Effects of viscoelasticity on the stability and bifurcations of nonlinear energy sinks

Due to the increasing use of passive absorbers to control unwanted vibrations,many studies have been done on energy absorbers ideally,but the lack of studies of real environmental conditions on these absorbers is felt.The present work investigates the effect of viscoelasticity on the stability and bifurcations of a system attached to a nonlinear energy sink(NES).In this paper,the Burgers model is assumed for the viscoelasticity in an NES,and a linear oscillator system is considered for investigating the instabilities and bifurcations.The equations of motion of the coupled system are solved by using the harmonic balance and pseudo-arc-length continuation methods.The results show that the viscoelasticity affects the frequency intervals of the Hopf and saddle-node branches,and by increasing the stiffness parameters of the viscoelasticity,the conditions of these branches occur in larger ranges of the external force amplitudes,and also reduce the frequency range of the branches.In addition,increasing the viscoelastic damping parameter has the potential to completely eliminate the instability of the system and gradually reduce the amplitude of the jump phenomenon.

The Dynamic Viscoelasticity of Polyethylene Based Montmorillonite Intercalated Nanocomposites

The viscoelastic behavior of melts for two systems composed of intercalated nanocomposites based on low density polyethylene and 3 wt% loading of cetyltrimethyl- ammonium bromide modified montmorillonite was studied. The results obtained through examining the dynamic storage module G' and dynamic loss module G' values of the composite revealed that the dynamic viscoelastic properties of composite strongly depended on intercalation of polymer, and exhibited dramatically change with altering intercalation conditions. Only when modified montmorillonite content was about 3 wt%, the composite showed a trend of pseudo-solidlike at lower frequencies.

Complex variable element-free Galerkin method for viscoelasticity problems

Based on the complex variable moving least-square （CVMLS） approximation, the complex variable element-free Galerkin （CVEFG） method for two-dimensional viscoelasticity problems under the creep condition is presented in this paper. The Galerkin weak form is employed to obtain the equation system, and the penalty method is used to apply the essential boundary conditions, then the corresponding formulae of the CVEFG method for two-dimensional viscoelasticity problems under the creep condition are obtained. Compared with the element-free Galerkin （EFG） method, with the same node distribution, the CVEFG method has higher precision, and to obtain the similar precision, the CVEFG method has greater computational efficiency. Some numerical examples are given to demonstrate the validity and the efficiency of the method.

Human umbilical cord blood-derived stem cells and brain-derived neurotrophic factor protect injured optic nerve:viscoelasticity characterization

The optic nerve is a viscoelastic solid-like biomaterial.Its normal stress relaxation and creep properties enable the nerve to resist constant strain and protect it from injury.We hypothesized that stress relaxation and creep properties of the optic nerve change after injury.Moreover,human brain-derived neurotrophic factor or umbilical cord blood-derived stem cells may restore these changes to normal.To validate this hypothesis,a rabbit model of optic nerve injury was established using a clamp approach.At 7 days after injury,the vitreous body received a one-time injection of 50 μg human brain-derived neurotrophic factor or 1 × 106 human umbilical cord blood-derived stem cells.At 30 days after injury,stress relaxation and creep properties of the optic nerve that received treatment had recovered greatly,with pathological changes in the injured optic nerve also noticeably improved.These results suggest that human brain-derived neurotrophic factor or umbilical cord blood-derived stem cell intervention promotes viscoelasticity recovery of injured optic nerves,and thereby contributes to nerve recovery.

Comparison of viscoelasticity between normal human sciatic nerve and amniotic membrane

Sciatic nerve tissue was obtained from the gluteus maximus muscle segment of normal human cadavers and amniotic membrane tissue was obtained from healthy human puerperant placentas. Both tissues were analyzed for their stress relaxation and creep properties to determine suitability for transplantation applications. Human amniotic membrane and sciatic nerve tissues had similar tendencies for stress relaxation and creep properties. The stress value of the amniotic membrane stress relaxation group decreased to a greater extent compared with the sciatic nerve stress relaxation group. Similarly, the stress value of the amniotic membrane creep group increased to a greater extent compared with the sciatic nerve creep group. The stress relaxation curve for human amniotic membrane and sciatic nerve showed a logarithm correlation, while the creep curve showed an exponential correlation. These data indicate that amniotic membrane tissue has better stress relaxation and creep properties compared with sciatic nerve tissue.

