11,616 research outputs found

    Analysis of the Size Effects on the Pseudoelastic Behavior of Shape Memory Alloy Micro-pillars

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    Size dependent properties of Shape Memory Alloys (SMAs) in micro and nano scales have gained an increasing attention due to the existing and potential applications of SMAs in microelectromechanical systems (MEMS) and small scale biomedical devices. Such applications exploit the pseudoelastic and shape memory properties the SMAs. In order to enhance the applicability of SMA micro and nano structures, the size dependency of the thermo-elastic behavior of SMAs should be understood. In this study, the dependency of the pseudoelastic behavior of Nickel-Titanium (NiTi) micro-pillars on their diameter was analyzed. Isothermal compression experiments from literature of bulk and micro-pillars were analyzed to determine the critical transformation stresses for different pillar diameters. The analysis of experimental data shows that the critical transformation stresses increase as the micro-pillar average diameter decreases. The relations between the critical transformation stresses and the average pillar diameter were represented using power functions. It was assumed that the elastic modulus and Poisson’s ratios of the austenite and martensite phases, the transformation strain parameters, and the stress influence coefficients were unaffected by the micro-pillar size. Parametric studies were performed using the finite element analysis to find the effects of the taper angle and the aspect ratio on the micro-pillars behavior. Comparisons of the results found from finite element simulations and experiments show that the model accurately predicts the pseudoelastic response of bulk and micro-pillars. The results of the parametric studies show that the hysteresis of the compression response decreases as the taper angle increases. The effect of the micro-pillar diameter on the compression response is less significant for micro-pillars of higher aspect ratios and higher taper angles

    Characterization of mechanical behavior of nanocrystalline layer induced by SMAT using micro-pillar compression technique coupled with finite element analysis

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    Micro-pillar compression tests were used to study the mechanical behavior of a stainless steel that has undergone SMAT (Surface Mechanical Attrition Treatment). Micro-pillars were machined using a Focused Ion Beam (FIB) on the cross-section of a SMATed specimen at different distances from the treated surface. These micro-pillars were thus located in different areas more or less affected by the SMAT. They were then compressed with a flat head mounted on a nanoindenter to obtain loading-displacement curves. These compression tests can give information on the mechanical gradient present from the top surface down to the bulk material after SMAT: a superficial nanocrystalline layer (from 10 to 50 micrometers thick and composed of grains with a diameter ranging from 10 to 50 nm) is indeed generated as well as a transition layer (between 200 and 300 micrometers thick and characterized by a grain size gradient from the nanometer to the micrometer scale as the distance from the surface increases) just below the nanocrystalline layer. These compression tests coupled with finite element analysis (FEA) can provide precious information at the mesoscopic scale on the mechanical behavior of the different layers present in the SMATed steel. FEA was used to study the effect of experimental parameters including taper angle (the angle between the tangent of wall and the axis of pillar), aspect ratio (the ratio of height and diameter of the pillar), and misalignment between the pillar axis and the compression direction. Based on the results of FEA, the constitutive behavior in the form of stress-strain curve was identified for the different layers beneath the treated surface including the nanocrystalline layer. According to the obtained stress-strain curves, the mechanical strength of the stainless steel is significantly improved after SMAT

    Characterizing Strength and Fracture of Wood Micropillars Under Uniaxial Compression

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    The structure of the actual wood cell wall is very complex and it consists of several layers. Some research has been done to measure the mechanical properties of wood cell wall. For example, the hardness and modulus of wood cell wall was estimated using a nanoindentation test. But the mechanical contribution of wood cell wall is not fully understood and documented in the literature. Understanding the micro mechanical properties of the wood cell wall are paramount because of the potential for applications in cellulose nano-composites research and development. The focus of this research was to investigate the essential of the strength and fracture of wood cell walls by uniaxial micro-compression test. Keranji and loblolly pine were chosen to perform the micro-compression tests. After initial sample preparation by microtoming, a novel method for sample preparation was adopted. The cylindrical shaped micro pillars were milled using a Focused Ion Beam (FIB) while each pillar was milled in a single wood cell wall. The beam voltage of this FIB system was 30 KV. After measuring the dimension of the micropillar through analyzing the SEM images by ImageJ software, the uniaxial compression test on the micro pillar was conducted using a Nano II Indenter system with a 10 micrometers diameter flat tip. The loading rate of 20 nm/s was used to obtain the load-displacement curves. As a result, the yield stress of keranji cell wall was 136.5 MPa and the compression strength was 160 MPa. The yield stress of loblolly pine cell wall was 111.3 MPa and the compression strength was 125 iv MPa. The fracture behavior of wood micropillar confirmed that wood cell wall also is a brittle type of material. KEY WORDS: wood, cell wall, loblolly pine, keranji, focused ion beam (FIB), scanning electron microscopy (SEM), micropillar, uniaxial micro-compression test, fracture behavior

    Investigations on micro-mechanical properties of polycrystalline Ti(C,N) and Zr(C,N) coatings

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    Micro-mechanical properties of Ti(C,N) and Zr(C,N) coatings deposited by chemical vapor deposition on a WC-Co cemented carbide substrate were examined by micro-compression testing using a nanoindenter equipped with a flat punch. Scanning Electron Microscopy, Focused Ion Beam, Electron Backscattered Diffraction and Finite Element Modeling were combined to analyze the deformation mechanisms of the carbonitride layers at room temperature. The results revealed that Ti(C,N) undergoes a pure intergranular crack propagation and grain decohesion under uniaxial compression; whereas the fracture mode of Zr(C,N) was observed to be inter/transgranular failure with unexpected plastic deformation at room temperature.Peer ReviewedPostprint (author's final draft

