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This article presents the design and fabrication of a soft gripper made of hyperelastic materials and driven by pneumatics for application in a bento box robot system. The soft fingers are designed based on material properties and morphological studies. The design prototypes are then simulated using the finite element method in Abaqus software to predict the actual behavior of the gripper. Finally, the design prototypes are fabricated using molding and 3D printing methods, going through 5 versions, and are tested to evaluate their ability to grasp various food items without causing spillage or damage, as required by the company. [ABSTRACT FROM AUTHOR]

In this study, an analytical solution has been developed to examine the mechanical behavior of an incompressible functionally graded hyperelastic cylinder subjected to simultaneous extension and torsion. The recently proposed exp-exp strain energy density is employed to predict the behavior of hyperelastic material, and its related material parameters are assumed to vary along the radial direction in an exponential fashion. Finite element analysis is conducted by preparing a userdefined UHYPER subroutine in ABAQUS to evaluate the proposed analytical solutions. FEM results and those of the analytical solution are in good agreement for various stretches and twists and reveal that the form of stress distributions and the maximum stress depend on the exponential power in the material variation function. In contrast to axial stretch, the effect of twist on the distribution of longitudinal stress is more complicated, and for large twists, two extrema in the stress distribution plot are observed, which move toward the center and outer surface of the cylinder on further twisting. Moreover, the longitudinal stress controls the variation of von-Mises and strain energy density throughout the radial direction. Additionally, considering an axial stretch, a point is identified where the axial force arising from torsion is compressive for stretches below this value, and it brings about the cylinder to elongate under twisting. However, this part of the total axial force varies from a tension state to a compression one for larger stretches, i.e., by increasing the twist, the cylinder first tends to shorten and then elongates on further twisting. [ABSTRACT FROM AUTHOR]

Some soft tissues of the human body and biomaterials are considered as multi-layer structures with layer-specific material properties. Identification of the material parameters of these distinct layers is challenging and has attracted considerable attention in recent decades. In this research, an inverse finite element method, which uses the data obtained from simulated suction experiments, is developed. The method employs a limited number of surface-measured data to recover the layer-specific hyperelastic material parameters of a two-layer structure. The Gauss-Newton method is employed to minimize a cost function, which is defined in terms of the difference between measured and calculated displacements of the surface points. The sensitivity analysis during the optimization process is carried out by the finite difference method. The effects of pipette diameter, noise in experimental data, the initial guess and the hyperelastic material model, on the robustness and efficiency of the inverse method are investigated. [ABSTRACT FROM AUTHOR]

Summary: This work proposes a higher‐order unified shell finite element for the analysis of cylinders made of compressible and nearly incompressible hyperelastic materials. The nonlinear governing equations are derived employing the Carrera unified formulation (CUF), thanks to which it is possible to build shell elements with the capability to capture three‐dimensional (3D) transverse and out‐of‐plane effects. The material and geometric nonlinearities are expressed in an orthogonal curvilinear reference system and the coupled formulation of hyperelastic constitutive law is considered. The principle of virtual work and a total Lagrangian approach is used to derive the nonlinear governing equations, which are solved by a Newton–Raphson scheme. The numerical investigations deal with a curved arch and both thick and thin cylinders subjected to line and point loadings. The obtained results are validated by comparing them with those from the literature. They demonstrate the reliability of the proposed method to analyze compressible and incompressible hyperelastic shell structures. [ABSTRACT FROM AUTHOR]

Pipelines have been increasingly used as an efficient and economic means for the transportation of large quantities of resources such as water, fuel and gases. While there are various modes of transporting resources, pipeline systems happen to be among the safest. In the present study, the integrity of different components of the distribution network is checked for the sustainability of its designated pressure. The rubber gasket used as a sealing element in the distribution system is made from ethylene-propylene diene monomer (EPDM) and is tested to determine its stress-strain behavior, which is further analyzed in finite element analysis. From the study, it is concluded that the Mooney-Rivlin nine-parameter model is best suited for EPDM, and full analysis of the joint shows that it can sustain the designated pressure without failure. In this way, a new class of pipe can be designed without an experimental setup, which is very costly and requires considerable space. This will revolutionize the distribution field and save the surrounding environment affected by leakage and failure. [ABSTRACT FROM AUTHOR]

