Sharma, Atul Kumar

In recent years, there has been increasing interest in hard-magnetic soft materials (HMSMs) due to their ability to retain high residual magnetization and undergo large deformations under external magnetic loading. The performance of these materials in the dynamic mode of actuation is significantly influenced by internal properties, such as entanglements, crosslinks, and the finite extensibility of polymer chains. This article presents a theoretical framework for modeling the dynamic behavior of a hard-magnetic soft material-based planar actuator. A physics-based nonaffine material model is utilized to consider the inherent properties of polymer chain networks. The governing equation for dynamic motion is derived using Euler–Lagrange's equation of motion for conservative systems. The devised dynamic model is utilized to examine the dynamic response, stability, periodicity, and resonance properties of a planar hard-magnetic soft actuator for different values of polymer chain entanglements, crosslinks, and finite extensibility parameters. The Poincaré maps and phase-plane plots are presented to analyze the stability and periodicity of the nonlinear vibrations of the actuator. The results reveal that transitions between aperiodic and quasi-periodic oscillations occur when the density of polymer chain entanglements and cross-linking changes. The findings from the present investigation can serve as an initial step towards the design and manufacturing of remotely controlled actuators for various futuristic applications.

In view of their unique shape morphing behaviour, dielectric elastomer-based minimum energy structures (DEMES) have received an increasing attention in the technology of electroactive soft transduction. Because several of them undergo a time-dependent motion during their operation, understanding their nonlinear dynamic behaviour is crucial to their effective design. Additionally, in the recent past, there has been a growing scientific interest in imparting anisotropy to the material behaviour of dielectric elastomers in view of ameliorating their actuation performance. Spurred with these ongoing efforts, this paper presents an analytical framework for investigating the nonlinear dynamic behaviour of aniso-visco-hyperelastic DEMES actuator with an elementary rectangular geometry. We use a rheological model comprising two Maxwell elements connected in parallel with two single spring elements for modelling the material behaviour of the DE membrane. The governing equations of motion for the underlying non-conservative system are then derived using the Euler–Lagrange equation. The proposed model is used for building insights into the attainable equilibrium states, periodicity of the response as well as the resonant behaviour of the DEMES actuator over a feasible range of anisotropy and viscosity parameters. Our results reveal that the DEMES with hyperelastic material properties exhibits a supercritical pitchfork bifurcation of equilibrium state which is further accelerated in terms of attained equilibrium angle due to membrane anisotropy. A significant enhancement in the equilibrium angle attained by the structure with the extent of membrane anisotropy parameter is observed, indicating a favourable impact of material anisotropy. Poincare maps and phase-portraits are presented for assessing the periodicity of the nonlinear oscillations. The frequency response of the actuator for a combined DC and AC load indicates an upsurge in the resonant frequency with an increase in anisotropy parameter. The underlying analytical model and the trends presented in this study can find their potential use in the design and development of the futuristic anisotropic DEMES actuators subjected to time-dependent actuation.

This article deals with the dissimilar joining of two different grade Cr-Mo steel (2.25Cr-1Mo: P22 and modified 9Cr-1Mo: P91) for power plant application. The dissimilar butt-welded joint was produced for conventional V groove design by using the gas tungsten arc welding (GTAW) process with the application of an ERNiCrMo-3 Ni-based super alloy filler. A microstructure characterization was performed to measure the inhomogeneity in the microstructure and element diffusion across the interface in a welded joint. The experiments were also performed to evaluate the mechanical properties of the dissimilar welded joint in as-welded (AW) and post-weld heat treatment (PWHT) conditions. An acceptable level of the mechanical properties was obtained for the AW joint. After PWHT, a significant level of the element diffusion across the interface of the weld metal and P22 steel was observed, resulting in heterogeneity in microstructure near the interface, which was also supported by the hardness variation. Inhomogeneity in mechanical properties (impact strength and hardness) was measured across the weldments for the AW joint and was reduced after the PWHT. The tensile test results indicate an acceptable level of tensile properties for the welded joint in both AW and PWHT conditions and failure was noticed in the weak region of the P22 steel instead of the weld metal.

