Investigations On Dynamic Response And Instability Of Electro/magneto Active Soft Material Structures

Soft active materials are intelligent materials that respond to external stimuli by undergoing large deformations and can achieve specific functions.They have broad applications in fields such as soft robotics,biomedical treatment,and intelligent wearable devices.However,due to the factors such as hyperelasticity and viscoelasticity,soft materials often exhibit complex nonlinear mechanical responses under multi-field coupling.Moreover,soft structures often experience instability,hysteresis and damping phenomena caused by the viscoelasticity of the materials.All these factors pose challenges to the stability and safety of soft structures in practical applications.Therefore,the research on the mechanical behavior of viscoelastic soft materials under multi-field coupling has crucial engineering applications value.It can provide valuable guidance for the design and development of intelligent soft structures,facilitating efficient and precise realization of specific functionalities in soft structures under multifield coupling.This thesis focuses on two typical soft active materials,namely dielectric elastomers and hard-magnetic soft materials.The dynamic responses and instability of soft structures under electromechanical coupling and magnetic-elastic coupling are investigated by theoretical analysis and numerical simulations.The analysis of the viscoelastic effects of materials on the large deformation and instability of structures under mechanical-electric or mechanical-magnetic interactions has been conducted.Furthermore,a nonlinear Proportional-Integral-Derivative(PID)controller is introduced to actively control the dynamic response of structures,enabling them to achieve desired outputs under electric or magnetic field excitations.The main research objectives are outlined as follows:Firstly,a numerical model for viscoelastic dielectric elastomers is developed,and the Snap-through path of structures under mechanical-electric coupling is determined.The study then focuses on the influence of viscoelastic effects on the instability of dielectric elastomer balloons under different loading conditions.Numerical results indicate that viscoelastic effects have a significant inhibitory effect on the instability of balloons.Under a single relaxation mechanism,an increase in relaxation time delays the onset of structural instability without affecting the critical stretch for instability.In contrast,multiple relaxation mechanisms enhance the critical stretch for structural instability.Then,a dynamic model for viscoelastic dielectric elastomer balloons considering multiple relaxation times is developed by combining the generalized Maxwell model.Using perturbation analysis,the natural frequencies of structures under mechanicalelectric coupling are determined.By comparing with existing experimental results,it is found that the model incorporating multiple relaxation mechanisms provides a more accurate description of the deformation behavior of viscoelastic dielectric elastomers.The analysis then places particular emphasis on the impact of relaxation times on the dynamic response of balloons.Results demonstrate that multiple relaxation times not only affect the amplitude and duration of the beat phenomenon but also lead to changes in the resonance frequencies and amplitudes of the structure.Additionally,by applying a nonlinear PID controller to viscoelastic dielectric elastomer balloons,the beat and super-harmonic resonance are eliminated,and the desired output can also be achieved.Subsequently,the dynamic model of a hard-magnetic soft actuator is constructed by the Euler-Lagrange equations and the ideal hard-magnetic soft material model.By combining perturbation methods,the influence of different structural dimensions on the natural frequencies of the hard-magnetic soft structure under magneto-elastic coupling is analyzed.The time response and amplitude-frequency curves of the structural vibration are provided for different aspect ratios,and the stability and periodicity of the vibration are analyzed using phase portraits and Poincaré maps.The effect of excitation frequency on the beat phenomenon in the dynamic response is considered,and it is found that the beat phenomenon weakens or even disappears when the excitation frequency is far from the resonance frequency.Taking into account the Rayleigh dissipation function,the influence of viscoelasticity on the dynamic response of the hard-magnetic soft actuator is further considered.Then,by using a nonlinear PID controller,the nonlinear responses such as beat in the vibration of the hard-magnetic soft structure are eliminated,and the desired output is achieved.Finally,a numerical model for viscoelastic hard-magnetic soft structures is developed based on the theory of non-equilibrium thermodynamics and the evolution equation of stress-type internal variables.The finite deformation and instability issues of typical structures of viscoelastic hard-magnetic soft materials are analyzed.With the consistent mode imperfection method,the buckling and post-buckling paths of hardmagnetic soft beams and spherical shells under magnetic field driving are determined.It is found that the loading rate significantly affects the critical buckling magnetic field and post-buckling path.When the loading rate is low,the structure undergoes an immediate buckling controlled by an equilibrium mechanism with complete relaxation.When the loading rate is high,the structure undergoes an elastic immediate buckling instability.Under moderate loading rates,the structure experiences delayed buckling due to creep deformation.Further analysis is conducted on the effects of inertial effects,relaxation time,defect magnitude,and external magnetic field on the buckling strength and delayed instability of viscoelastic hard magnetic soft structures.Moreover,a displacement control loading method is used to capture the Snap-through and Snapback instabilities occurring in the hard magnetic soft spherical shell during the loading and unloading phases under magnetic field excitation.Methods for achieving large,reversible deformations and designing bistable structures under magnetic field excitation are presented.