Professor Bruck - Academia.edu (original) (raw)

Papers by Professor Bruck

Fracture, Fatigue, Failure and Damage Evolution , Volume 3, 2021

Mechanics of Composite, Hybrid and Multifunctional Materials, Fracture, Fatigue, Failure and Damage Evolution, Volume 3, 2022

Mechanics of Composite, Hybrid and Multifunctional Materials, Volume 5, 2018

Recent advances in additive manufacturing have enabled the realization of concepts in layered mat... more Recent advances in additive manufacturing have enabled the realization of concepts in layered materials that were not previously viable. We have created a layered manufacturing process using robots to fabricate actuators using prefabricated materials, e.g. solar cells and batteries. This has allowed us to design and fabricate multifunctional structures that can encompass new capabilities for applications, such as pneumatic actuators for robotics. One way to enhance the functionality of these actuators is to use layers that are not fused together to create "variable stiffness" components when a vacuum is drawn. Studying the mechanics of these components is important in understanding how to configure the layers of the jamming multifunctional actuators. We have prototyped and characterized the mechanics of a layered jamming multifunctional actuator to model the effects of layering on the multifunctional performance of these new actuation materials. The proposed model is for a soft actuator with opposing jamming layers, and the predictions of actuation performance were found to be consistent with measurements from the prototype structures. Three different types of composite jamming materials were investigated, consisting of polyurethane rubber, silicone, or paper with paper. It was found that the order of magnitude difference in the stiffness of the polyurethane rubber and silicone with paper reduced the predicted curvature of the more compliant material under transverse load by approximately 50%, which increases the shear strain between the stiffer materials due to the reduced shear stiffness of the more compliant material. Two prototype actuation structures were fabricated: (1) a programmable array of layered jamming actuators to control the 3D shape of a flexible solar cell for energy harvesting, and (2) a multi-mode actuator capable of bending and extensional deformations. We have also demonstrated the viability of these actuators on a robotic platform known as "ArmadilloBot" that is capable of walking, and then morphing into a rolled-up structure, just like a real Armadillo.

A new Materials by Design approach to creating energetic materials using Functionally Graded Mate... more A new Materials by Design approach to creating energetic materials using Functionally Graded Materials (FGMs) concepts has recently been developed in a joint collaboration between the University of Maryland (UMD) and Indian Head-Naval Surface Warfare Center (IH-NSWC) through the Center for Energetic Concepts Development (CECD). This approach has been facilitated by previous efforts at IH-NSWC to apply a new process, known as Twin Screw Extrusion (TSE), for continuously manufacturing energetic polymer composites. It takes advantage of the continuous nature and superior mixing characteristics of the TSE process to manufacture a new concept for propellants and explosives: Functionally Graded Energetic Materials (FGEMs). For example, conventional geometricallycomplex homogeneous grains for solid rocket motors can be replaced by a geometrically-simpler cylindrical FGEM configuration with an axial gradient in the energetic material formulation. The simpler geometry does not have the undes...

International Journal of Solids and Structures, 2006

Pressureless sintering of powder-processed functionally graded materials is being pursued to econ... more Pressureless sintering of powder-processed functionally graded materials is being pursued to economically produce metal-ceramic composites for a variety of high-temperature (e.g., thermal protection) and energy-absorbing (e.g., armor) applications. During sintering, differential shrinkage induces stresses that can compromise the integrity of the components. Because the strength evolves as the component is sintered, it is important to model how the evolution of the differential shrinkage governs the stress distribution in the component in order to determine when the strength will be exceeded and cracking initiated. In this investigation, a model is proposed that describes the processing/microstructure/property/ performance relationship in pressurelessly sintered functionally graded plates and rods. This model can be used to determine appropriate shrinkage rates and gradient architectures for a given component geometry that will prevent the component from cracking during pressureless sintering by balancing the evolution of strength, which is assumed to be a power law function of the porosity, with the evolution of stress. To develop this model, the powder mixture is considered as a three-phase material consisting of voids, metal particles, and ceramic particles. A micromechanical thermal elasticviscoplastic constitutive model is then proposed to describe the thermomechanical behavior of the composite microstructure. The subsequent evolution of the thermomechanical properties of the matrix material during sintering is assumed to obey a power law relationship with the level of porosity, which is directly related to the shrinkage strain, and was refined to account for the evolving interparticle cohesion of the matrix phase due to sintering. These thermomechanical properties are incorporated into a 2-D thermomechanical finite element analysis to predict the stress distributions and distortions that arise from the evolution of differential shrinkage during the pressureless sintering process. Differential shrinkage results were verified quantitatively through comparison with the shape profile for a pressurelessly sintered functionally graded nickel-alumina composite plate with a cylindrical geometry, and the stress distribution results verified from qualitative observations of the absence or presence of cracking as well as the location in specimens with different gradient architectures. The cracking was mitigated using a reverse gradient at one end of the specimen, and the resulting distortions associated with the shape profile were determined to be no more than 15% reduced from the predictions. The effects of geometry were also studied out-of-plane by transforming the plate into a rod through an increase in thickness, while in-plane effects were studied by comparing the results from the cylindrical specimen with a specimen that has a square

Conference Proceedings of the Society for Experimental Mechanics Series, 2011

Page 1. Mechanical Behavior of Bio-inspired Sandwich Composites Sandip Haldar, Jachimike K. Imo a... more Page 1. Mechanical Behavior of Bio-inspired Sandwich Composites Sandip Haldar, Jachimike K. Imo and Hugh A. Bruck Department of Mechanical Engineering, Univ. of Maryland, College Park, MD 20742 Abstract Nature is ...

