Characterization Of Viscoelastic Materials Research Articles

Ellipsometry of surface acoustic waves using 3D vibrometry for viscoelastic material characterization by the estimation of complex Lamé coefficients versus the frequency

Surface acoustic waves (SAW) are adequate regarding material characterization because they have low geometric attenuation compared to bulk waves. SAW can be generated easily by normal excitation using contact transducers or power lasers and have also a unique elliptic polarization, characterized by two parameters: the ellipticity (H/V) ratio between the horizontal and the vertical components of the elliptic motions and the orientation angle (θ) between the horizontal axis of the ellipse and the surface. In the case of a viscoelastic isotropic material, a complete characterization is achieved by the association of the quantitative measurement of the polarization and the propagative characteristics, the complex wavenumber, of the SAW. In practice, this operation is performed using 3D lased vibrometry for propagation monitoring in space and time. The post-processing is carried out by Quaternion Fourier Transform, the Prony algorithm and the complex Lamé coefficients identification for the theoretical model of propagation on the material. Good agreement is observed between the obtained results and the ones of the pulse-echo method.

Analysis of the Scenarios of Use of an Innovative Technology for the Fast and Nondestructive Characterization of Viscoelastic Materials in the Tires Field.

The properties of tires related to their viscoelastic behavior have a significant impact in the field of vehicle dynamics. They affect the performance and safety of a vehicle based on how they change when the tire performs in variable thermal conditions, interacts with various kinds of road surfaces, and accumulates mileage over time. To analyze and understand such properties of viscoelastic materials, destructive tests like dynamic mechanical analysis (DMA) are used, which make the tire unusable after the test; these are usually carried out on specimens cut from the zone of interest. The development of an innovative testing methodology connected to a hardware device called VESevo allows the characterization of the viscoelastic properties of tire compounds belonging to tread or other parts in a fast and nondestructive way. This new device provides valuable information about the evolution of the tire's viscoelastic properties, allowing it to monitor them throughout the whole lifecycle. In the paper, an overview of the possible sensitivities that can be investigated thanks to the VESevo is provided: The tread viscoelasticity was characterized and monitored for several tire tread compounds, over tire mileage, over tread thermal curing cycles, and as an index of the tread quality and uniformity in production. Preliminary results were collected and are presented. In the final paragraph, further recent applications developed from the tire field, which are not directly related, are reported.

Data-driven characterization of viscoelastic materials using time-harmonic hydroacoustic measurements

Any numerical procedure in mechanics requires choosing an appropriate model for the constitutive law of the material under consideration. The most common assumptions regarding linear wave propagation in a viscoelastic material are the standard linear solid model, (generalized) Maxwell, Kelvin-Voigt models or the most recent fractional derivative models. Usually, once the frequency-dependent constitutive law is fixed, the intrinsic parameters of the mathematical model are estimated to fit the available experimental data with the mechanical response of that model. This modelling methodology potentially suffers from the epistemic uncertainty of an inadequate a priori model selection. However, in this work, the mathematical modelling of linear viscoelastic materials and the choice of their frequency-dependent constitutive laws is performed based only on the available experimental measurements without imposing any functional frequency dependence. This data-driven approach requires the numerical solution of an inverse problem for each frequency. The acoustic response of a viscoelastic material due to the time-harmonic excitations has been calculated numerically. In these numerical simulations, the non-planar directivity pattern of the transducer has been taken into account. Experimental measurements of insertion loss and fractional power dissipation in underwater acoustics have been used to illustrate the data-driven methodology that avoids selecting a parametric viscoelastic model.