Rheological Behavior for Polymer Melts and Concentrated Solutions——Part Ⅶ: A Quantitative Verification for the Molecular Theory of Non-linear Viscoelasticity with Entanglement Constraints in Polymer Melts

Based on the molecular theory of non-linear viscoelasticity with constrained entanglements in polymer melts, the material functions in simple shear flow were formulated, the theoretical relations between and shear rate (), and topologically constrained dimension number and a were derived. Linear viscoelastic parameters and and topologically constrained dimension number and as a function of the primary molecular weight, molecular weight between entanglements and the entanglement sites sequence distribution in polymer chain were determined. A new method for determination of viscoelastic parameters G and topologically constrained dimension number a and and molecular weight and from the shear flow measurements was proposed. It was used to determine those parameters and structures of HDPE, making a good agreement between these values and those obtained by other methods. The agreement affords a quantitative verification for the molecular theory of nonlinear viscoelasticity with constrained entanglement in polymer melts.

The viscoelasticity of lipid shell and the hysteresis of subharmonic in liquid containing microbubbles

The viscoelasticity and subharmonic generation of a kind of lipid ultrasound contrast agent are investigated. Based on the measurement of the sound attenuation spectrum, the viscoelasticity of the lipid shell is estimated by use of an optimization method. Shear modulus Gs=10MPa and shear viscosity μs=1.49N.S/m^2 are obtained. The nonlinear oscillation of the encapsulated microbubble is studied with Church＇s model theoretically and experimentally. Especially, the dependence of subharmonic on the incident acoustic pressure is studied. The results reveal that the development of the subharmonic undergoes three stages, i.e. occurrence, growth and saturation, and that hysteresis appears in descending ramp insonation.

Fluid discrimination incorporating viscoelasticity and frequency-dependent amplitude variation with offsets inversion

Frequency-dependent amplitude versus offset(FAVO)inversion is a popular method to estimate the frequency-dependent elastic parameters by using amplitude and frequency information of pre-stack seismic data to guide fluid identification.Current frequency-dependent AVO inversion methods are mainly based on elastic theory without the consideration of the viscoelasticity of oil/gas.A fluid discrimination approach is proposed in this study by incorporating the viscoelasticity and relevant FAVO inversion.Based on viscoelastic and rock physics theories,a frequency-dependent viscoelastic solid-liquid decoupling fluid factor is initially constructed,and its sensitivity in fluid discrimination is compared with other conventional fluid factors.Furthermore,a novel reflectivity equation is derived in terms of the viscoelastic solid-liquid decoupling fluid factor.Due to the introduction of viscoelastic theory,the proposed reflectivity is related to frequency,which is more widely applicable than the traditional elastic reflectivity equation directly derived from the elastic reflectivity equation on frequency.Finally,a pragmatic frequency-dependent inversion method is introduced to verify the feasibility of the equation for frequency-dependent viscoelastic solid-liquid decoupling fluid factor prediction.Synthetic and field data examples demonstrate the feasibility and stability of the proposed approach in fluid discrimination.

Dynamic Viscoelasticity of Polyester/Rubber Composites under Cyclic Loading

This paper focuses on the influence of dynamic viscoelasticity and surface temperature on the fatigue mechanism and fatigue lifetime of polyester/rubber composites. Rubber composites show significant viscoelasticity during fatigue process. The variations of dynamic elastic modulus, mechanical loss angle, loss energy per cycle exhibit different trend in fatigue initial stage and final stage. Due to high viscoelasticity high heat generation occurs under cyclic loading, which leads to a high surface temperature. It is found that the variation of specimen surface temperature depends strongly on cycling frequency and stress amplitude. SEM (scanning electron microscopy) observation and static residual stiffness studies reveal that the surface temperature affects fracture morphology and fatigue lifetime of rubber composites strongly because of heat aging.

THE EFFECTS OF BLOOD VISCOELASTICITY ON THE PULSE WAVE IN ARTERIES

Based on Womersley' s theory, the frequency equation satisfied by a complex wave velocity of a pulse wave in arteries war generalized to viscoelastic blood, a general formula of the complex wave velocity with regard to both linearly viscoelastic arteries and linearly viscolelastic blood was obtained, and the effects of the viscoelastic property of blood on the phase velocity and the wave attenuation of the pulse wave using the formula systematically was discussed. It is concluded that the influence of the blood elasticity on the wave propagation of a pulse wave in arteries is weaker than that of the arterial viscosity and may be neglected in larger arteries.

