| 1. |
- Aihara, Aya, et al.
(författare)
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A Vibration Estimation Method for Wind Turbine Blades
- 2017
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Ingår i: Experimental mechanics. - : Springer Science and Business Media LLC. - 0014-4851 .- 1741-2765. ; 57:8, s. 1213-1224
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Tidskriftsartikel (refereegranskat)abstract
- This paper reports the development of a vibration monitoring system for wind turbine blades. This system is used to estimate the deflection at the tip blade on a wind turbine tower. Technical accidents of wind turbine blades have become increasingly common. Thus, regular monitoring of the blades is very important to prevent breakdowns, especially in cases when the blades begin to vibrate excessively. The monitoring system developed in this study satisfies two main objectives for practicality. First, our system is easy to install on existing wind turbines. Second, blade deflection is measured in real time. Our system can be operated using a few strain gages attached at the blade root, and the deflection is calculated based on the monitored stress. Thus, direct measurement of deflection at the blade tip is unnecessary. An estimation algorithm for this purpose is adopted based on the experimental modal analysis. This paper focuses on the evaluation of the estimation algorithm to investigate the feasibility of our system. Basic experiments were conducted using a simple blade model of a 300 W scaled wind turbine under rotation. Signals from the strain gages were acquired by a sensor network and sent to a computer through a wireless connection. The results show that the estimation accuracy is acceptably high. Therefore, we conclude that our proposed system is practical.
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| 2. |
- Asp, Leif E., et al.
(författare)
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A biaxial thermomechanical disk test for glassy polymers
- 1997
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Ingår i: Experimental mechanics. - 0014-4851 .- 1741-2765. ; 37:1, s. 96-101
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Tidskriftsartikel (refereegranskat)abstract
- Failure criteria for polymers need to include effects from the stress state. For this reason, biaxial test results are of interest. However, biaxial test methods usually require expensive equipment. In the test method presented here, a disk of epoxy is bonded between a steel ring and a steel disk. The temperature is then lowered until fracture is observed. Experiments were performed on three different glassy epoxy polymers. The biaxial stress state was analyzed by finite element analysis and by an approximate analytical model. Experimental observations support the ability of the method to provide material property data. An approximate analytical model was found sufficiently accurate for stress analysis and determination of the stress state at failure.
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| 3. |
- Ben David, O., et al.
(författare)
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Evaluation of the Mechanical Properties of PMMA Reinforced with Carbon Nanotubes - Experiments and Modeling
- 2014
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Ingår i: Experimental Mechanics. - : Springer Science and Business Media LLC. - 1741-2765 .- 0014-4851. ; 54:2, s. 175-186
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Tidskriftsartikel (refereegranskat)abstract
- The effective mechanical properties of polymethyl-methacrylate (PMMA) reinforced with carbon nanotubes (CNTs) were evaluated by means of two approaches: experiments and a micromechanical model. With various concentrations of CNTs, two specimen fabrication processes were examined: hot pressing (HP) and injection molding (IM). Experiments included a series of uniaxial tensile tests guided by an ASTM standard. Using displacement control, tests were carried out while images were taken of the gage area. The in-plane displacement fields were evaluated by means of Digital Image Correlation (DIC). A MATLAB program was then used to calculate strains, create stress-strain and strain-force curves and determine Young's modulus E, Poisson's ratio , the ultimate tensile stress and the strain to failure . In addition, simulations were carried out using a micromechanical model (High-Fidelity Generalized Method of Cells or HFGMC). A Repeating Unit Cell (RUC) consisting of one CNT and PMMA surrounding it was modeled and analyzed in order to determine the effective mechanical properties of the composite. This method allows for imperfect bonding between the phases which is controlled by two parameters. These damage parameters decrease the stress-strain response of the material. However, the increase of the volume fraction increases the composite response. These two conflicting effects appear to provide the observed decrease in Young's modulus for low volume fractions as discussed. The effects of CNT concentration, geometry and orientation, as well as the interface between the phases, were examined. It was seen from the experimental results, for HP specimens, that for low concentrations of CNTs, E initially decreases and then increases significantly as the weight fraction increases. This behavior of E was quantitatively predicted by the HFGMC model. For IM specimens, Young's modulus is nearly constant for low weight fractions of CNTs and then increases with weight fraction.
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| 4. |
- Chen, Bin, et al.
(författare)
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Meshfree Digital Image Correlation Using Element Free Galerkin Method : Theory, Algorithm and Validation
- 2023
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Ingår i: Experimental mechanics. - : Springer Nature. - 0014-4851 .- 1741-2765. ; 63:3, s. 517-528
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Tidskriftsartikel (refereegranskat)abstract
- Background: The association of advanced digital image correlation (DIC) and numerical simulation has been widely used for inverse parameter identification. Objective: It is attractive to develop an accurate DIC method sharing the common features with numerical simulation, which can lead to better synergy between experiments and simulations. Methods: A new meshfree digital image correlation (MF-DIC) using element free Galerkin method (EFGM) is proposed for deformation measurement. The EFGM is a classical meshfree method in numerical studies, and it is directly used to construct the shape function in MF-DIC from a set of scattered nodes for image matching. The MF-DIC is principally different from the classical local DIC and global DIC since it does not rely on the concept of a subset or an element. Results: In MF-DIC, the C1-continuous displacement for every point is constructed based on a group of scattered nodes in a small support domain surrounding it. The continuous strain map can then be directly derived from the displacement, instead of using an additional smoothing technique as required in classical local DIC or post-processing used in global DIC. A performance assessment based on the Metrological Efficiency Indicator (MEI), as defined in DIC Challenge 2.0, shows that the proposed MF-DIC yields an excellent balance between spatial resolution and measurement resolution for both displacement and strain measurements. Conclusions: Given that the proposed MF-DIC shares common features with the classical meshfree method in computational mechanics, it paves the way for an enhanced synergy between experiments and simulations required for robust inverse parameter identification methods.
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| 5. |
- Engqvist, Jonas, et al.
(författare)
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Multi-scale Measurement of (Amorphous) Polymer Deformation: Simultaneous X-ray Scattering, Digital Image Correlation and In-situ Loading
- 2014
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Ingår i: Experimental Mechanics. - : Springer Science and Business Media LLC. - 1741-2765 .- 0014-4851. ; 54:8, s. 1373-1383
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a method to investigate the behaviour of polymers on different scales during deformation using simultaneously collected synchrotron X-ray scattering, digital image correlation (DIC) and tensile loading. The method is demonstrated through experiments made on specimens of amorphous polycarbonate. Deformation is measured in-situ, simultaneously across different scales from the macroscopic deformation, measured using sensors on the tensile machine, to the full-field mesoscopic deformation, measured using DIC, down to the deformation of the nano-scale structure, studied using small and wide angle X-ray scattering (SAXS/WAXS). The DIC reveals highly inhomogeneous deformations that render conventional techniques for measuring deformation, such as extensiometers, virtually useless. The X-ray scattering is measured in several spatial points during continuous loading giving the evolution of the microstructure with respect to both spatial location and load level. The spatial mapping of the scattering reveals characters that would not be observed when only measuring at the centre point or measuring on a large area of the specimen, e.g. wide beam SAXS/WAXS or small angle neutron scattering (SANS). With these data, the macroscopic and the mesoscopic deformation can be correlated to the behaviour of the microstructure providing relevant information when developing micro-mechanical based constitutive models. The experimental results shown here indicate a direct correlation between the major principal strain direction and the maximum anisotropy direction of the SAXS patterns. The current approach can be extended to any kind of polymeric materials or polymer-based nano-composites.