Volume 5A: 38th Mechanisms and Robotics Conference, 2014

Flapping wing miniature aerial vehicles (FWMAVs) offer advantages over traditional fixed wing or ... more Flapping wing miniature aerial vehicles (FWMAVs) offer advantages over traditional fixed wing or quadrotor MAV platforms because they are more maneuverable than fixed wing aircraft and are more energy efficient than quadrotors, while being quieter than both. Currently, autonomy in FWMAVs has only been implemented in flapping vehicles without independent wing control, limiting their level of control. We have developed Robo Raven IV, a FWMAV platform with independently controllable wings and an actuated tail controlled by an onboard autopilot system. In this paper, we present the details of Robo Raven IV platform along with a control algorithm that uses a GPS, gyroscope, compass, and custom PID controller to autonomously loiter about a predefined point. We show through simulation that this system has the ability to loiter in a 50 meter radius around a predefined location through the manipulation of the wings and tail. A simulation of the algorithm using characterized GPS and tail resp...

ABSTRACT A novel biaxial microtensile tester has been designed and built to assess the mechanical... more ABSTRACT A novel biaxial microtensile tester has been designed and built to assess the mechanical properties of thin films. Loading is achieved along two normal axes , whose components are suspended on air bearings , permitting independent and/or simultaneous frictionless loading by nanometric motion capable motors . Available load cells allow a choice of three maximum tensile load levels: 1, 5 and 10 Newtons . A video camera is integral to an Atomic Force Microscope (AFM) and is used for specimen alignment and positioning. The AFM is used to assess specimen surface topography before and after specimen load application. Digital Image Correlation (DIC) is capable of resolving nanoscale full-field displacements over the area of interest. Preliminary testing is focused in assessing the microtensile tester capabilities, while pursuing an understanding of the microscopic deformation of sputtered copper thin films.

Experimental Mechanics of Composite, Hybrid, and Multifunctional Materials, Volume 6, 2013

The flight time of miniature air vehicles (MAVs) is limited by the need for a portable, light wei... more The flight time of miniature air vehicles (MAVs) is limited by the need for a portable, light weight power source. The development of multifunctional, power generating wings has the capability of extending flight time without compromising overall flight performance. This paper seeks to investigate the feasibility of integrating flexible solar cells onto the flapping wings of a MAV to create “compliant multifunctional wing structures”. Data is collected for bird-inspired miniature air vehicle wings designed with carbon fiber spars, and for comparable wings designed with a monolithic compliant component. Both of the designs are tested with wing bodies composed of plain Mylar foil with flexible, lightweight solar cells integrated onto them. The test setup is designed to simulate MAV operation under zero forward velocity. A motor that controls the wing flapping scheme is fixed to a rigid test stand. A 6 degree of freedom (DOF) load cell is used to measure aerodynamic lift as a function of time for a synchronized flapping scheme during wind tunnel testing. A second experiment is conducted to verify the functionality of solar cells as a regenerative energy source in real flight. A single compliant wing and a single regular wing are consecutively fixed to the test setup used in the first experiment, and the test is conducted under direct sunlight. The voltage generated by the solar cells is collected as a function of time, while the wing is flapping. The lift data is used to estimate flight characteristics, while the voltage data is used to estimate the viability of energy harvesting.

Volume 6B: 37th Mechanisms and Robotics Conference, 2013

Flapping wing air vehicles offer many useful flight characteristics due to their versatility, as ... more Flapping wing air vehicles offer many useful flight characteristics due to their versatility, as proven by flying animals. Wing design significantly influences the performance. However designing successful wings presents significant challenges. Efficient matching of the drive motors to the flapping wings is necessary to overcome the highly constrained weight budget. Simulating detailed information about the force response due to flapping is challenging due to complex fluid-structural interactions of the wings resulting in non-linear force response to flapping motion. To overcome this challenge, we conducted an experimental study of flapping wings to provide detailed temporal force response data for flapping wings. A prototype was built by synthesizing lightweight manufacturing techniques with the results of the experimental study. Our experimental investigations enabled us to select the flapping angle range and flapping frequency.

Strain, 2011

A methodology has been developed for accurately measuring the mechanical properties of materials ... more A methodology has been developed for accurately measuring the mechanical properties of materials used on the micro-scale. The direct tension test method using a dog bone-type specimen has been employed, as it is the most effective and straightforward method to obtain results including a full stress-strain curve. The goal of this investigation was to develop a universal, yet simple and reliable, methodology to be used for accurate characterisation of mechanical properties for a wide variety of materials. Specimens from single crystal silicon were fabricated using photolithography by means of deep reactive ion etching. This material was chosen as it is expected that on both the micro-and macro-scales, Young's modulus will have the same value. Hence, the accuracy of the methodology may be unambiguously examined. The test setup includes a small test machine containing a load cell whose maximum capacity is 5 N and is capable of direct gripping and displacement control. The specimens were found to have a trapezoidal cross-section that was accurately measured using a scanning electron microscope. The strains were obtained by means of digital image correlation using images obtained via optical microscopy. The quantities measured include Young's modulus E, the fracture strength r f and the fracture strain f. The average value of E obtained in the micro-tests agrees well with the reference value obtained on the macro-scale.

