Nonlinear Viscoelastic Parameters Research Articles

Probing metal-carboxylate interactions in cellulose nanofibrils-based hydrogels using nonlinear oscillatory rheology

Cellulose nanofibrils (CNFs) have gained much attention as part of biocompatible soft hydrogels used in various biomedical applications such as biodegradable scaffolds, biomedicine, tissues, and regenerative medicine. The CNF hydrogels were mediated with metal cations for improved mechanical strength and structural reversibility. Intermolecular interactions in these CNF hydrogels are controlled by metal cation-carboxylate coordination bonding, leading to the creation of interconnected three-dimensional nanofibril structures that produce high structural reversibility. The nonlinear inter- and intra-cycle were investigated viscoelastic responses of these CNF hydrogels by quantitative nonlinear viscoelastic parameters and transient responses. The dynamic and transitional analyses conducted indicate that the structural deformation and recovery characteristics of the CNF hydrogels are affected by the valency number of the metal cations. This property can be carefully chosen to tune the intermolecular interactions between the cellulose nanofibrils to create an efficient interwoven network structure with high structural reversibility that can go through repeated cycles of reformation and yielding.

Linear and nonlinear viscoelastic parameters of asphalt binders modified with softening agents

Asphalt binder modification using various chemically processed synthetic or bio-based additives can be used to reduce cracking potential in asphalt concrete (AC), especially with recycled materials. These modifiers affect rheological properties of asphalt binders. State-of-the-practice parameters obtained within the linear viscoelastic range (LVER) have not successfully predicted their performance. Using a large-strain amplitude and/or novel small-strain parameters at extended aging conditions (double and triple pressure aging vessel cycles) could help. In this study, parameters for unmodified and modified asphalt binders were studied at various aging conditions. The discussed data supports that: (1) Extended aging allows binder distinction beyond performance grading (2) Intermediate-temperature small-strain rheological parameters of binders may be predicted using low-temperature small-strain parameters, and vice versa. (3) Stiffness-dependent parameters must be interpreted based on temperature and frequency of testing. (4) Δ|G*|peak τ is not correalted to small-strain parameters.

Research on nonlinear rheological properties of waxy crude oil based on large amplitude oscillatory shearing

The rheological properties of waxy crude oil are important for its economical and safe pipeline transportation. In actual pipeline transportation, waxy crude oil often exhibits nonlinear rheological characteristics, among which viscoelastic-thixotropic characteristics are the main ones, while there are currently few related studies. This article carried out a study of large amplitude oscillatory shearing (LAOS) for typical Daqing waxy crude oil, and conducted analysis of the nonlinear rheological characteristics of the low temperature waxy crude oil based on the periodic strain loading mode. Using the Fourier transform rheology, Lissajous curve, and stress decomposition method, the effects of strain amplitude and frequency on the nonlinear rheological characteristics under LAOS conditions are analyzed. Results show that when the Fourier transform method analyzes the non-linear rheological characteristics area, the relative intensities of frequency triple and frequency quintuple increase with the increase of strain amplitude and frequency, and it cannot accurately reflect the non-linear rheological properties of waxy crude oil under high-frequency conditions. The Lissajous curve method is more suitable for the study of the nonlinear rheological characteristics of waxy crude oil under low frequency conditions and establishing a prediction formula of dissipated energy density. In addition, the nonlinear viscoelastic parameters obtained by the stress decomposition method can accurately characterize the nonlinear viscoelasticity of waxy crude oil. This research deeply explores the nonlinear rheological properties of waxy crude oil, which can provide theoretical guidance for practice.