Multichannel resonant acoustic rheometry system for quantification of coagulation of multiple human plasma samples

Resonant Acoustic Rheometry (RAR), a newly developed ultrasound-based technique for non-contact characterization of soft viscoelastic materials, has shown promise for quantitative viscoelastic assessment of temporally changing soft biomaterials in real time, and may be used to monitor blood coagulation process. Here, we report the development of a novel, multichannel RAR (mRAR) system for simultaneous measurements of multiple temporally evolving samples and demonstration of its use for monitoring the coagulation of multiple small-volume plasma samples. The mRAR system was constructed using an array of 4 custom-designed ultrasound transducers at 5.0 MHz and a novel electronic driving system that controlled the generation of synchronized ultrasound pulses for real time assessment of multiple samples simultaneously. As a proof-of-concept of the operation of the mRAR system, we performed tests using pooled normal human plasma samples and anti-coagulated plasma samples from patients treated with warfarin with a range of International Normalized Ratio (INR) values as well-characterized samples with different coagulation kinetics. Our results show that simultaneous tracking of dynamic changes in 4 plasma samples triggered by either kaolin or tissue factor was achieved for the entire duration of coagulation. The mRAR system captured distinct changes in the samples and identified parameters including the clotting start time and parameters associated with the stiffness of the final clots that were consistent with INR levels. Data from this study demonstrate the feasibility of the mRAR system for efficient characterization of the kinetic coagulation processes of multiple plasma samples.

A Rheological Model with Integer and Non-Integer Orders Nonlinearities for Predicting Stress Relaxation Behavior in Viscoelastic Materials

Viscoelastic materials are widely used as devices for vibration control in modern engineering applications. They exhibit both viscous and elastic characteristic when undergoing deformation. They are mainly characterized by three time-dependent mechanical properties such as creep, stress relaxation and hysteresis. Among them, stress relaxation is one of the most important features in the characterization of viscoelastic materials. This phenomenon is defined as a time-dependent decrease in stress under a constant strain. Due to the inherent nonlinearity shown by the material response over a certain range of strain when viscoelastic materials are subjected to external loads, nonlinear rheological models are needed to better describe the experimental data. In this study, a singlenonlinear differential constitutive equation is derived froma nonlinear rheological model composed of a generalized nonlinear Maxwell fluid model in parallel with a nonlinear spring obeying a power law for the prediction of the stress relaxation behavior in viscoelastic materials. Under a constant strain-history, the time-dependent stress is analytically derived in the cases where the positive power law exponent, α ˂ 1 and α ˃1. The Trust Region Method available in MATLAB Optimization Toolbox is used to identify the material parameters. Significant correlations are found between the experimental relaxation data taken from literature and exact analytical predictions. The obtained results show that the developed rheological model with integer and non-integer orders nonlinearities accurately describes the experimental relaxation data of some viscoelastic materials.

Thermographic and rheological characterization of viscoelastic materials for hot-extrusion 3D food printing

Thermoreversible food materials are suitable for hot-extrusion 3D food printing (HE-3DFP) to customize food designs and enable on-demand food production. A challenge of HE-3DFP is to control the material phase transition such that it melts to allow flow and extrusion and rapidly solidifies afterwards to obtain stable printed structures. We here report on the use of thermal imaging to simultaneously monitor material cooling and deformation of common thermoreversible food materials during HE-3DFP. Thermographic and rheological measurements show that the structural deformation is driven by slow material cooling and prolonged printing time. The surface temperature of printed objects is a good indicator for structural stability. Solidification mechanisms such as cross-linking or strong particle jamming are required to prevent deformation in time (i.e. creep) during printing. Thus we recommend to set the printing temperature just above material's gelation temperature to ensure proper extrudability and structural stability of the printed foods.

A new way to investigate the damping properties of NR/CIIR blends characterized by the rebound behaviors

AbstractIn this work, effects of natural rubber (NR) content on the vulcanization and damping properties of chlorinated butyl rubber (CIIR) were studied. A new method to characterize the damping performance of the NR/CIIR blends by the rebound behavior of rubber ball was proposed. And an optical laser‐based measurement was used to follow the movement of the rubber sample. The DMA results showed that better damping properties of the blends was obtained at higher proportion of CIIR in the blends at room temperature (20°C) and a frequency of 0.1–100 Hz. Furthermore, a propose was made that the damping properties of the blends can be characterized by the rebound behavior of the rubber ball, and two methods were developed to characterize the damping properties of the blends according to the rebound experimental results of the rubber ball. The functional relationship between the height of the nth bounce of the rubber ball and NR content was obtained by experimental fitting. The damping index (DI) was defined by combining the parameters such as bounce height, bounce times and bounce time of the rubber ball. It was expected that this study could provide a new idea for the characterization and testing of viscoelastic damping materials.

