Variations In Viscoelastic Properties Research Articles

Exact solution for hygro-thermo-mechanical creep and recovery of viscoelastic laminated beam

In order to predict the creep and recovery behaviors of for viscoelastic laminated beam in hygro-thermo-mechanical (HTM) coupled condition, an exact analytical solution is proposed. This solution considers two effect mechanisms: temperature and humidity, including the expansion difference and the variation of viscoelastic properties. In the analytical model, the stresses and displacements of each lamina are described based on the hygro-thermo-elasticity theory combined with the Boltzmann superposition. By employing series expansion, the transfer matrix method, and Laplace transform, the solution is determined analytically. Notably, the entire process of Laplace inverse transformation is carried out analytically without relying on any numerical approximations. The comparison analysis indicates that the proposed solution is more computationally efficient than the finite element counterpart and is more accurate than the solutions based on transverse shear deformation assumption. By analyzing the components of the proposed solution, a modified superposition principle applicable to HTM coupled conditions and a superposition principle for creep recovery are derived. In the end, the HTM coupled behavior, the bending creep and recovery as well as the optimization of expansion coefficient are investigated in detail.

Reconstructing the source condition for focused shear wave beams in soft elastic media

Shear waves are employed in medical ultrasound imaging because they reveal variations in viscoelastic properties of soft tissue. Focused shear waves produced by a longitudinally vibrating piston at several hundred hertz are investigated here experimentally in soft tissue phantoms and using an analytical model for shear wave beam generation and propagation. Experiments employed a spherically concave piston shaped to focus the shear wave beam at a depth of 4 cm in a soft tissue phantom. Analytical modeling of this data provided a good fit, but required treating the focal length and piston diameter as adjustable parameters. The largest source of error in the modeling is suspected to be that the source condition is assumed to exhibit zero displacement surrounding the piston, whereas the actual condition in the source plane is a traction-free surface surrounding the piston. Here, in order to elucidate the actual source condition, numerical back propagation of field measurements is employed. Simulations show that the shear source condition can be accurately recovered with information related only to the shear wave, i.e., without compressional wave information. Back propagation of measured beams is discussed in the context of more accurately modeling the source condition for future device optimization.

Viscoelastic properties of decellularized and freeze-dried human dermis between −90°C and 40°C

BACKGROUND: Human donor skin is processed to make the acellular dermis matrix (ADM) for tissue repair and regeneration. There is no data on the viscoelastic properties of ADM at room and subzero temperatures. OBJECTIVE: The work evaluated the temperature dependence of viscoelastic properties of freeze-dried ADM. MATERIALS AND METHODS: Donor skin was de-epidermized, de-cellularized and freeze-dried with trehalose as the lyo-protectant. Glass transition of freeze-dried ADM was measured by differential scanning calorimeter (DSC), and viscoelastic properties were examined by dynamic mechanical analyzer (DMA). RESULTS: At the low moisture range (1.4 ± 0.5%), the glass transition temperature (Tg) of freeze-dried ADM was 90°C to 100°C. As the moisture content increased, the Tg decreased steadily. At the high moisture range (10.8 ± 2.9%), the Tg was 40°C to 60°C. There were large donor-to-donor variations in viscoelastic properties of freeze-dried ADM as demonstrated by the changes in storage modulus (G’), loss modulus (G”) and damping factor tan δ(G”/G’). However, the trends of the temperature dependence for G’, G” and tan δ were similar among all 8 donors. For each donor, changes in G’ and G” were relatively small between −90°C and 40°C, and G’ was at least one order of magnitude greater than G”. Two viscoelastic relaxations were observed in freeze-dried ADM, one at −20°C and the other at −60°C respectively. CONCLUSION: Freeze-dried ADM was protected in the glassy carbohydrate matrix. DMA observed two viscoelastic relaxations (i.e., α process at −20°C and β process at −60°C). Overall changes in G’ and G” of freeze-dried ADM were relatively small within one order of magnitude between −90°C and 40°C.