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| 6. |
- Fadil, Hassan, et al.
(författare)
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On the Measurement of two Independent Viscoelastic Functions with Instrumented Indentation Tests
- 2018
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Ingår i: Experimental mechanics. - : Springer. - 0014-4851 .- 1741-2765. ; 58:2, s. 301-314
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Tidskriftsartikel (refereegranskat)abstract
- In the present paper, a methodology for complete characterization of linear isotropic viscoelastic material with spherical instrumented indentation test is proposed. The developed method allows for measuring two independent viscoelastic functions, shear relaxation modulus and time-dependent Poisson's ratio, from the indentation test data obtained at non-decreasing loading, but otherwise arbitrary. Finite element modelling (FEM) is relied upon for validating the proposed methodology and for quantifying the influence of experimental variables on the measurements accuracy. Spherical indentation experiments are performed on several viscoelastic materials: polyoxymethylene, bitumen and bitumen-filler mastics. The viscoelastic material functions obtained with the indentation tests are compared with the corresponding results from the standard mechanical tests. Numerical and experimental results presented indicate that the methodology proposed allows mitigating the machine compliance and loading rate effects on the accuracy of the viscoelastic indentation tests.
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| 7. |
- Fitzka, M., et al.
(författare)
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High Speed In Situ Synchrotron Observation of Cyclic Deformation and Phase Transformation of Superelastic Nitinol at Ultrasonic Frequency
- 2020
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Ingår i: Experimental Mechanics. - : Springer Science and Business Media LLC. - 0014-4851 .- 1741-2765. ; 60:3, s. 317-328
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Tidskriftsartikel (refereegranskat)abstract
- The near equi-atomic intermetallic Ni Ti alloy Nitinol is used for medical implants, notably in self-expanding stent grafts and heart valve frames, which are subjected to several hundred million load cycles in service. Increasing the testing frequency to the ultrasonic range would drastically shorten the testing times and make the very-high cycle regime experimentally accessible. Such tests are, however, only meaningful if the material response at ultrasonic frequency is identical to that observed in conventional fatigue tests. A novel fatigue testing setup where superelastic Nitinol dog bone specimens are loaded at ultrasonic cycling frequency is presented. Loading conditions resemble in vivo loading (i.e., repeated cyclic loading with relatively small strain amplitudes, specimens in a pre-strained multi-phase state). Strains and phase transformations during ultrasonic frequency cycling are quantitatively measured in an X-ray diffraction (XRD) synchrotron experiment and compared to the material response at low frequency. The XRD experiment confirms that forward and reverse stress-induced phase transformation from austenite to martensite via the intermediate R-phase occurs during low frequency (0.1 Hz, strain rate ε˙ ≈ 10−3 s−1) and ultrasonic frequency (20 kHz, ε˙ ≈ 102 s−1) cycling. Since the same deformation mechanisms are active at low and ultrasonic frequency, these findings imply a general applicability of the ultrasonic fatigue testing technique to Nitinol.
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| 8. |
- Fällström, Karl-Evert, et al.
(författare)
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Transient bending waves in anisotropic plates studied by hologram interferometry
- 1989
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Ingår i: Experimental mechanics. - 0014-4851 .- 1741-2765. ; 29:4, s. 409-413
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Tidskriftsartikel (refereegranskat)abstract
- Propagating bending waves are studied in plates made of glass-fiber reinforced polyester. The waves are generated by the impact of a ballistic pendulum. Hologram interferometry, with a double pulsed ruby laser as light source, is used to record the out of plane motion of the waves. The interferograms have an elliptic-like symmetry for an orthotropic plate, while the wave pattern for a symmetric angle-ply reinforced plate has a symmetry about the axes of reinforcements. Experimental data are compared on one hand to analytical results obtained by assuming that the orthotropic plate can be described as if isotropic along the main axes, and on the other hand to numerical results from calculations using the finite-element method. The effective Young's modulus raised to power 1/4 is shown to be an important parameter for the description of the dispersive wave pattern. A defect in the plate alters the wave pattern in the interferograms significantly. This may have technical use.
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| 9. |
- Fällström, Karl-Evert, et al.
(författare)
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Transient bending waves in plates studied by hologram interferometry
- 1989
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Ingår i: Experimental mechanics. - 0014-4851 .- 1741-2765. ; 29:4, s. 378-387
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Tidskriftsartikel (refereegranskat)abstract
- Propagating bending waves are studied in plates made of aluminum and wood. The waves are generated by the impact of a ballistic pendulum. Hologram interferometry, with a double pulsed ruby laser as the light source, is used to record the out of plane motion of the waves. Elliptic-like fringes visualize differences in wave speed for different directions in the anisotropic plate and circular ones are obtained for the isotropic plate. The experimental data for the isotropic plate compare favorably with analytical results derived from the Kirchhoff-plate equation with a point impact of finite duration. A similarity variable is found when starting conditions are modeled as a Dirac pulse in space and time, that brings new understanding to the importance of specific parameters for wave propagation in plates. A formal solution is obtained for a point force with an arbitrary time dependence. For times much larger than the contact time, the plate deflection is shown to be identical to that from a Dirac pulse applied at the mean contact time. A method for determining material parameters, and the mean contact time, from the interferograms is hence developed.
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| 10. |
- Giannadakis, Konstantinos, et al.
(författare)
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Mechanical Performance of a Recycled Carbon Fibre/PP Composite
- 2011
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Ingår i: Experimental mechanics. - : Springer Science and Business Media LLC. - 0014-4851 .- 1741-2765. ; 51:5, s. 767-777
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Tidskriftsartikel (refereegranskat)abstract
- A composite made of recycled carbon fibres in recycled polypropylene matrix is studied experimentally to describe the features of the elastic and time dependent nonlinear mechanical behaviour. The properties of the developed material have a large variability to be addressed and understood. It was found that the stress-strain curves in tension are rather nonlinear at low strain rate and the strength is sensitive to strain rate. The elastic properties' reduction for this composite after loading to high strains is rather limited. More important is that even in the "elastic region" due to viscoelastic effects the slope of loading-unloading curve is not the same and that at higher stress large viscoplastic strains develop and creep rupture is typical. The time and stress dependence of viscoplastic strains was analysed and described theoretically. The viscoelastic response of the composite was analysed using creep compliance, which was found to be slightly nonlinear. © 2010 Society for Experimental Mechanics.
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| 11. |
- Gozzi, Jonas, et al.
(författare)
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Experimental investigation of the behavior of extra high strength steel
- 2005
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Ingår i: Experimental mechanics. - 0014-4851 .- 1741-2765. ; 45:6, s. 533-540
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Tidskriftsartikel (refereegranskat)abstract
- This paper comprises a study concerning the mechanical behavior of extra high strength steel. This is investigated by means of biaxial testing of flat cross-shaped specimens in the full σ1-σ2 plane, a concept developed earlier at Steel Structures, Luleå University of Technology. Furthermore, new specimen designs had to be developed to enable testing of a material with high yield strength and low ultimate over yield strength ratio, such as the extra high strength steel Weldox 1100. The tests are performed in two steps: one initial loading followed by unloading and a subsequent loading in a new direction. The test results, containing data from 15 biaxial tests, are characterized by a slightly anisotropic initial yield criterion where the proof stress in compression is consequently somewhat higher compared to the results in tension. In the subsequent loading the observed phenomena are a Bauschinger effect in the direction opposite the initial loading direction and that the transition from elastic to plastic state in subsequent loadings is gradual and direction-dependent.
