Experimental and numerical investigations on the reinforced hyperelastic composites for soft actuator applications

Abstract

Dielectric elastomer actuators (DEAs) are widely used in soft robotics as they are flexible, light weight, resembles natural muscle, induce large strain, have high energy density and quick response time. The DEA is a capacitive structure consists of a pre-stretched elastomeric film sandwiched between two compliant electrodes. On applying voltage, compressive stresses are developed due to the charge accumulation at top and bottom electrode and induce deformation in sandwiched elastomeric film. The deformation is converted into desired actuation using varying actuator configurations such as beam bend, diaphragm, roll actuator, bow-tie etc. Fundamentally, the actuation performance of DEA is governed by the induced Maxwell stress (σ = 1/2 εoεEf 2) and stiffness of the elastomeric film. A stiffer material has lower deformation at a given σ and thus adversely affects the actuation capacity of DEA. Consequently, dielectric constant (ε) and elastic modulus (Y) are the two material parameters which provide a handle to tailor the response of DEA under applied electric field Ef. The key issues here are the low dielectric constant of existing soft elastomeric material, poor tearing strength and resistance to crack propagation. Generating complex actuations with out of plane deformation is also required for many engineering applications. This work explores the use of nano- and continuous fiber reinforced polydimethylsiloxane (PDMS) composites as an elastomeric layer in soft actuators using multitude of experimental and simulation tools. The choice of these materials is motivated by one of the most amazing creation of nature, the human skin. It has high tearing resistance and fracture toughness and can undergo large deformation without snapping. These properties are primarily attribted to the composition of skin where anisotropic fibers (collagen) are embedded in soft isotropic hyperelastic matrix (elastin). To achieve the objectives, experimental investigations are first carried out to quantify the effect of carbon nanotubes (CNTs) addition on the actuation performance of PDMS based soft dielectric elastomer actuator (DEA). Comparative analysis of experimental results shows that incorporation of optimum CNT concentration (0.05 wt%) significantly enhances the tip displacement (two times) and efficiency (three times) of pure PDMS based DEA. Increasing the CNT concentration beyond optimum level degrades the tip displacement and efficiency of bend actuator. It is shown that at optimum CNT concentration, the induced Maxwell stress compensates for the increase in stiffness of DEA. Even though use of CNT reinforced PDMS augments the actuation performance, out of plane deformation is not achieved in fabricated DEA. This is due to the isotropic nature of CNT/PDMS composites caused by the random orientation of CNTs in PDMS matrix. To exploit the coupling exists in fiber reinforced composites in DEA, PDMS matrix is infiltrated by continuous glass fibers (GF). A detailed electromechanical analysis using FE simulations is performed to optimize the fiber orientation and spacing for maximum out of plane actuation. It is observed that 45 º fiber orientations provide maximum twist of 8º. After optimizing the composition of DEA, finite element (FE) simulations are performed to investigate the effect of fiber-induced anisotropy on the notch behavior in GF/PDMS composites by modelling them as anisotropic hyperelastic skin type materials. The modified anisotropic (MA) model is used to define the constitutive behavior in FE simulations through Abaqus user defined material model UMAT. A parametric study is carried out to examine the effect of fiber orientation, notch root radius and sample geometry on the stress field ahead of the notch tip. It is shown that fibre orientation significantly influences the stress state and Jintegral at the notch.It has been proposed that the response of soft tissues and skin type materials critically depends on the orientation and re-orientation of reinforcing fiber. Here, we quantify the effect of fiber orientation on the mechanical and fracture properties of anisotropic hyperelastic glass fiber (GF)/polydimethylsiloxane (PDMS) composites through a unified experimental and computational analysis. Experimental results show that the tensile strength and fracture toughness of skin type materials depends on the initial orientation of fiber families and is maximum for 0-90 fiber orientation. The re-orientation of fibers under tensile loading is experimentally demonstrated. The experimental findings are well complimented by the finite element (FE) simulations performed using bilinear strain stiffening fiber and matrix (BLFM) material model for soft tissues. The material model parameters are obtained by fitting the experimental data for a meaningful comparison. It is observed that fiber rotation along the loading direction indeed enhances the resistance to crack growth in skin type materials. The size of process zone (RC) ahead of the crack tip and relative contribution of anisotropic energy is characterized using J integral and average strain energy density (ASED) criteria. The RC and anisotropic energy contribution (ϕaniso-avg/ ϕavg) depends on the fiber orientation with higher values for 0-90 fiber orientation. The larger process zone enhances the fracture toughness for 0-90 orientation in comparison to other orientations. Moreover, RC and ϕanisoavg/ ϕavg are found to be independent of initial crack length and therefore, they can be considered as material parameters for a given fiber orientation. It is envisaged that the findings of the present investigation will be a significant step in designing artificial skin type materials for soft actuator, gripper and bio-medical applications with enhanced toughness and tearing strength.