Sensors and Actuators A: Physical, 2011

Polymer, 2008

The addition of multi-walled carbon nanotubes (MWCNTs) or carbon nanofibers (CNFs) to polymeric m... more The addition of multi-walled carbon nanotubes (MWCNTs) or carbon nanofibers (CNFs) to polymeric melts offers a convenient route to obtain highly conductive plastics. However, when these materials are melt processed, their conductivity can be lost. Here, it is shown that conductivities can be recovered through melt annealing at temperatures above the polymer's glass transition temperature (T g). We demonstrate these results for both MWCNT and CNF-based composites in polystyrene (PS). The mechanism behind the conductivity increase is elucidated through modeling. It involves a transition from aligned, unconnected particles prior to annealing to an interconnected network after annealing through viscoelastic relaxation of the polymer. Such rearrangement is directly visualized for the case of the CNFbased composites using confocal microscopy. The annealing-induced increase in particle connectivity is also reflected in dynamic rheological measurements on both MWCNT and CNF composites as an increase in their elastic moduli at low frequencies.

Nanotechnology, 2007

For the first time, an interpenetrating phase polymer nanocomposite formed by the percolation of ... more For the first time, an interpenetrating phase polymer nanocomposite formed by the percolation of multiwalled carbon nanotubes (MWCNTs) in polystyrene (PS) has been quantitatively characterized through electrical conductivity measurements and melt rheology. Both sets of measurements, in conjunction with scanning electron microscopy (SEM) images, indicate the presence of a continuous phase of percolated MWCNTs appearing at particle concentrations exceeding 2 vol% MWCNTs in PS. To quantify the amount of this continuous phase present in the PS/MWCNT composite, electrical conductivity data at various MWCNT concentrations, β, are correlated with a proposed degree of percolation,C(β), developed using a conventional power-law formula with and without a percolation threshold. To quantify the properties of the interpenetrating phase polymer nanocomposite, the PS/MWCNT composite is treated as a combination of two phases: a continuous phase consisting of a pseudo-solid-like network of percolated MWCNTs, and a continuous PS phase reinforced by non-interacting MWCNTs. The proposed degree of percolation is used to quantify the distribution of MWCNTs among the phases, and is then used in a rule-of-mixtures formulation for the storage modulus, G (β,C(β), ω), and the loss modulus, G (β,C(β), ω), to quantify the properties of the continuous phase consisting of percolated MWCNTs and the continuous PS phase reinforced by non-interacting MWCNTs from the experimental melt rheology data. The properties of the continuous phase of percolated MWCNTs are indicative of a scaffold-like microstructure exhibiting an elastic behavior with a complex modulus of 360 kPa at lower frequencies and viscoplastic behavior with a complex viscosity of 6 kPa s rad −1 at higher frequencies, most likely due to a stick-slip friction mechanism at the interface of the percolated MWCNTs. Additional evidence of this microstructure was obtained via scanning electron microscopy. This research has important implications in providing a new methodology based on the electrical and rheological properties of the polymer nanocomposite for quantifying the continuous phase formed by the percolation of new functionalized nanostructures being developed for: (a) controlling the percolation of the nanostructures through self-assembly, (b) enhancing their interaction with the continuous reinforced polymer phase, (c) enhancing the cohesion between nanostructures.

Macromolecules, 2007

A systematic electrical and rheological characterization of percolation in commercial polydispers... more A systematic electrical and rheological characterization of percolation in commercial polydisperse polystyrene (PS) nanocomposites containing multiwall carbon nanotubes (MWCNTs) is presented. The MWCNTs confer appreciable electrical conductivities (up to ca. 1 S/m) to these nanocomposites at a concentration of 8 vol %. In addition to enhancing the electrical properties, even at small concentrations (ca. 2 vol %), MWCNTs significantly enhance the rheological properties of PS melts. At concentrations exceeding 2 vol %, a plateau appears in the storage modulus G′ at low frequencies, indicating the formation of a percolated MWCNT network that responds elastically over long timescales. Network formation, in turn, implies a diverging complex viscosity vs complex modulus curve. A focus of this study is on the correlation between electrical and rheological properties at the onset of percolation. The experimental results indicate that the elastic load transfer and electrical conductivity are far more sensitive to the onset of percolation than the viscous dissipation in the nanocomposite. Sensitivity of the electrical and rheological percolations to two different solvents used in processing the nanocomposites has also been characterized.

Journal of Manufacturing Science and Engineering, 2010

In-mold assembly can be used to create mesoscale articulating polymeric joints that enable the mi... more In-mold assembly can be used to create mesoscale articulating polymeric joints that enable the miniaturization of devices, reduction in production costs, and increase in throughput. One of the major challenges in miniaturizing devices using the in-mold assembly is to develop appropriate characterization techniques and modeling approaches for the interaction between polymer melt flow fronts and premolded components. When a high speed, high temperature second stage melt comes in contact with a premolded mesoscale component that has similar melting temperatures, the premolded component can experience a significant plastic deformation due to the thermal softening and the force associated with impingement of the melt flow front. In our previous work, we developed methods to inhibit the plastic deformation by supporting the ends of the mesoscale premolded components. In this paper, we present an alternative strategy for controlling premolded component deformations. This involves a mesosca...

Experimental Mechanics, 2004

Interfaces in heterogeneous structures are typically engineered for optimal strength through the ... more Interfaces in heterogeneous structures are typically engineered for optimal strength through the control of surface roughness and the choice of adhesives. Advances in manufacturing technologies are now making it possible to also tailor the geometries of interfaces from the nanoscale to the macroscale to create geometrically complex interfaces that exhibit enhanced performance characteristics. However, the impact of geometric complexity on the mechanical behavior of interfaces has not yet been ascertained. In this investigation, the first step is taken towards understanding the effects of geometric complexity on interfacial strength. A new multistage, multi-piece molding process is used to create heterogeneous polymer structures with geometrically complex interfaces consisting of rectangular and circular interlocking features. The structural integrity of these heterogeneous structures is characterized through interfacial tension testing.The full-field deformation measurement technique known as digital image correlation is also used during the testing to visualize the deformation fields around the geometrically complex features. Through this characterization, it is determined that the complex geometries increase the interfacial strength by approximately 20-25%, while reducing the statistical variation by 50%. These effects are attributed to a transition in the failure mechanism from interfacial fracture to homogeneous ligament failure. Results also indicate that geometrically complexity can be used on completely debonded interfaces to increase the strength to at least 25-35% of the bonded interface. Based on these results, some simple design rules have been proposed that enable geometrically complex interfaces to be engineered with enhanced strengths approaching the weaker of the two base materials. These design rules can also be used in the engineering of interfaces to facilitate the development of heterogeneous structures using new design paradigms, such as design for recyclability and the design of products based on bio-inspired concepts.