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| 12. |
- Gradin, Per, et al.
(författare)
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The Use of Acoustic Emission Monitoring to Rank Paper Materials With Respect to their Fracture Toughness
- 2008
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Ingår i: Experimental mechanics. - : Springer Science and Business Media LLC. - 0014-4851 .- 1741-2765. ; 48:1, s. 133-137
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Tidskriftsartikel (refereegranskat)abstract
- In this study, simplified Acoustic Emission (AE) equipment, in essence an AE signal conditioner and a USB (Universal Serial Bus) data acquisition system, are used to study what happens in paper structures during mechanical loading. By the use of such equipment, some parameters that can be extracted are e.g. the stress and strain at onset of AE, the stress and strain at the onset of rapid AE (defined in some appropriate way), the emission rate at the first stage of loading and the stress and strain at final rupture. In this study however, the interest is focused on one particular parameter i.e. the elastic strain energy density Wc at onset of AE. This is a parameter with a clear physical meaning and in this study, the correlation between this parameter and a fracture toughness measure, is investigated. The conclusion is that when nine different paper materials (with a large span regarding properties) are considered, there is a correlation (however not linear) between these two parameters.
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| 13. |
- Gupta, Priyank, et al.
(författare)
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Characterization of the Constitutive Behavior of a Cathode ActiveLayer in Lithium-Ion Batteries Using a Bending Test Method
- 2020
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Ingår i: Experimental mechanics. - : Springer. - 0014-4851 .- 1741-2765. ; 60, s. 847-860
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Tidskriftsartikel (refereegranskat)abstract
- Presently used experimental techniques for the characterization of tensile and compressive behavior of active layers in lithiumionbatteries have limitations of different kinds. This is particularly true for measurements of compressive properties.Furthermore, the characterizations of time-dependent stress-strain behavior are largely missing. In order to characterize thestress-strain relationship for a dry cathode active layer in lithium-ion batteries, a mechanical testing method is presented thatpreviously has been applied to the testing of optical fibers. The method is based on U-shaped bending of single-side coatedaluminum foils, which enables separate measurements of tensile and compressive properties. In particular, the method has clearadvantages for measurements of compressive properties in comparison to previously reported techniques. Relaxation experimentsare also conducted in order to characterize the time-dependent properties of the dry active layer and to check if these effectscould explain the measured hysteresis. It is found that the elastic modulus in compression is significantly larger than the elasticmodulus in tension and that the compressive modulus increases with strain level. Contrary, the tensile modulus is approximatelyindependent of strain. Furthermore, hysteresis effects are present at loading-unloading measurements, both for tension andcompression. The low values of the measured elastic moduli show that the electrode properties are largely controlled by thebinder and carbon additives. It is concluded that the development of particle-particle contacts most likely is the reason for thehigher modulus in compression in comparison to tension. The time-dependent effects are significant, primarily for shorter timescales, which explains the relaxation behavior, but they cannot fully explain the hysteresis effects. Most likely non-linear micromechanismsdo contribute as well.
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| 14. |
- Hagman, Anton, 1984-, et al.
(författare)
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Thermographical Analysis of Paper During Tensile Testing and Comparison to Digital Image Correlation
- 2017
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Ingår i: Experimental mechanics. - : Springer. - 0014-4851 .- 1741-2765. ; 57:2, s. 325-339
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Tidskriftsartikel (refereegranskat)abstract
- The thermal response in paper has been studied by thermography. It was observed that an inhomogeneous deformation pattern arose in the paper samples during tensile testing. In the plastic regime a pattern of warmer streaks could be observed in the samples. On the same samples digital image correlation (DIC) was used to study local strain fields. It was concluded that the heat patterns observed by thermography coincided with the deformation patterns observed by DIC. Because of its fibrous network structure, paper has an inhomogeneous micro-structure, which is called formation. It could be shown that the formation was the cause of the inhomogeneous deformations in paper. Finite element simulations was used to show how papers with different degrees of heterogeneity would deform. Creped papers, where the strain at break has been increased, were analysed. For these paper it was seen that an overlaid compaction of the paper was created during the creping process. During tensile testing this was recovered as the paper network structure was strained.
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| 15. |
- Harihara Subramonia Iyer, Anand, 1990, et al.
(författare)
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Cyclic Deformation of Microcantilevers Using In-Situ Micromanipulation
- 2021
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Ingår i: Experimental Mechanics. - : Springer Science and Business Media LLC. - 1741-2765 .- 0014-4851. ; 61:9, s. 1431-1442
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Tidskriftsartikel (refereegranskat)abstract
- Background: The trend in miniaturisation of structural components and continuous development of more advanced crystal plasticity models point towards the need for understanding cyclic properties of engineering materials at the microscale. Though the technology of focused ion beam milling enables the preparation of micron-sized samples for mechanical testing using nanoindenters, much of the focus has been on monotonic testing since the limited 1D motion of nanoindenters imposes restrictions on both sample preparation and cyclic testing. Objective/Methods: In this work, we present an approach for cyclic microcantilever bending using a micromanipulator setup having three degrees of freedom, thereby offering more flexibility. Results: The method has been demonstrated and validated by cyclic bending of Alloy 718plus microcantilevers prepared on a bulk specimen. The experiments reveal that this method is reliable and produces results that are comparable to a nanoindenter setup. Conclusions: Due to the flexibility of the method, it offers straightforward testing of cantilevers manufactured at arbitrary position on bulk samples with fully reversed plastic deformation. Specific microstructural features, e.g., selected orientations, grain boundaries, phase boundaries etc., can therefore be easily targeted.
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| 16. |
- Hellström, Lisbeth, et al.
(författare)
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A laboratory wood chipper for chipping under realistic conditions
- 2011
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Ingår i: Experimental mechanics. - : Springer Science and Business Media LLC. - 0014-4851 .- 1741-2765. ; 51:8, s. 1309-1316
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Tidskriftsartikel (refereegranskat)abstract
- In order to be able to study the damage mechanisms and in general the mechanisms active when a wood chip is created during the wood chipping process, it is crucial to have access to an experimental equipment in which chips can be produced under realistic conditions. In this paper is presented a laboratory chipper, which has been developed to admit chipping at rates that can be varied in a large interval i.e. at rates ranging from zero to 50 m/s. The knife used to cut the chips is mounted in a knife holder, which is instrumented in such a way that forces in three orthogonal directions can be measured. Since the actual force and the measured force differs due to inertia effects, a simple mathematical model is developed and used to evaluate the force acting on the knife. Some results are shown from the force measurements and it is concluded that the laboratory chipper is a versatile tool in the process of increasing the understanding of the chipping process.
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| 17. |
- Hosseini, Seyed, 1981, et al.
