287 research outputs found

    Mechanical analog of temperature for the description of force distribution in static granular packings

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    The mechanical analog of temperature for the description of force distribution in static granular packings was discussed. The constraint on entropy was found to be considered as measure of the degree of retained disorderness in the system. The role of mechanical temperature as a parameter controlling the mixity between energy minimization and entropy maximization in the equilibrium condition was also elaborated.published_or_final_versio

    Nanomechanical Characterization of Soft Materials

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    This chapter reviews the creep or viscoelastic deformation behavior of soft materials under nanoindentation-type testing. Analysis protocols of nanoindentation based on the Hertzian elastic contact theory, linear viscoelasticity analyses, and a more recent rate-jump method, are described and assessed. In addition to continuous viscoelasticity, a special type of discrete creep deformation, often observed in a wide range of materials during nanomechanical testing, is also highlighted.postprin

    The effects of creep on elastic modulus measurement using nanoindentation

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    During the unloading segment of nanoindentation, time dependent displacement (TDD) accompanies elastic deformation. Consequently the modulus calculated by the Oliver-Pharr scheme can be overestimated. In this paper we present evidences for the influence of the measured modulus by TDD. A modification method is also presented to correct for the effects of TDD by extrapolating the TDD law in the holding process to the beginning of the unloading process. Using this method, the appropriate holding time and unloading rate can be estimated for nanoindentation test to minimise the effects of TDD. The elastic moduli of three materials computed by the modification method are compared with the results without considering the TDD effects.published_or_final_versio

    A rate-jump method for characterization of soft tissues using nanoindentation techniques

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    The biomechanical properties of soft tissues play an important role in their normal physiological and physical function, and may possibly relate to certain diseases. The advent of nanomechanical testing techniques, such as atomic force microscopy (AFM), nano-indentation and optical tweezers, enables the nano/micro-mechanical properties of soft tissues to be investigated, but in spite of the fact that biological tissues are highly viscoelastic, traditional elastic contact theory has been routinely used to analyze experimental data. In this article, a novel rate-jump protocol for treating viscoelasticity in nanomechanical data analysis is described. © 2012 The Royal Society of Chemistry.postprin

    Effects of pore-channel ordering on the mechanical properties of anodic aluminum oxide nano-honeycombs

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    The mechanical properties of anodic aluminum oxide (AAO) nano-honeycombs with different spatial ordering of pore-channels are investigated by nanoindentation. The pore-channel ordering is systematically varied by carefully adjusting the orientation of the aluminum used for anodization. The results indicate that the strength of AAO structures increases significantly with the regularity of their pore-channel arrangement, whereas the elastic modulus is less sensitive to pore-channel regularity. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.postprin

    Indentation strain burst phenomenon induced by grain boundaries in niobium

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    Using depth-sensing indentation, a pop-in phenomenon induced by grain boundaries, namely, a sudden indenter displacement jump when indented near a grain boundary segment, was observed in polycrystalline niobium. This grain-boundary type of pop-in occurs at a larger force than the initial elasto-plastic pop-in, which is observed with and without a grain boundary nearby. The experimental results show that this pop-in effect has a close relationship with the misorientation across the grain boundary. The occurrence of this pop-in phenomenon is rationalized in terms of slip transmission across the grain boundary. © 2004 Materials Research Society.published_or_final_versio

    Viscoelastic effects during unloading in depth-sensing indentation

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    With polypropylene as a prototype viscoelastic material at room temperature, it was found that a "nose" may appear in the unloading segment of the load-displacement curve during nanoindentation when the holding time at peak load is short and/or the unloading rate is small, and when the peak load is high enough. The load at which the nose appears was also found to decrease linearly with decreasing unloading rate. A linear viscoelasticity analysis was performed to interpret this effect. The analysis predicts a linear variation between the nose load and the unloading rate, and the slope of such a linear variation is also shown to be proportional to the viscosity parameter of the material. Thus, by measuring the slope of the nose-load versus unloading rate plot at a given temperature, the viscosity parameter of the specimen can be found. This is a new way of measuring the viscosity parameter of a material in addition to the existing method of force modulation and noting the frequency response of the displacement.published_or_final_versio

    Effects of boron doping on the grain growth behaviour of γ/γ nickel-aluminium alloy

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    Effects of 0.5 at. 9c boron doping on Nigs Al y/f superalloys were investigated for the first !ime. yielding a number of observations not previously observed in the literature. First, the grant growth kinetics of both the doped and undoped alloys were found to disobey the simple Nielsen law d = Of. but instead follow an equation of the type d = C ln(r) + C0. The constants C and Co were found to be respectively 10.2 and 23.2 for the boron-free alloy, and 6.2 and 14.2 for ire boron-doped alloy, i.e., the grain growth rate was retarded significantly upon boron doping. Such a retarding effect is thought to be due to the formation of a boron-nickel cosegregated zone observed at the grain boundaries of the doped alloy: the width of the zone, in juni's, is two to three orders of magnitude larger than the boron induced disordered layer found in nickel rich Ni3Al. compounds doped with boron. Other associated effects of the cosegregated zone include a -harp increase in toughness, much better slip transmission across grains and reduced workhardening rates. Another intriguing point is that the Y precipitates were found to segregate to the grain boundaries in the boron-free alloy after cold rolling, but no such segregation of y precipitates, has been observed in the boron-doped alloy. The different deformation microstructures and the retarded grain growth rates upon boron doping will be discussed.published_or_final_versionlink_to_subscribed_fulltex
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