Experimental Mechanics, 2004

Structures are being actuated by embedding shape memory alloy (SMA) wires into compliant material... more Structures are being actuated by embedding shape memory alloy (SMA) wires into compliant materials, such as polyurethane. To achieve bending actuation, these wires are placed in opposing wire configurations, where multiple wires are often employed to enhance the amplitude of the bending actuation response. In this investigation, a procedure has been developed for fabricating polyurethanes with a symmetrically graded distribution of SMA wires. The effects of grading the distribution of one-way SMA wires have been characterized using full-field displacement deformation measurements obtained with the digital image correlation (DIC) technique. These measurements have been used in a onedimensional (1D) model of bending actuation to determine the "equivalent two-way shape memory effect (SME)" of the graded wire distribution. To utilize the 1D actuation model, the constitutive properties of the polyurethane structure predicted by rule-of-mixture formulations were reduced to account for the differences in strain between the SMA wires and the polyurethane matrix. The graded wire distribution was also found to significantly stiffen the polyurethane structure. The level of equivalent two-way SME therefore became limited by the maximum recovery stress of the SMA wires, with a maximum level that was approximately 75% less than previously measured levels in an opposing wire Configuration. However, the bending actuation behavior was more symmetric, and the actuated bending deflections were similar to those observed when using more compliant materials. It was also predicted that the symmetrically graded wire distribution would exhibit abetter balance between actuation amplitude and uniformity, which combined with the more symmetric actuation behavior makes the graded wire distribution potentially more desirable for achieving higher actuation frequencies with distributed actuation concepts in new applications, such as miniaturized double diaphragm pumping devices. KEY WORDS-Functionally graded materials, smart structures, digital image correlation, bending actuation, rule-ofmixtures, equivalent two-way shape memory effect

Experimental Mechanics, 2009

Compliant wing designs have the potential of improving flapping wing Micro-Air Vehicles (MAVs). D... more Compliant wing designs have the potential of improving flapping wing Micro-Air Vehicles (MAVs). Designing compliant wings requires a detailed understanding of the effect of compliance on the generation of thrust and lift forces. The low force and high-frequency measurements associated with these forces necessitated a new versatile test stand design that uses a 250 g load cell along with a rigid linear air bearing to minimize friction and the dynamic behavior of the test stand while isolating only the stationary thrust or lift force associated with drag generated by the wing. Moreover, this stand is relatively inexpensive and hence can be easily utilized by wing designers to optimize the wing compliance and shape. The frequency response of the wing is accurately resolved, along with wing compliance on the thrust and lift profiles. The effects of the thrust and lift force generated as a function of flapping frequency were also determined. A semi-empirical aerodynamic model of the thrust and lift generated by the flapping wing MAV on the new test stand was developed and used to evaluate the measurements. This model accounted for the drag force and the effects of the wing compliance. There was good correlation between the model predictions and experimental measurements. Also, the increase in average thrust due to increased wing compliance was experimentally quantified for the first time using the new test stand. Thus, our measurements for the first time reveal the detrimental influence of excessive compliance on drag forces during high frequency operation. In addition, we were also able to observe the useful effect of compliance on the generation of extra thrust at the beginning and end of upstrokes and downstrokes of the flapping motion.

Experimental Mechanics, 2010

Palmetto wood is garnering growing interest as a template for creating biologically-inspired poly... more Palmetto wood is garnering growing interest as a template for creating biologically-inspired polymer composites due to its historical use as an energy absorbing material in protective structures. In this study, quasi-static three-point bend tests have been performed to characterize the mechanical behavior of Palmetto wood. Full-field deformation measurements are obtained using Digital Image Correlation (DIC) to elucidate on the strain fields associated with the mechanical response. By analyzing strain fields at multiple length scales, it is possible to study the more homogeneous mechanical behavior at the macroscale associated with the global load-deformation response; while at the microscale the mechanical behavior is more inhomogeneous due to microstructural failure mechanisms. Thus, it was possible to determine that, despite the presence of discontinuous macro-fiber reinforcement, at the macroscale the response is associated with classical bending and progressive failure processes that are adequately described by Weibull statistics proceeding from the tensile side of the specimen. At the microscale, however, the failure mechanisms giving rise to the macroscopic response consist of both shear-dominated debonding between the fiber and matrix, and inter-fiber matrix failure due to pore collapse. These microscale mechanisms are present in both the compressive and tensile regions of the specimen, most likely due to local macro-fiber bending, which is independent of the global bending state. The pore collapse mechanism observed during mechanical loading appears to improve the energy absorption of the matrix material, thereby, transferring less energy and shear strain to the macro-fibermatrix interface for initiation of debonding. However, the pore collapse mechanism can also accumulate substantial shear strain, which results in matrix shear cracking. Through these complex failure mechanisms, Palmetto wood exhibits a high resistance to catastrophic failure after damage initiation, an observation that can be used as inspiration for creating new polymer composite materials.