(författare)
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A Methodology for Temperature Correction When Using Two-Color Pyrometers : Compensation for Surface Topography and Material
- 2014
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Ingår i: Experimental mechanics. - : Springer Science and Business Media LLC. - 0014-4851 .- 1741-2765. ; 54:3, s. 369-377
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Tidskriftsartikel (refereegranskat)abstract
- In this investigation, the applicability of the two-color pyrometer technique for temperature measurements in dry hard turning of AISI 52100 steel was studied, where both machined surfaces as well as cutting tools were considered. The impacts of differing hard turned surface topography on the two-color pyrometer readings was studied by conducting temperature measurements on reference samples created using cutting tools with different degrees of tool flank wear. In order to conduct measurements in a controlled environment, a specially designed furnace was developed in which the samples were heated step-wise up to 1,000 °C in a protective atmosphere. At each testing temperature, the temperatures measured by the two-color pyrometer were compared with temperatures recorded by thermocouples. For all materials and surfaces as studied here, the two-color pyrometer generally recorded significantly lower temperatures than the thermocouples; for the hard turned surfaces, depending on the surface topography, the temperatures were as much as 20 % lower and for the CBN cutting tools, 13 % lower. To be able to use the two-color pyrometer technique for temperature measurements in hard turning of AISI 52100 steel, a linear approximation function was determined resulting in three unique equations, one for each of the studied materials and surfaces. By using the developed approximation function, the measured cutting temperatures can be adjusted to compensate for differing materials or surface topographies for comparable machining conditions. Even though the proposed equations are unique for the hard turning conditions as studied here, the proposed methodology can be applied to determine the temperature compensation required for other surface topographies, as well as other materials. © 2013 Society for Experimental Mechanics.
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| 18. |
- Hosseini, Seyed, 1981, et al.
(författare)
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Determination of Stresses and Retained Austenite in Carbon Steels by X-rays - A Round Robin Study
- 2011
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Ingår i: Experimental Mechanics. - : Springer Science and Business Media LLC. - 1741-2765 .- 0014-4851. ; 51:1, s. 59-69
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Tidskriftsartikel (refereegranskat)abstract
- Residual stresses and retained austenite are twoimportant process-related parameters which need to becontrolled and monitored carefully during production andheat treatment of products. X-ray diffraction techniques arenormally used in this context, and the purpose of thepresent study was to investigate the reproducibility andaccuracy of these methods for medium and high carbonsteels. The work was carried out as a round robin studyincluding nine different laboratories in Sweden and Finland.Stress measurements were carried out on three specimens etched to three different depths, 0 μm, 230 μm and 515 μm. Retained austenite measurements were carried out on three specimens containing approximately 11, 17 and 30 vol.-% of this phase. The stress measurements showed good reproducibility with standard deviations of typically 4% on flat and smooth surfaces and not more than about 8% on etched surfaces. Estimations revealed that specimen misalignment and improper X-ray spot location were the main sources behind the variation in the stress recordings. The determination of retained austenite showed a standard deviation of typically 15% between the different contributors. However, by using identical evaluation methodsfor all raw data, the data spread could be narrowed by afactor of 3 to 4 despite the fact that different experimental settings were used in the individual laboratories.
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| 19. |
- Huang, Hui, et al.
(författare)
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The Dependency of Shear Zone Length on the Shear Strength Profiles in Paperboard
- 2012
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Ingår i: Experimental mechanics. - : Springer Science and Business Media LLC. - 0014-4851 .- 1741-2765. ; 52:8, s. 1047-1055
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Tidskriftsartikel (refereegranskat)abstract
- In this work, the notched shear strength test (NST) has been further improved. In order to simplify and accelerate the testing procedure, the notches with declined slopes were used. With the proposed procedure, the shear strength profile in the thickness direction of a paperboard can be measured using one sheet only. By using the test setup, the dependency of shear zone length on shear strength was investigated. Experimental results show that both the measured shear strength values as well as the shear strength profile varied significantly with different shear zone length. Longer shear zone gave lower shear strength values and flatter profiles, while a shorter shear zone gave higher strength values and more pronounced shear strength profiles that better followed the paperboard ply structure. This proposed new method was also compared with the NST, strip shear test (SST) and rigid shear test (RST) method by using the same test material.
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| 20. |
- Hägglund, R, et al.
(författare)
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Some Aspects on the Zero-span tensile test
- 2004
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Ingår i: Experimental mechanics. - : Springer Science and Business Media LLC. - 0014-4851 .- 1741-2765. ; 44:4, s. 365-374
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we present some analytical and numerical results concerning the zero-span testing method, frequently used for quality control of cellulose fiber for papermaking. Of particular interest is the relationship between an apparent modulus obtained from the zero-span testing method and the elastic properties of the fibers. The apparent elasticity modulus is estimated using two energy theorems in elasto-statics in which the role of span length is explored. Analytical results, derived under the assumption that slippage between specimen and clamps does not occur, clearly show that the apparent modulus strongly depends on the span length. This is verified by the numerical results obtained using the finite element method. In addition to the above analysis, the effect of slippage is investigated, also by utilizing the finite element method, and it is found that for a specific case, the contribution from slippage to the total displacement depends strongly on the length of the span. Tensile tests at nominal zero span were conducted in an effort to further validate the analysis with relevant experimental data and it was concluded that there is qualitative agreement between the experimental results and the result of the analysis.
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| 21. |
- Isaksson, Per, et al.
(författare)
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A Simplified Treatise of the Scott Bond Testing Method
- 2010
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Ingår i: Experimental mechanics. - : Springer Science and Business Media LLC. - 0014-4851 .- 1741-2765. ; 50:6, s. 745-751
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Tidskriftsartikel (refereegranskat)abstract
- The Scott bond test method has been used extensively in the paper industry over the years as a means to assess the bond strength of paper. The method has been a subject of some controversy lately since it does not always correlate to the sensitivity of the material to fracture by delamination. To gain some further insight into which parameters govern the fracture process in a Scott bond test a simplified approach has been chosen in order to formulate an analytical mathematical/mechanical model of the test. The model is dynamic in the sense that inertia effects are included. The material model utilised is a simple cohesive theory that assumes a linear behaviour between stress and crack opening when the material has started to degrade. This choice of material model makes the mathematical model very nonlinear. In fact, a system of three coupled nonlinear second order partial differential equations have to be solved and adjusted to the correct initial conditions. The material parameters needed for the model are the elastic modulus in the thickness direction, the transverse shear (elastic) modulus, the tensile strength (in the thickness direction) and the fracture work (per unit area) for a delamination crack. To investigate the ability of the model, a Scott bond testing apparatus have been equipped with a piezoelectric load sensor. The load cell was mounted on the apparatus' pendulum so that the load acting on the sample holder could be recorded during the whole impact stage. This was done for a number of different initial velocities of the pendulum and it is found that the model gives a fair prediction of the contact load.
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| 22. |
- Jelagin, Denis, et al.
(författare)
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Measurement of the Viscoelastic Properties of Bitumen Using Instrumented Spherical Indentation
- 2013
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Ingår i: Experimental mechanics. - : Springer Science and Business Media LLC. - 0014-4851 .- 1741-2765. ; 53:7, s. 1233-1244
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Tidskriftsartikel (refereegranskat)abstract
- Indentation testing as a tool for determination of the viscoelastic mechanical properties of bitumen is examined in some detail using theoretical, numerical as well as experimental methods. In particular Brinell indentation is analysed and simple but rigorous formulae for a complete characterization of linear viscoelastic materials are presented. Numerical methods (finite element methods) are used in order to verify and substantiate these relations for an experimental situation. Indentation experiments are then performed on bitumen and special efforts are made in order to avoid size effects, i. e. anomalous results due to the fact that the indented specimens are too small and as a result, far field boundary conditions will influence the interpretation of the experimental output. The mechanical properties determined experimentally by indentation are compared with corresponding results from standard mechanical tests, and the results are encouraging considering the fact that non-linear effects are also influencing the outcome of the experiments.