Nonlinear viscoelastic rheological behavior of bentonite and sepiolite drilling fluids under large amplitude oscillatory shear

Common models used to describe rheological behavior of drilling fluids are built on pure viscous flow assumption. Full rheological characterization of drilling fluid under this assumption is not feasible since drilling fluids are viscoelastic materials and exhibit both elastic and viscous features. Gel strength development, yield stress value at near zero shear rates, and sag tendency in drilling fluids are the strong function of viscoelastic responses. Linear and nonlinear viscoelastic responses should be measured to provide complete viscoelastic analysis. Two water-based drilling fluids, sepiolite and bentonite drilling muds, each in four states, were subjected to testing using rheometer. Nonlinear viscoelastic parameters and their indications for both fluid systems were characterized for the first time. Large amplitude oscillation sweep tests were conducted as a function of strain and strain rate at four temperatures and frequencies. Stress response was decomposed through Fourier transform into elastic and viscous stress components. Lissajous-Bowditch loops were assessed as rheological fingerprints to detect the initiation of nonlinear region and the nature of nonlinearity. Results revealed that bentonite fluid systems provided stronger gel strength, a more stable network, and high mechanical stability compared to sepiolite fluid samples up to 50 °C. However, the sepiolite fluid systems outperformed at high temperatures (100 and 150 °C). The elastic nonlinear response was determined to be strain/strain rate softening, and the nonlinear viscous response was shear rate thinning at large strain rates for both fluid systems. Contrary to common knowledge found in literature (shear rate thinning), it was revealed that both fluid systems demonstrated shear rate thickening behavior at low to moderate strain rates. This finding is of importance in understanding gel strength development and evaluating sag tendency in drilling operations. • Nonlinear viscoelastic properties of drilling fluids under LAOS were investigated for the first time. • Thermo-structural stability of drilling fluids was evaluated regarding nonlinear viscoelasticity. • Lissajous-Bowditch curves were established to visualize nonlinear viscoelastic characters of drilling fluids. • LAOS parameters quantified nonlinearity based on LAOS protocols for drilling fluids.

Evaluation of Nonlinear Viscoelastic Non-isochoric viscoplastic Characteristics of Thermoplastic

The present study evaluates the mechanical characteristic of polybutylene terephthalate based on nonlinear viscoelasticity and non-isochoric viscoplasticity by creep recovery test and uniaxial test. A series-connected model of Schapery nonlinear viscoelasticity and Perzyna viscoplasticity is used to express the mechanical behavior. The nonlinear viscoelastic parameters and the viscoplastic parameters are identified by creep recovery test and by uniaxial tensile test, respectively. As the results, the mechanical behavior in creep recovery test and uniaxial tensile test can be reproduced by using the series-connected model. However, the identified initial yield stress is larger than the loading stress in creep recovery test. This fact means that further consideration is needed the viscoplastic model with physically meaningful parameters.

Exploring the Biomechanical Properties of the Human Cornea In Vivo Based on Corvis ST.

Purpose: The aim of this study was to provide a method to determine corneal nonlinear viscoelastic properties based on the output data of corneal visualization Scheimpflug technology (Corvis ST). Methods: The Corvis ST data from 18 eyes of 12 healthy humans were collected. Based on the air-puff pressure and the corneal displacement from the Corvis ST test of normal human eyes, the work done by the air-puff attaining the whole corneal displacement was obtained. By applying a visco-hyperelastic strain energy density function of the cornea, in which the first-order Prony relaxation function and the first-order Ogden strain energy were employed, the corneal strain energy during the Corvis ST test was calculated. Then the work done by the air-puff attaining the whole corneal displacement was completely regarded as the strain energy of the cornea. The identification of the nonlinear viscoelastic parameters was carried out by optimizing the sum of difference squares of the work and the strain energy using the genetic algorithm. Results: The visco-hyperelastic model gave a good fit to the data of corneal strain energy with time during the Corvis ST test (R 2 > 0.95). The determined Ogden model parameter μ ranged from 0.42 to 0.74 MPa, and α ranged from 32.76 to 55.63. The parameters A and τ in the first-order Prony function were 0.09–0.36 and 1.21–1.95 ms, respectively. Conclusion: It is feasible to determine the corneal nonlinear viscoelastic properties based on the corneal contour information and air-puff pressure of the Corvis ST test.