Fracture, Fatigue, Failure and Damage Evolution , Volume 3, 2021

Mechanics of Composite, Hybrid and Multifunctional Materials, Fracture, Fatigue, Failure and Damage Evolution, Volume 3, 2022

Mechanics of Composite, Hybrid and Multifunctional Materials, Volume 5, 2018

Recent advances in additive manufacturing have enabled the realization of concepts in layered mat... more Recent advances in additive manufacturing have enabled the realization of concepts in layered materials that were not previously viable. We have created a layered manufacturing process using robots to fabricate actuators using prefabricated materials, e.g. solar cells and batteries. This has allowed us to design and fabricate multifunctional structures that can encompass new capabilities for applications, such as pneumatic actuators for robotics. One way to enhance the functionality of these actuators is to use layers that are not fused together to create "variable stiffness" components when a vacuum is drawn. Studying the mechanics of these components is important in understanding how to configure the layers of the jamming multifunctional actuators. We have prototyped and characterized the mechanics of a layered jamming multifunctional actuator to model the effects of layering on the multifunctional performance of these new actuation materials. The proposed model is for a soft actuator with opposing jamming layers, and the predictions of actuation performance were found to be consistent with measurements from the prototype structures. Three different types of composite jamming materials were investigated, consisting of polyurethane rubber, silicone, or paper with paper. It was found that the order of magnitude difference in the stiffness of the polyurethane rubber and silicone with paper reduced the predicted curvature of the more compliant material under transverse load by approximately 50%, which increases the shear strain between the stiffer materials due to the reduced shear stiffness of the more compliant material. Two prototype actuation structures were fabricated: (1) a programmable array of layered jamming actuators to control the 3D shape of a flexible solar cell for energy harvesting, and (2) a multi-mode actuator capable of bending and extensional deformations. We have also demonstrated the viability of these actuators on a robotic platform known as "ArmadilloBot" that is capable of walking, and then morphing into a rolled-up structure, just like a real Armadillo.

A new Materials by Design approach to creating energetic materials using Functionally Graded Mate... more A new Materials by Design approach to creating energetic materials using Functionally Graded Materials (FGMs) concepts has recently been developed in a joint collaboration between the University of Maryland (UMD) and Indian Head-Naval Surface Warfare Center (IH-NSWC) through the Center for Energetic Concepts Development (CECD). This approach has been facilitated by previous efforts at IH-NSWC to apply a new process, known as Twin Screw Extrusion (TSE), for continuously manufacturing energetic polymer composites. It takes advantage of the continuous nature and superior mixing characteristics of the TSE process to manufacture a new concept for propellants and explosives: Functionally Graded Energetic Materials (FGEMs). For example, conventional geometricallycomplex homogeneous grains for solid rocket motors can be replaced by a geometrically-simpler cylindrical FGEM configuration with an axial gradient in the energetic material formulation. The simpler geometry does not have the undes...

International Journal of Solids and Structures, 2006

Pressureless sintering of powder-processed functionally graded materials is being pursued to econ... more Pressureless sintering of powder-processed functionally graded materials is being pursued to economically produce metal-ceramic composites for a variety of high-temperature (e.g., thermal protection) and energy-absorbing (e.g., armor) applications. During sintering, differential shrinkage induces stresses that can compromise the integrity of the components. Because the strength evolves as the component is sintered, it is important to model how the evolution of the differential shrinkage governs the stress distribution in the component in order to determine when the strength will be exceeded and cracking initiated. In this investigation, a model is proposed that describes the processing/microstructure/property/ performance relationship in pressurelessly sintered functionally graded plates and rods. This model can be used to determine appropriate shrinkage rates and gradient architectures for a given component geometry that will prevent the component from cracking during pressureless sintering by balancing the evolution of strength, which is assumed to be a power law function of the porosity, with the evolution of stress. To develop this model, the powder mixture is considered as a three-phase material consisting of voids, metal particles, and ceramic particles. A micromechanical thermal elasticviscoplastic constitutive model is then proposed to describe the thermomechanical behavior of the composite microstructure. The subsequent evolution of the thermomechanical properties of the matrix material during sintering is assumed to obey a power law relationship with the level of porosity, which is directly related to the shrinkage strain, and was refined to account for the evolving interparticle cohesion of the matrix phase due to sintering. These thermomechanical properties are incorporated into a 2-D thermomechanical finite element analysis to predict the stress distributions and distortions that arise from the evolution of differential shrinkage during the pressureless sintering process. Differential shrinkage results were verified quantitatively through comparison with the shape profile for a pressurelessly sintered functionally graded nickel-alumina composite plate with a cylindrical geometry, and the stress distribution results verified from qualitative observations of the absence or presence of cracking as well as the location in specimens with different gradient architectures. The cracking was mitigated using a reverse gradient at one end of the specimen, and the resulting distortions associated with the shape profile were determined to be no more than 15% reduced from the predictions. The effects of geometry were also studied out-of-plane by transforming the plate into a rod through an increase in thickness, while in-plane effects were studied by comparing the results from the cylindrical specimen with a specimen that has a square

Conference Proceedings of the Society for Experimental Mechanics Series, 2011

Page 1. Mechanical Behavior of Bio-inspired Sandwich Composites Sandip Haldar, Jachimike K. Imo a... more Page 1. Mechanical Behavior of Bio-inspired Sandwich Composites Sandip Haldar, Jachimike K. Imo and Hugh A. Bruck Department of Mechanical Engineering, Univ. of Maryland, College Park, MD 20742 Abstract Nature is ...