Force-Displacement Analysis in Diaphragm-Embedded Fiber Bragg Grating Sensors.

This paper presented the force and displacement analyses of a diaphragm-embedded fiber Bragg grating (FBG) sensor. In the first step, a numerical analysis (via finite element method) was performed considering linear elastic materials, where there is a linear variation on the strain in the optical fiber for both displacement and force (or pressure). In the second step, the experimental analysis was performed using two approaches: (i) controlling the displacement applied in the diaphragm-embedded FBG (while the force is also measured). (ii) Controlling the force applied in the sensor (also with the measurement of the displacement). Results showed reflected optical power variations and wavelength shift following the application of displacement and force. The sensitivities of both wavelength shift and optical power were different (and non-proportional) when displacement and force were compared. However, a higher correlation, determination coefficient (R2) of 0.998, was obtained in the analysis of the wavelength shift as a function of the displacement, which indicated that the strain transmission in the optical fiber is directly related to the strain in the diaphragm, whereas the force has an indirect relation with the strain and depends on the material features. Then, the possibility of simultaneous estimation of force and displacement was investigated, where the linear relation of both parameters (displacement and force) with the wavelength shift and the optical power were obtained in a limited range of displacement and force. In this range, root mean squared errors of 0.37 N and 0.05 mm were obtained for force and displacement, respectively. In addition, the force variation with a step displacement input also shows the possibility of using the proposed FBG device for the characterization of the materials’ viscoelastic features such as phase delay, creep, and stress relaxation, which can be employed for in situ characterization of different viscoelastic materials.

Some open challenges in polymer physics*

AbstractThree subjects in polymer and soft matter physics are outlined in the present article. The first relates to concepts of an ideal glass transition. We describe work on ultra‐stable glass, either a 20‐million‐year‐old amber or a vapor deposited amorphous fluoropolymer, which examines the temperature dependence of the dynamics in a window between a low fictive temperature and the glass transition temperature. From this “finesse” of the problem of the geological time scales in sub‐glass temperature systems strong evidence that the divergence of the relaxation times at finite temperature from WLF‐types of extrapolation is not correct. The second topic is polymer nonlinear viscoelasticity as it relates to dynamic heterogeneity. We show results from mechanical hole burning experiments that suggest that dynamic heterogeneity arises from the nature of specific relaxation mechanisms rather than heterogeneous changes in, for example, the fictive temperature. Included is a discussion of large amplitude oscillatory shear testing and how it relates to characterization of nonlinear viscoelastic materials. The last topic addressed is the rheology of circular macromolecules. Here, we take a historical perspective and describe important new results from several laboratories that seem inconsistent. New works from our own studies on circular DNA are also discussed.

Visco-elastic material characterization by means of full field Lamb waves

In structural design and structural health monitoring (SHM), the identification of the visco-elastic stiffness tensor of composite materials is considered fundamental, however, the reconstruction of the stiffness coefficients is not straightforward due to the inherent presence of anisotropy and heterogeneity. In the current study, an inverse procedure is proposed based on full field wave dispersion curves to characterize the (18) visco-elastic stiffness parameters of an orthotropic composite plate. The procedure is illustrated by means of finite element method simulations conducted with broadband signals. First, the FEM simulation data, consisting of the in-plane and out-of-plane velocities on the 2D top surface of a composite plate, were converted into the frequency-complex wavenumber domain, including information on elasticity and viscosity, by using the matrix pencil method. The converted data set was then considered as input to a surrogate optimization inversion algorithm, using the semi analytical finite element method (SAFE) as the forward model owing to its accuracy, efficiency and robustness. The full tensorial visco-elastic stiffness parameters were calculated and updated by minimizing the error between the simulated and the calculated complex wavenumbers. The final reconstructed stiffness properties in case of an aluminum plate and of a unidirectional composite laminate show good agreement with the exact input stiffness values, with average errors below 5% for all 18 visco-elastic stiffness parameters.