The effects of hydrocolloids on the thermomechanical, viscoelastic and microstructural properties of whole wheat flour dough

To use hydrocolloids for improving the breadmaking performance of whole wheat flour dough, relationships between hydrocolloid addition and dough thermomechanical, viscoelastic and microstructural properties were investigated. The responses of dough thermomechanical and viscoelastic properties to hydrocolloid addition depended on the hydrocolloid type. A power-law gel model fitted well to the linear and non-linear viscoelastic parameters, i.e., G'(ω), G''(ω) and J(t), of doughs. The model parameters gel strength (S) and exponent (n) were well indicative of hydrocolloid-induced changes in dough strength and relaxation behavior. The torque-scale mixolab parameters C2, C3 and C5, showed a good linear relationship with hydrocolloid addition. These parameters were also well correlated with S and n. Hydrocolloids played a crucial role in the modification for dough microstructure by forming a more continuous gluten network and better connection between starch granules and protein matrix.

Characterization of Desulfurized Crumb Rubber/Styrene-Butadiene-Styrene Composite Modified Asphalt Based on Rheological Properties.

With the growing interest in bituminous construction materials, desulfurized crumb rubber (CR)/styrene–butadiene–styrene (SBS) modified asphalts have been investigated by many researchers as low-cost environmental-friendly road construction materials. This study aimed to investigate the rheological properties of desulfurized CR/SBS composite modified asphalt within various temperature ranges. Bending beam rheometer (BBR), linear amplitude sweep (LAS), and multiple stress creep recovery (MSCR) tests were performed on conventional CR/SBS composite modified asphalt and five types of desulfurized CR/SBS modified asphalts. Meanwhile, Burgers’ model and the Kelvin–Voigt model were used to derive nonlinear viscoelastic parameters and analyze the viscoelastic mechanical behavior of the asphalts. The experimental results indicate that both the desulfurized CR/SBS composite modifier and force chemical reactor technique can enhance the crosslinking of CR and SBS copolymer, resulting in an improved high-, intermediate-, and low-temperature performance of desulfurized CR/SBS composite modified asphalt. Burgers’ model was found to be apposite in simulating the creep stages obtained from MSCR tests for CR/SBS composite modified asphalts. The superior high-temperature performance of desulfurized CR/SBS modified asphalt prepared with 4% SBS, 20% desulfurized rubber, and a force chemical reactor time of 45 min contributes to the good high-temperature elastic properties of the asphalt. Therefore, this combination is recommended as an optimal preparation process. In summary, the desulfurization of crumb rubber and using the force chemical reactor technique are beneficial to composite asphalt performance and can provide a new way of utilizing waste tire rubber.

Static and dynamic hygrothermal postbuckling analysis of sandwich cylindrical panels with an FG-CNTRC core surrounded by nonlinear viscoelastic foundations

In this article, analytical and semi-analytical methods are used to analyze the nonlinear static and dynamic hygrothermal postbuckling response of sandwich cylindrical panels with functionally graded carbon nanotube-reinforced composite (FG-CNTRC) core. The sandwich cylindrical panels with an FG-CNTRC core are resting on generalized nonlinear viscoelastic foundations which are consisted of a Winkler and Pasternak foundation parameters augmented by a Kelvin-Voigt viscoelastic model and a nonlinear cubic stiffness. The sandwich cylindrical panels with four types of FG-CNTRC core are considered such that the CNTs distribution types consist of the FG-O, FG-Λ, FG-X, and UD. Regarding to the classical shell theory and geometrical nonlinearity in von Kármán-Donnell sense, the governing equation is obtained and discretized utilizing the Galerkin method. The nonlinear dynamic hygrothermal (NDHT) postbuckling is analyzed by means of the fourth-order Runge-Kutta method. The influences of material parameters, various geometrical characteristics, and nonlinear viscoelastic foundation parameters are studied on the nonlinear static hygrothermal (NSHT) postbuckling and NDHT postbuckling analysis of sandwich cylindrical panels with an FG-CNTRC core. The results show that the different types of CNTs distribution and nonlinear viscoelastic foundation parameters have a strong effect on the hygrothermal postbuckling behaviors of sandwich cylindrical panels with an FG-CNTRC core.