Volume 5A: 38th Mechanisms and Robotics Conference, 2014

Flapping wing miniature aerial vehicles (FWMAVs) offer advantages over traditional fixed wing or ... more Flapping wing miniature aerial vehicles (FWMAVs) offer advantages over traditional fixed wing or quadrotor MAV platforms because they are more maneuverable than fixed wing aircraft and are more energy efficient than quadrotors, while being quieter than both. Currently, autonomy in FWMAVs has only been implemented in flapping vehicles without independent wing control, limiting their level of control. We have developed Robo Raven IV, a FWMAV platform with independently controllable wings and an actuated tail controlled by an onboard autopilot system. In this paper, we present the details of Robo Raven IV platform along with a control algorithm that uses a GPS, gyroscope, compass, and custom PID controller to autonomously loiter about a predefined point. We show through simulation that this system has the ability to loiter in a 50 meter radius around a predefined location through the manipulation of the wings and tail. A simulation of the algorithm using characterized GPS and tail resp...

ABSTRACT A novel biaxial microtensile tester has been designed and built to assess the mechanical... more ABSTRACT A novel biaxial microtensile tester has been designed and built to assess the mechanical properties of thin films. Loading is achieved along two normal axes , whose components are suspended on air bearings , permitting independent and/or simultaneous frictionless loading by nanometric motion capable motors . Available load cells allow a choice of three maximum tensile load levels: 1, 5 and 10 Newtons . A video camera is integral to an Atomic Force Microscope (AFM) and is used for specimen alignment and positioning. The AFM is used to assess specimen surface topography before and after specimen load application. Digital Image Correlation (DIC) is capable of resolving nanoscale full-field displacements over the area of interest. Preliminary testing is focused in assessing the microtensile tester capabilities, while pursuing an understanding of the microscopic deformation of sputtered copper thin films.

Experimental Mechanics of Composite, Hybrid, and Multifunctional Materials, Volume 6, 2013

The flight time of miniature air vehicles (MAVs) is limited by the need for a portable, light wei... more The flight time of miniature air vehicles (MAVs) is limited by the need for a portable, light weight power source. The development of multifunctional, power generating wings has the capability of extending flight time without compromising overall flight performance. This paper seeks to investigate the feasibility of integrating flexible solar cells onto the flapping wings of a MAV to create “compliant multifunctional wing structures”. Data is collected for bird-inspired miniature air vehicle wings designed with carbon fiber spars, and for comparable wings designed with a monolithic compliant component. Both of the designs are tested with wing bodies composed of plain Mylar foil with flexible, lightweight solar cells integrated onto them. The test setup is designed to simulate MAV operation under zero forward velocity. A motor that controls the wing flapping scheme is fixed to a rigid test stand. A 6 degree of freedom (DOF) load cell is used to measure aerodynamic lift as a function of time for a synchronized flapping scheme during wind tunnel testing. A second experiment is conducted to verify the functionality of solar cells as a regenerative energy source in real flight. A single compliant wing and a single regular wing are consecutively fixed to the test setup used in the first experiment, and the test is conducted under direct sunlight. The voltage generated by the solar cells is collected as a function of time, while the wing is flapping. The lift data is used to estimate flight characteristics, while the voltage data is used to estimate the viability of energy harvesting.

Volume 6B: 37th Mechanisms and Robotics Conference, 2013

Flapping wing air vehicles offer many useful flight characteristics due to their versatility, as ... more Flapping wing air vehicles offer many useful flight characteristics due to their versatility, as proven by flying animals. Wing design significantly influences the performance. However designing successful wings presents significant challenges. Efficient matching of the drive motors to the flapping wings is necessary to overcome the highly constrained weight budget. Simulating detailed information about the force response due to flapping is challenging due to complex fluid-structural interactions of the wings resulting in non-linear force response to flapping motion. To overcome this challenge, we conducted an experimental study of flapping wings to provide detailed temporal force response data for flapping wings. A prototype was built by synthesizing lightweight manufacturing techniques with the results of the experimental study. Our experimental investigations enabled us to select the flapping angle range and flapping frequency.

Strain, 2011

A methodology has been developed for accurately measuring the mechanical properties of materials ... more A methodology has been developed for accurately measuring the mechanical properties of materials used on the micro-scale. The direct tension test method using a dog bone-type specimen has been employed, as it is the most effective and straightforward method to obtain results including a full stress-strain curve. The goal of this investigation was to develop a universal, yet simple and reliable, methodology to be used for accurate characterisation of mechanical properties for a wide variety of materials. Specimens from single crystal silicon were fabricated using photolithography by means of deep reactive ion etching. This material was chosen as it is expected that on both the micro-and macro-scales, Young's modulus will have the same value. Hence, the accuracy of the methodology may be unambiguously examined. The test setup includes a small test machine containing a load cell whose maximum capacity is 5 N and is capable of direct gripping and displacement control. The specimens were found to have a trapezoidal cross-section that was accurately measured using a scanning electron microscope. The strains were obtained by means of digital image correlation using images obtained via optical microscopy. The quantities measured include Young's modulus E, the fracture strength r f and the fracture strain f. The average value of E obtained in the micro-tests agrees well with the reference value obtained on the macro-scale.

