The Experts below are selected from a list of 7986 Experts worldwide ranked by ideXlab platform
T.g. Nieh - One of the best experts on this subject based on the ideXlab platform.
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flow Serration in a zr based bulk metallic glass in compression at low strain rates
Intermetallics, 2008Co-Authors: S X Song, J Wadsworth, T.g. NiehAbstract:Abstract It is demonstrated that at slow strain rates (∼10−4 s−1) in compression, the dominant room temperature macroscopic deformation mode in a ductile Zr-based bulk metallic glass is single shear along the principal shear plane. The stress–strain curve exhibited serrated flow in the plastic region. Scanning electron micrographs of the deformed samples revealed regularly spaced striations on the shear surface. A detailed analysis of the observed Serrations disclosed that they were intimately related to the striations on the shear surface, suggesting that the Serrations were mainly caused by intermittent sample sliding. Further investigations were conducted using in situ compression experiments; video images showed that there was indeed a one-to-one correspondence between the intermittent sliding and flow Serration. The current study therefore suggests that flow Serration is a result of intermittent sample sliding. This result also implies that the principal shear plane, once formed, is the preferential site for additional shear band formation.
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Analyses of shear band emission in a Mg-based bulk metallic glass deformed at different nanoindentation rates
Intermetallics, 2008Co-Authors: S X Song, Jason S.c. Jang, T.g. NiehAbstract:Abstract Nanoindentation behavior of Mg 57 Cu 31 Y 6.6 Nd 5.4 bulk metallic glass was characterized at different loading rates. The load–displacement curves exhibit significant displacement Serrations, apparently associated with discrete shear band emission, at low loading rates but disappear at high rates. Analyses based on displacement Serration, strain rate Serration and hardness Serration were carried out to determine the critical strain rate beyond which the transition from inhomogeneous to homogeneous deformation actually took place. It was concluded that the hardness Serration analysis probably provides the most reasonable result as the other two were limited by the instrument noises. Based on a shear band nucleation model, the critical nucleus size was estimated to be a sphere of about 25 nm in diameter.
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rate dependence of serrated flow during nanoindentation of a bulk metallic glass
Journal of Materials Research, 2002Co-Authors: Christopher A Schuh, T.g. Nieh, Yoshihito KawamuraAbstract:Plastic deformation of Pd-40Ni-20P bulk metallic glass (BMG) was investigated by instrumented nanoindentation experiments over a broad range of indentation strain rates. At low rates, the load-displacement curves during indentation exhibited numerous Serrations or pop-ins, but these Serrations became less prominent as the indentation rate was increased. Using the tip velocity during pop-in as a gauge of Serration activity, we found that serrated flow is only significant at indentation strain rates below about 1-10/s. This result suggests a transition in plastic flow behavior at high strain rates, in agreement with prior studies of BMGs under different modes of loading.
J W Qiao - One of the best experts on this subject based on the ideXlab platform.
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temperature effects on deformation and Serration behavior of high entropy alloys heas
JOM, 2014Co-Authors: James Antonaglia, J W Qiao, Yong Zhang, Xie Xie, Z Tang, C W Tsai, M O Laktionova, E D Tabachnikova, Jienwei Yeh, O N SenkovAbstract:Many materials are known to deform under shear in an intermittent way with slip avalanches detected as acoustic emission and Serrations in the stress–strain curves. Similar Serrations have recently been observed in a new class of materials, called high-entropy alloys (HEAs). Here, we discuss the Serration behaviors of several HEAs from cryogenic to elevated temperatures. The experimental results of slow compression and tension tests are compared with the predictions of a slip-avalanche model for the deformation of a broad range of solids. The results shed light on the deformation processes in HEAs. Temperature effects on the distributions of stress drops and the decrease of the cutoff (i.e., of the largest observed slip size) for increasing temperature qualitatively agree with the model predictions. The model is used to quantify the Serration characteristics of HEAs, and pertinent implications are discussed.
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Serration Dynamics in a Zr-Based Bulk Metallic Glass
Metallurgical and Materials Transactions A, 2014Co-Authors: Zhihua Wang, J W Qiao, Peter K. Liaw, H. J. Yang, C. J. HuangAbstract:Intermittent or serrated plastic flows have been widely observed in irreversible deformation through shear banding in bulk metallic glasses (BMGs). The strain-rate-dependent plasticity under uniaxial compression at 2 × 10−3, 2 × 10−4, and 2 × 10−5 s−1 in a Zr-based BMG is investigated. Serration events have a typical time scale at a relatively higher strain rate (2 × 10−3 s−1), while at lower strain rates, there is a lack of typical time scale. During Serrations, the stress is falling rapidly, and the amplitude of the stress drop between the neighboring Serrations is approximately equal. The stress drop vs time satisfies the exponential decay rule during jerk flows. Due to the serrated flow corresponding to the internal shear process, the free-volume model and stick–slip model are introduced to explain how the shear bands form and propagate and the cooperation of multiple shear bands. The mechanism is explained by relating the atomic-scale deformation with the macroscopic shear-band behavior, offering key ingredients to fundamentally cognize Serrations in jerk flows.