Ultraviolet Light Radiation Effects on Viscoelastic Property Variation in Polymers

Viscoelastic materials exhibit a variable mechanical behavior generally investigated through Dynamic Mechanical Analysis (DMA). Such kind of test is carried out on standardized specimens, which have to be specifically produced or extracted from the component to be characterized, causing its destruction. This work, taking advantage of an innovative non-invasive testing procedure, based on an instrumented evaluation of indentation, aims to experimentally study the viscoelastic properties changes in polymer compounds, caused by exposure to UV rays. The analysis has been executed on two different polymer compounds designed for tire tread. An extensive use of UV radiation is performed to generate changes in rubber compound properties, but as concerns tire compounds application, there are only a few studies in literature and the same can be stated regarding the monitoring of properties over time using non-destructive procedures. In order to evaluate the viscoelastic properties variation, a proper test bench has been developed employing an UV lamp coupled with an innovative testing device. A test campaign has been carried out on the different tire compound specimens, tested in different conditions in terms of exposure time, exposure face and after various time ranges from the exposition. The tested compound samples exhibit a different chemical composition in terms of polymer blend ratio. The overall results highlight: the ability of the proposed technique to measure the variation in the mechanical behavior of the materials induced by exposure to UV rays and significant variations in viscoelastic properties induced on the compounds as a function of the time of exposure to UV rays and of the chemical composition.

ANKLE LIGAMENT REPAIR FOR RESTORATION OF FUNCTION: EXPLORING THE APPLICATION OF DECELLULARIZED GRAFTS

Ligament integrity is directly associated with ankle stability. Nearly 40% of ankle sprains result in chronic ankle instability, affecting biomechanics and potentially causing osteoarthritis. Ligament replacement could restore stability and avoid this degenerative pathway, but a greater understanding of ankle ligament behaviour is required. Additionally, autograft or allograft use is limited by donor-site morbidity and inflammatory responses respectively. Decellularised porcine grafts could address this, by removing cellular material to prevent acute immune responses, while preserving mechanical properties.This project will characterise commonly injured ankle ligaments and damage mechanisms, identify ligament reconstruction requirements, and investigate the potential of decellularised porcine grafts as a replacement material.Several porcine tendons were evaluated to identify suitable candidates for decellularisation. The viscoelastic properties of native tissues were assessed using dynamic mechanical analysis (DMA), followed by ramp to ‘sub-rupture’ at 1% strain/s, and further DMA. Multiple samples (n=5) were taken along the graft to assess variation along the tendon.When identifying suitable porcine tendons, a lack of literature on human ankle ligaments was identified. Inconsistencies in measurement methods and properties reported makes comparison between studies difficult.Preliminary testing on porcine tendons suggested there is little variation in viscoelastic properties along the length of tendon. Testing also suggested strain rates of 1%/s sub-rupture was not large enough to affect viscoelastic properties (no changes in storage or loss moduli or tanẟ). Further testing is underway to improve upon low initial sample numbers and confirm these results, with varying strain rates to identify suitable sub-rupture sprain conditions.This work highlights need for new data on human ankle ligaments to address knowledge gaps and identify suitable replacement materials. Future work will generate this data and decellularise porcine tendons of similar dimensions. Collagen damage will be investigated using histology and lightsheet microscopy, and viscoelastic changes through DMA.

Longitudinal motion of focused shear wave beams in soft elastic media.