    Extraction of crystal plasticity parameters of IN718 using high temperature micro-compression

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    Ni-based superalloys are widely utilized in structural applications in aeroengine and power generation industries, owing to their exceptional high temperature mechanical properties in corrosive and oxidizing environments. Depending on the application fields, different types of superalloys with disparate microstructures are employed. For the turbine discs and some static components, forged or cast polycrystalline Ni-based superalloys are usually used. In the present investigation, site-specific micropillars with the diameter varying from 1 um to 18 um were milled out by Focused Ion Beam (FIB) from a polycrystalline IN718 superalloy specimen and then measured with high temperature micro-compression techniques up to 575 ÂşC. The effects of pillar size, pillar orientation, strain rate and temperatures on the micro-compression behavior were quantitatively assessed. The measurements show that there is a small size effect with the pillar diameter spanning from 3 mm to 18 mm, a large effect of crystal orientation on yield strength, a small strain rate sensitivity at room temperature and high temperature as well as a small yield strength drop when the testing temperature increases to 575 ÂşC. The different hardening behavior for single-slip, co-planar and non co-planar double slip as well as multiple-slip conditions were investigated. The micro-compression testing results were used to determine the crystal plasticity parameters of a phenomenological crystal plasticity (CP) model of IN718 superalloy, by comparing the experimental measurements with finite element (FE) simulations. The extracted crystal plasticity parameters were then used in a polycrystalline finite element model, in which the actual microstructure is explicitly accounted by a Representative Volume Element (RVE). This new model was able to predict, without fitting any parameter, the experimental macroscopic compression test with an error below 5%

    \u3cem\u3eIn Situ\u3c/em\u3e TEM Micropillar Compression Testing in Irradiated Oxide Dispersion Strengthened Alloys

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    The objective of this study is to determine the validity of in situ transmission electron microscopy (TEM) micro-compression of pillars in as received and ion-irradiated Fe-9%Cr oxide dispersion strengthened (ODS) alloy. The growing role of charged particle irradiation in the evaluation of nuclear reactor candidate materials requires the development of novel methods to assess mechanical properties in near-surface irradiation damage layers just a few micrometers thick. In situ TEM mechanical testing is one such promising method, yet size effects must be understood to validate the technique. In this work, a micro-compression pillar fabrication method is developed. Yield strengths measured directly from TEM in situ compression tests are within expected values, and are consistent with predictions based on the irradiated microstructure. Measured elastic modulus values, once adjusted for deformation and deflection in the base material, are also within the expected range. A pillar size effect is only observed in samples with minimum dimension ≤ 100 nm due to the low inter-obstacle spacing in the as received and irradiated material. By comparing the microstructural obstacle spacing with specimen dimensions, size effects can be understood and TEM in situ micropillar compression tests can be used to quantitatively determine mechanical properties of shallow ion-irradiated layers

    Using coupled micropillar compression and micro-Laue diffraction to investigate deformation mechanisms in a complex metallic alloy Al13Co4

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    In this investigation, we have used in-situ micro-Laue diffraction combined with micropillar compression of focused ion beam milled Al13Co4 complex metallic alloy to study the evolution of deformation in Al13Co4. Streaking of the Laue spots showed that the onset of plastic flow occured at stresses as low as 0.8 GPa, although macroscopic yield only becomes apparent at 2 GPa. The measured misorientations, obtained from peak splitting, enabled the geometrically necessary dislocation density to be estimated as 1.1 x 1013 m-2

    Compression of micron-sized pillars of anodic aluminium oxide nano-honeycomb

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    Micro-pillars of anodic aluminium oxide with nano-sized honeycomb channels along the pillar axis exhibit compressive stressstrain response with large excursions corresponding to discrete, inhomogeneous deformation events. Each excursion is found to associate with the severe distortion of a material layer at the pillar's head, whereas the remaining of the pillar remains intact. The stresses at which these excursions occur do not exhibit any significant dependence on the pillar size. A simple model is proposed to describe the response of pillars under compression, which energetically, as well as kinetically, explains as to why the localized deformation always takes place at the pillar head. Predictions on the occurrence of instability events from this model also quantitatively agree with the experimental observations. © 2010 2010 Elsevier Ltd. All rights reserved.postprin

    Micro-pillar testing of amorphous silica

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    International audienceAmorphous silica exhibits a complex mechanical response. The elastic regime is highly non linear while plastic flow does not conserve volume, re- sulting in densification. As a result the quantification of a reliable constitutive equation is a difficult task. We have assessed the potential of micro-pillar compression testing for the investigation of the micromechanical properties of amorphous silica. We have calculated the response of amorphous silica mi- cropillars as predicted by Finite Element Analysis. The results were compared to preliminary micro-compression tests. In the calculations an advanced con- stitutive law including plastic response, densification and strain hardening was used. Special attention was paid to the evaluation of the impact of substrate compliance, pillar misalignment and friction conditions. We find that amor- phous silica is much more amenable than some metals to microcompression experiments due to a comparatively high ratio between yield stress and elastic modulus. The simulations are found to be very consistent with the experimen- tal results. However full agreement cannot be obtained without allowance for the non linear response of amorphous silica in the elastic regime
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