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| 23. |
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| 24. |
- Johansson, S., et al.
(författare)
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3D Strain Field Evolution and Failure Mechanisms in Anisotropic Paperboard
- 2021
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Ingår i: Experimental Mechanics. - : Springer Science and Business Media LLC. - 0014-4851 .- 1741-2765. ; 61:3, s. 581-608
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Tidskriftsartikel (refereegranskat)abstract
- Background: Experimental analyses of the 3D strain field evolution during loading allows for better understanding of deformation and failure mechanisms at the meso- and microscale in different materials. In order to understand the auxetic behaviour and delamination process in paperboard materials during tensile deformation, it is essential to study the out-of-plane component of the strain tensor that is, in contrast to previous 2D studies, only achievable in 3D. Objective: The main objective of this study is to obtain a better understanding of the influence of different out-of-plane structures and in-plane material directions on the deformation and failure mechanisms at the meso- and microscale in paperboard samples. Methods: X-ray tomography imaging during in-situ uniaxial tensile testing and Digital Volume Correlation analysis was performed to investigate the 3D strain field evolution and microscale mechanical behaviour in two different types of commercial paperboards and in two material directions. The evolution of sample properties such as the spatial variation in sample thickness, solid fraction and fibre orientation distribution were also obtained from the images. A comprehensive analysis of the full strain tensor in paperboards is lacking in previous research, and the influence of material directions and out-of-plane structures on 3D strain field patterns as well as the spatial and temporal quantification of the auxetic behaviour in paperboard are novel contributions. Results: The results show that volumetric and deviatoric strain, dominated by the out-of-plane normal strain component of the strain tensor, localize in the out-of-plane centre already in the initial linear stress-strain regime. In-plane strain field patterns differ between samples loaded in the Machine Direction (MD) and Cross Direction (CD); in MD, strain localizes in a more well-defined zone close to the notches and the failure occurs abruptly at peak load, resulting in angular fracture paths extending through the stiffer surface planes of the samples. In CD, strain localizes in more horizontal and continuous bands between the notches and at peak load, fractures are not clearly visible at the surfaces of CD-tested samples that appear to fail internally through more well-distributed delamination. Conclusions: In-plane strain localization preceded a local increase of sample thickness, i.e. the initiation of the delamination process, and at peak load, a dramatic increase in average sample thickening occurred. Different in-plane material directions affected the angles and continuity of the in-plane strain patterns as well as the sample and fracture properties at failure, while the out-of-plane structure affected how the strain fields distributed within the samples.
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| 25. |
- Johansson, Sten, et al.
(författare)
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Errors in crack closure measurements caused by flexure test fixture support effects
- 2001
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Ingår i: Experimental mechanics. - 0014-4851 .- 1741-2765. ; 41:1, s. 47-51
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Tidskriftsartikel (refereegranskat)abstract
- Flexural tests are sometimes used for the evaluation of fatigue and fracture properties, especially for thin cross sections such as plate material where compact tension specimens cannot be used, or where material thickness is limited in the direction required for data generation. The method is also useful for evaluation of crack closure provided the force transfer occurs correctly. This paper shows how the influence of incorrect test fixture supports, which are outside the standard testing arrangement for such test specimens, can affect the force-displacement curves as evidenced using the back-face strain method, The results show that incorrect test fixture supports may cause deviations in the force-displacement trace as an artifact of the testing conditions. This effect can severely influence crack closure measurements. The effect was also modeled with finite element analysis.
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| 26. |
- Kroon, Martin, et al.
(författare)
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Anisotropic Elastic-Viscoplastic Properties at Finite Strains of Injection-Moulded Low-Density Polyethylene
- 2018
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Ingår i: Experimental mechanics. - : Springer New York LLC. - 0014-4851 .- 1741-2765. ; 58:1, s. 75-86
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Tidskriftsartikel (refereegranskat)abstract
- Injection-moulding is one of the most common manufacturing processes used for polymers. In many applications, the mechanical properties of the product is of great importance. Injection-moulding of thin-walled polymer products tends to leave the polymer structure in a state where the mechanical properties are anisotropic, due to alignment of polymer chains along the melt flow direction. The anisotropic elastic-viscoplastic properties of low-density polyethylene, that has undergone an injection-moulding process, are therefore examined in the present work. Test specimens were punched out from injection-moulded plates and tested in uniaxial tension. Three in-plane material directions were investigated. Because of the small thickness of the plates, only the in-plane properties could be determined. Tensile tests with both monotonic and cyclic loading were performed, and the local strains on the surface of the test specimens were measured using image analysis. True stress vs. true strain diagrams were constructed, and the material response was evaluated using an elastic-viscoplasticity law. The components of the anisotropic compliance matrix were determined together with the direction-specific plastic hardening parameters. © 2017 The Author(s)
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| 27. |
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| 28. |
- Lindfeldt, Erik, 1982, et al.
(författare)
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Using DIC to Identify Microscale Strain Fields from in-situ SEM Images of a Pearlitic Steel
- 2014
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Ingår i: Experimental Mechanics. - : Springer Science and Business Media LLC. - 1741-2765 .- 0014-4851. ; 54:9, s. 1503-1513
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Tidskriftsartikel (refereegranskat)abstract
- Digital Image Correlation (DIC) is used to analyze in-situ obtained SEM images of a pearlitic steel. Rather than using a synthetic speckle the microstructure of the material (cementite lamellae embedded in a ferrite matrix) is used as a natural speckle. The impact of the DIC method parameters on the identified motion (displacements and strains) is studied and it is shown that the method is robust, in the sense of being insensitive to the subset size, when it comes to determining the local subset displacements. However, a sufficiently large subset size is required in order for the local subset strains to converge.
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| 29. |
- Linvill, Eric, et al.
(författare)
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The Combined Effects of Moisture and Temperature on the Mechanical Response of Paper
- 2014
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Ingår i: Experimental mechanics. - : Springer-Verlag New York. - 0014-4851 .- 1741-2765. ; 54:8, s. 1329-1341
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Tidskriftsartikel (refereegranskat)abstract
- To model advanced 3-D forming strategies for paper materials, the effects of environmental conditions on the mechanical behavior must be quantitatively and qualitatively understood. A tensile test method has been created, verified, and implemented to test paper at various moisture content and temperature levels. Testing results for one type of paper for moisture contents from 6.9 to 13.8 percent and temperatures from 23 to 168 degrees Celsius are presented and discussed. Coupled moisture and temperature effects have been discovered for maximum stress. Uncoupled effects have been discovered for elastic modulus, tangent modulus, hardening modulus, strain at break, tensile energy absorption (TEA), and approximate plastic strain. A hyperbolic tangent function is also utilized which captures the entire one-dimensional stress-strain response of paper. The effects of moisture and temperature on the three coefficients in the hyperbolic tangent function may be assumed to be uncoupled, which may simplify the development of moisture- and temperature-dependent constitutive models. All parameters were affected by both moisture and temperature with the exception of TEA, which was found to only be significantly dependent on temperature.