Shear waves are employed in medical ultrasound imaging because they reveal variations in viscoelastic properties of soft tissue. Frequencies below 1 kHz are required due to the substantially higher attenuation and lower propagation speeds than for compressional waves. Shear waves exhibiting particle motion in the direction of propagation, referred to as longitudinal shear waves, can be generated with longitudinal motion of a circular disk on the surface of a soft elastic medium. This approach permits imaging of the longitudinal shear wave with a conventional ultrasound transducer that is coaxial with the source of the shear wave. Presented here is a mathematical model describing the complete wave field generated by displacement at the surface of an isotropic elastic half-space. Numerical simulations are shown for longitudinal, transverse, torsional, and radial source polarizations, with emphasis on focused longitudinal shear waves. Predictions are consistent with measurements of light beams revealing that the longitudinal electric field component produces a smaller focal spot than the transverse field component [Dorn, Quabis, and Leuchs, Phys. Rev. Lett. 91, 233901 (2003)]. Simulations are compared with preliminary measurements of a focused longitudinal shear wave beam generated in a soft tissue phantom by longitudinal motion of a spherically concave piston.

Intratree Variation in Viscoelastic Properties of Cell Walls of Masson Pine (Pinus Massoniana Lamb)

In this study, Pinus massoniana Lamb at different heights, across the annual rings, and between earlywood and latewood was measured by X-ray diffraction and the chemical composition was analyzed by chemical treatment. Results indicated that the microfibril angle (MFA) decreased and the chemical composition changed little with the increase in height from 1 m to 9 m. In the radial direction, the MFA decreased and the chemical composition changed little with an increase in annual rings. The cellulose content of latewood was higher than that of earlywood. The viscoelastic changes of wood cell walls at different heights, across the annual rings by the method of quasi-static nanoindentation and dynamic modulus mapping techniques. Results indicated that the wood cell walls’ elastic modulus increased, and the creep rate and creep compliance decreased with the increase in height; The elastic modulus and hardness increased with the annual rings. The cell walls’ storage modulus increased and the loss modulus gradually decreased with an increase in height; the storage modulus gradually increased and the loss modulus decreased with the annual rings.

Comparative analysis of thermal aging behavior and comprehensive performance of high viscosity asphalt (HVA) from cohesion, adhesion and rheology perspectives

High viscosity asphalt (HVA) has been widely utilized in the paving of large-void asphalt pavement, and it will be affected by thermal aging and exhibit different pavement performance. In this study, through a series of macroscopic and microscopic experiments, the change behaviors of cohesion, adhesion, high- and low-temperature rheological properties, microscopic phase structure and functional groups of HVA under different thermal aging conditions were first analyzed—in comparison with the three asphalts viz. styrene-butadienestyrene (SBS) modified asphalt (SBSMA), crumb rubber modified asphalt (CRMA) and neat asphalt (NA). Further, the relationship between macro-micro properties of asphalt binders was investigated by grey correlation theory, and the comprehensive performance of HVA at different aging stages and its aging resistance were contrastively evaluated via using radar chart analysis method. Results showed that aging degraded the cohesion and adhesion of asphalt binders as well as the water stability of asphalt-aggregate systems, and weakened the low-temperature property of asphalt binders—compared to the unaged sample, the short- and long-term aging reduced the cohesion work of HVA by 1.10% and 3.41%, respectively, and decreased the water stability index of HVA-limestone system by 0.54% and 1.86%, respectively. However, the variations of viscoelastic properties, temperature sensitivity, creep recovery ability and rutting resistance of HVA under aging were inconsistent with those of SBSMA, CRMA and NA. Aging decreased the microscopic surface roughness Ra of HVA and altered the intensity of functional groups at different wavenumbers, and the short- and long-term aging diminished its Ra by 21.26% and 37.85% compared to the unaged sample, respectively. Grey correlation analysis showed that the microscopic surface roughness Ra and the functional group indexes at different wavenumbers correlated well with several macroscopic indexes, which successfully explicated the variations of macroscopic properties of HVA from the micro perspective. Radar chart analysis displayed that the comprehensive performance of HVA in the original and short-term aging stages was second only to CRMA, but its comprehensive performance after long-term aging was only superior to NA; overall, HVA exhibited the most favorable anti-aging performance.