Sensors and Actuators A: Physical, 2011

Polymer, 2008

The addition of multi-walled carbon nanotubes (MWCNTs) or carbon nanofibers (CNFs) to polymeric m... more The addition of multi-walled carbon nanotubes (MWCNTs) or carbon nanofibers (CNFs) to polymeric melts offers a convenient route to obtain highly conductive plastics. However, when these materials are melt processed, their conductivity can be lost. Here, it is shown that conductivities can be recovered through melt annealing at temperatures above the polymer's glass transition temperature (T g). We demonstrate these results for both MWCNT and CNF-based composites in polystyrene (PS). The mechanism behind the conductivity increase is elucidated through modeling. It involves a transition from aligned, unconnected particles prior to annealing to an interconnected network after annealing through viscoelastic relaxation of the polymer. Such rearrangement is directly visualized for the case of the CNFbased composites using confocal microscopy. The annealing-induced increase in particle connectivity is also reflected in dynamic rheological measurements on both MWCNT and CNF composites as an increase in their elastic moduli at low frequencies.

Nanotechnology, 2007

For the first time, an interpenetrating phase polymer nanocomposite formed by the percolation of ... more For the first time, an interpenetrating phase polymer nanocomposite formed by the percolation of multiwalled carbon nanotubes (MWCNTs) in polystyrene (PS) has been quantitatively characterized through electrical conductivity measurements and melt rheology. Both sets of measurements, in conjunction with scanning electron microscopy (SEM) images, indicate the presence of a continuous phase of percolated MWCNTs appearing at particle concentrations exceeding 2 vol% MWCNTs in PS. To quantify the amount of this continuous phase present in the PS/MWCNT composite, electrical conductivity data at various MWCNT concentrations, β, are correlated with a proposed degree of percolation,C(β), developed using a conventional power-law formula with and without a percolation threshold. To quantify the properties of the interpenetrating phase polymer nanocomposite, the PS/MWCNT composite is treated as a combination of two phases: a continuous phase consisting of a pseudo-solid-like network of percolated MWCNTs, and a continuous PS phase reinforced by non-interacting MWCNTs. The proposed degree of percolation is used to quantify the distribution of MWCNTs among the phases, and is then used in a rule-of-mixtures formulation for the storage modulus, G (β,C(β), ω), and the loss modulus, G (β,C(β), ω), to quantify the properties of the continuous phase consisting of percolated MWCNTs and the continuous PS phase reinforced by non-interacting MWCNTs from the experimental melt rheology data. The properties of the continuous phase of percolated MWCNTs are indicative of a scaffold-like microstructure exhibiting an elastic behavior with a complex modulus of 360 kPa at lower frequencies and viscoplastic behavior with a complex viscosity of 6 kPa s rad −1 at higher frequencies, most likely due to a stick-slip friction mechanism at the interface of the percolated MWCNTs. Additional evidence of this microstructure was obtained via scanning electron microscopy. This research has important implications in providing a new methodology based on the electrical and rheological properties of the polymer nanocomposite for quantifying the continuous phase formed by the percolation of new functionalized nanostructures being developed for: (a) controlling the percolation of the nanostructures through self-assembly, (b) enhancing their interaction with the continuous reinforced polymer phase, (c) enhancing the cohesion between nanostructures.

Macromolecules, 2007

A systematic electrical and rheological characterization of percolation in commercial polydispers... more A systematic electrical and rheological characterization of percolation in commercial polydisperse polystyrene (PS) nanocomposites containing multiwall carbon nanotubes (MWCNTs) is presented. The MWCNTs confer appreciable electrical conductivities (up to ca. 1 S/m) to these nanocomposites at a concentration of 8 vol %. In addition to enhancing the electrical properties, even at small concentrations (ca. 2 vol %), MWCNTs significantly enhance the rheological properties of PS melts. At concentrations exceeding 2 vol %, a plateau appears in the storage modulus G′ at low frequencies, indicating the formation of a percolated MWCNT network that responds elastically over long timescales. Network formation, in turn, implies a diverging complex viscosity vs complex modulus curve. A focus of this study is on the correlation between electrical and rheological properties at the onset of percolation. The experimental results indicate that the elastic load transfer and electrical conductivity are far more sensitive to the onset of percolation than the viscous dissipation in the nanocomposite. Sensitivity of the electrical and rheological percolations to two different solvents used in processing the nanocomposites has also been characterized.

Journal of Manufacturing Science and Engineering, 2010

In-mold assembly can be used to create mesoscale articulating polymeric joints that enable the mi... more In-mold assembly can be used to create mesoscale articulating polymeric joints that enable the miniaturization of devices, reduction in production costs, and increase in throughput. One of the major challenges in miniaturizing devices using the in-mold assembly is to develop appropriate characterization techniques and modeling approaches for the interaction between polymer melt flow fronts and premolded components. When a high speed, high temperature second stage melt comes in contact with a premolded mesoscale component that has similar melting temperatures, the premolded component can experience a significant plastic deformation due to the thermal softening and the force associated with impingement of the melt flow front. In our previous work, we developed methods to inhibit the plastic deformation by supporting the ends of the mesoscale premolded components. In this paper, we present an alternative strategy for controlling premolded component deformations. This involves a mesosca...

Experimental Mechanics, 2004

Interfaces in heterogeneous structures are typically engineered for optimal strength through the ... more Interfaces in heterogeneous structures are typically engineered for optimal strength through the control of surface roughness and the choice of adhesives. Advances in manufacturing technologies are now making it possible to also tailor the geometries of interfaces from the nanoscale to the macroscale to create geometrically complex interfaces that exhibit enhanced performance characteristics. However, the impact of geometric complexity on the mechanical behavior of interfaces has not yet been ascertained. In this investigation, the first step is taken towards understanding the effects of geometric complexity on interfacial strength. A new multistage, multi-piece molding process is used to create heterogeneous polymer structures with geometrically complex interfaces consisting of rectangular and circular interlocking features. The structural integrity of these heterogeneous structures is characterized through interfacial tension testing.The full-field deformation measurement technique known as digital image correlation is also used during the testing to visualize the deformation fields around the geometrically complex features. Through this characterization, it is determined that the complex geometries increase the interfacial strength by approximately 20-25%, while reducing the statistical variation by 50%. These effects are attributed to a transition in the failure mechanism from interfacial fracture to homogeneous ligament failure. Results also indicate that geometrically complexity can be used on completely debonded interfaces to increase the strength to at least 25-35% of the bonded interface. Based on these results, some simple design rules have been proposed that enable geometrically complex interfaces to be engineered with enhanced strengths approaching the weaker of the two base materials. These design rules can also be used in the engineering of interfaces to facilitate the development of heterogeneous structures using new design paradigms, such as design for recyclability and the design of products based on bio-inspired concepts.