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| 30. |
- Lundberg, Bengt, et al.
(författare)
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Analysis of elastic waves from two-point strain measurement : strain, particle velocity, power transmission and related quantities can be determined at an arbitrary section of a cylindrical rod from measurement of strains at two different rod sections
- 1977
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Ingår i: Experimental mechanics. - 0014-4851 .- 1741-2765. ; 17:6, s. 213-218
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Tidskriftsartikel (refereegranskat)abstract
- Provided that the one-dimensional wave equation applies, strain measurement at two sections of a linearly elastic cylindrical rod makes it possible to determine a number of important quantities at an arbitrary section of the rod; for example, strain, particle velocity and power transmission. The equations needed are derived, and the design of an analogue real-time analyzer is presented. The influence of some principal sources of error is analyzed and it is shown that it should be possible to perform accurate evaluation (errors less than a few percent) during a time interval which is not very long compared to the travelling time for a wave between the two gage positions. Comparisons are made between direct measurement and digital evaluation of strain, and between digital and analogue evaluation of particle velocity and power transmission. The discrepancies are typically less than ten percent during a time interval of 20 travelling times between gages. Although these results do not represent what is achievable, the accuracy is sufficient in several applications and demonstrates the feasibility of the method used.
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| 31. |
- Magnusson, Mikael S., et al.
(författare)
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Experimental Evaluation of the Interfibre Joint Strength of Papermaking Fibres in Terms of Manufacturing Parameters and in Two Different Loading Directions
- 2013
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Ingår i: Experimental mechanics. - : Springer Science and Business Media LLC. - 0014-4851 .- 1741-2765. ; 53:9, s. 1621-1634
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Tidskriftsartikel (refereegranskat)abstract
- The strength properties of interfibre bonds play a vital role in the response of fibrous materials, such as paper and paperboard, under mechanical loading. To help tailor the properties of such materials by chemical or mechanical treatments of the fibre wall and fibre surfaces, improved understanding of the microscopic damage and failure mechanisms of interfibre joints is desirable. In this paper, a method for manufacturing and testing of interfibre joint specimens in two principally different modes of loading is presented. The method was applied to investigate the strength of Kraft pulp interfibre joints with different geometries and in two different modes of loading: the conventional shearing mode and also a peeling mode of loading. The method was also used to investigate the influence of drying pressure, defined as the nominal pressure between two Teflon surfaces or between a rubber surface and a Teflon one as well as a simple comparison of a pulp with two different degrees of refining. The results are presented in terms of rupture force and using different methods of normalization such as nominal overlap area, length, and width of the joint region, measured using a microscope. It was shown in this study that normalising the force at rupture by either geometric parameter reduced the scatter of the strength measurements slightly, but, neither were unambiguously more successful than the other. The results of tests done with the peeling type of loading were about 20 % of those done with the conventional shearing type of loading.
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| 32. |
- Nässelqvist, Mattias, et al.
(författare)
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Bearing load measurement in a hydropower unit using strain gauges installed inside pivot pin
- 2012
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Ingår i: Experimental mechanics. - : Springer Science and Business Media LLC. - 0014-4851 .- 1741-2765. ; 52:4, s. 361-369
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Tidskriftsartikel (refereegranskat)abstract
- To determine a machine's mechanical condition it is of importance to know the radial bearing forces in the machine. Radial forces are caused by magnetic pull forces in the generator, clamped shafts, mass unbalance and flow properties around the turbine. Measuring the shaft displacement in the bearing or the bearing housing acceleration is not sufficient for status determination of a vertical hydropower unit. It is the magnitude and frequencies of the radial forces in combination with structure properties which give information as to whether a measured value is harmful or not. This paper presents an alternative method for measurement of radial bearing load in a hydropower unit. The method presented in this paper is based on strain measurements on pivot pins. The pivot pins are placed behind the bearing pad and the radial loads acting on the pad propagate through the pivot pin. New pivot pins were purchased and equipped with strain gauges. The new pivot pins were calibrated and a transfer function between applied load and measured output voltage was identified for each pivot pin. After calibration the pivot pins were installed in a vertical hydropower unit. Measurements were performed for several different operating modes of the hydropower unit. To verify that the measured load levels were of right order of magnitude, the radial bearing loads were calculated from numerical simulations of bearing properties and shaft eccentricity measurements. The two methods for determining bearing load showed almost the same results. This indicates that either method can be used to determine bearing load
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| 33. |
- Olofsson, Kenneth, et al.
(författare)
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Holographic interferometry measurements of transient bending waves in tubes and rings
- 1993
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Ingår i: Experimental mechanics. - 0014-4851 .- 1741-2765. ; 33:4, s. 308-313
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Tidskriftsartikel (refereegranskat)abstract
- Propagation bending waves are studied in a tube of steel and in a ring of aluminum. The waves are generated by the impact of a ballistic pendulum. Holographic interferometry, with a double-pulsed ruby laser as light source, is used to record the waves. A conical mirror is placed axially inside the tube. Axial illumination and axial observation directions, make it possible to view all sides of the tube simultaneously with a high sensitivity to radial deformation. The interferograms, which have an unusual perspective, are captured with a CCD-camera and then spatially transformed into an unwrapped strip of the tube wall. This makes the interpretation of the measurements simpler. The geometry of the tube causes the wave pattern to propagate with different speed and amplitude along and across the tube, even when the material itself is isotropic. A finite-element simulation of the impact is compared to the corresponding experiment. An impact on a ring with a defect is performed in order to study the effect on the wave pattern. The proposed method could be used in nondestructive testing of pipes.
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| 34. |
- Olofsson, Kenneth, et al.
(författare)
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Laser generated and recorded transient bending waves in composite tubes
- 1996
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Ingår i: Experimental mechanics. - 0014-4851 .- 1741-2765. ; 36:3, s. 224-231
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Tidskriftsartikel (refereegranskat)abstract
- Propagating bending waves are studied in three different composite tubes by holographic interferometry. A conical mirror is placed axially inside the tubes. Axial illumination and observation directions make it possible to view the circumference of the tube, with a high sensitivity to radial deformation. It is shown how the deformation field can be numerically evaluated using a phase stepping and unwrapping technique. Transient bending waves in the tubes are both generated and recorded by the same pulsed laser, which makes the experiments easy to perform. Finite element simulations of the impacted tubes are compared to corresponding experiments. Both the geometry and the material properties of the tubes affect the wave propagation. For unidirectional composite tubes, the 0-deg and 90-deg directions have different dynamic behavior. The proposed method could be used in nondestructive testing of tubes.
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| 35. |
- Rohde, S. E., et al.
(författare)
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Measuring Contact Mechanics Deformations Using DIC through a Transparent Medium
- 2017
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Ingår i: Experimental mechanics. - : Springer New York LLC. - 0014-4851 .- 1741-2765. ; 57:9, s. 1445-1455
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes the experimental methodology used to study the contact mechanics of a rigid, rough surface and a compliant, nominally flat surface using digital image correlation (DIC). The rough surface was produced by 3-D printing PMMA and the flat surface was produced with transparent PDMS (silicone rubber). The deformation of the speckled top surface (contact) of the PDMS was measured via DIC viewed through the transparent media. Four different PDMS formulations with moduli ranging from 64 to 2120 kPa were used in the experiment program to cover a wide range of modulus normalized loads. The deformation of the contact surface and depth of penetration versus normalized load were measured. The results were overlaid with previous measurements of contact area and complemented them extremely well. Additionally, it was shown that scaling laws associated with such contact mechanics problems extend many length scales.