A new technique for the characterization of viscoelastic materials: Theory, experiments and comparison with DMA

In this paper we present a theoretical and experimental study aimed at characterizing the hysteretic properties of viscoelastic materials. In the last decades viscoelastic materials have become a reference for new technological applications, which require lightweight, deformable but ultra-tough structures. The need to have a complete and precise knowledge of their mechanical properties, hence, is of utmost importance. The presented study is focused on the dynamics of a viscoelastic beam, which is both experimentally investigated and theoretically characterized by means of an accurate analytical model. In this way it is possible to fit the experimental curves to determine the complex modulus. Our proposed approach enables the optimal fitting of the viscoelastic modulus of the material by using the appropriate number of relaxation times, on the basis of the frequency range considered. Moreover, by varying the length of the beams, the frequency range of interest can be changed/enlarged. Our results are tested against those obtained with a well established and reliable technique as compared with experimental results from the Dynamic Mechanical Analysis (DMA), thus definitively establishing the feasibility, accuracy and reliability of the presented technique.

Viscoelastic Material Characterization: A Realistic Approach to Stress Analysis

This paper presents an advanced method in materials characterization for the mold compound material in semiconductor packages to build models that can technically explain the actual warpage or stress observations under different thermal conditions and time history. In the study, the mold compound material characterization was conducted using Dynamic Mechanical Analyzer (DMA) followed by curve fitting to obtain parameters for the computer modeling input requirement. Thermo-mechanical modeling using viscoelastic material properties was conducted on a bi-material test sample model. Results showed that the new characterized viscoelastic material properties exhibited dependence on time and temperature. Slow cool down from post mold cure (PMC) to room temperature resulted in lower warpage or stress. This observed rate dependent response was explained using viscoelastic material properties in contrast to the usual linear elastic material simplification. Thus, a realistic result from stress or warpage analysis could be achieved using viscoelastic material characterization.

Optimal Design Parameters for a Phased-Array-Based Ultrasonic Polar Scan.

In the context of designing a next-generation ultrasonic polar scan (UPS) measurement system for viscoelastic material characterization, a novel approach is proposed, which draws on a set of cylindrically focused emitters in conjunction with a circular phased array (C-PA) receiver in order to create a portable measurement system while improving the data quality and ease the data interpretation. To explore the potential of the new approach and determine its optimal design parameters, a 3-D analytical model is presented to numerically simulate UPS experiments with the proposed system. Furthermore, a postprocessing procedure is worked out to treat the acquired raw data with the aim to deal with the integrating effect of finite size transducers and directly reconstruct the angle-dependent plane wave reflection coefficients of the sample under study. As the accuracy of the reconstruction heavily depends on various design parameters, a parameter study focusing on the influence of three main experimental parameters is performed to guide the optimal design. For each of these parameter studies, the UPS simulation results have been inverted, and the errors on the estimated C-tensor parameters have been deduced. First, it is shown that, for a given frequency, the radius of the C-PA must be large enough to capture both the specular and nonspecular reflected field, which is crucial to assure a correct reconstruction of the plane wave characteristics and find proper estimates of the C-tensor parameters. Second, the impact of the emitter and receiver lengths on the quality of the reconstruction and the C-tensor parameters has been investigated, yielding superior results upon increasing either of them. Finally, a dedicated study of the pitch of the C-PA elements and the angular range of the cylindrically focused emitters shows that aliasing effects disturb the results if the pitch is too large. However, this effect can somewhat be mitigated by employing multiple emitters with a restricted angular range. Using the knowledge of the abovementioned parameter studies, a simulated UPS experiment using a proper set of design parameters is performed for a cross-ply carbon epoxy laminate. The postprocessed reconstruction based on these data shows an excellent agreement with the theoretical plane wave results.

