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Composite materials consisting of negative-stiffness inclusions in positive-stiffness matrix may exhibit anomalous effective coupled-field properties through the interactions of the positive and negative phases, giving rise to extremely large or small effective properties. In this work, effective viscoelastic properties of a continuum composite system under the effects of negative inclusion Young's modulus ratio λ E = E inc / E matrix are studied with the finite element method. Furthermore, effective coupled-field properties, such as thermal expansion coefficient, dielectric constants and piezeoelectric constants, are numerically calculated under the effects of negative inclusion bulk modulus ratio λ K = K inc / K matrix . Stability boundaries are determined by applying small dynamic perturbation to the systems through boundary surfaces, and the system is unstable if its field variables become divergent in time. For viscoelastic composite systems containing small volume fractions, less than V i = 1.5 % , the systems can be stable up to λ E ≈ - 0.3 in 0.3 s under 10 Hz driving frequency. For V i = 5.1 % case, its stability boundary is around λ E ≈ 0 . Larger inclusion volume fraction reduces allowable negative stiffness in the viscoelastic system. All anomalous peaks found in the coupled-field properties are in the unstable regime, except for the piezoelectric and thermal-expansion anomalies in the composite system with electrically insulated inclusions and large inclusion volume fraction V i = 26.81 % . Insulated inclusions may cause charge accumulation at the inclusion–matrix interface and boundary surface effects may serve as stabilizing agents to the composite system. Since it is known that negative-stiffness composite is unstable in the purely elastic system in statics, stability enhancement found here in the negative-stiffness systems with viscoelastic and coupled-field effects may be considered as multiphysics-induced stabilization. [ABSTRACT FROM AUTHOR]

Polyamide exhibits hygroscopic nature and can absorb up to 10% of moisture relative to its dry weight. The absorbed moisture increases the mobility of the molecular chains and causes a reduction in the glass transition temperature. Thus, depending on the moisture distribution, a polyamide component can show different stiffness and relaxation times. Moreover, the moisture distribution also depends on the mechanical loading of the material as the volumetric deformation results in a change of the available free volume for the moisture. Thus, a strongly coupled model is required to describe the material behaviour. In this work, a thermodynamically consistent coupled model within the framework of mixture theory is developed. The mechanical deformation of polyamide 6 (PA6) is based on a linear viscoelastic material model, and the moisture transport is based on a nonlinear diffusion model. The stiffness and the relaxation time of the viscoelastic model change with the moisture concentration. Furthermore, the moisture transport is affected by the pressure gradient generated by the mechanical loading of the material. This strongly coupled model has been implemented using the finite element method, and simulation results are presented for a three-point bending experiment. [ABSTRACT FROM AUTHOR]

Micromechanical models can be used to calculate the mechanical properties of short glass fiber reinforced thermoplastics. In the present work, a three-step framework is used to validate a three-phases micromechanical model (RDI model) in the time domain, since the analysis of technical components by the finite element method is usually carried out in the time domain. The framework includes mechanical characterization, the implementation of the RDI model and a finite element analysis. The characterization delivers necessary information about the material phases of the composite. A dynamic mechanical analysis is performed to characterize the matrix material in order to obtain the linear viscoelastic properties. The mechanical properties of the matrix-fiber interphase are determined with an inverse calculation. In the second step, the RDI model is used to calculate the frequency depended effective stiffness of the composite. A new developed approach transforms the effective stiffness from the frequency domain into the time domain thus avoiding an explicit inverse Laplace-Carson transformation. In the third step, the RDI model is experimentally validated. [ABSTRACT FROM AUTHOR]