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| 36. |
- Saldner, Henrik O., et al.
(författare)
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Applications of reciprocity, electronic holography and shearography in statics
- 1994
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Ingår i: Experimental mechanics. - 0014-4851 .- 1741-2765. ; 34:4, s. 334-339
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Tidskriftsartikel (refereegranskat)abstract
- This paper illustrates the use of reciprocity in electronic-holography experiments to determine unknown forces or couples acting on a mechanical structure. Electronic holography and electronic shearography are simple, fast and contact-free methods for the measurement of deformation fields or gradients of deformation fields, both for static and sinusoidal loadings. From a first measurement, the deformation field of the structure for a known force is determined. In a second experiment, the deformation field for the unknown quantity is measured. Reciprocity is then used to determine the unknown quantity. These optical methods are useful tools for the understanding of reciprocity.
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| 37. |
- Storgärds, Erik, et al.
(författare)
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Crack Length Evaluation for Cyclic and Sustained Loading at High Temperature Using Potential Drop
- 2015
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Ingår i: Experimental mechanics. - : Society for Experimental Mechanics (SEM). - 0014-4851 .- 1741-2765. ; 55:3, s. 559-568
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Tidskriftsartikel (refereegranskat)abstract
- Crack length evaluations for cyclic loading and sustained loading at high temperature and a mix of both have been conducted on Inconel 718 surface crack specimens at 550 C-a similar to. The choice of method for evaluating the crack length is seen to have a non-negligible impact on the resulting crack propagation rate values. In this paper, some aspects regarding how to evaluate such testing when using the potential drop technique are presented, with the aim of giving a firm explanation on how to proceed for the best possible result.
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| 38. |
- Subasic, Nihad, universitetsadjunkt, 1964-, et al.
(författare)
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Mechanical Characterization of Fatigue and Cyclic Plasticity of 304L Stainless Steel at Elevated Temperature
- 2023
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Ingår i: Experimental mechanics. - : Springer Nature. - 0014-4851 .- 1741-2765. ; 63:8, s. 1391-1407
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Tidskriftsartikel (refereegranskat)abstract
- Background: The mechanical characterization of the cyclic elastoplastic response of structural materials at elevated temperatures is crucial for understanding and predicting the fatigue life of components in nuclear reactors. Objective: In this study, a comprehensive mechanical characterization of 304L stainless steel has been performed including metallography, tensile tests, fatigue tests, fatigue crack growth tests and cyclic stress-strain tests. Methods: Isothermal tests were conducted at both room temperature and 300 °C for both the rolling direction and the transverse direction of the hot rolled steel. Mechanical properties were extracted from the uniaxial experiments by fitting relevant material models to the data. The cyclic plasticity behavior has been modelled with a radial return-mapping algorithm that utilizes the Voce nonlinear isotropic hardening model in combination with the Armstrong-Frederick nonlinear kinematic hardening model. The plasticity models are available in commercial FE software and accurately capture the stabilized hysteresis loops, including a substantial Bauschinger effect. Results: The material exhibits near isotropic properties, but its mechanical performance is generally reduced at high temperatures. Specifically, in the rolling direction, the Young’s modulus is reduced by 16 % at 300 °C, the yield strength at 0.2 % plastic strain is lower by 23 %, and the ultimate tensile strength is lower by 30 % compared to room temperature. Fatigue life is also decreased, leading to an accelerated fatigue crack growth rate compared to room temperature. A von Mises radial return mapping algorithm proves to be effective in accurately modelling the cyclic plasticity of the material. The algorithm has also been used to establish a clear correlation between energy dissipation per cycle and cycles to failure, leading to the proposal of an energy-based fatigue life prediction model. Conclusions: The material exhibits reduced mechanical performance at elevated temperatures, with decreased monotonic strength, compared to room temperature. Fatigue life is also compromised, resulting in accelerated fatigue crack growth. The material’s hardening behavior differs at room temperature and elevated temperature, with lower peak stress values observed at higher temperatures. The radial return mapping algorithm can be used to determine the dissipated energy per cycle which together with fatigue testing has been used to propose a low cycle fatigue life prediction model at both temperatures.
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| 39. |
- Sundin, Karl-Gustaf, et al.
(författare)
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A stiff and compact impact-force transducer based on strain measurement
- 1985
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Ingår i: Experimental mechanics. - 0014-4851 .- 1741-2765. ; 25:1, s. 48-53
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Tidskriftsartikel (refereegranskat)abstract
- A stiff and compact transducer for both static and impact compressive forces has been developed. The principle of the transducer is based on measurement of tangential strain around the periphery of an axially loaded short and axisymmetric body. It is shown that for an appropriate choice of transducer geometry the output is nearly insensitive to the distribution of the force over the loaded surface. In an experimental investigation, a prototype of the transducer was subjected to both static and impact loads of different distributions. The results show only a few percent variation in transducer sensitivity for the load distributions used. The transducer consists essentially of a homogeneous body and is simple to make. The size and material can be chosen to fit special applications
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| 40. |
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| 41. |
- Tan, X.C., et al.
(författare)
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Buckling of slender string in cylindrical tube under axial load : experiments and theoretical analysis
- 1995
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Ingår i: Experimental mechanics. - 0014-4851 .- 1741-2765. ; 35:1, s. 55-60
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Tidskriftsartikel (refereegranskat)abstract
- Laboratory buckling tests of strings was reported and the results were compared with theoretical formulas. The findings implied that the process of string buckling can be divided into two stages, specifically plane buckling and spatial buckling. In the spatial buckling, the string-buckling configuration can be illustrated by helical buckling theories. The friction between the string and the wall of the confining tube rendered an irregular buckling shape and prohibited further buckling at some critical loads. The buckling theories do not account for this effects. Thus, an approximate formula for estimating the friction force was proposed, which provided more exact results.
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| 42. |
- Tarigopula, Venkapati, et al.
(författare)
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A study of large plastic deformations in dual phase steel using digital image correlation and FE analysis
- 2008
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Ingår i: Experimental mechanics. - : Springer Science and Business Media LLC. - 0014-4851 .- 1741-2765. ; 48:2, s. 181-196
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Tidskriftsartikel (refereegranskat)abstract
- Large plastic deformation in sheets made of dual phase steel DP800 is studied experimentally and numerically. Shear testing is applied to obtain large plastic strains in sheet metals without strain localisation. In the experiments, full-field displacement measurements are carried out by means of digital image correlation, and based on these measurements the strain field of the deformed specimen is calculated. In the numerical analyses, an elastoplastic constitutive model with isotropic hardening and the Cockcroft - Latham fracture criterion is adopted to predict the observed behaviour. The strain hardening parameters are obtained from a standard uniaxial tensile test for small and moderate strains, while the shear test is used to determine the strain hardening for large strains and to calibrate the fracture criterion. Finite Element (FE) calculations with shell and brick elements are performed using the non-linear FE code LS - DYNA. The local strains in the shear zone and the nominal shear stress-elongation characteristics obtained by experiments and FE simulations are compared, and, in general, good agreement is obtained. It is demonstrated how the strain hardening at large strains and the Cockcroft - Latham fracture criterion can be calibrated from the in-plane shear test with the aid of non-linear FE analyses.
