1,721,045 research outputs found
Mechanics of indentation of plastically graded materials - I: Analysis
No full textThe introduction of controlled gradients in plastic properties is known to influence the resistance to damage and cracking at contact surfaces in many tribological applications. In order to assess potentially beneficial effects of plastic property gradients in tribological applications, it is essential first to develop a comprehensive and quantitative understanding of the effects of yield strength and strain hardening exponent on contact deformation under the most fundamental contact condition: normal indentation. To date, however, systematic and quantitative studies of plasticity gradient effects on indentation response have not been completed. A comprehensive parametric study of the mechanics of normal indentation of plastically graded materials was therefore undertaken in this work by recourse to finite element method (FEM) computations. On the basis of a large number of computational simulations, a general methodology for assessing instrumented indentation response of plastically graded materials is formulated so that quantitative interpretations of depth-sensing indentation experiments could be performed. The specific case of linear variation in yield strength with depth below the indented surface is explored in detail. Universal dimensionless functions are extracted from FEM simulations so as to predict the indentation load versus depth of penetration curves for a wide variety of plastically graded engineering metals and alloys for interpretation of, and comparisons with, experimental results. Furthermore, the effect of plasticity gradient on the residual indentation pile-up profile is systematically studied. The computations reveal that pile-up of the graded alloy around the indenter, for indentation with increasing yield strength beneath the surface, is noticeably higher than that for the two homogeneous reference materials that constitute the bounding conditions for the graded material. Pile-up is also found to be an increasing function of yield strength gradient and a decreasing function of frictional coefficient. The stress and plastic strain distributions under the indenter tip with and without plasticity gradient are also examined to rationalize the predicted trends. In Part 11 of this paper, we compare the predictions of depth-sensing indentation and pile-up response with experiments on a specially made, graded model Ni-W alloy with controlled gradients in nanocrystalline grain size. (c) 2007 Elsevier Ltd. All rights reserved.N
Electromigration-induced via failure assisted by neighboring clusters
No full textWith different locations of a cluster, stress evolutions at a via are simulated. Via fails fastest at a specific location of a cluster, which is named fastest stress enhancing polygranular cluster position (FaSEPP). Suggested model and simulated results show that FaSEPP decreases with increasing current density but does not vary with temperature. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.N
An epifluidic electronic patch with spiking sweat clearance for event-driven perspiration monitoring
No full textSensory neurons generate spike patterns upon receiving external stimuli and encode key information to the spike patterns, enabling energy-efficient external information processing. Herein, we report an epifluidic electronic patch with spiking sweat clearance using a sensor containing a vertical sweat-collecting channel for event-driven, energy-efficient, long-term wireless monitoring of epidermal perspiration dynamics. Our sweat sensor contains nanomesh electrodes on its inner wall of the channel and unique sweat-clearing structures. During perspiration, repeated filling and abrupt emptying of the vertical sweat-collecting channel generate electrical spike patterns with the sweat rate and ionic conductivity proportional to the spike frequency and amplitude over a wide dynamic range and long time (> 8 h). With such ‘spiking’ sweat clearance and corresponding electronic spike patterns, the epifluidic wireless patch successfully decodes epidermal perspiration dynamics in an event-driven manner at different skin locations during exercise, consuming less than 0.6% of the energy required for continuous data transmission. Our patch could integrate various on-skin sensors and emerging edge computing technologies for energy-efficient, intelligent digital healthcare
Transforming One-Dimensional Nanowalls to Long- Range Ordered Two- Dimensional Nanowaves: Exploiting Buckling Instability and Nanofi bers Effect in Holographic Lithography
No full textTwo-dimensional nanowaves with long-range order are fabricated by exploiting swelling-induced buckling of one-dimensional (1D) nanowalls with nanofibers formed in-between during holographic lithography of the negative-tone photoresist SU-8. The 1D film goes through a constrained swelling in the development stage, and becomes buckled above the critical threshold. The degree of lateral undulation can be controlled by tuning the pattern aspect ratio (height/width) and exposure dosage. At a high aspect ratio (e.g., 6) and a high exposure dosage, nanofibers (30-50 nm in diameter) are formed between the nanowalls as a result of overlapping of low crosslinking density regions. By comparing experimental results with finite-element analysis, the buckling mechanism is investigated, which confirms that the nanofibers prevent the deformed nanowalls from recovery to their original state, thus, leading to long-range ordered two-dimensional (2D) wavy structures. The film with nanowaves show weaker reflecting color under an ambient light and lower transmittance compared to the straight nanowalls. Using double exposure through a photomask, patterns consisting of both nanowaves and nanowalls for optical display are created.N
Design of super-conformable, foldable materials via fractal cuts and lattice kirigami