On the frequency dependence of viscoelastic material characterization with intermittent-contact dynamic atomic force microscopy: avoiding mischaracterization across large frequency ranges.

Atomic force microscopy (AFM) is a widely use technique to acquire topographical, mechanical, or electromagnetic properties of surfaces, as well as to induce surface modifications at the micrometer and nanometer scale. Viscoelastic materials, examples of which include many polymers and biological materials, are an important class of systems, the mechanical response of which depends on the rate of application of the stresses imparted by the AFM tip. The mechanical response of these materials thus depends strongly on the frequency at which the characterization is performed, so much so that important aspects of behavior may be missed if one chooses an arbitrary characterization frequency regardless of the materials properties. In this paper we present a linear viscoelastic analysis of intermittent-contact, nearly resonant dynamic AFM characterization of such materials, considering the possibility of multiple characteristic times. We describe some of the intricacies observed in their mechanical response and alert the reader about situations where mischaracterization may occur as a result of probing the material at frequency ranges or with probes that preclude observation of its viscoelastic behavior. While we do not offer a solution to the formidable problem of inverting the frequency-dependent viscoelastic behavior of a material from dynamic AFM observables, we suggest that a partial solution is offered by recently developed quasi-static force–distance characterization techniques, which incorporate viscoelastic models with multiple characteristic times and can help inform dynamic AFM characterization.

A mechanistic framework for field response assessment of asphalt pavements

Pavement analysis encompasses the use of different assumptions for material assessment that can in turn induce variations in the predicted pavement critical strains and affect the outcome of pavement evaluation. This research deals with the incorporation of mechanistic principles into asphalt pavements structural assessment, considering variable mechanical behavior (elastic and viscoelastic) for the Asphalt Concrete (AC) materials. A field experiment along a new asphalt pavement structure was performed including Non-Destructive Testing (NDT) and coring. The study focused on the tensile strains within the AC layers that usually trigger rehabilitation actions. For strain calculations, the standard practice of Multi-Layered Elastic Theory (MLET) and the rheological Huet-Sayegh model were both implemented into the analysis. Results indicated statistically significant discrepancies between elastic and viscoelastic calculations that may become considerable in terms of decision-making. Thus, pavement engineers should not proceed with the decision-making process based solely on a unique approach. In addition, the issue of performing a new pavement’s evaluation based on the desi gn assumptions was discussed. A substantial material differentiation pinpointed the need to extract cores in order to perform viscoelastic material characterization. Moving forward with a regression analysis, a practical approach was developed for viscoelastic strains prediction from NDT data in order to avoid extensive coring. Overall, the implementation of the framework at a new structure is grounded on that the methodology followed for pavement evaluation throughout its service life can predefine the way of decision-making in terms of consistently interpreting the analysis output and assessing future maintenance needs.

A Novel Approach to Describe the Time-Temperature Conversion among Relaxation Curves of Viscoelastic Materials.

Time and temperature, besides pressure in a lesser extent, represent the most significant variables influencing the rheological behavior of viscoelastic materials. These magnitudes are each other related through the well-known Time–Temperature Superposition (TTS) principle, which allows the master curve referred to relaxation (or creep) behavior to be derived as a material characteristic. In this work, a novel conversion law to interrelate relaxation curves at different temperatures is proposed by assuming they to be represented by statistical cumulative distribution functions of the normal or Gumbel family. The first alternative responds to physical considerations while the latter implies the fulfillment of extreme value conditions. Both distributions are used to illustrate the suitability of the model when applied to reliable derivation of the master curve of Polyvinil–Butyral (PVB) from data of experimental programs. The new approach allows not only the TTS shift factors to be estimated by a unique step, but the whole family of viscoelastic master curves to be determined for the material at any temperature. This represents a significant advance in the characterization of viscoelastic materials and, consequently, in the application of the TTS principle to practical design of viscoelastic components.