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serrated flow kinetics in a zr based bulk metallic glass
Intermetallics, 2010Co-Authors: J W Qiao, P K LiawAbstract:Abstract At different strain rates, the stress-flow Serrations in the compressive stress-time curves of a Zr-based bulk metallic glass (BMG) exhibit different magnitudes and time scales. A critical strain rate can be identified, at which the serrated flow kinetics is characterized to be homogeneous in both time and magnitude. Below the critical strain rate, the Serrations are lack of homogeneous magnitudes and time scales because the excessive elastic energy results in distinct softening within the viscous fluid-like shearing layer due to the adiabatic heating. As a consequence, the tiny disturbances, such as the vibration of test machines, affect the shear banding behavior. A periodic function is established for the critical instability, and two pertinent parameters, the mean stress drop magnitude and the mean waiting time (MWT), can be determined, which may be associated with the deformation micromechanism, the mechanical property, and the glass-forming ability of BMGs. The present approach may provide a new way to study the Serration-deformation behavior of BMGs.
Daniele Ragni - One of the best experts on this subject based on the ideXlab platform.
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Pressure fluctuations from large-scale PIV over a serrated trailing edge
Experiments in Fluids, 2020Co-Authors: Lourenco T. Lima Pereira, Daniele Ragni, Francesco Avallone, Fulvio ScaranoAbstract:Measurements of distributed surface pressure fluctuations over trailing-edge Serrations at a Reynolds number $${\text{Re}}_{\theta }$$ Re θ = 4900 are performed with time-resolved 3D particle imaging velocimetry using helium filled soap bubbles as flow tracers. The sparse velocity vector field obtained with Lagrangian particle tracking is densely reconstructed using VIC+, a data assimilation technique based on the vortex-in-cell method. The instantaneous pressure distribution is inferred by invoking the momentum equation. Experiments are performed first over a flat plate, as assessment of the technique, where the properties of the convecting turbulent boundary layer are assessed and surface pressure fluctuations are validated against synchronous surface microphone measurements. The analysis of the flow over the trailing-edge Serration focuses on the characterization of the spatial distribution and spectral coherence of surface pressure fluctuations, i.e. the flow features responsible for the acoustic scattering. Present results indicate that the measurement technique is suited to describe the spatio-temporal development of the pressure fluctuations over the Serration surface at the proposed scale of the experiments. Graphic abstract
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Trailing-edge Serrations effect on the performance of a wind turbine
Renewable Energy, 2020Co-Authors: Elena Llorente, Daniele RagniAbstract:An experimental study focusing on the change of the aerodynamic performance of a wind turbine with the employment of trailing-edge Serrations is presented. The design procedure which starts from the Serration design, together with the experimental wind tunnel testing and the installation in an already operating wind turbine is discussed. A prediction methodology to estimate the aerodynamic performance change of the machine with the blade add-on is validated by in-field measurements. Wind tunnel experiments for the validation of the original Serration design have been carried out on a Nordex ADO30 airfoil with a relative thickness of 30% and on a NACA643418 airfoil with 18% thickness. The two airfoils are respectively used in this study due to their peculiar characteristics. While the first has been designed for relatively high Cl/Cd performance versus structural integrity (i.e. relatively high thickness), the second one is a typical reference airfoil used in wind turbines for its interesting Cl/Cd performance in both laminar and rough conditions. Clean and rough conditions have been tested in order to prepare a database for the analysis of the full turbine in several wind conditions. Aerodynamic forces with the serrated trailing-edge extensions are measured for different angles of attack and Serration flap angles. The results are further discussed and employed for the analysis of the full rotor. Results already show that an increase in the flap angle is typically associated with an increment in lift, but not necessarily in drag. This has a beneficial effect on the operational regime of the machine when taken into account. The influence of the trailing-edge Serrations on an operating wind turbine has been quantified in terms of total loads and energy production. The power curves with and without the trailing-edge installations are further analyzed and compared with the theoretical predictions. • Experimental tests show big impact of Serrations on airfoil's aerodynamic properties • Predictive law used for extending the Serrations influence to the whole blade • Theoretically, Serrations have positive impact on power production of wind turbines • Theoretically, Serrations increase wind turbine loads and reduce its life-time • Positive impact of Serrations on power production of real machines has been probed
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Measurements of near-wall pressure fluctuations for trailing-edge Serrations and slits