The capability of Dielectric Elastomers (DEs) undergoing large deformation is limited by electromechanical instability arising because of the positive feedback between the applied electric field and the reduction in thickness of the elastomer. This paper theoretically analyzes the stability of a dielectric elastomer fiber composite made up of two soft incompressible DE phases in the quasi-static and dynamic modes of actuation. An energy method, which relies on the energy balance at the position of maximum overshoot in an oscillation cycle, is used for extracting the critical electric field and stretch in the dynamic mode. The material behaviors of the fiber and matrix phases are characterized by neo-Hookean energy density function of the elastomer. The results demonstrate that DE composite with higher fiber volume fraction exhibits higher electric field at the onset of instability in both static and DC dynamic modes of actuation. The results of the present investigation can find potential applications in the development and design of the soft electroactive actuators subjected to dynamic loading.

The hard-magnetic soft materials (HMSMs) belong to the magnetoactive category of smart polymers that undergo large actuation strain under an externally applied magnetic field and can sustain a high residual magnetic flux density. Because of these remarkable characteristics, HMSMs are promising candidates for the remotely controlled actuators. The magnetic actuation behavior of the hard-magnetic soft actuators (HMSAs) is considerably affected by the viscoelastic material behavior of HMSMs. In this article, we aim at developing an analytical dynamic model of a typical planar model of HMSAs concerning the viscoelasticity of HMSMs. A Zener rheological model in conjunction with an incompressible neo-Hookean model of hyperelasticity and Rayleigh dissipation function is employed for defining the constitutive behavior of the viscoelastic HMSA. The governing equations of dynamic motion are deduced by implementing the nonconservative form of the Euler- Lagrange equation. The established dynamic model is utilized for providing preliminary insights pertaining to the effect of the viscoelasticity on the nonlinear oscillations of the actuator. The phase-plane portraits, Poincaré maps, and the time-history response are plotted to investigate the stability, resonant behavior, and periodicity of the actuator. The results and inferences reported here should provide the initial step toward the design and the development of modern actuators for diverse futuristic applications in the medical and engineering fields.

This work presents the dynamic modeling and analysis of a particle-reinforced and prestressed electro-magneto-viscoelastic plate actuator. The actuator belongs to a smart actuator category and is made of an electro-magneto-Active polymer filled with a particular volume fraction of suitable fillers. An energy-based electro-magneto-viscoelastic model is developed to predict the actuator response and interrogate the impact of particle reinforcement on the dynamic oscillations of a pre-stressed condition of the actuator. An Euler-Lagrange equation of motion is implemented to deduce the governing dynamic equation of the actuator. The findings of the model solutions provide preliminary insights on the alteration of the nonlinear behavior of the actuator driven by DC and AC dynamic modes of actuation. It is observed that the enrichment in the particle reinforcement characterized by the amount of fillers strengthens the polymer and depleted the associated level ofdeformation. Also, the depletion in the intensity of oscillation and enhancement in the frequency of excitation is perceived with an increase in the particle reinforcement. In addition, the time-history response, Poincare plots, and phase diagrams are also plotted to assess the stability, periodicity, beating phenomenon, and resonant behavior of the actuator. In general, the current study provides initial steps toward the modern actuator designs for various futuristic applications in the engineering and medical field. [DOI: 10.1115/1.4051911].

The installed power capacity of grid-connected wind energy conversion systems (WECSs) has increased exponentially around the world. The size of each unit is also increasing as they are becoming more affordable. The grid is a highly non-linear system due to wide spread use of power electronically switched loads and devices. Hence, the grid disturbances affect the output of the WECSs. This paper presents a study of the effects of grid disturbances such as switching of loads, feeder tripping, outage and synchronization of conventional generators on the output parameters of the grid-connected wind generators. The voltage, current and power of wind generator at the point of common coupling have been analysed. Further, presence of disturbances has also been investigated using the discrete wavelet transform (DWT). The results are validated using a 1.5 MW doubly fed induction generator based wind energy conversion system integrated with a test system in MATLAB/Simulink environment.