Interface Structure and Dynamics in Polymer‐Nanoparticle Hybrids: A Review on Molecular Mechanisms Underlying the Improved Interfaces

AbstractIn a nano‐assembly of soft polymer chains and hard filler nanoparticles (NPs), the intermolecular interactions at the polymer‐filler interface lead to variations in viscoelastic properties in the vicinity of the NP surface. However, unravelling the molecular parameters controlling the entropic and enthalpic interactions, including polymer chain conformation, dynamics, dispersion mechanisms and confinement effects across the interfacial region, has been remote and challenging. Even though the polymer‘s molecular structure and the NPs surface chemistry contribute to these interfacial interactions, the underlying mechanism behind the enhanced dynamic‐mechanical performance is still a hot topic of debate. By controlling the chemistry of NPs and polymer matrix in model nanocomposites, many efforts have been made in elucidating the physical origin of the structure and dynamics of the interfacial polymer layer with estimates varying from 1 to 10 nm around the spherical NPs. Therefore, understanding the interface structure and dynamics influenced by polymer‐NP interaction is of great technological importance. In this article, the latest polymer‐particle interface literature developments are reviewed where computational simulations, calorimetry, and dielectric spectroscopy studies have been used to examine the constraints levied by the interface structure on segmental relaxation dynamics of polymer chains in the vicinity of NP surface.

Mechanism of chain formation under shearing forces in magneto-rheological fluids

The present article focuses on the basic physics of structure formation chain formation and chain dynamics that make variation in viscoelastic properties of magnetorheological (MR) fluids. The formation and deformation of chain like structures in MR fluids at room temperature has been studied under the application of shearing force and externally applied magnetic field. The morphological investigation of MR fluid particles has been studied using SEM. A rheomicroscopy set-up was used to record the in-situ videos under magnetic field’s influence and a sequence of images were taken whose analysis has been performed using ImageJ. The plot of chain length versus shear rate at constant magnetic fields reveals the competitive nature of dipole–dipole interactions and shear forces acting simultaneously.

Viscoelastic properties of mitral valve leaflets: An analysis of regional variation and frequency-dependency.

The aim of this study was to determine the regional variation in viscoelastic properties of mitral valve leaflets over a range of physiological and patho-physiological frequencies. This included comparisons to be made between anterior and posterior leaflets, anterior leaflet clear and rough zones, and radial and circumferential leaflet orientation. Dynamic mechanical analysis was used to determine frequency-dependent viscoelastic properties. The valve leaflets were dissected from eight porcine hearts. The leaflets were loaded under a sinusoidal tensile displacement, with a mean dynamic peak to trough strain of 11%, applied to all leaflet samples at nine different frequencies, ranging from 0.5 to 10 Hz. The anterior leaflet has higher storage and loss stiffness than the posterior leaflet. The storage stiffness of circumferential tissue is greater than that of radially oriented valve tissue (2.0 ± 1.6 N/mm cf. 1.7 ± 0.9 N/mm; p < 0.05); however, the loss stiffness is greater for radial tissue (0.15 ± 0.07 cf. 0.14 ± 0.09 N/mm; p < 0.05). Likewise, the storage stiffness of the anterior leaflet clear zone is greater than that of the rough zone (2.4 ± 1.6 cf. 2.1 ± 1.2; p < 0.05), but the loss stiffness is greater for the rough zone (0.17 ± 0.09 N/mm cf. 0.14 ± 0.08 N/mm; p < 0.05). In conclusion, the viscoelastic properties of porcine mitral valve leaflets have regional variations, with dynamic stiffness being dependent on circumferential or radial orientation and on location at a clear or rough zones.

