfy all design constraints. A wide selection of high-temperature materials has been investigated. Comparisons of cylindrical and rectangular combustors are made for the leading candidates. It is established that the cylindrical designs allow both lighter optimal structures as well as greater robustness and fuel economy.

Organic substrates for flip chip design: a thermo-mechanical model that accounts for heterogeneity and anisotropy

Journal paper
L. Valdevit, V. Khanna, A. Sharma, S. Sri-Jayantha, D. Questad, K. Sikka
Microelectronics Reliability, 48 (2008), 245-260
Publication year: 2008

Abstract

We present a thermo-mechanical characterization of organic substrates that accounts for heterogeneity both in the in-plane and out-of-plane directions. Systematic observation of the board files of a number of substrates of commercial interest reveals primarily three recurrent topological arrangements of copper and polymer; for each arrangement, the in-plane effective thermo-elastic properties are calculated via appropriate composite materials models. The averaging process in the out-of-plane direction (i.e. the stacking effect) is performed using standard laminated plate theory. The model is successfully applied to various regions of three organic substrates of interest (mainly differing in core thickness): the analytically calculated effective Young’s moduli (E) and coefficients of thermal expansion (CTE) are shown to be typically within 10% of the experimental measurements. An important attribute of this model is its ability to provide substrate description at various levels of complexity: a few effective properties are outputted that can be useful for further purely analytical investigations; at the same time, the model provides the full stiffness matrix for each region of the substrate, to be used for more detailed finite elements simulations of higher-level structures (e.g. silicon die/underfill/substrate/cooling solution assemblies). Preliminary application of this model to the warp analysis of a flip-chip is presented in the end.

A materials selection protocol for lightweight actively cooled panels

Journal paper
L. Valdevit, N. Vermaak, F. W. Zok, A. G. Evans
Journal of Applied Mechanics 75 (2008) 061022 (15p)
Publication year: 2008

Abstract

This article provides a materials selection methodology applicable to lightweight actively cooled panels, particularly suitable for the most demanding aerospace applications. The key ingredient is the development of a code that can be used to establish the capabilities and deficiencies of existing panel designs and direct the development of advanced materials. The code is illustrated for a fuel-cooled combustor liner of a hypersonic vehicle, optimized for minimum weight subject to four primary design constraints (on stress, temperatures, and pressure drop). Failure maps are presented for a number of candidate high-temperature metallic alloys and ceramic composites, allowing direct comparison of their thermostructural performance. Results for a Mach 7 vehicle under steady-state flight conditions and stoichiometric fuel combustion reveal that, while C–SiC satisfies the design requirements at minimum weight, the Nb alloy Cb752 and the Ni alloy Inconel X-750 are also viable candidates, albeit at about twice the weight. Under the most severe heat loads (arising from heat spikes in the combustor), only Cb752 remains viable. This result, combined with robustness benefits and fabrication facility, emphasizes the potential of this alloy for scramjets. 

Structural performance of near-optimal sandwich panels with corrugated cores

Journal paper
L. Valdevit, Z. Wei, C. Mercer, F. W. Zok, A. G. Evans
International Journal of Solids and Structures, 43 (2006), 4888-4905
Publication year: 2006

Abstract

An experimental and computational study of the bending response of steel sandwich panels with corrugated cores in both transverse and longitudinal loading orientations has been performed. Panel designs were chosen on the basis of failure mechanism maps, constructed using analytic models for failure initiation. The assessment affirms that the analytic models provide accurate predictions when failure initiation is controlled by yielding. However, discrepancies arise when failure initiation is governed by other mechanisms. One difficulty is related to the sensitivity of the buckling loads to the rotational constraints of the nodes, as well as to fabrication imperfections. The second relates to the compressive stresses beneath the loading platen. To address these deficiencies, existing models for core failure have been expanded. The new results have been validated by experimental measurements and finite element simulations. Limit loads have also been examined and found to be sensitive to the failure mechanism. When face yielding predominates, appreciable hardening follows the initial non-linearity, rendering robustness. Conversely, for designs controlled by buckling (either elastic or plastic) failure initiation is immediately followed by softening. The implication is that, when robustness is a key requirement, designs within the face failure domain are preferred.

Optimal active cooling performance of metallic sandwich panels with prismatic cores

Journal paper
L. Valdevit, A. Pantano, H. A. Stone, A. G. Evans
Int. Journal of Heat and Mass Transfer, 49 (2006), 3819-3830
Publication year: 2006

Abstract

All-metallic sandwich panels with prismatic cores are being currently investigated for combined structural and active cooling performance. We present a new approach to active cooling performance, and use it to optimize the panel geometry for four different systems: aluminum-air, aluminum-water, aluminum-gasoline and titanium-gasoline. The results show that some geometric parameters can be fixed without much detriment in thermal performance. Moreover, while optimal core densities are typically 25–50%, near-optimal results can be obtained with densities as low as 10%. These findings provide considerable geometric flexibility when attempting combined thermal and structural optimization.

Structurally optimized sandwich panels with prismatic cores

Journal paper
L. Valdevit, J. W. Hutchinson, A. G. Evans
International Journal of Solids and Structures, 41 (2004), 5105-5124
Publication year: 2004

Abstract

Multifunctional sandwich panels with corrugated and prismatic diamond cores have been analyzed and their behavior compared with panels designed using truss and honeycomb cores. Failure mechanism maps have been devised that account for interactions between core and face members during buckling. The optimal dimensions and the minimum weight have been evaluated. The load capacities predicted for near-optimal designs have been validated by conducting selected finite element calculations. Designs that use diamond prismatic cores (with corrugation order 4) are slightly more weight efficient than trusses, when optimized for a specific loading direction. Honeycomb cores, while somewhat more weight efficient, especially at lower load capacities, are not amenable to the fluid flows needed for cooling. We conclude that the diamond prismatic topology is the most weight efficient among designs amenable to simultaneous load bearing and active cooling.