Experiments in Fluids, 2018Co-Authors: Daniele Ragni, Francesco Avallone, W.c.p. Van Der Velden, Damiano CasalinoAbstract:Abstract: Pressure fluctuations on the suction side of a NACA 0018 with trailing-edge add-ons are obtained from integration of time-resolved stereoscopic and tomographic particle image velocimetry data and compared to the ones computed from Lattice–Boltzmann simulations. The airfoil is retrofitted with solid and slitted serrated trailing edges and measured at 0° and at 12° angles of attack. At 0° angle of attack, the boundary-layer thickness and the intensity of the pressure fluctuations are found to decrease along the edge of the Serration from its root to its tip. The spectra of the pressure fluctuations additionally show a change of decay in frequency along the Serration edge. This last finding has important repercussions for noise-prediction models, which usually assume the turbulence and the slope of the pressure spectra to be “frozen” in the streamwise direction. Results from this study also indicate that the pressure-fluctuation modification along the Serrations scales with the local boundary-layer parameters, which can be obtained from experimental and numerical data. In particular, the pressure spectra collapse into a single profile when the local boundary-layer thickness and skin-friction coefficient is employed, instead of the parameters of the incoming flow. The analysis is further extended to flow fields at positive angle of attack, where Serrations are known to exhibit lower performance in noise reduction. At incidence angle, the scaling with the local parameters shows that the spatial distribution of boundary-layer thickness and pressure fluctuations is uniform along the Serration. This evidence might indicate a positive correlation between the noise-reduction performance of Serrations and the spatial change of pressure spectra (and local boundary-layer thickness) along their edge. Graphical abstract: [Figure not available: see fulltext.].
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Noise reduction mechanisms of sawtooth and combed-sawtooth trailing-edge Serrations
Journal of Fluid Mechanics, 2018Co-Authors: Francesco Avallone, Daniele Ragni, W.c.p. Van Der Velden, Damiano CasalinoAbstract:Trailing-edge Serrations are add ons retrofitted to wind-turbine blades to mitigate turbulent boundary-layer trailing-edge noise. This manuscript studies the physical mechanisms behind the noise reduction by investigating the far-field noise and the hydrodynamic flow field. A conventional sawtooth and a combed-sawtooth trailing-edge Serration are studied. Combed-sawtooth Serrations are obtained by filling the empty space between the teeth with combs (i.e. solid filaments). Both Serration geometries are retrofitted to a NACA 0018 aerofoil at zero degree angle of attack. Computations are carried out by solving the explicit, transient, compressible lattice Boltzmann equation, while the acoustic far field is obtained by means of the Ffowcs Williams and Hawkings analogy. The numerical results are validated against experiments. It is confirmed that the combed-sawtooth Serrations reduce noise more than the conventional sawtooth ones for the low- and mid-frequency range. It is found that the presence of combs affects the intensity of the scattered noise but not the frequency range of noise reduction. For both configurations, the intensity of the surface pressure fluctuations decreases from the root to the tip, and noise sources are mainly located at the Serrations root for the low- and mid-frequency range. The presence of the filaments generates a more uniform distribution of the noise sources along the edges with respect to the conventional Serration. The installation of combs mitigates the interaction between the two sides of the aerofoil at the trailing edge and the generation of a turbulent wake in the empty space between teeth. As a result, the inward (i.e. from the Serration edge to the centreline) and outward (i.e. from the Serration centreline to the edge) flow motions, due to the presence of the teeth, are mitigated. It is found that the installation of Serrations affects the surface pressure fluctuations integral parameters. Both the spanwise correlation length and convective velocity of the surface pressure fluctuations increase with respect to the baseline straight configuration. When both quantities are similar to the one obtained for the straight trailing edge, the effect of the slanted edge is negligible, thus corresponding to no noise reduction. It is concluded that the changes in sound radiation are mainly caused by destructive interference of the radiated sound waves for which a larger spanwise correlation length is beneficial. Finally, the difference between measurements and the literature is caused by an incorrect modelling of the spanwise correlation length, which shows a different decay rate with respect to the one obtained for a straight trailing edge.
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Acoustic Emissions of Semi-Permeable Trailing Edge Serrations
Acoustics Australia, 2018Co-Authors: Carlos Arce León, Daniele Ragni, Stefan Pröbsting, Roberto Merino-martínez, Francesco AvalloneAbstract:The trailing edge of a NACA 0018 airfoil is modified through the attachment of Serrations with different degrees of permeability. Acoustic beamforming is used to inspect the turbulent boundary layer-trailing edge noise emissions from the unmodified and serrated trailing edges. Different freestream velocities and angles of attack are investigated. The Serration permeability is prescribed by having slits cut into the solid surface of the Serrations in two different configurations. The results indicate that a certain benefit in noise reduction is obtained from a mixed solid/slitted configuration, while a fully slitted configuration loses most of the noise reduction performance.
