Person: Vlassak, Joost
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Vlassak
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Joost
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Vlassak, Joost
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Publication Precipitation and thermal fatigue in Ni–Ti–Zr shape memory alloy thin films by combinatorial nanocalorimetry(Elsevier BV, 2011-08) Vlassak, Joost; McCluskey, Patrick J.; Zhao, Chunwang; Kfir, OferThin-film samples of Ni-Ti-Zr shape memory alloys were studied by combinatorial nanocalorimetry to determine the effects of high-temperature (900°C) heat treatments and low- temperature (450°C) thermal cycling on the characteristics of the martensite transformation. The response of the samples to heat treatments depends on composition and is controlled by a precipitation mechanism. Two precipitate types, a Ti2Ni base phase at low Zr concentration and a Ni10Zr7 base phase at high Zr concentration, affect the martensite transformation characteristics by altering the composition and the stress state of the shape memory phase. Thermal fatigue behavior, induced by thermal cycling, is improved compared to previous results. The most stable sample demonstrates a transformation temperature reduction of just 11°C for 100 cycles. The improved stability of the samples is attributed to the very small grain size of approximately 5-20 nm. The high heating and cooling rates characteristic of nanocalorimeters allowed this study to be performed in a high-throughput manner with efficiencies not previously achieved.Publication Dislocation climb in two-dimensional discrete dislocation dynamics(AIP Publishing, 2012-05-15) Vlassak, Joost; Davoudi, Kamyar M.; Nicola, LuciaIn this paper, dislocation climb is incorporated in a two-dimensional discrete dislocation dynamics model. Calculations are carried out for polycrystalline thin films, passivated on one or both surfaces. Climb allows dislocations to escape from dislocation pile-ups and reduces the strain- hardening rate, especially for fully passivated films. Within the framework of this model, climb modifies the dislocation structures that develop during plastic deformation and results in the formation of pile-ups on slip planes that do not contain any dislocation sources.Publication Transforming the Dynamic Response of Robotic Structures and Systems Through Laminar Jamming(Institute of Electrical and Electronics Engineers (IEEE), 2018-04) Narang, Yashraj; Degirmenci, Alperen; Vlassak, Joost; Howe, RobertResearchers have developed variable-impedance mechanisms to control the dynamic response of robotic systems and improve their adaptivity, robustness, and efficiency. However, these mechanisms have limitations in size, cost, and convenience, particularly for variable damping. We demonstrate that laminar jamming structures can transform the dynamic response of robotic structures and systems while overcoming these limitations. In laminar jamming, an external pressure gradient is applied to a laminate of compliant material, changing its stiffness and damping. In this latter, we combine analysis, simulation, and characterization to formulate a lumped-parameter model that captures the nonlinear mechanical behavior of jamming structures and can be used to rapidly simulate their dynamic response. We illustrate that by adjusting the vacuum pressure, the fundamental features of the dynamic response (i.e., frequency, amplitude, decay rate, and steady-state value) can be tuned on command. Finally, we demonstrate that jamming structures can be integrated into soft structures and traditional rigid robots to considerably alter their response to impacts. With the models and demonstrations provided here, researchers may move further toward building versatile and transformative robots. Index Terms: Soft material robotics, compliant joint/mechanism, dynamics, compliance and impedance control, aerial systems: mechanics and control.Publication Ultra-sensitive and resilient compliant strain gauges for soft machines(Springer Science and Business Media LLC, 2020-11-11) Araromi, Oluwaseun A.; Graule, Moritz; Dorsey, Kristen; Castellanos, Samantha; Foster, Jonathan; Hsu, Wen-Hao; Passy, Arthur; Vlassak, Joost; Weaver, James; Walsh, Conor; Wood, RobertSoft machines are a promising new design paradigm for human-centric devices and systems required to interact gently with their environment. For soft machines to respond intelligently to their surroundings, compliant sensory feedback mechanisms are needed. Specifically, soft alternatives to strain gauges – possessing high resolution at low strain ranges (< 5%), could unlock promising new capabilities in soft systems. However, currently available sensing mechanisms typically possess either high strain sensitivity or high mechanical resilience, but not both. The lack of robustness of available compliant and highly sensitive sensing mechanisms has confined their operation to laboratory settings, inhibiting their widespread deployment. Here we present a versatile and compliant transduction mechanism for high-sensitivity strain detection with high mechanical resilience. The mechanism relies upon changes in ohmic contact between stiff, micro-structured, anisotropically conductive meanders encapsulated by stretchable films. The mechanism achieves high sensitivity, with gauge factors greater than 85,000, while being adaptable to use with high-strength conductors, producing sensors resilient to adverse loading conditions. The sensing mechanism also exhibits high linearity, as well as insensitivity to bending and twisting deformations - important features for soft device applications. The potential impact of our technology is shown through the construction of a sensor-integrated, lightweight, textile-based arm sleeve, which is able to recognize gestures without encumbering the hand. We demonstrate predictive tracking and classification of discrete gestures and continuous hand motions via detection of small muscle movements in the arm. The sleeve demonstration shows the potential of the SCARS technology for the development of unobtrusive, wearable biomechanical feedback systems and human-computer interfaces.Publication Kinetic Role of Carbon in Solid-State Synthesis of Zirconium Diboride using Nanolaminates: Nanocalorimetry Experiments and First-Principles Calculations(American Chemical Society (ACS), 2015) Lee, Dongwoo; Sim, Gi-dong; Zhao, Kejie; Vlassak, JoostReactive nanolaminates afford a promising route for the low-temperature synthesis of zirconium diboride, an