• Advances in aircraft and spacecraft science
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• v.10 no.3
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• pp.257-279
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• 2023

This manuscript is dedicated to deriving the closed form solutions of free vibration of viscoelastic nanobeam embedded in an elastic medium using nonlocal differential Eringen elasticity theory that not considered before. The kinematic displacements of Euler-Bernoulli and Timoshenko theories are developed to consider the thin nanobeam structure (i.e., zero shear strain/stress) and moderated thick nanobeam (with constant shear strain/stress). To consider the internal damping viscoelastic effect of the structure, Kelvin/Voigt constitutive relation is proposed. The perforation geometry is intended by uniform symmetric squared holes arranged array with equal space. The partial differential equations of motion and boundary conditions of viscoelastic perforated nonlocal nanobeam with elastic foundation are derived by Hamilton principle. Closed form solutions of damped and natural frequencies are evaluated explicitly and verified with prestigious studies. Parametric studies are performed to signify the impact of elastic foundation parameters, viscoelastic coefficients, nanoscale, supporting boundary conditions, and perforation geometry on the dynamic behavior. The closed form solutions can be implemented in the analysis of viscoelastic NEMS/MEMS with perforations and embedded in elastic medium.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.40 no.4
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• pp.353-362
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• 2016

An accurate hysteresis model of an elastomer is important for quantifying viscoelastic energy loss. We suggest a highly nonlinear hyper-viscoelastic constitutive model of elastomers. The model captures a nonlinear viscoelastic characteristic by combining Yeoh's hyperelastic model and Hoofatt's hysteresis model used Neo-Hookean hyperelastic model. Analytical and numerical models were generated from uniaxial cyclic tests of an elastomer under a sinusoidal load with a mean strain of 150%, amplitudes of 20~80%, and frequencies of 0.02~0.2Hz. The viscoelastic model can highly capture the viscoelastic energy loss up to a strain of 230%.

• Journal of Veterinary Clinics
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• v.14 no.1
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• pp.56-64
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• 1997

These studies were performed to investigate the complications affecting the vision after extracapsular lens extraction(ECE), the effects of an I/A (irrigation & aspiration)device and a viscoelastic material used on the vision, the occurrence of complications and the effective corneal incision method to reduce the corneal opacity in dogs. ECE was performed bilaterally with 3 different methods using clinically normal twele mixed dogs; the method in which I/A device and viscoelastic material were not used, the method in which I/A device was used but viscoelastic material not, and the method in which I/A device and viscoelastic material were used. Postoperative complications were observed as followed; conjunctival injection, uveitis, corneal opacity due to endothelial cell loss, hyphemia, remnants of lens cortex, vitreous loss, synechia and capsular opacity. Preservation rate of vision was lower significantly in the cases showing signs of synechia, capsular opacity, or remnants of lens cortex than the cases not showing the above signs(p<0.01). There were significant reduction of the complications such as corneal opacity, clot in anterior chamber in the group using I/A device compared to the group in which I/A device was not used(p<0.01). Groups using I/A device showed slightly higher vision than the group not using I/A device (75%; 42%). There were no significant differences in the occurrence rate of complications and the preservative rate of vision between the groups with and without viscoelastic material. The present study indicated that the postoperative complications of posterior synechia, capsular opacity, uveitis and vistreous loss were important factors affecting the vision and that I/A device was applicable to extract the lens cortex and effective to elevate the success rate after ECE in dogs.

• Journal of the Institute of Convergence Signal Processing
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• v.13 no.3
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• pp.119-129
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• 2012

In the medical field, the hardening of tissues is one of important informations used in diagnosis or understanding progress of disease, a quantitative measuring method of hardening is important for objective diagnosis. It has been proposed MRE(Magnetic Resonance Elastography) method that measures an index of hardening, viscoelastic properties in a noninvasive. Because the S/N ratio of MRE images go down when measuring viscoelastic properties from local wavelength and local damping factor of a propagating wave in MRE method, methods using multiple phase MRE images have been examined to decrease the effect of noise. We propose a method measuring viscoelastic properties after Fitting a function for multiple phase MRE images in this research. This proposed method has a advantage to set up arbitrarily the variation rate of a space direction of viscoelastic properties or the spatial resolution of measuring values according to changing of the noise included in images, though it applies viscoelastic wave for multiple phase MRE images. We confirmed the effectiveness of a proposed method by experiment using simulation images and experiment using silicone-gel phantom.

• Elastomers and Composites
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• v.46 no.1
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• pp.54-59
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• 2011

One of the viscoelastic flow behaviors during profile extrusion is the swelling of extrudate. In this study, die swell of rubber compound in the capillary die have been investigated through experiment and computer simulation. Simplified viscoelastic model and non-linear differential viscoelastic model such as PTT model have been used in the computer simulation. The simulation results have been compared with experimental data. Experiment and simulation have been performed using fluidity tester and commercial CFD code, Polyflow respectively. Die swells predicted by two models showed good agreement with experimental results. Pressure and velocity distribution, and circulation flow at the corner of reservoir have been well predicted by PTT model. Simplified viscoelastic model can not predict circulation flow at the corner of reservoir. However this model has an advantage in computation time compare with full viscoelastic model, PTT model.