No full textMaterials that can expand and collapse, fold, and transform into a variety of shapes have attracted significant interest and have applications in the design of flexible electronics, color displays, smart windows, actuators, sensors, and both photonic and phononic devices. But how can we render a rigid device super-flexible so that it can wrap around a sphere without bending and stretching? How can flat surfaces be transformed into any desired three-dimensional (3D) structure without disruptive or catastrophic deformation? The key lies in cuts. Here, we review recent research progress in the design of super-conformable and foldable materials by employing fractal cutting and lattice-based kirigami elements that combine cutting and folding. By prescribing cuts with different motifs, identifying edges in the right geometry, and by programming the folding directions, we show that a single flat sheet can be transformed into a variety of targeted 2D and 3D structures-a pluripotent platform for new technologies.N
On the effect of Ag content on the deformation behavior of ultrafine-grained Pd-Ag alloys
No full textPd and Pd-Ag alloys with grain size smaller than 500 mn were studied using nanoindentation to investigate the effect of alloy content on the deformation behavior. The grain size decreased with increasing Ag content. While the hardness increases with decreasing grain size, the strain rate sensitivity decreases, which is opposite to what is expected for fine-grained face-centered cubic metals. Solid solution strengthening and changes in the stacking fault density are suggested to contribute to the hardness increase and the deformation behavior. (c) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.N
Highly durable and flexible dye-sensitized solar cells fabricated on plastic substrates: PVDF-nanofiber-reinforced TiO2 photoelectrodes
No full textIn this study, we developed a novel nanostructured polymer nanofiber/TiO2 nanoparticle composite photoelectrode with high bendability by a spray-assisted electrospinning method. The composite film is used as the photoelectrode in plastic dye-sensitized solar cells (DSCs). The polymer/TiO2 composite photoelectrode has a structure similar to that of a fiber-reinforced composite; the matrix of the composite photoelectrode contains TiO2 nanoparticles, and PVDF nanofibers are embedded in this matrix. Compared to conventional DSCs, composite-based DSCs show outstanding bending stability because the polymer nanofibers prevent delamination of the electrode by relieving the external stress and effectively retarding crack generation and propagation. Moreover, the efficiency of the cell containing composite electrodes is comparable to that of a cell containing only TiO2, suggesting that the proposed PVDF-nanofiber-reinforced photoelectrode is a promising candidate for a bendable photoelectrode in high-efficiency flexible plastic DSCs
Validity of the reduced modulus concept to describe indentation loading response for elastoplastic materials with sharp indenters
No full textRecent computational parametric studies have developed reverse algorithms to extract material properties of elastoplastic materials using experimental sharp nanoindentation. These methods used reduced modulus in their parameters to include the effect of indenter compliance. To investigate the validity of using reduced modulus, we conducted experimental indentation of a couple of representative cases for elastoplastic metals with a diamond and a sapphire Berkovich tip. Then, we performed a finite element study for sharp indentation of the same material systems. Both computational and experimental results indicate that the use of reduced modulus is invalid to describe indentation loading response for elastoplastic materials in a certain material regime. Our results show that indenter compliance is overestimated by the previous predictions using reduced modulus. This overestimation leads to underestimation of indenter curvature and causes error in extracting material properties by reverse algorithms.N
Mechanics of indentation of plastically graded materials - II: Experiments on nanocrystalline alloys with grain size gradients
No full textA systematic study of depth-sensing indentation was performed on nanocrystalline (nc) Ni-W alloys specially synthesized with controlled unidirectional gradients in plastic properties. A yield strength gradient and a roughly constant Young's modulus were achieved in the nc alloys, using electrodeposition techniques. The force vs. displacement response from instrumented indentation experiments matched very well with that predicted from the analysis of Part I of this paper. The experiments also revealed that the pile-up of the graded alloy around the indenter is noticeably higher than that for the two homogeneous reference alloys that constitute the bounding conditions for the graded material. These trends are also consistent with the predictions of the indentation analysis. (c) 2007 Elsevier Ltd. All rights reserved.N
Mechanical Properties and Piezoresistivity of Tellurium Nanowires
No full textAmong elemental semiconductors, tellurium (Te) exhibits unique mechanical and electromechanical properties due to its highly anisotropic crystal structure and mixed interatomic bonding modes. A lack of experimental investigations of these properties inhibits its adoption in new applications both in bulk form as well as at the nanoscale. In this study, uniaxial tensile tests were conducted in a scanning electron microscope (SEM) on [0001] orientated Te nanowires (NWs) with diameters ranging from 15 to 35 nm. An average elastic modulus is estimated to be 38.6 +/- 4.7 GPa. Both elastic and elastic-plastic behaviors are observed in tested NWs, with a large fracture strain of up to 18% achieved in the latter case. Regardless of the deformation type, electromechanical tests of Te NWs show a trend of decreasing resistance with increasing strain at low-to-moderate tensile strains (0-4%). This piezoresistive effect provides for new opportunities for tellurium to be utilized either in nanoscale devices or in systems that can utilize the extraordinary properties of single-crystal tellurium.N
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