ultrahigh-temperature ceramic with metallic properties. Although the addition of carbon is known to facilitate sintering of ZrB2, its effect on the kinetics of the formation reaction has not been elucidated. We have employed a combined approach of nanocalorimetry and first-principles theoretical studies to investigate the kinetic role of carbon in the synthesis of ZrB2 using B4C/Zr reactive nanolaminates. Structural characterization of the laminates by XRD and TEM reveal that the reaction proceeds via interdiffusion of the B4C and Zr layers, which produces an amorphous Zr3B4C alloy. This amorphous alloy then crystallizes to form a supersaturated ZrB2(C) compound. A kinetic analysis shows that carbon lowers the energy barriers for both interdiffusion and crystallization by more than 20%. Energetic calculations based on first-principles modeling suggest that the reduction of the diffusion barrier may be attributed to the stronger bonding between Zr and C as compared to the bonding between Zr and B.Publication Determination of critical cooling rates in metallic glass forming alloy libraries through laser spike annealing(Nature Publishing Group UK, 2017) Bordeenithikasem, Punnathat; Liu, Jingbei; Kube, Sebastian A.; Li, Yanglin; Ma, Tianxing; Scanley, B. Ellen; Broadbridge, Christine C.; Vlassak, Joost; Singer, Jonathan P.; Schroers, JanThe glass forming ability (GFA) of metallic glasses (MGs) is quantified by the critical cooling rate (R C). Despite its key role in MG research, experimental challenges have limited measured R C to a minute fraction of known glass formers. We present a combinatorial approach to directly measure R C for large compositional ranges. This is realized through the use of compositionally-graded alloy libraries, which were photo-thermally heated by scanning laser spike annealing of an absorbing layer, then melted and cooled at various rates. Coupled with X-ray diffraction mapping, GFA is determined from direct R C measurements. We exemplify this technique for the Au-Cu-Si system, where we identify Au56Cu27Si17 as the alloy with the highest GFA. In general, this method enables measurements of R C over large compositional areas, which is powerful for materials discovery and, when correlating with chemistry and other properties, for a deeper understanding of MG formation.Publication An Apparatus for Performing Microtensile Tests at Elevated Temperatures inside a Scanning Electron Microscope(Elsevier, 2013) Sim, Gi-dong; Park, Jun-Hyub; Uchic, Michael D.; Shade, Paul A.; Lee, Soon-Bok; Vlassak, JoostIn this paper, we introduce an apparatus to perform microtensile tests at elevated temperatures inside a scanning electron microscope. The apparatus has a stroke of 250 μm with a displacement resolution of 10 nm and a load resolution of 9.7 μN. Measurements at elevated temperatures are performed through use of two silicon-based micromachined heaters that support the sample. Each heater consists of a tungsten heating element that also serves as a temperature gauge. To demonstrate the testing capabilities, tensile tests were performed on submicron Cu films at various temperatures up to 430 °C. Stress–strain curves show a significant decrease in yield strength and initial slope for the samples tested at elevated temperature, which we attribute to diffusion-facilitated grain boundary sliding and dislocation climb.Publication Bauschinger effect in thin metal films: Discrete dislocation dynamics study(American Institute of Physics (AIP), 2014) Davoudi, Kamyar M.; Nicola, Lucia; Vlassak, JoostThe effects of dislocation climb on plastic deformation during loading and unloading are studied using a two-dimensional discrete dislocation dynamics model. Simulations are performed for polycrystalline thin films passivated on both surfaces. Dislocation climb lowers the overall level of the stress inside thin films and reduces the work hardening rate. Climb decreases the density of dislocations in pile-ups and reduces back stresses. These factors result in a smaller Bauschinger effect on unloading compared to simulations without climb. As dislocations continue to climb at the onset of unloading and the dislocation density continues to increase, the initial unloading slope increases with decreasing unloading rate. Because climb disperses dislocations, fewer dislocations are annihilated during unloading, leading to a higher dislocation density at the end of the unloading step.Publication Scanning AC Nanocalorimetry Study of Zr/B Reactive Multilayers(American Institute of Physics, 2013) Lee, Dongwoo; Sim, Gi-dong; Xiao, Kechao; Seok Choi, Yong; Vlassak, JoostThe reaction of Zr/B multilayers with a 50 nm modulation period has been studied using scanning AC nanocalorimetry at a heating rate of approximately \(10^3 K/s\). We describe a data reduction algorithm to determine the rate of heat released from the multilayer. Two different exothermic peaks are identified in the nanocalorimetry signal: a shallow peak at low temperature (200–650°C) and a sharp peak at elevated temperature (650–800°C). TEM observation shows that the first peak corresponds to heterogeneous inter-diffusion and amorphization of Zr and B while the second peak is due to the crystallization of the amorphous Zr/B alloy to form \(ZrB_2\).Publication Scanning AC nanocalorimetry combined with in-situ x-ray diffraction(American Institute of Physics (AIP), 2013) Xiao, Kechao; Gregoire, John M.; McCluskey, Patrick; Dale, Darren; Vlassak, JoostMicromachined nanocalorimetry sensors have shown excellent performance for high-temperature and high-scanning rate calorimetry measurements. Here, we combine scanning AC nanocalorimetry with in-situ x-ray diffraction (XRD) to facilitate interpretation of the calorimetry measurements. Time-resolved XRD during in-situ operation of nanocalorimetry sensors using intense, high-energy synchrotron radiation allows unprecedented characterization of thermal and structural material properties. We demonstrate this experiment with detailed characterization of the melting and solidification of elemental Bi, In, and Sn thin-film samples, using heating and cooling rates up to 300 K/s. Our experiments show that the solidification process is distinctly different for each of the three samples. The experiments are performed using a combinatorial device that contains an array of individually addressable nanocalorimetry sensors. Combined with XRD, this device creates a new platform for high-throughput mapping of the composition dependence of solid-state reactions and phase transformations.