• Elastomers and Composites
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• v.41 no.4
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• pp.223-230
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• 2006

Rubber compounds have a high viscoelastic property. One of the viscoelastic behaviors during profile extrusion is the swelling of extrudate, and the amount of swelling varies with operational conditions in extrusion. It is well recognized that the elastic portion in the viscoelastic property plays an important role in the extrudate swell. In this study computer simulation of the die swell at the capillary die for several rubber compounds has been performed using commercial CFD code, Polyflow. A non-linear differential viscoelastic model, Phan-Thien-Tanner (PTT) model, was used in the computer simulation. Non-isothermal behavior was considered in the calculation. Distribution of pressure, velocity and temperature in the reservoir and capillary die, and extrudate profiles were predicted through the simulation. The amount of the die swell fur the different rubber compounds was investigated for various flow rates and three types of length to diameter of the capillary die. It is concluded that the PTT model successfully represented viscoelastic behavior of rubber compounds.

• Steel and Composite Structures
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• v.25 no.2
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• pp.141-155
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• 2017

This paper presents an investigation into the magneto-thermo-mechanical vibration and damping of a viscoelastic functionally graded-carbon nanotubes (FG-CNTs)-reinforced curved microbeam based on Timoshenko beam and strain gradient theories. The structure is surrounded by a viscoelastic medium which is simulated with spring, damper and shear elements. The effective temperature-dependent material properties of the CNTs-reinforced composite beam are obtained using the extended rule of mixture. The structure is assumed to be subjected to a longitudinal magnetic field. The governing equations of motion are derived using Hamilton's principle and solved by employing differential quadrature method (DQM). The effect of various parameter like volume percent and distribution type of CNTs, temperature change, magnetic field, boundary conditions, material length scale parameter, central angle, viscoelastic medium and structural damping on the vibration and damping behaviors of the nanocomposite curved microbeam is examined. The results show that with increasing volume percent of CNTs and considering magnetic field, material length scale parameter and viscoelastic medium, the frequency of the system increases and critically damped situation occurs at higher values of damper constant. In addition, the structure with FGX distribution type of CNTs has the highest stiffness. It is also observed that increasing temperature, structural damping and central angle of curved microbeam decreases the frequency of the system.

• Proceedings of the Membrane Society of Korea Conference
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• 2001.05a
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• pp.88-92
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• 2001

• Korea-Australia Rheology Journal
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• v.18 no.2
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• pp.67-81
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• 2006

Using a strain-controlled rheometer, the dynamic viscoelastic properties of aqueous xanthan gum solutions with different concentrations were measured over a wide range of strain amplitudes and then the linear viscoelastic behavior in small amplitude oscillatory shear flow fields was investigated over a broad range of angular frequencies. In this article, both the strain amplitude and concentration dependencies of dynamic viscoelastic behavior were reported at full length from the experimental data obtained from strain-sweep tests. In addition, the linear viscoelastic behavior was explained in detail and the effects of angular frequency and concentration on this behavior were discussed using the well-known power-law type equations. Finally, a fractional derivative model originally developed by Ma and Barbosa-Canovas (1996) was employed to make a quantitative description of a linear viscoelastic behavior and then the applicability of this model was examined with a brief comment on its limitations. Main findings obtained from this study can be summarized as follows: (1) At strain amplitude range larger than 10%, the storage modulus shows a nonlinear strain-thinning behavior, indicating a decrease in storage modulus as an increase in strain amplitude. (2) At strain amplitude range larger than 80%, the loss modulus exhibits an exceptional nonlinear strain-overshoot behavior, indicating that the loss modulus is first increased up to a certain strain amplitude(${\gamma}_0{\approx}150%$) beyond which followed by a decrease in loss modulus with an increase in strain amplitude. (3) At sufficiently large strain amplitude range (${\gamma}_0>200%$), a viscous behavior becomes superior to an elastic behavior. (4) An ability to flow without fracture at large strain amplitudes is one of the most important differences between typical strong gel systems and concentrated xanthan gum solutions. (5) The linear viscoelastic behavior of concentrated xanthan gum solutions is dominated by an elastic nature rather than a viscous nature and a gel-like structure is present in these systems. (6) As the polymer concentration is increased, xanthan gum solutions become more elastic and can be characterized by a slower relaxation mechanism. (7) Concentrated xanthan gum solutions do not form a chemically cross-linked stable (strong) gel but exhibit a weak gel-like behavior. (8) A fractional derivative model may be an attractive means for predicting a linear viscoelastic behavior of concentrated xanthan gum solutions but classified as a semi-empirical relationship because there exists no real physical meaning for the model parameters.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2007.05a
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• pp.191-194
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• 2007

In hot embossing lithography which has shown to be a good method to fabricate polymeric patterns for IT and bio components, it is very important to determine the proper process conditions of pressure, temperature, and time. It is also a key factor for predicting the optical properties of final product to calculate residual stress distribution after the embossing process. Therefore, to design the optimum process with right conditions, the ability to predict viscoelastic behavior of polymer during and after the hot embossing process is required. The objective of the present investigation is to establish simulation technique based on constitutive modeling of polymer with experiments. To analyze deformation behavior of viscoelastic polymer, the large strain material properties were obtained from quasi-static compression tests at different strain rates and temperatures and also stress relaxation tests were executed. With this viscoelastic material model, finite element simulation of hot embossing was executed and stress distribution is obtained. Proper process pressure is very important to predict the defect and incomplete filling.