Ring and Tails: Exploring the Intimacy of Cyclodextrin - Membrane Interactions

Cyclodextrins (CDs) are cyclic, ring-shaped molecules made of sugar moieties, consisting of a hydrophobic cavity and a hydrophilic outer part. Such a structure allows CDs to form inclusion complexes with hydrophobic molecules, where the guest molecule is physically entrapped in the host's (CD) cavity. This feature finds a wide range of applications in drug delivery, pharmaceutical and food industries, as inclusion complexes with CDs show enhanced bioavailability and longer circulation times.Recently, αCDs have been reported to directly interact with aliphatic tails of phospholipids from biological and model bilayer membranes. As the tails are not directly accessible for molecules from the aqueous solution, this observation suggests complex interplays and possible membrane remodeling occurring at the CD/membrane/water interface. Interestingly, despite the wide applications of CDs, the essentials of this process and its possible consequences in vivo remain poorly understood.Herein, we use a combination of complementary biophysical techniques to uncover the inner workings of interactions between CDs and model lipid membranes, focusing on CDs’ membrane-perturbing properties. Specifically, we employ Quartz Crystal Microbalance (QCMB-D) to reveal variations in viscoelastic properties of a lipid membrane induced by the presence of αCD. Furthermore, the application of confocal microscopy and CryoTEM allows us to explore the role of membrane curvature-mediated interactions between αCDs and the lipid membrane.

Mechanical, thermomechanical, and creep performance of CNT embedded epoxy at elevated temperatures: An emphasis on the role of carboxyl functionalization

ABSTRACTIn contrast to polymeric composites, the role of interface/interphase has been widely acknowledged to govern their overall properties and performance. Environmental temperature has substantial effects on the interfacial durability of polymer nanocomposites. In this regard, present investigation has been carried out to study the mechanical performance of pristine (UCNT) and carboxylic functionalized CNT (FCNT) embedded epoxy nanocomposites under different elevated temperatures. Higher flexural strength and modulus of FCNT‐EP nanocomposite were recorded over UCNT‐EP and neat epoxy at room temperature environment. Flexural testing at elevated temperatures revealed a higher rate of strength degradation in polymer nanocomposites over neat epoxy. Postfailure analysis of specimens has been conducted to understand the alteration in failure micro‐mechanisms upon UCNTs and FCNTs addition in epoxy. Variation in viscoelastic properties with temperature has been studied from dynamic mechanical thermal analysis and significant reduction in glass transition temperature (Tg) is observed for nanocomposites. In the studied temperature and stress combinations, FCNT‐EP nanocomposites exhibited better creep resistance over UCNT‐EP and neat epoxy. Room temperature strengthening, elevated temperature strength degradations, improved creep resistance and reduction in Tg in nanocomposites over neat polymer have been discussed in terms of dynamic nature and gradient structure of CNT/epoxy interphase. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44851.

Investigation of multilayer printed circuit board (PCB) film warpage using viscoelastic properties measured by a vibration test

Woven glass fabric/BT (bismaleimide triazine) composite laminate (BT core), copper (Cu), and photoimageable solder resist (PSR) are the most widely used materials for semiconductors in electronic devices. Among these materials, BT core and PSR contain polymeric materials that exhibit viscoelastic behavior. For this reason, these materials are considered to have time- and temperature-dependent moduli during warpage analysis. However, the thin geometry of multilayer printed circuit board (PCB) film makes it difficult to identify viscoelastic characteristics. In this work, a vibration test method was proposed for measuring the viscoelastic properties of a multilayer PCB film at different temperatures. The beam-shaped specimens, composed of a BT core, Cu laminated on a BT core, and PSR and Cu laminated on a BT core, were used in the vibration test. The frequency-dependent variation of the complex bending stiffness was determined using a transfer function method. The storage modulus (E′) of the BT core, Cu, and PSR as a function of temperature and frequency were obtained, and their temperature-dependent variation was identified. The obtained properties were fitted using a viscoelastic model for the BT core and the PSR, and a linear elastic model for the Cu. Warpage of a line pattern specimen due to temperature variation was measured using a shadow Moiré analysis and compared to predictions using a finite element model. The results provide information on the mechanism of warpage, especially warpage due to temperature-dependent variation in viscoelastic properties.