Nonlinear hygrothermal vibration and buckling analysis of imperfect FG-CNTRC cylindrical panels embedded in viscoelastic foundations

This paper investigates the nonlinear free vibration and nonlinear dynamic hygrothermal buckling behavior of imperfect functionally graded carbon nanotube-reinforced composite (FG-CNTRC) cylindrical panels under hygrothermal environment. The nonlinear temperature distribution is assumed along the thickness direction. The structure is resting on a generalized nonlinear viscoelastic foundation which is composed of a two-parameter Winkler-Pasternak foundation augmented by a Kelvin-Voigt viscoelastic model with a nonlinear cubic stiffness and damping considerations. The nonlinear von Kármán strain-displacement relations and the classical shell theory are considered to derive the governing equations of the cylindrical panels. The discretized equations of motion are obtained using the Galerkin method. The fourth order Runge-Kutta method is utilized to analyze the nonlinear dynamic hygrothermal buckling. The influences of the four CNTs distribution types, consist of the FG-O, FG-Λ, FG-X, and UD, are considered for the cylindrical panel. The effects of temperature, moisture, nonlinear viscoelastic foundations, initial imperfection, and material parameters on the nonlinear free vibration and nonlinear dynamic hygrothermal buckling behavior of the system are investigated.

Unveiling Temporal Nonlinear Structure-Rheology Relationships under Dynamic Shearing.

Understanding how microscopic rearrangements manifest in macroscopic flow responses is one of the central goals of nonlinear rheological studies. Using the sequence-of-physical-processes framework, we present a natural 3D structure–rheology space that temporally correlates the structural and nonlinear viscoelastic parameters. Exploiting the rheo-small-angle neutron scattering (rheo-SANS) techniques, we demonstrate the use of the framework with a model system of polymer-like micelles (PLMs), where we unveil a sequence of microscopic events that micelles experience under dynamic shearing across a range of frequencies. The least-aligned state of the PLMs is observed to migrate from the total strain extreme toward zero strain with increasing frequency. Our proposed 3D space is generic, and can be equally applied to other soft materials under any sort of deformation, such as startup shear or uniaxial extension. This work therefore provides a natural approach for researchers to study complex out-of-equilibrium structure–rheology relationships of soft materials.

Finite element analysis of mechanical response of cellulosic fiber-reinforced composites

An investigation is carried out on natural cellulosic fibers which have gained interest in the composite field due to their superior specific properties as well as their eco-friendly environment character and biodegradability. A multi-scale finite element (FE) model of natural fiber composite materials is developed. The study is performed for multiple fibers in hexagonal packings with 10, 20, and 30% of fiber mass fractions. The mechanical behavior of the composite, processed by means of hand lay-up method and examined thanks to scanning electron microscopy (SEM), is simulated at macro-scale as well as mesoscopic scale. In particular, the response to tensile and three-point bending test is studied. Linear material properties are obtained by using pure strain assumptions in the implicit analysis of the composite, while the non-linear behavior and viscoelastic parameters require the explicit dynamic analysis. Simulation is performed thanks to ABAQUS finite element software. Comparison of experimental and FEM tensile and three-point bending strength shows very good agreement. From the results, it has been found that the prepared natural fiber composite materials can be used for structural engineering applications.