This paper presents static modelling and simulation of a spherically-shaped hyperelastic balloon actuator subjected to an angular rotation with an internally applied air pressure. These actuators are extensively used in soft robotics because its safe and flexible nature. The balloon actuator is a pneumatic-type actuator made of a polymeric material. A continuum mechanics-based analytical modelling and Finite element method-based simulation are performed to predict the response of the actuator for a given angular rotation with internally applied air pressure. The proposed modelling framework is subsequently utilised to perform the parametric studies for varying pressure, thickness, and rotational speed of the actuator. Various elastic instability curves are also obtained to examine the critical inflation of the rubber balloon actuator. The analytical findings agree well with the FEM simulations. [ABSTRACT FROM AUTHOR]

We proposed an approach for interpretable data‐driven modeling of an isotropic incompressible hyperelastic membrane deformation. The approach is based on the response functions in terms of the Laplace stretch and the finite element method, where response functions are partial derivatives of a hyperelastic potential with respect to the chosen strain measure. The Laplace stretch as the strain measure allows us to recover directly the response functions from experimental data and construct automatically data‐driven constitutive relations. All needed formulas were obtained explicitly. We tested our approach for membrane inflation with data‐driven constitutive relations based on the perforated membrane extension tests. [ABSTRACT FROM AUTHOR]

Fracture of compliant materials is preceded by large deformations that reshape initially sharp cracks into rounded defects. This phenomenon, known as elastic crack blunting, is peculiar of rubber‐like polymers and soft biological tissues, such as skin, vessel walls, and tendons. With this work, we aim to provide a discussion on crack‐tip blunting and its implications in terms of tearing resistance and flaw tolerance of soft elastic materials. The characteristic features of the crack‐tip zone in the framework of nonlinear elasticity are reviewed analytically and with the help of finite element analyses on pure shear cracked geometries. Specifically, the strain‐hardening behavior typical of soft biological tissues is addressed, and we illustrate its effect on crack‐tip blunting, in terms of a local radius of curvature at the crack tip. A simplified geometrically nonlinear model, proposed to describe the progressive blunting at the crack tip and its effect on flaw tolerance, is validated through finite element analyses and experimental tests on silicone samples. We show how this can lead to a simplified criterion to define the fracture condition in nonlinear soft materials. [ABSTRACT FROM AUTHOR]

This study describes the basic design, material selection, fabrication, and evaluation of soft pneumatic actuators (SPA) for lower-limb rehabilitation compression therapy. SPAs can be a promising technology in proactive pressure delivery, with a wide range of dosages for treating venous-related diseases. However, the most effective design and material selection of SPAs for dynamic pressure delivery have not been fully explored. Therefore, a SPA chamber with two elastomeric layers was developed for this study, with single-side inflation. The 3D deformation profiles of the SPA chamber using three different elastomeric rubbers were analyzed using the finite element method (FEM). The best SPA-compliant behavior was displayed by food-grade silicone A10 Shore with a maximum deformation value of 25.34 mm. Next, the SPA chamber was fabricated using A10 Shore silicone and experimentally validated. During the simulation in FEM, the air pressure was applied on the inner wall of the chamber (i.e., the affected area). This is to ensure the applied pressure was evenly distributed in the inner wall while the outer wall of the chamber remained undeformed for all compression levels. During the inflation process, pressure will be applied to the SPA chamber, causing exerted pressure on the skin which is then measured for comparison. The simulation and experimental results show an excellent agreement of pressure transmission on the skin for the pressure range of 0–120 mmHg, as depicted in the Bland–Altman plots. The findings exhibited promising results in the development of the SPA chamber using low-cost and biocompatible food-grade silicone. [ABSTRACT FROM AUTHOR]

• Three amplifier shapes and their impact on the dynamics and efficiency of EH are proposed. • Model analyzes are performed in a wide range of excitation parameters. • A method for calculating the EH efficiency index is proposed. • EH efficiency is increased by 2 to 6 times compared with the classic TEH design. This paper presents research on a new type of energy harvester consisting of two parts – the classical tristable system based on a flexible beam and permanent magnets, supplemented by the second part: an additional nonlinear support made of ethylene propylene diene monomer (EPDM). This support is used to amplify the vibration amplitude. As part of this work, laboratory tests on the EPDM material are performed to determine the strain curve. Then, finite element modeling (FEM) software is applied to determine the characteristics of the designed three shapes of the amplifiers and the efficiency of energy harvesting from the vibrations is tested based on the developed dimensionless model. The results are presented with regard to the effective value of the voltage induced on the piezoelectric electrodes and the efficiency factor that compares the tristable energy harvester with the structure developed in this paper. The obtained results allow for the conclusion that the average values of harvested energy over the entire frequency range ω characterize the system, in which the amplifier is two to six times more effective than the system without it. The best results are achieved for a solution based on a full cross-section. Moreover, the introduction of a nonlinear mechanical amplifier caused the classic system based on a magnetic section to obtain the so-called broadband effect over a wide spectrum of frequencies. [ABSTRACT FROM AUTHOR]

