Linear and Nonlinear Viscoelastic Behavior of MR Fluids: Effect of Temperature

This study aims to investigate the influence of temperature on the linear and nonlinear rheological behavior of a MR fluid, MRF 132DG, using a rotational rheometer. The experiments were designed to obtain properties of the fluid under oscillatory shear strain in the amplitude and frequency sweep modes, while maintaining different constant temperatures (−5, 0, 20 and 50 °C). The data were used to evaluate the storage and loss moduli under different levels of magnetic flux density considering the linear as well as nonlinear viscoelastic regions. The critical strain amplitudes were further obtained. Results showed enhanced linear viscoelastic region with increasing magnetic field density. Moreover, the effects of temperature and magnetic field on the frequency dependency of the fluid properties are illustrated for small and large amplitudes of shear strains.

Effect of Variation of Hard Segment Content and Graphene-Based Nanofiller Concentration on Morphological, Thermal, and Mechanical Properties of Polyurethane Nanocomposites

This study describes the development of a new class of high-performance polyurethane elastomer nanocomposites containing reduced graphene oxide (RGO) or graphene nanoplatelets (GNP). Two types of polyurethane elastomers with different contents of hard segments (HS) were used as a polymer matrix. The developed nanocomposites were characterized by thermal analysis (DSC, TG), dynamic mechanical testing (DMA), hardness testing, mechanical properties, rheology, FTIR spectroscopy, XRD, and microscopy investigation (TEM, SEM). Morphological investigation confirmed better compatibility of RGO with the polyurethane (PU) matrix compared to GNP. Both applied nanofillers influenced melting and crystallization of the PU matrix. The nonlinear viscoelastic behavior of the nanocomposites (Payne effect) was studied, and the results were compared with theoretical predictions.

Constitutive Equations and Finite Element Implementation of Isochronous Nonlinear Viscoelastic Behavior

We present a phenomenological three-dimensional (3D) nonlinear viscoelastic constitutive model for time-dependent analysis. Based on Schapery's single integral constitutive law, a solution procedure has been provided to solve nonlinear viscoelastic behavior. This procedure is applicable to 3D problems and uses time- and stress-dependent material properties to characterize the nonlinear behavior of material. The equations describing material behavior are chosen based on the measured material properties in a short test time frame. This estimation process uses the Prony series material parameters, and the constitutive relations are based on the nonseparable form of equations. Material properties are then modified to include the long-term response of material. The presented model is suitable for the development of a unified computer code that can handle both linear and nonlinear viscoelastic material behavior. The proposed viscoelastic model is implemented in a user-defined material algorithm in abaqus (UMAT), and the model validity is assessed by comparison with experimental observations on polyethylene for three uniaxial loading cases, namely short-term loading, long-term loading, and step loading. A part of the experimental results have been conducted by (Liu, 2007, “Material Modelling for Structural Analysis of Polyethylene,” M.Sc. thesis, University of Waterloo, Waterloo, ON Canada), while the rest are provided by an industrial partner. The research shows that the proposed finite element model can reproduce the experimental strain–time curves accurately and concludes that with proper material properties to reflect the deformation involved in the mechanical tests, the deformation behavior observed experimentally can be accurately predicted using the finite element simulation.