A Straightforward Procedure to Characterize Nonlinear Viscoelastic Response of Asphalt Concrete at High Temperatures

This paper proposes a straightforward procedure to characterize the nonlinear viscoelastic response of asphalt concrete materials. Furthermore, a model is proposed to estimate the nonlinear viscoelastic parameters as a function of the triaxiality ratio, which accounts for both confinement and deviatoric stress levels. The simplified procedure allows for easy characterization of linear viscoelastic (LVE) and nonlinear viscoelastic (NVE) responses. First, Schapery’s nonlinear viscoelastic model is used to represent the viscoelastic behavior. Dynamic modulus tests are performed to calibrate LVE properties. Repeated creep-recovery tests at variable deviatoric stress levels (RCRT-VS) were designed and conducted to calibrate the nonlinear viscoelastic properties of four types of mixtures used in the Federal Aviation Administration’s National Airport Pavement and Materials Research Center test sections. The RCRT-VS were conducted at 55°C, 140 kPa initial confinement pressure, and wide range of deviatoric stress levels; mimicking the stress levels induced in a pavement structure under traffic. Once calibrated, the model was validated by comparing the model predictions and experimental measurements at different deviatoric stress levels. The predictions indicate that the proposed method is capable of characterizing NVE response of asphalt concrete materials.

Introducing a stress-dependent fractional nonlinear viscoelastic model for modified asphalt binders

Asphalt binders generally behave in a linear viscoelastic manner under conventional stress and strain level. There are several researches and technical protocols consistent with linear viscoelastic theorem. However, two main reasons can affect this well-known behavior of asphalt binders: 1) modification of asphalt binder with some additives causing a change in its microstructure and 2) higher order stress and strain level induced to asphalt binder under heavy traffic or climate loading. It is important to consider non-linear viscoelastic behavior of asphalt binder and its effect on conventional characteristics of asphalt especially two constitutive functions of creep compliance and relaxation modulus. In this research, nonlinear viscoelastic properties of asphalt binders modified with crumb rubber, styrene-butadiene-styrene and polyphosphoric acid are investigated to evaluate the variation in non-linear viscoelastic parameters due to addition of these modifiers. For this purpose, a specified research program was defined to consider non-linear behavior of one neat asphalt binder as well as nine modified ones. Based on a robust literature review and some prior experiences, three different extents selected for each modifier and then specimens were made. Implementing a simple creep-recovery test with creep loading time of 1 s and 100 s and 999 s unloading to simulate traffic load by using a dynamic shear rheometer at different stress levels of 0.1, 1, 10, 20 and 30 kPa, non-linear viscoelastic behavior of neat and modified asphalt binder was captured. The generalized fractional nonlinear viscoelastic model is introduced and successfully implemented to simulate creep and recovery behavior of neat and modified asphalt binders. Addition of all modifiers to the neat asphalt binder alters its non-linear viscoelastic behavior and properties. For short time creep loading, crumb rubber and polyphosphoric acid amplify non-linear behavior while SBS control it and for long creep loading time all asphalt modifiers increases non-linearity of asphalt binder’s viscoelastic behavior.

Linear and nonlinear viscoelastic and viscoplastic analysis of asphalt binders with warm mix asphalt additives

ABSTRACTWarm-Mix Asphalt (WMA) is a widely used product, which proved a contribution to the reduction in asphalt mixing and compaction temperatures. This reduction leads to lower fuel consumption and smoke emission in asphalt plants. Most of the characterisation of binders used in WMA has focused in the past on measuring linear viscoelastic properties and associated Superpave parameters. Several studies have shown that the average stresses and strains of the asphalt mixture remain mostly within the linear viscoelastic response. However, localised strains in the binder phase of the mixture could reach values high enough to induce nonlinear viscoelastic and viscoplastic deformations. Therefore, this study focuses on an experimental and analytical evaluation of linear, nonlinear viscoelastic and viscoplastic responses of selected binders modified for use in WMA. The first part of the paper analyses the linear viscoelastic material properties and their ability to evaluate permanent deformation resistance. Then, the non-recoverable creep compliance parameter obtained from the Multiple Stress Creep Recovery (MSCR) test is analysed to assess the nonlinear response and permanent deformation of asphalt binders. The paper utilises a nonlinear plasto-viscoelastic (NPVE) approach to assess and quantify the nonlinear plasto-viscoelastic response of binders by separating the recoverable and irrecoverable strains measured in the MSCR test. Two WMA additives were included in this study by mixing them with polymer-modified and unmodified asphalt binders. Analysis of results showed that the NPVE approach captured a higher percentage of recovery than the NLVE approach. However, binder’s performance evaluation and ranking did not change by adopting the NPVE approach. The nonlinear viscoelastic parameters provided insight on the behaviour of asphalt binders mixed with WMA additives during loading cycles. Sasobit showed higher influence than Advera on binders in resisting permanent deformation by increasing the recoverable strain during the unloading phase.