Variation in viscoelastic properties of bovine articular cartilage below, up to and above healthy gait-relevant loading frequencies.

The aim of this study was to determine the variation in viscoelastic properties of femoral head bovine articular cartilage, on-bone, over five orders of magnitude of loading frequency. These frequencies ranged from below, up to and above healthy gait-relevant frequencies, using<1, 1–5 and 10 Hz, respectively. Dynamic mechanical analysis was used to measure storage and loss stiffness. A maximum compressive force of 36 N was applied through a chamfered-end, 5.2-mm-diameter, indenter. This induced a maximum nominal stress of 1.7 MPa. The ratio of storage to loss stiffness increased from near parity (2.5) at low frequencies to 11.4 at 10 Hz. This was the result of a significant logarithmic increase (p < 0.05) in storage stiffness with frequency, from 367 N/mm (0.001 Hz) up to 1460 N/mm (10 Hz). In contrast, the loss stiffness remained approximately constant. In conclusion, viscoelastic properties of articular cartilage measured at frequencies below those of gait activities are poor predictors of its relevant dynamic mechanical behaviour.

Modulation of physical properties of supramolecular hydrogels based on a hydrophobic core.

We demonstrate herein the variation in viscoelastic properties of supramolecular hydrogels (SMGs) composed of two amphiphiles, N-Palmitoyl-Gly-His (PalGH) and sodium palmitate (PalNa). PalGH molecules in water form lamellar-like assemblies, which stack into sheet-shaped aggregates, resulting in the evolution of three-dimensional network structures. Once PalNa is added to PalGH, the alkyl groups of PalNa incorporate themselves into the hydrophobic cores of PalGH lamellar-like assemblies, resulting in a change in the assembly from lamellar-like to fibrous micelle-like. Consequently, sheet-shaped aggregates turn into flexible fibrils, which form bundles, resulting in network structures. Mixed hydrogel network structures differ in morphology from those in homogenous PalGH and PalNa hydrogels. Changes in the network structure eventually alter the bulk viscoelastic properties of hydrogels. These results demonstrate that the viscoelastic properties of supramolecular hydrogels can be tuned by controlling the aggregation states.

Effect of Mode of Loading on Viscoelastic and Damage Properties of Asphalt Concrete

The objective of this research was to study the effect of the mode of loading on viscoelastic properties and damage characteristics of asphalt concrete. Four 12.5-mm nominal maximum aggregate size mixtures were designed according to Superpave® specifications. The specimens prepared with these mixtures were tested under uniaxial, biaxial, and flexure modes to obtain viscoelastic and damage properties. Viscoelastic and damage properties depended on loading mode and testing frequency. Statistically significant differences were found between mean values of measured modulus under different modes of loading. Also, regions of overlap in phase angle measurements were observed. A systematic variation in viscoelastic properties was found between different loading modes. The specimen underwent damage at a faster rate under the uniaxial mode than under the flexure loading mode. At a given value of normalized pseudostiffness, the damage parameter ratio, computed with parameters obtained under different modes of loading, remained approximately the same and was a function of material properties.

Viscoelastic Properties and Physico-Functional Characterization of Six High Yielding Cassava Mosaic Disease-Resistant Cassava (Manihot Esculenta Crantz) Genotypes

