| 1. |
- Mahata, Kaushik, et al.
(författare)
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On the estimation of complex modulus and Poisson's ratio using longitudinal wave experiments
- 2006
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Ingår i: Mechanical systems and signal processing. - : Elsevier BV. - 0888-3270 .- 1096-1216. ; 20:8, s. 2080-2094
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we consider different least-squares-based approaches for estimating the complex Young's modulus and the complex Poisson's ratio of a viscoelastic material using a longitudinal wave propagation experiment. We present a statistical analysis of different estimation approaches and compare their performances. The analytical covariance expressions are validated using experimental data.
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| 2. |
- Mahata, Kaushik, et al.
(författare)
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On the use of flexural wave propagation experiments for identification of complex modulus
- 2001
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- In this paper, we investigate the estimation of the complex modulus of a viscoelastic material from flexural wave experiments. A bar specimen of uniform cross-section is subjected to lateral impact by a steel ball giving rise to flexural waves traveling along the bar. The strains due to wave propagation are registered as functions of time using strain gauges at different sections. The measured strains are transformed in to the frequency domain. A non-parametric estimation of the complex modulus is carried out for each frequency. An analysis of the quality of the non-parametric estimate is carried out. The validity of the theoretical results are confirmed by numerical studies and experimental tests.
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| 3. |
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| 4. |
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| 5. |
- Mousavi, Saed, et al.
(författare)
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Identification of complex shear modulus from measured shear strains on a circular disc subjected to transient torsion at its centre
- 2008
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Ingår i: Journal of Sound and Vibration. - : Elsevier BV. - 0022-460X .- 1095-8568. ; 313:3-5, s. 567-580
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Tidskriftsartikel (refereegranskat)abstract
- A method for identification of complex shear modulus from measured shear strains on a circular disc subjected to a transient torque at its centre has been established. It is based on the evolution of an outgoing shear wave between two radial positions at which the associated shear strains are measured. The two-dimensional shear wave solutions used are exact in the sense of three-dimensional theory. Therefore, in principle, there is no frequency beyond which they are not valid. The method requires a minimum disc size, which is related to the duration of the load. The non-parametric results become inaccurate at frequencies near zero and at certain problematic frequencies where the excitation of the disc is weak or non-existent. These frequencies may be moved outside the frequency range of interest by sufficiently decreasing the duration of the load. If there are problematic frequencies within this range, the results of parametric identification become more accurate than those of non-parametric identification. Parametric results from experimental tests with loads having different amplitudes and durations agree well with each other in accord with the assumed linearity of the tested polypropylene material.
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| 6. |
- Mousavi, Saed, 1970-
(författare)
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Identification of Viscoelastic Materials by Use of Wave Propagation Methods
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Complex moduli and Poisson’s ratio have been estimated using extensional and torsional wave experiments. The data were used for assessment of linearity and isotropy of two polymers, polymethyl methacrylate (PMMA) and polypropylene (PP). The responses of both materials were found to be close to linear and isotropic. A statistical analysis of different estimation approaches for complex modulus and Poisson’s ratio was conducted. It was shown that a joint estimation of complex modulus and Poisson’s ratio improves the estimated results. Considerable improvement was achieved in the frequency range 5-15 kHz for Poisson’s ratio. A non-equilibrium split Hopkinson pressure bar (SHPB) procedure for identification of complex modulus has been developed. Two simplified procedures were also established. Both overestimated the magnitude of the complex modulus. The complex modulus of PP was identified using PMMA and aluminium bars, and the estimated complex modulus was in good agreement with published results. The procedure was found to be accurate regardless of the specimen size or the specimen-to-bar impedance ratio. The procedure was also used to analyze the mechanical response of four compacted pharmaceutical tablet materials. A Debye-like relaxation was observed for all tested materials. Utilizing SHPB effectively requires knowledge about the impact process that is normally used for excitation. Therefore the impact between a cylindrical striker and a long cylindrical bar of viscoelastic material was studied theoretically and experimentally. Strains measured at three locations along a PMMA bar impacted by strikers of the same material agreed well with the theoretical results. A method for identification of complex shear modulus from measured shear strains on a disc subjected to a transient torque at its centre has been established. The two-dimensional wave solutions used are exact in the sense of three-dimensional theory. The results from experimental tests with different load amplitudes and durations agree well with each other.
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| 7. |
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| 8. |
- Mousavi, Saed, et al.
(författare)
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Non-equilibrium split Hopkinson pressure bar procedure for non-parametric identification of complex modulus
- 2005
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Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 31:9, s. 1133-1151
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Tidskriftsartikel (refereegranskat)abstract
- A split Hopkinson pressure bar procedure was developed for non-parametric identification of complex modulus under conditions of non-equilibrium and axially non-uniform stress. Two simplified procedures were also established. The first requires low frequency and/or short specimen. The second, identical to a classical procedure based on equilibrium, requires that also the specimen-to-bar characteristic impedance ratio be low. Both overestimate the magnitude of the complex modulus, the second even at low frequencies. Tests were carried out with polymethyl methacrylate and aluminium bars and with polypropylene specimens having diameter 20 mm and lengths 10, 20, 50 and 100 mm. The complex moduli identified are in good to fair agreement with published results up to 10 kHz for all specimens with polymethyl methacrylate bars and for the 10–50 mm specimens with aluminium bars. The quality of the results is sensitive to truncation and to imperfect contact at the bar-specimen interfaces.
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| 9. |
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| 10. |
- Rensfelt, Agnes, et al.
(författare)
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Optimal sensor locations for nonparametric identification of viscoelastic materials
- 2008
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Ingår i: Automatica. - : Elsevier BV. - 0005-1098 .- 1873-2836. ; 44:1, s. 28-38
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Tidskriftsartikel (refereegranskat)abstract
- The problem of optimal sensor locations in nonparametric identification of viscoelastic materials is considered. Sensor locations have previously been assigned in an ad hoc manner, resulting in a sub-optimal experiment design with a comparatively high variance of the estimates. Different scalar criteria of the covariance matrix, connected to A- and D-optimal experiment design, are considered and evaluated. The results indicate that the accuracy of the estimates can be greatly improved by the use of optimally placed sensors. The theoretical study has been verified by experiments. The presented results can be used to design experiments with improved accuracy of the estimates.
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