A systematic approximation of discrete relaxation time spectrum from the continuous spectrum

Most of viscoelastic models contain linear and nonlinear viscoelastic parameters and the linear viscoelastic parameters correspond to relaxation time spectra of materials. The relaxation spectra can be classified into continuous and discrete ones. Discrete relaxation time spectrum is more convenient to simulate multi-mode models than continuous one because it demands shorter calculation time. It has been demonstrated that the continuous spectrum is uniquely determined in views of theoretical (Fuoss and Kirkwood, 1941; Davies and Anderssen, 1997) [1,23] as well as empirical approaches (McDougall et al., 2014) [5]. Whereas, it is reported that different algorithms for discrete spectrum infer the different results from the same data (Malkin and Masalova, 2001) [14]. This is the study on a systematic method for discrete spectrum on the basis that a discrete spectrum must be consistent with the continuous one. We suggest a simple method to extract discrete relaxation spectrum as a systematically approximated continuous spectrum by means of the Levenberg–Marquardt method. The new algorithm is tested and compared with previous algorithms using synthesized model spectra and experimental data.

Atomic force microscopy indentation and inverse analysis for non-linear viscoelastic identification of breast cancer cells

Breast cancer cells (MCF-7 and MCF-10A) are studied through indentation with spherical borosilicate glass particles in atomic force microscopy (AFM) contact mode in fluid. Their mechanical properties are obtained by analyzing the recorded reaction force–time response. The analysis is based on comparing experimental data with predictions from finite element (FE) simulation. Here, FE modeling is employed to simulate the AFM indentation experiment which is neither a displacement nor a force controlled test. This approach is expected to overcome many underlying problems of the widely used models such as Hertz contact model due to its capability to capture the contact behaviors between the spherical indentor and the cell, account for cell geometry, and incorporate with large strain theory. In this work, a non-linear viscoelastic (NLV) model in which the viscoelastic part is described by Prony series terms is used for the constitutive model of the cells. The time-dependent material parameters are extracted through an inverse analysis with the use of a surrogate model based on a Kriging estimator. The purpose is to automatically extract the NLV properties of the cells with a more efficient process compared to the iterative inverse technique that has been mostly applied in the literature. The method also allows the use of FE modeling in the analysis of a large amount of experimental data. The NLV parameters are compared between MCF-7 and MCF-10A and MCF-10A treated and untreated with the drug Cytochalasin D to examine the possibility of using relaxation properties as biomarkers for distinguishing these types of breast cancer cells. The comparisons indicate that malignant cells (MCF-7) are softer and exhibit more relaxation than benign cells (MCF-10A). Disrupting the cytoskeleton using the drug Cytochalasin D also results in a larger amount of relaxation in the cell’s response. In addition, relaxation properties indicate larger differences as compared to the elastic moduli like instantaneous shear modulus. These results may be useful for disease diagnosing purposes.

Characterizing SBS modified asphalt with sulfur using multiple stress creep recovery test

The effect of cross-linking agent and SBS content on SBS modified asphalt were investigated using multiple stress creep recovery (MSCR) test. Nonlinear viscoelastic characterization of MSCR curve was also performed to derive nonlinear viscoelastic parameters. Cross-linking agent can prominently improve high temperature properties of SBS modified asphalt, especially at lower polymer content. The effect of increasing SBS content is more prominent for binders at lower SBS content. MSCR test failed to distinguish 5.0% and 5.5% SBS modified asphalt in this limited study. All binders modified from 3.0% to 5.5% SBS with sulfur have sufficient delayed elastic response according to AASHTO TP 70. When cross-linking agent was not used, only high dosage SBS modified binders are considered to be modified with an acceptable elastomeric polymer in this study. In addition, there is strong linear relationship between percent recovery and Jnr and such relationship is not dependent on stress level, but is concerned with test temperature. The nonlinear parameter G2 derived from MSCR curve can be used to identify the existence of cross-linking agent and characterize the elastic response of SBS modified binder. The parameter PS% shows potentiality in evaluating modified binder’s ability to resist permanent deformation. However, PS% used in this study should be further investigated to confirm the applicability for highly modified asphalt.