• A total Lagrangian Galerkin free element method is proposed for finite deformation. • Hyperelastic materials are considered calculated by the Galerkin free element method. • 2D and 3D nonlinear problems are solved to verify the accuracy. • Favorable results can be obtained using the proposed method for the nearly incompressible materials. In this research, a total Lagrangian Galerkin free element method (GFrEM) is proposed for the analysis of finite deformation in hyperelastic materials. This method derives the total Lagrangian formulation using the initial configuration as the reference. The mechanical behavior of hyperelastic materials is modeled by the non-Hookean strain energy function. Since Lagrangian isoparametric elements are freely formed in GFrEM by collocation nodes with their surrounding nodes, intrinsic boundary conditions can be imposed simply as in the finite elements method. In addition, the Galerkin method was used to ensure the stability of the results when constructing the equations for each collocation node. The validity and convergence of the proposed method are verified by several two- and three-dimensional numerical examples that include bending, compression, and torsion of hyperelastic materials. The example of nearly incompressible material shows that GFrEM remains highly accurate even with large deformations where the FEM cannot converge. [ABSTRACT FROM AUTHOR]

The deformation behavior of an artificial heart valve was analyzed using the explicit dynamic finite element method. Time variations of the left ventricle and the aortic pressure were considered as the mechanical boundary conditions in order to reproduce the opening and closing movements of the valve under the full cardiac cycle. The valve was assumed to be made from a medical polymer and hence, a hyperelastic Mooney–Rivlin model was assigned as the material model. A simple formula of the damage mechanics was also introduced into the theoretical material model to express the hysteresis response under the unloading state. Effects of the hysteresis on the valve deformation were characterized by the delay of response and the enlargement of displacement. Most importantly, the elastic vibration observed in the pure elastic response under the full close state was dramatically reduced by the conversion of a part of elastic energy to the dissipated energy due to hysteresis. [ABSTRACT FROM AUTHOR]

• Finite-element analysis offers insights into needle-TM interactions for middle-ear interventions. • Combined 2D and 3D FE modeling strategy reveals TM deformation during needle insertion. • First-order Ogden hyperelastic model accurately captures TM mechanical nonlinearity. • TM thickness, needle geometry, and material impact needle insertion force and success. • Methodology aids in developing therapeutic and diagnostic technologies for hearing and balance pathologies. The tympanic membrane (TM) is one of the most common routes to access the middle ear and inner ear for the treatment of hearing and balance pathologies. Since the TM is a soft thin biological tissue with small dimensions, using needles seems to be among the most practical interventional approaches. In this study, we proposed a finite-element (FE) analysis of needle-TM interactions that combines a 3D model of the TM and other main middle-ear structures in gerbil, and a 2D model of needle insertion into the TM based on the cohesive zone method (CZM). The TM was modelled using a 1st-order Ogden hyperelastic material and its properties were obtained by fitting to the experimental force-displacement plots of large deformation in the TM under needle indentation. The cohesive parameters were also acquired by calibrating the puncture force against the experimental data of needle insertion into the TM. These FE models were then used to obtain the deformation behaviour of the TM and other middle-ear structures due to the insertion force applied at different locations on the TM. Moreover, we investigated the effect of the TM thickness, the geometry of the needle (i.e., diameter and tip angle), and needle material on the insertion of needles into the TM. We also studied the penetration success of deformable needles. [ABSTRACT FROM AUTHOR]

Elastomers are widely used in various engineering applications due to their huge elasticity and good dynamic behavior. One of the elastomeric elements is rubber blocks, which are used in many applications, such as vibration isolators, bumpers, shock absorbers, dampers, etc. In this research, the impact on the ride index due to the deflated state of the air spring with various kinds of hyperelastic materials is analyzed by the finite element method. The stress–energy function of rubber materials is first diagnosed, and Poisson's ratio defined the volumetric terms. The rubber isolator's stress is studied based on the finite element model, and the structure is improved. It is observed that for the deflated state of the air spring, laminated rubber isolation diminishes the muscular amount of structural responses compared to the conventional rubber base structures, and improves the ride comfort. This study would assist in doing finite element analysis of the secondary suspension system and other vibration-damping components. [ABSTRACT FROM AUTHOR]

