Extreme viscoelastic and coupled-field properties of negative-stiffness composite materials

  • 柯 智欽

Student thesis: Doctoral Thesis


Negative-stiffness (NS) composite materials consist of inclusions with negative bulk modulus Young’s modulus or shear modulus that are embedded in positive matrix Elastic isotropy is assumed for the inclusion Experimental evidence from composites having ferroelastic inclusions such as barium titanate in the vicinity of solid-solid phase transformation supports the use of negative stiffness in theoretical and numerical modeling Extreme effective properties such as extremely large or small effective viscoelastic moduli damping and coupled-field coefficients of the composite materials can be realized if negative stiffness in inclusions is allowed hence bounds of composite materials can be broken Stability of the NS composites requires considerations in thermodynamics continuum mechanics and coupled-field theories as well as the Lyapunov stability analysis In addition to using the theory of composite materials to predict their effective properties under the effects of negative stiffness in this dissertation numerical calculations based on finite element methods are employed to study the effective elastic viscoelastic thermoelastic piezoelectric and dielectric properties of the NS composite systems Furthermore their stability is determined through monitoring divergence of field variables when the system is under small sinusoidal perturbation on the boundary in the time domain analysis Connections between material stability and numerical stability are established in the finite element time-domain analysis Stability boundaries in terms of allowable amounts of negative stiffness are identified for all of the studied effective properties including the coupled-field properties It is found that all of the extreme properties occur in the unstable regime except for the piezoelectric and thermal expansion properties The multiphysics effects in the coupled-field properties may provide a stabilizing mechanisms when inclusion volume fraction is large and inclusions are electrically insulated In addition stability of the NS viscoelastic systems is superior to their purely elastic counterparts due to damping mechanisms When time-dependent material properties are considered the divergent rate of the instability phenomena can be controlled by damping in the composite systems Hence the extreme properties of the NS composites may be considered as metastable In addition to research in the negative-stiffness composite materials conventional composite materials that simultaneously exhibit high stiffness and high damping are studied through the combination of ‘microstructured’ soft metal and polymer In small deformation the composite exhibits large damping due to the viscoelasticity of polymer In large deformation the soft metal provides energy-dissipation capabilities through plastic deformation Through suitable design the viscoelastic composite material may be used as a beam-column connector in civil engineering
Date of Award2015 Aug 19
Original languageEnglish
SupervisorYun-Che Wang (Supervisor)

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