Large amplitude oscillatory shear rheology of three different shear-thickening particle dispersions

We present a large amplitude oscillatory shear rheology (LAOS) investigation of three different shear-thickening particle dispersions - fumed silica in polyethylene oxide (FLOC), fumed silica in polypropylene glycol (HydroC), and cornstarch in water (JAM). These systems shear-thicken by three different mechanisms - shear-induced formation of particle clusters flocculated by polymer bridging, hydrocluster formation, and jamming. The viscoelastic non-linearities of the three fluids were studied as a function of strain and strain-rate space through the use of Lissajous-Bowditch curves and local nonlinear viscoelastic moduli of an oscillatory shear cycle. The nonlinear behaviors of the three fluids were compared and contrasted to understand the nonlinear shear-thickening mechanism of each. Both HydroC and JAM dispersions were found to exhibit strong strain stiffening of the elastic moduli and strain thickening of the loss moduli behavior associated with possible hydrocluster formation and particle jamming. However, the FLOC dispersion, in contrast, showed strong strain softening and strain thinning behavior at large strain amplitudes associated with yielding of the microstructure. The expected thickening of the loss modulus of FLOC in LAOS with increasing strain was not observed even though viscosity of FLOC was found to shear-thicken in steady-shear measurements. This disagreement is likely due to very large strain amplitudes required for shear-thickening to occur by shear-induced polymer bridging mechanism. The hypothesis was confirmed through stress growth experiments. Conversely, the HydroC and JAM dispersions required relatively small applied strains for shear-thickening to occur by hydrocluster and jamming mechanism. The comparison of local intra-cycle nonlinearity through Lissajous-Bowditch plots and nonlinear viscoelastic parameters indicated that the elastic nonlinearities of all three systems are primarily driven by a strong dependence on the magnitude of the applied strain-rates within an oscillatory cycle rather than the amplitude of the applied strain. A close inspection of the LAOS data reveals strong differences in the viscoelastic nonlinearities of these three different shear-thickening dispersions which can be used to create a nonlinear rheological fingerprint for each and offers valuable new insights into the nonlinear dynamics associated with each of the shear-thickening mechanisms.

Development of a Multibody Model to Predict the Settling Point and Interfacial Pressure Distribution in a Seat–Occupant System

The location of the hip-joint (H-Point) of a seat occupant is an important design specification which directly affects the seat static comfort. Most car seats are made of polyurethane foam and so the location of the H-Point is dependent on the quasi-static behavior of foam. In this research, a previously developed model of the seat–occupant system is refined by incorporating an improved foam model which is used to study seat and occupant interactions and the location of occupant’s H-Point. The seat is represented by a series of discrete nonlinear viscoelastic elements that characterize the seating foam behavior. The nonlinear elastic behavior of these elements is expressed by a higher order polynomial while their viscoelastic behavior is described by a hereditary type model with parameters that are functions of the compression rate. The nonlinear elastic and viscoelastic model parameters were estimated previously using data obtained from a series of quasi-static compression tests on a car seat foam sample. The occupant behavior is described by a constrained two-dimensional multibody model with five degrees of freedom. A Lagrangian formulation is used to derive the governing equations for the seat–occupant model. These differential equations are solved numerically to obtain the H-Point location. These results are then used to calculate the force distribution at the seat and occupant interfaces. The force distribution at the seat–occupant interface is also investigated experimentally and is found to match qualitatively with the results obtained using the seat–occupant model.