P K Liaw - One of the best experts on this subject based on the ideXlab platform.
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tuned critical avalanche scaling in bulk metallic glasses
Scientific Reports, 2015Co-Authors: James Antonaglia, P K Liaw, Yong Zhang, Xie Xie, Jun Wei Qiao, Gregory Schwarz, Matthew Wraith, Jonathan T Uhl, Karin A. DahmenAbstract:Ingots of the bulk metallic glass (BMG), Zr64.13Cu15.75Ni10.12Al10 in atomic percent (at. %), are compressed at slow strain rates. The deformation behavior is characterized by discrete, jerky stress-drop bursts (Serrations). Here we present a quantitative theory for the Serration behavior of BMGs, which is a critical issue for the understanding of the deformation characteristics of BMGs. The mean-field interaction model predicts the scaling behavior of the distribution, D(S), of avalanche sizes, S, in the experiments. D(S) follows a power law multiplied by an exponentially-decaying scaling function. The size of the largest observed avalanche depends on experimental tuning-parameters, such as either imposed strain rate or stress. Similar to crystalline materials, the plasticity of BMGs reflects tuned criticality showing remarkable quantitative agreement with the slip statistics of slowly-compressed nanocrystals. The results imply that material-evaluation methods based on slip statistics apply to both crystalline and BMG materials.
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serrated flow kinetics in a zr based bulk metallic glass
Intermetallics, 2010Co-Authors: J W Qiao, P K LiawAbstract:Abstract At different strain rates, the stress-flow Serrations in the compressive stress-time curves of a Zr-based bulk metallic glass (BMG) exhibit different magnitudes and time scales. A critical strain rate can be identified, at which the serrated flow kinetics is characterized to be homogeneous in both time and magnitude. Below the critical strain rate, the Serrations are lack of homogeneous magnitudes and time scales because the excessive elastic energy results in distinct softening within the viscous fluid-like shearing layer due to the adiabatic heating. As a consequence, the tiny disturbances, such as the vibration of test machines, affect the shear banding behavior. A periodic function is established for the critical instability, and two pertinent parameters, the mean stress drop magnitude and the mean waiting time (MWT), can be determined, which may be associated with the deformation micromechanism, the mechanical property, and the glass-forming ability of BMGs. The present approach may provide a new way to study the Serration-deformation behavior of BMGs.
S X Song - One of the best experts on this subject based on the ideXlab platform.
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flow Serration in a zr based bulk metallic glass in compression at low strain rates
Intermetallics, 2008Co-Authors: S X Song, J Wadsworth, T.g. NiehAbstract:Abstract It is demonstrated that at slow strain rates (∼10−4 s−1) in compression, the dominant room temperature macroscopic deformation mode in a ductile Zr-based bulk metallic glass is single shear along the principal shear plane. The stress–strain curve exhibited serrated flow in the plastic region. Scanning electron micrographs of the deformed samples revealed regularly spaced striations on the shear surface. A detailed analysis of the observed Serrations disclosed that they were intimately related to the striations on the shear surface, suggesting that the Serrations were mainly caused by intermittent sample sliding. Further investigations were conducted using in situ compression experiments; video images showed that there was indeed a one-to-one correspondence between the intermittent sliding and flow Serration. The current study therefore suggests that flow Serration is a result of intermittent sample sliding. This result also implies that the principal shear plane, once formed, is the preferential site for additional shear band formation.
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Analyses of shear band emission in a Mg-based bulk metallic glass deformed at different nanoindentation rates
Intermetallics, 2008Co-Authors: S X Song, Jason S.c. Jang, T.g. NiehAbstract:Abstract Nanoindentation behavior of Mg 57 Cu 31 Y 6.6 Nd 5.4 bulk metallic glass was characterized at different loading rates. The load–displacement curves exhibit significant displacement Serrations, apparently associated with discrete shear band emission, at low loading rates but disappear at high rates. Analyses based on displacement Serration, strain rate Serration and hardness Serration were carried out to determine the critical strain rate beyond which the transition from inhomogeneous to homogeneous deformation actually took place. It was concluded that the hardness Serration analysis probably provides the most reasonable result as the other two were limited by the instrument noises. Based on a shear band nucleation model, the critical nucleus size was estimated to be a sphere of about 25 nm in diameter.
