Papers by Keyword: Viscoelasticity

Abstract: The damping property is a material's energy dissipation capacity, indicating its ability to resist vibrations. The parameters of damping characteristics can be evaluated using the traditional Fast Fourier Transformation (FFT) technique, which suffers from the loss of time. Therefore, Hilbert Transform (HT) and Wavelet Transform (WT) have been developed to overcome such problems and help comprehend damping properties precisely with time and frequency. This study evaluates and compares damping ratio assessment using HT, WT, and Log Decrement in linear and non-linear viscoelastic material models. To test the adapted HT and WT methods, we developed a homemade MATLAB code to evaluate the damping ratio of two data sets. Analytical data obtained from solving a linear viscoelastic material model and numerical data attained from the FE-model of a non-linear viscoelastic material were both subjected to vibration. The error percentages of the damping ratio estimated by HT and WT were 6.1 and 11.75, respectively, compared to 43 for Log Decrement. These results confirm that HT and WT can accurately predict the damping ratio of non-linear viscoelastic material models.

Abstract: Torsional vibration analysis of the axially functionally graded carbon nanotubes has been carried out. Nonlocal stress gradient elasticity theory has been used in continuum mechanics model of the carbon nanotube. Variation of the material properties of the axially graded nanostructure has been assumed in exponential form. Differently from the majority of literature works, viscous damping and nonlocal parameters have been assumed in grading form. Energy functional for the carbon nanotube has been achieved with minimum potential energy principle and weak form solution has been obtained with the Ritz Method. Effects of material grading, nonlocality and viscoelasticity to the torsional dynamics of axially graded carbon nanotube have been investigated. Results of the present work could be useful in modeling and production of axially functionally graded nanostructures.

Abstract: We present our recent study on adhesive contacts of viscoelastic materials sliding against rigid substrates. Ultimately, the theory addresses the combined effect of viscoelasticity and adhesion in sliding contacts, with specific focus on the sliding frictional behavior. Compared to the adhesiveless case, we show that a significant enhancement of hysteretic friction occurs in the presence of adhesion, in agreement with long-standing experimental evidence. The presented formulation allows to investigate the effect of sliding velocities ranging from extremely slow to very high, thus taking into for local viscoelasticity, occurring at the edges of the contacts (crack tips), and bulk viscoelasticity, occurring in the bulk deformable material.

Abstract: One of the ways to increase the bearing capacity and stability of a water-saturated base by introducing a sand pile vertically reinforced along the contour with geosynthetic material (geogrid SSP 30 / 30-2.5) is experimentally substantiated. This constructive solution is used in low-rise construction. For the theoretical substantiation of the suggested method, it is proposed to model the interaction of a weak foundation and a reinforced sand pile on the basis of the linear theory of viscoelasticity. Calculation of vertical displacements of the pile and comparison with the results of in situ experiments is presented.

Abstract: This work is devoted to the formulation and search of an analytical solution for the problem of the interaction of a rigid cylindrical body and a pipe with an inner coating in the case when the cylinder is placed inside such a pipe. It is assumed that the pipe coating can have a strong nonuniformity, and its thickness depends on the longitudinal coordinate. A special approach used in this work allows obtaining analytical solutions in which functions related to the properties and profile of the coating are separated by separate terms and factors. This allows us to provide efficient calculations even in cases where coating characteristics are described by complex functions. Other known methods lead to significant calculation errors.

Abstract: Thermoset adhesives convert from liquid to solid due to chemical reactions. Once cured, these adhesives carry the potential to create strong load-bearing joints, resisting even severe detrimental service conditions. In the progress of curing of a thermoset adhesive the viscoelastic properties of the resin and hardener formulation change as the chemical reaction proceeds. Gelation occurs once a continuous 3-dimensional network of polymer chains has been created. After gelation, the microstructure of the resin is fixed and further cure is affected by diffusion limitations [1]. Mastering of the curing kinetics and the physicochemical changes in the transition from the liquid to the solid-state is essential to reliably process adhesives in industrial applications. Rheological experiments in parallel plate configuration have become a well-established practice in investigating viscoelastic properties in the progress of curing. In practice, it has shown to be challenging to access the full range of viscoelastic parameters of thermoset resins with a low initial viscosity from the very beginning of the curing reaction to the post-cure consolidation of the vitrified polymer. This paper will discuss experimental methods and criteria for the viscoelastic analysis of curing thermoset adhesives and present experimental data of the time-, temperature-, and frequency-dependent viscoelastic properties of a curing thermoset epoxy in relation to the features of its time-temperature-transformation-diagram.

Abstract: The assembly of graphene and other two-dimensional (2D) materials into artificial crystals termed van-der-Waals stacks has great potential to produce new materials without precedence in nature and develop novel electronic devices. To reliably assemble 2D materials into such structures, however, a better understanding of the transfer process is required. Here we report a quantitative approach to examining the adhesion behavior during viscoelastic stamping of 2D materials. By measuring the adhesion of graphene to different carrier substrates and varying the peeling speed we have identified the range of adhesion of samples. The result shows that the adhesion occurs between graphene-graphene and graphene-SiO₂ substrate have a higher value than the ability of polydimethylsiloxane (PDMS) stamp to pick up. The impact of surface modification and alternative substrates is investigated and our results provide guidelines to realize an effective fabrication method for two-dimensional heterostructure devices.

Abstract: The chopped aramid fiber reinforced rubber composite (AFRC) has been widely used in the tire treads for its excellent characteristics. The viscoelasticity which is an important mechanical property for rubber matrix would be influenced by the adding of aramid fiber. In this study, the dynamic and quasi-static viscoelasticity were investigated via the dynamic thermal analysis experiment and the mullins experiment with multi-step relaxation, respectively. The frequency and temperature scanning were employed for AFRC with different fiber volume fractions, fiber aspect ratios and fiber orientation distributions. The effects of constituent parameters on the dynamic viscoelasticity were studied as well as the general rule of reinforcing effect of aramid fiber on rubber materials was presented. The stress relaxtion for AFRC were analysed basing on the experimental results. In addition, The ability of the viscoelastic constitutive model to describe the quasi-static viscoelastic behavior of AFRC were explored.

Abstract: Expressed viscoelasticity of polymeric materials which can develop over a long period of time prevents their widespread. Some types of polymers, such as epoxy resins, can be used to connect various structural elements. The destruction in this case can be caused by the growth of tangential stresses in the adhesive joint and their achievement of some critical value τ_adhezive, at which the adhesive joint is destroyed.

Abstract: To predict the nonlinear mechanical behavior of components made of short fiber-reinforced plastics (SFRP) under long term and cyclic loading, coupled process and component simulations are required. The injection molding process leads to locally varying fiber orientations within the component. This varying microstructure [1] significantly influences the viscoelastic and fatigue behavior. The interaction between the microstructure [2] and the nonlinear macroscopic properties is resolved by a coupled fast Fourier transformation and finite element two-scale method (FFT-FEM), where the fiber orientation tensor is obtained by analyzing μCT images or by the corresponding process simulation. The aim of this work is to reduce the numerical costs of such a multiscale method. In a first step, the highly efficient micro-scale solver FeelMath [3,4] using an FFT-based preconditioner is presented. Afterwards, a numerical scheme based on a precomputed database trained with FeelMath simulations on the microscale and a model order reduction algorithm, is discussed. The combination of these ideas reduces the numerical effort, such that the method is applicable for industrial problems. Comparative studies of the fully coupled and reduced model document the high accuracy of this approach. The overall performance of this methodology is demonstrated by three-dimensional, industrial applications.