Investigations were conducted to characterize six high yielding cassava mosaic disease (CMD) resistant cassava varieties (Ampong, Broni bankye, Sika, Otuhia, Amakuma and Bankye fitaa) that have been developed by the Crop Research Institute of Ghana in collaboration with the International Institute for Tropical Agriculture for their differences and similarities in viscoelastic properties and physico-functional characteristics. The viscoelastic properties (pasting temperature, peak viscosity, final viscosity, breakdown viscosity and setback viscosity), and physico-functional characteristics (swelling power, solubility and water binding capacity) were determined using standard analytical methods. The results showed wide variations in viscoelastic properties with values ranging from 270.67-380.67 BU for peak viscosity, 37.17-260 BU for final viscosity, 199.83-282.33 BU for breakdown viscosity, 21.83-98.66 BU for setback viscosity and 2.48-10.51 min time to pasting temperature. Similarly, variations in swelling power, solubility and water binding capacity were noted with values ranging between 14.34-17.04%, 73.04-79.98% and 234.53- 276.63% respectively for all the different cassava genotypes. Statistical analysis showed significant differences (P < 0.05) amongst the studied cassava genotypes with Sika (improved variety) having exceptionally high viscoelastic characteristics. The differences noted in the viscoelastic properties and physico-functional characteristics of the six CMD resistant cassava genotypes could be used in their selection for specific food and industrial processing applications.

Viscoelastic properties of adsorbed and cross-linked polypeptide and protein layers at a solid–liquid interface

The real-time changes in viscoelasticity of adsorbed poly(L-lysine) (PLL) and adsorbed histone (lysine rich fraction) due to cross-linking by glutaraldehyde and corresponding release of associated water were investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D) and attenuated total reflection Fourier transform infrared spectroscopy (ATR/FTIR). The kinetics of PLL and histone adsorption were measured through changes in mass adsorbed onto a gold-coated quartz surface from changes in frequency and dissipation and using the Voigt viscoelastic model. Prior to cross-linking, the shear viscosity and shear modulus of the adsorbed PLL layer were ∼ 3.0 × 10 −3 Pa s and ∼ 2.5 × 10 5 Pa , respectively, while after cross-linking, they increased to ∼ 17.5 × 10 −3 Pa s and ∼ 2.5 × 10 6 Pa , respectively. For the adsorbed histone layer, shear viscosity and shear modulus increased modestly from ∼ 1.3 × 10 −3 to ∼ 2.0 × 10 −3 Pa s and from ∼ 1.2 × 10 4 to ∼ 1.6 × 10 4 Pa , respectively. The adsorbed mass estimated from the Sauerbrey equation (perfectly elastic) and the Voigt viscoelastic model differ appreciably prior to cross-linking whereas after cross-linking they converged. This is because trapped water molecules were released during cross-linking. This was confirmed experimentally via ATR/FTIR measurements. The variation in viscoelastic properties increased substantially after cross-linking presumably due to fluctuation of the randomly cross-linked network structure. An increase in fluctuation of the viscoelastic properties and the loss of imbibed water could be used as a signature of the formation of a cross-linked network and the amount of cross-linking, respectively.

Effects of viscous behavior of geosynthetic reinforcement and foundation soils on the performance of reinforced embankments

The combined effects of viscoelastic behavior of geosynthetic reinforcement and viscoplastic nature of rate-sensitive foundation soils on the performance of reinforced embankments are investigated. The variation of viscoelastic properties is examined using geosynthetic products that are made of polyester, polypropylene and polyethylene. The foundation soils consist of two soft clay deposits with different strain-rate sensitivities. Embankment construction is numerically simulated to identify the magnitude of creep deformation of the reinforcement and the foundation soil under both limit state and working stress conditions and the consequent effects on the stability of the embankment. It is shown that the creep of geosynthetic reinforcement and foundation soil can decrease the short-term stability of embankments and that the mobilized reinforcement stiffness and corresponding tensile force can be significantly lower than that measured from standard laboratory test. The isochronous stiffness can reasonably represent the mobilized reinforcement stiffness at the critical stage of the embankment. During post-construction periods, reinforcement strain can increase substantially and it has shown that the increase in reinforcement strain is largely due to the viscoplastic behavior of foundation soils. The mobilized reinforcement strain under working stress conditions with respect to reinforcement stiffness are also discussed.