Photoacoustic viscoelasticity imaging dedicated to mechanical characterization of biological tissues

Since changes in mechanical properties of biological tissues are often closely related to pathology,the viscoelastic properties are important physical parameters for medical diagnosis.A photoacoustic(PA)phase-resolved method for noninvasively characterizing the biological tissue viscoelasticity has been proposed by Gao et al.[G.Gao,S.Yang,D.Xing,\Viscoelasticity imaging of biological tissues with phase-resolved photoacoustic measurement,"Opt.Lett.36,3341–3343(2011)].The mathematical relationship between the PA phase delay and the viscosity–elasticity ratio has been theoretically deduced.Moreover,systems of PA viscoelasticity(PAVE)imaging including PAVE microscopy and PAVE endoscopy were developed,and high-PA-phase contrast images re°ecting the tissue viscoelasticity information have been successfully achieved.The PAVE method has been developed in tumor detection,atherosclerosis characterization and related vascular endoscopy.We reviewed the development of the PAVE technique and its applications in biomedical¯elds.It is believed that PAVE imaging is of great potential in both biomedical applications and clinical studies.

Oxidized low density lipoprotein (Ox-LDL) impacts on erythrocyte viscoelasticity and its molecular mechanism

Aim:The oxidized low-density lipoprotein(OxLDL) plays an important role in atherosclerosis yet it remains unclear if it damages circulating erythrocytes. Method: In this study。

A Novel Blood Coagulation Measuring Method Based on the Viscoelasticity of Non-Newtonian Fluid

There are complex and perfect coagulation, anticoagulation and fibrinolysis systems in the human body and their fine regulatory mechanisms. Once the coagulation system and its regulatory mechanisms are destroyed, bleeding or thrombosis will occur very soon. In the blood coagulation, the blood viscoelasticity changes. Therefore, the thrombus elasticity measurement technology can be used to continuously monitor the changing blood viscoelasticity in order to study the process of coagulation. The results of the interaction among the various components of the blood can be obtained from coagulation to fibrinolysis by bedside detection. The traditional electromagnetic induction sensors, based on conventional coil inductance, are manufactured complexly, high cost and non-linear. Therefore, this paper proposes a non-Newtonian fluid viscoelasticity measurement method based on the piezoelectric effect. We use the piezoelectric bimorphs with the diameter of 21 mm and the total thickness of 0.38 mm and DSM coupling probes with the length of 3 mm, 5 mm and 7 mm to design the piezoelectric bimorphs driver. The viscoelasticity of different non-Newtonian fluids is tested. The vibration amplitudes of the piezoelectric bimorphs and liquid surfaces range from 0.43 μm to 3.52 μm. Consequently, the feasibility of in vitro detection of thrombus is confirmed in principle and the above scheme is validated theoretically and experimentally, which provides the basis for the measurement of blood viscoelasticity, the in vitro detection of thrombus and the manufacture of blood coagulation instrument.

Effect of viscoelasticity on skin pain sensation

Pain sensation may appear under long-lasting mechanical stimulation. Although people have the experience that pain sensation generally decreases with time while the stimulation remains, the underlying mechanism remains elusive. We experimentally studied the thermal and strain rate- dependent viscoelastic behavior of skin in uniaxial stretch and numerically investigated the effects of temperature and strain rate on pain sensation. The results indicate that the viscosity of skin tissue decreases with increasing temperature and reducing strain rate, which subsequently decreases the discharge frequency of skin nociceptor and thus relieves the pain sensation. The results would contribute to the understanding of pain relief mechanism and optimizing for mechanical treatment.

Measurement of elastic modulus and evaluation of viscoelasticity of foundry green sand

Elastic modulus is an important physical parameter of molding sand; it is closely connected with molding sand’s properties. Based on theories of rheology and molding sand microdeformation, elastic modulus of molding sand was measured and investigated using the intelligent molding sand multi-property tester developed by ourselves. The measuring principle was introduced. Effects of bentonite percentage and compactibility of the molding sand were experimentally studied. Furthermore, the essential viscoelastic nature of green sand was analyzed. It is considered that viscoelastic deformation of molding sand consists mainly of that of Kelvin Body of clay membrane, and elastic modulus of molding sand depends mainly on that of Kelvin Body which is the elastic component of clay membrane between sands. Elastic modulus can be adopted as one of the property parameters, and can be employed to evaluate the viscoelastic properties of molding sand.