In this paper, we present a computationally efficient cellular mathematical model that accounts for the boundary collective behavior of a cell group by hepatocyte growth factor. The large cell group is modeled using continuum-based finite elements with incompressible hyperelastic materials for the nonlinear elastic behaviors. The total Lagrangian formulation is used enabling for large deformations, and the explicit time integration scheme without the Newton-Raphson iterative solution required for a time step is adopted to model the dynamics of the collective cell migration. With the explicit time integration and low order finite elements under the total Lagrangian framework, the proposed model is much computationally efficient for modeling the dynamic mechanical behavior of a cell colony. Detailed comparison to the experimental data shows that the proposed mathematical model provides a quantitatively accurate description of the collective cell motion in three different concentrations of hepatocyte growth factor. [ABSTRACT FROM AUTHOR]

• A robust beam element is developed based on the 3D exact deformation field. • The generalized hyperelastic Rivlin model is integrated into the FE model. • A general unit cell (UC) is designed to simulate all hexagonal lattice structures. • The geometrical parameters effect of UC on the mechanical properties are studied. • UC with a broad range of large and negative Poisson's ratios are studied. • The lattices under large deformation are analyzed using experiment and FE model. This paper addresses the intricate analysis of lattice structures, pivotal components in engineering applications, confronted by challenges arising from their diverse unit cells (UCs) and complex behavior across micromechanical and macro-scale dimensions. The present study deals with a comprehensive analysis of lattice structure with honeycomb and re-entrant auxetic unit cells under large deformation via both a robust finite element analysis (FEA) and experimental tests. The proposed FEA is developed based on the hyperelastic Mooney–Rivlin strain energy function and the novel exact motion field that can fully describe the projection of cross-section. The results demonstrate that mechanical parameters such as Young's modulus, Poisson's ratio, and shear modulus have significant nonlinear behaviors with respect to UC geometrical parameters that are crucial for optimization across varied operational conditions. Additionally, the lattices made of TPU material and fabricated by Fused Deposition Modeling are tested under three-point bending and compression considering contact interaction. The results reveal highly nonlinear responses due to instabilities in some links and material nonlinearity. Furthermore, the behavior of lattice structures is exceedingly dependent on the orientations and types of UCs. It also can be found that both of the proposed FEA and constitutive model are in good agreement with experimental data. [Display omitted] [ABSTRACT FROM AUTHOR]

Rubber is one of the most important materials of modern industry and have a wide range of usage owing to a number of superior properties. In this study, the mechanical behaviour of the metal-rubber combination pulley part transferring motion in the engine through the timing belt was examined by the finite element method, confirmed experimentally, and optimized. Uniaxial tension and pure shear tests were carried out physically to develop a hyperelastic material model. 3 parameter Mooney-Rivlin hyperelastic material constants were calculated by using force-elongation values obtained from uniaxial tension and, pure shear tests. Afterwards, pulley geometry was modelled, and displacements, stresses, reaction moments were examined under various torsional loadings. To confirm the accuracy of the pulley analysis, a servomotor driven pulley torsion testing machine was designed and manufactured. Then, torque and angle values were measured by testing the pulley parts at the same torsional angles. By this study, it was proven that mechanical behaviour of pulley can be expressed numerically. After validation of the material model and the results obtained using analysis method, the pulley geometry was further improved the stress levels on the rubber material were reduced 28% under the same loading conditions by using optimization tools. [ABSTRACT FROM AUTHOR]

In automotive industries, leakage is one of the major problem reducing the efficiency in hydraulic and pneumatic system. The leakage in a device can be identified only during the physical test, once after the product is developed, leading to increased development time and cost. The leakage is purely based on the type of sealing element (O-rings) and sealing pressure. Since the sealing elements are hyperelastic and exhibit highly nonlinear behavior, there is no standard formulation available to predict the sealing pressure. It can be predicted using finite element analysis (FEA) in the design stage itself. One of the main inputs for the finite element analysis is the exact material parameter of the sealing element. This article aims at determining the sealing element material parameter using stress–strain data generated from uniaxial compression test and sealing pressure considering different hardness using finite element analysis. To generate the stress–strain data, compression force is applied on the test specimen at the rate of 12 mm/min and compressed up to 25% of its initial height with help of uniaxial compression test machine as per ASTM D 575. In this article, O-ring is considered as sealing element with hardness ranging from 40 IRHD to 90 IRHD. [ABSTRACT FROM AUTHOR]