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| 43. |
- Widehammar, Svante
(författare)
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Stress-strain relationships for spruce wood : Influence of strain rate, moisture content and loading direction
- 2004
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Ingår i: Experimental mechanics. - 0014-4851 .- 1741-2765. ; 44:1, s. 44-48
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Tidskriftsartikel (refereegranskat)abstract
- The influence of strain rate, moisture content and loading direction on the stress-strain relationships for spruce wood has been investigated. The strain rates were approximately 8 x 10(-3) s(-1), 17 s(-1) and 1000 s(-1), and the states of moisture content were those corresponding to oven dry, fiber saturated and fully saturated. Compressive loads were applied along the principal directions of the stem of the tree, i.e., radially, tangentially and axially. The low and medium strain-rate tests were performed with the aid of a servohydraulic testing machine, while the high strain-rate tests were carried out using the split Hopkinson pressure bar (SHPB) technique. Magnesium or steel bars were used in the different SHPB tests in order to reduce impedance mismatch for the different directions of the wood specimens. The strain rate was found to have large influence on the behavior of the wood, especially under the condition of full saturation, where water transport in the deforming specimen is of major importance.
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| 44. |
- Widehammar, Svante
(författare)
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Stress-strain relationships for spruce wood : influence of strain rate, moisture content and loading direction
- 2004
-
Ingår i: Experimental mechanics. - 0014-4851 .- 1741-2765. ; 44:1, s. 44-48
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Tidskriftsartikel (refereegranskat)abstract
- The influence of strain rate, moisture content and loading direction on the stress-strain relationships for spruce wood has been investigated. The strain rates were approximately 8×10−3 s−1, 17s−1 and 1000 s−1, and the states of moisture content were those corresponding to oven dry, fiber saturated and fully saturated. Compressive loads were applied along the principal directions of the stem of the tree, i.e., radially, tangentially and axially. The low and medium strain-rate tests were performed with the aid of a servohydraulic testing machine, while the high strain-rate tests were carried out using the split Hopkinson pressure bar (SHPB) technique. Magnesium or steel bars were used in the different SHPB tests in order to reduce impedance mismatch for the different directions of the wood specimens. The strain rate was found to have large influence on the behavior of the wood, especially under the condition of full saturation, where water transport in the deforming specimen is of major importance.
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| 45. |
- Åberg, Jonas, et al.
(författare)
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Uniaxial material damping measurements using a fiber optic lattice: a discussion of its performance envelope
- 2004
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Ingår i: Experimental mechanics. - 0014-4851 .- 1741-2765. ; 44:1, s. 33-36
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Tidskriftsartikel (refereegranskat)abstract
- Damping is the internal transfer of kinetic energy to other forms of energy. Today, most methods use either bending or torsional vibration to measure damping. This means that the strain field in the specimen is nonhomogeneous. If the damping of the tested material is linear, strain-independent, the values acquired with these traditional methods will be equal to the intrinsic material damping of the material. If, however, the damping is strain-dependent, nonlinear, the measured value will be an average of the damping of the specimen, and not equal to its intrinsic material damping. To address this problem, a method is required to experimentally determine the damping in uniaxial tension in order to produce the same strain level in all parts of the test specimen and hence obtain a measurement of the intrinsic material damping. Using such a method, it is possible to view the material damping as the phase angle between the stress and the strain in a harmonic oscillation. In this paper, a method is suggested for measuring this phase shift in uniaxial tension to determine the material damping properties. It uses a tensile test machine, an optical fiber Bragg grating technique and a lock-in amplifier. Measurements with the phase shift technique have been suggested previously, but its performance envelope has been overestimated. In this paper, the performance envelope is discussed and restricted. It is shown that the envelope depends on the specimen length, loss factor and test frequency. An optical strain measurement method is also believed to help avoid many electrical measurement problems seen with the originally proposed method.
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| 46. |
- Östlund, Magnus, et al.
(författare)
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Experimental determination of residual stresses in paperboard
- 2005
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Ingår i: Experimental mechanics. - 0014-4851 .- 1741-2765. ; 45:6, s. 493-497
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Tidskriftsartikel (refereegranskat)abstract
- A method for the experimental determination of the through-thickness residual stress distribution in paper-board is presented.The successive removal of thin layers from strips of board through surface grinding changes the stress-state and the bending stiffness resulting in a changed curvature, which is measurable. From tests of strips in both in-plane directions, stress distributions can then be evaluated using the Treuting-Read method. Geometrically nonlinear effects at the large deformations taking place are avoided through a proper choice of strip dimensions. Typical results are presented and factors influencing the accuracy of the determination are thoroughly discussed.
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| 47. |
- Özdür, N.A., et al.
(författare)
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Residual Intensity as a Morphological Identifier of Twinning Fields in Microscopic Image Correlation
- 2021
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Ingår i: Experimental mechanics. - : Springer Nature. - 0014-4851 .- 1741-2765. ; 61:3, s. 499-514
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Tidskriftsartikel (refereegranskat)abstract
- Background: In the microscopic observation of deforming metals, it is well known that crystallite defects that accommodate strain can occasionally become visible, namely, they introduce image contrast to their locality. For microscopic digital image correlation (DIC) applications, this is typically known as a disturbance. Objective: Here, we explore a potential upside of these image-intensity offsets, to present a new mode of differential imaging that exclusively displays the underlying plasticity agents. Methods: For this, the intensity-offset signal is isolated with residual intensity, essentially the differential between reference- and deformed-configuration intensity of each material point. The premise is showcased over an autocatalytic twin band in Magnesium AZ31, with an advanced DIC instrument that utilizes bright-field optical microscopy. With robust area-scanning that utilizes in-situ corrective measures, a material field of around 5000 grains (13 μm average size) is sampled with a maximal intragranular resolution (~250 data points per grain) for this technique. For added robustness against the intensity alterations, a DIC algorithm (Augmented Lagrangian DIC) that enforces global kinematic compatibility constraints is utilized. Results: The calculated residual intensity map yields a detailed image of the twin networks that show a strong positional alignment with the strain localizations. At the band boundary, the twins (and their accompanying strain localization) protrude into the dormant material in a comb-like pattern. Conclusions: With a combination of high-resolution optics and defects that alter the surface topography, residual intensity presents a new in-situ microscopy mode that is tied to the DIC analysis. This principle also offers potential micro-deformation imaging capabilities for various other material-microscopy combinations.
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| 48. |
- Tryding, Johan
(författare)
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Crack-growth-rate Estimation on a Three-point Bend Specimen During Low-cycle Fatigue
- 1991
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Ingår i: Experimental Mechanics. - 1741-2765. ; 31:1, s. 42-46
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Tidskriftsartikel (refereegranskat)abstract
- The ratio of the change of the clip-gage measured crack-opening displacement of a three-point bend specimen to the change of the displacement at the point of application of the cyclic load is shown to vary linearly with the ratio of the crack length to the specimen height. With the aid of this relation, the crack-growth rate is obtained by numerical differentiation of the crack length with respect to the number of load cycles. A clip-gage correction factor is introduced in order to compensate for the use of external edges to position the clip-gage during the experiment.
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