Experimental Mechanics, 2004

Structures are being actuated by embedding shape memory alloy (SMA) wires into compliant material... more Structures are being actuated by embedding shape memory alloy (SMA) wires into compliant materials, such as polyurethane. To achieve bending actuation, these wires are placed in opposing wire configurations, where multiple wires are often employed to enhance the amplitude of the bending actuation response. In this investigation, a procedure has been developed for fabricating polyurethanes with a symmetrically graded distribution of SMA wires. The effects of grading the distribution of one-way SMA wires have been characterized using full-field displacement deformation measurements obtained with the digital image correlation (DIC) technique. These measurements have been used in a onedimensional (1D) model of bending actuation to determine the "equivalent two-way shape memory effect (SME)" of the graded wire distribution. To utilize the 1D actuation model, the constitutive properties of the polyurethane structure predicted by rule-of-mixture formulations were reduced to account for the differences in strain between the SMA wires and the polyurethane matrix. The graded wire distribution was also found to significantly stiffen the polyurethane structure. The level of equivalent two-way SME therefore became limited by the maximum recovery stress of the SMA wires, with a maximum level that was approximately 75% less than previously measured levels in an opposing wire Configuration. However, the bending actuation behavior was more symmetric, and the actuated bending deflections were similar to those observed when using more compliant materials. It was also predicted that the symmetrically graded wire distribution would exhibit abetter balance between actuation amplitude and uniformity, which combined with the more symmetric actuation behavior makes the graded wire distribution potentially more desirable for achieving higher actuation frequencies with distributed actuation concepts in new applications, such as miniaturized double diaphragm pumping devices. KEY WORDS-Functionally graded materials, smart structures, digital image correlation, bending actuation, rule-ofmixtures, equivalent two-way shape memory effect

Experimental Mechanics, 2009

Compliant wing designs have the potential of improving flapping wing Micro-Air Vehicles (MAVs). D... more Compliant wing designs have the potential of improving flapping wing Micro-Air Vehicles (MAVs). Designing compliant wings requires a detailed understanding of the effect of compliance on the generation of thrust and lift forces. The low force and high-frequency measurements associated with these forces necessitated a new versatile test stand design that uses a 250 g load cell along with a rigid linear air bearing to minimize friction and the dynamic behavior of the test stand while isolating only the stationary thrust or lift force associated with drag generated by the wing. Moreover, this stand is relatively inexpensive and hence can be easily utilized by wing designers to optimize the wing compliance and shape. The frequency response of the wing is accurately resolved, along with wing compliance on the thrust and lift profiles. The effects of the thrust and lift force generated as a function of flapping frequency were also determined. A semi-empirical aerodynamic model of the thrust and lift generated by the flapping wing MAV on the new test stand was developed and used to evaluate the measurements. This model accounted for the drag force and the effects of the wing compliance. There was good correlation between the model predictions and experimental measurements. Also, the increase in average thrust due to increased wing compliance was experimentally quantified for the first time using the new test stand. Thus, our measurements for the first time reveal the detrimental influence of excessive compliance on drag forces during high frequency operation. In addition, we were also able to observe the useful effect of compliance on the generation of extra thrust at the beginning and end of upstrokes and downstrokes of the flapping motion.

Experimental Mechanics, 2010

Palmetto wood is garnering growing interest as a template for creating biologically-inspired poly... more Palmetto wood is garnering growing interest as a template for creating biologically-inspired polymer composites due to its historical use as an energy absorbing material in protective structures. In this study, quasi-static three-point bend tests have been performed to characterize the mechanical behavior of Palmetto wood. Full-field deformation measurements are obtained using Digital Image Correlation (DIC) to elucidate on the strain fields associated with the mechanical response. By analyzing strain fields at multiple length scales, it is possible to study the more homogeneous mechanical behavior at the macroscale associated with the global load-deformation response; while at the microscale the mechanical behavior is more inhomogeneous due to microstructural failure mechanisms. Thus, it was possible to determine that, despite the presence of discontinuous macro-fiber reinforcement, at the macroscale the response is associated with classical bending and progressive failure processes that are adequately described by Weibull statistics proceeding from the tensile side of the specimen. At the microscale, however, the failure mechanisms giving rise to the macroscopic response consist of both shear-dominated debonding between the fiber and matrix, and inter-fiber matrix failure due to pore collapse. These microscale mechanisms are present in both the compressive and tensile regions of the specimen, most likely due to local macro-fiber bending, which is independent of the global bending state. The pore collapse mechanism observed during mechanical loading appears to improve the energy absorption of the matrix material, thereby, transferring less energy and shear strain to the macro-fibermatrix interface for initiation of debonding. However, the pore collapse mechanism can also accumulate substantial shear strain, which results in matrix shear cracking. Through these complex failure mechanisms, Palmetto wood exhibits a high resistance to catastrophic failure after damage initiation, an observation that can be used as inspiration for creating new polymer composite materials.