IDENTIFICATION OF QUASI-STATIC DISPLACEMENT FOR CONSTITUTIVE MODELS OF VISCOELASTICITY

An explicit relation between constitutive parameters and quasi-static displacement of viscoelasticity is derived under a kind of boundary condition, and an iterative form of optimized identification is presented. Viscoelastic constitutive models are identified from a two order differential model, and effects of information errors on results of inverse analysis are discussed.

Hybrid natural element method for viscoelasticity problems

A hybrid natural element method（HNEM） for two-dimensional viscoelasticity problems under the creep condition is proposed. The natural neighbor interpolation is used as the test function, and the discrete equation system of the HNEM for viscoelasticity problems is obtained using the Hellinger–Reissner variational principle. In contrast to the natural element method（NEM）, the HNEM can directly obtain the nodal stresses, which have higher precisions than those obtained using the moving least-square（MLS） approximation. Some numerical examples are given to demonstrate the validity and superiority of this HNEM.

VISCOELASTICITY AND IONIC CONDUCTIVITY OF TWO-COMPONENT EPOXY NETWORK CONTAINING LITHIUM PERCHLORATE

Polymeric solid electrolyte system composed of triglycidyl ether of glycerol (TGEG), diglycidyl ether of polyethylene glycol (DGEPEG)and LiClO_4 salt were synthesized. In this' system the electrolyte has a pecularity that not merely can the LiClO_4 provide ionic carriers, but also catalyze the crosslinking reaction without adding an usual curing agent. The effect of salt content and degree of crosslinking on the viscoelasticity and ionic conductivity were studied. Both WLF and VTF equations were used to treat the experimental data in order to elucidate the mechanism of ionic conduction. It was found that the ionic conductivity of the system is carded out through the segmental motion mechanism. However, the data must be treated with care. For example, in evaluating WLF parameters, the contribution concerned with ionic carrier generation with temperature to the conductivity must be differentiated from that concerned with segmental motion. Besides, the temperature range suitable to WLF equation must also be considered. For VTF equation, it might be inapplicable ff the temperature is too low and close to the glass transition temperature of the specimen. Further study is needed in order to have a quantitative information on the limitation of these equations.

FLUID MECHANICS OF MICROVASCULAR VASOMOTION AND THE EFFECTS OF BLOOD VISCOELASTICITY

This paper deals with blood flow caused by microvascular vasomotion with the focus on the effects of blood viscoelasticity on the pressure rise and wall resistance. It is shown that microvascular vasomotion plays a role of the 'second heart' of the body which is of importance in conveying blood, and that the effects of blood viscoelasticity greatly depend on the Weissenberg number and mean flow rate.

Relationship between Dermal Structural Changes on Ultrasonographic Images and Skin Viscoelasticity in Overweight and Obese Japanese Males

It has been reported that overweight Japanese males have poorly organised dermis and their skin may be fragile to external forces because of decreased dermal echogenicity. However, it is unknown whether the changes in the dermal structure actually affect the dermal function. The objective of this study was to clarify the relationship between dermal structural parameters and dermal functional parameters in overweight and obese Japanese males. A cross-sectional, observational study was conducted on Japanese male volunteers. Two ultrasound scanners, a 20-MHz Dermascan C® and an 18-MHz MylabTM five® were used to evaluate dermal structure. Echogenicity of the lower dermis and dermal thickness of the participants’ abdomens and thighs were measured. A Cutometer® MPA580 was used to evaluate skin viscoelasticity, skin deformation (R0) and elasticity (R2, R7). The correlation between dermal structural parameters and skin viscoelasticity were validated using Pearson’s correlation coefficient or Spearman’s correlation coefficient by rank test. A total of 79 male volunteers were recruited of which 43 were control subjects with BMI <25 (age, 22 - 63 years), 25 were overweight subjects with BMI ≥ 25 to <30 (age, 23 - 64 years) and the 11 obese subjects had a BMI ≥ 30 (age, 26 - 47 years). There was no correlation between dermal structural parameters and skin viscoelasticity in the abdomens or thighs of all 79 participants. On the other hand, in the 36 overweight and obese participants, thigh dermal echogenicity was significantly and negatively correlated with R0 (r = -0.456, p < 0.01) and dermal thickness in the abdomen was significantly and positively correlated with R0 (r = 0.464, p < 0.01). The dermal functional parameter was significantly correlated with dermal structural parameters in overweight and obese males;in other words, a decrease in dermal function may be caused by structural changes in the dermis during obesity.