Dielectric elastomer minimum energy structure (DEMES) formed by clinging a pre-stretched dielectric elastomer (DE) membrane to the compliant frame possess both material and geometrical nonlinearity. The viscous and hyperelastic nature of the DE membrane and coupling between the compliant frame and membrane forms the basis of these nonlinearities. Practically, DE membrane based DEMES actuator executes the transient motion which is significantly affected by the viscoelastic behavior of the DE membrane. Hence, for an efficient design of such devices, it is very important to analyse the effect of membrane viscoelasticity on the dynamic response of DEMES. In the present work, we have developed an analytical model to analyse the viscoelastic effect of DE membrane on the nonlinear dynamic behavior of the DEMES. In order to incorporate the viscous effect, the Zener rheological model consisting of a spring element connected in parallel to a Maxwell element is employed. The neo-Hookean material model based on the additive decomposition of the isotropic strain energy density into equilibrium and viscous parts is considered. The governing differential equation representing the dynamic behavior of DEMES actuator is derived using Euler–Lagrange equation of motion for non-conservative system. The isotropic viscous stretch is obtained by using the thermodynamically consistent evolution equation. The developed dynamic model predicts the initial shape, DC and AC response, periodicity of the DEMES for different values of viscosity parameter. The result reveals that the onset of equilibrium state delays as viscosity parameter increases. Further, the bending angle of DEMES actuator is significantly affected by the applied electric field and viscoelastic behavior of the DE membrane. Poincaré maps along with phase diagrams are presented to analyse the periodicity of nonlinear oscillation of the system. Further, the structure executes a stability transition (stable-unstable-stable) as the value of viscosity parameter increases. The obtained results can help in robust and efficient designing of DEMES based actuators subjected to dynamic loading.

This work presents a finite element framework for simulating the quasi-static response of dielectric elastomer-based minimum energy structure (DEMES). The DEMES is an actuator formed by combining an inextensible frame and pre-stretched dielectric membrane that exhibits the unique shape-morphing characteristics of the actuator. A continuum strain energy-based model is implemented to investigate the impact of the different geometrical parameters on the performance of the DEMES actuator. Finite element analyses are performed using user-defined element (UEL) in ABAQUS for determining the equilibrium shape of the actuators and further investigating their electromechanical response. Experiments are performed using the commercially available VHB-4910 acrylic tape and the PET frames. 3D-printed reinforcements are used to impart anisotropy in the specimen. The findings of the model solutions provide preliminary insights on the alteration of the initial and final configurations of the DEMES affected by different geometrical parameters. It is observed that the shape of the electrode (rectangular, circular and triangular), compliant frame (rectangular, circular and triangular) and implemented stiffeners appreciably alter the attained initial configuration, final configuration and actuation range of the DEMES actuator. In general, this investigation can find its potential use in designing the futuristic DEMES through topological optimization of the compliant electrode and frame geometry together with material anisotropy of the elastomer.

Soft dielectric elastomers (SDEs) represent a category of intelligent electroactive materials utilized in electro-mechanical actuation technology. The dynamic performance of these materials during actuation is notably affected by intrinsic factors like crosslinks, entanglements and the limited extensibility of polymer chains. In this paper, we provide a theoretical framework for modeling the dynamic behavior of a balloon actuator made up of soft dielectric elastomer. To account for the inherent characteristics of polymer chain networks, we employ a physics-based nonaffine material model proposed by Davidson and Goulbourne. The governing equation for dynamic motion is established using Euler-Lagrange's equation of motion for conservative systems. The reported dynamic modeling framework is then utilized to explore the transient response, stability, periodicity and resonance properties of a dielectric elastomer balloon (DEB) actuator for varying levels of polymer chain crosslinks, entanglements and finite extensibility parameters. To assess the periodicity and stability of the nonlinear vibrations exhibited by the DEB actuator, we present Poincaré maps and phase-plane plots. The results demonstrate that changes in the density of polymer chain entanglements lead to transitions between quasi-periodic and aperiodic vibrations. These findings represent an essential initial step toward the design and production of intelligent soft actuators with diverse applications in future technologies.