Damping control in viscoelastic beam dynamics

Viscoelasticity plays a key role in many practical applications and in different research fields, such as in seals, sliding-rolling contacts, and crack propagation. In all these contexts, a proper knowledge of the viscoelastic modulus is very important. However, experimental characterization of the frequency-dependent modulus, carried out through different standard procedures, still presents some complexities; then possible alternative approaches are desirable. For example, experimental investigation of viscoelastic beam dynamics would be challenging, especially for the intrinsic simplicity of this kind of test. This is why a deep understanding of damping mechanisms in viscoelastic beams is found to be a quite important task to better predict their dynamics. With the aim to enlighten damping properties in such structures, an analytical study of the transversal vibrations of a viscoelastic beam is presented in this article. Some dimensionless parameters are defined, depending on the material properties and the beam geometry, which enable to accurately design the beam dynamics. In this way, by properly tuning such disclosed parameters, for example the dimensionless beam length or a chosen material, it is possible to enhance or suppress some resonant peaks, one at a time or more simultaneously. This is a remarkable possibility to efficiently control damping in these structures, and the results presented in this article may help in elucidating experimental procedures for the characterization of viscoelastic materials.

Spectroscopic photonic force optical coherence elastography.

We demonstrate spectroscopic photonic force optical coherence elastography (PF-OCE). Oscillations of microparticles embedded in viscoelastic hydrogels were induced by harmonically modulated optical radiation pressure and measured by phase-sensitive spectral-domain optical coherence tomography. PF-OCE can detect microparticle displacements with pico- to nano-meter sensitivity and millimeter-scale volumetric coverage. With spectroscopic PF-OCE, we quantified viscoelasticity over a broad frequency range from 1Hz to 7kHz, revealing rich microstructural dynamics of polymer networks across multiple microrheological regimes. Reconstructed frequency-dependent loss moduli of polyacrylamide hydrogels were observed to follow a general power scaling law G''∼ω0.75, consistent with that of semiflexible polymer networks. Spectroscopic PF-OCE provides an all-optical approach to microrheological studies with high sensitivity and high spatiotemporal resolution, and could be especially beneficial for time-lapse and volumetric mechanical characterization of viscoelastic materials.

Dynamic mechanical characterization of viscoelastic materials using bending wave correlation vibrometry

Direct solid-borne sound measurements allow incorporation of phase in characterizing viscoelastic materials. In the absence of excessively large samples, reflections from specimen boundaries obscure this valuable data. Circularly crested bending waves generated in plates provide a means of preserving phase information while maintaining reasonable sample sizes. The coherence of signals measured equidistant from the source and the lack thereof concerning reflections, enables isolation of the direct sound. This talk presents developments in a laser Doppler vibrometer based dynamic mechanical characterization method aimed at broadband classification of viscoelastic materials. The applicability of this technique to in-situ applications is demonstrated. Material properties are shown to correspond with results from existing methods.

CMMSE 2017 – a numerical method based on genetic algorithms for the characterization of viscoelastic materials

ABSTRACTViscoelastic materials play a key role in mechanical designs due to its numerous application. Even though numerical models already exist and are included in commercial codes, the precision of the calculation depends strongly on the material characterization and the parameters identification. In our research, we propose the characterization of the viscoelasticity of a material, whose nonlinear elasticity was previously defined with a strain–stress test, and then we perform a relaxation test. Also three rheological models are studied in order to fit this test. The optimization problem is solved checking two optimization techniques: a deterministic local search method as Newton and a non-deterministic global search method as genetic algorithm. Both methods are compared with a known solution and a real test. Finally, we perform a load–unload test and compare the results with a Finite Element simulation where the viscoelastic material is defined using the well-known Prony series.