• Closed-form analytical solution for extension-torsion of a hyperelastic cylinder. • Analytical approaches for invariant and stretch based strain energy functions. • Finite element method is implemented in extension-torsion of hyperelastic material. • Exponential-exponential constitutive model employed in extension-torsion. • Comparing various forms of strain energy functions. In this paper, the mechanical response of an incompressible isotropic nonlinear elastic solid circular cylinder is investigated under combined extension and torsion. Since the deformation tensor in extension-torsion is non-diagonal, implementing stretch based energy functions are complex. Hence, in this study, an analytical solution is proposed for both invariant- and stretch-based models. Moreover, finite element analysis of extension-torsion of hyperelastic materials is carried out using both UHYPER and VUMAT user-defined subroutine in ABAQUS to verify the presented analytical methods. Both stretch and invariant-based exponential forms of strain energy function are employed, and its corresponded material parameters are calibrated for silicon-rubber. The finite element results for stress distribution show a good agreement with analytical findings which confirms the validity and accuracy of the proposed method. Results show that the both invariant- and stretch-based exp-exp models are less conservative for a relatively small stretch and twist while for large stretches they yield higher stresses than the other models and the rate of stress variation increases significantly, especially for a stretch-based exp-exp model. For both Invariant- and stretch-based exp-exp models, an alteration axial stretch is identified where the torsion induced axial force alters from tensile to compressive. For stretches smaller than this alteration point, the cylinder always elongates while for the larger stretches, the cylinder tends to shorten for small twists and then elongates on further twisting. Also, as the axial stretch increases the alteration point appears in larger twists. [ABSTRACT FROM AUTHOR]

Summary: A numerical multifield methodology is developed to address the large deformation problems of hyperelastic solids based on the 2D nonlinear elasticity in the compressible and nearly incompressible regimes. The governing equations are derived using the Hu‐Washizu principle, considering displacement, displacement gradient, and the first Piola‐Kirchhoff stress tensor as independent unknowns. In the formulation, the tensor form of equations is replaced by a novel matrix‐vector format for computational purposes. In the solution strategy, based on the variational differential quadrature (VDQ) technique and a transformation procedure, a new numerical approach is proposed by which the discretized governing equations are directly obtained through introducing derivative and integral matrix operators. The present method can be regarded as a viable alternative to mixed finite element methods because it is locking free and does not involve complexities related to considering several DOFs for each element in the finite element exterior calculus. Simple implementation is another advantage of this VDQ‐based approach. Some well‐known examples are solved to demonstrate the reliability and effectiveness of the approach. The results reveal that it has good performance in the large deformation problems of hyperelastic solids in compressible and nearly incompressible regimes. [ABSTRACT FROM AUTHOR]

In this paper, a novel higher stabilization-free virtual element method is proposed for compressible hyper-elastic materials in 2D. Different from the most traditional virtual element formulation, the method does not need any stabilization. The main idea is to modify the virtual element space to allow the computation of a higher-order polynomial L 2 projection of the gradient. Based on that the stiffness matrix can be obtained directly which greatly simplifies the analysis process, especially for nonlinear problems. Hyper-elastic materials are considered and some benchmark nonlinear problems are solved to verify the capability and accuracy of the stabilization-free virtual element method. [ABSTRACT FROM AUTHOR]

Highlights • Rupture tests are conducted on sharp V-notched styrene-butadiene rubbers (SBRs). • The averaged strain energy density (ASED) criterion is extended to notched rubbers. • The ASED criterion is then employed to assess rupture condition in tested SBRs. • Good consistency between the ASED estimations and the experiments is obtained. Abstract The rupture behavior of styrene-butadiene rubbers (SBR) in the presence of a V-shape notch is investigated for the first time both experimentally and theoretically. In the experiments, V-notched samples of SBR are tested under tensile loading and their rupture displacements are determined. Afterwards and in the analytical field, the rupture loads of tested rubbers are predicted using the averaged strain energy density (ASED) criterion. The key idea of this criterion (i.e. the almost uniaxial state of stress field near the notch tip) is verified through non-linear finite element modeling. It is shown that good agreement exists between the predictions of the ASED criterion and the experimental results obtained for SBR. Moreover, the microscopic study of the ruptured surfaces of the notched SBR demonstrates its high roughness which can be attributed to the resistance of the rubber chains against the crack growth. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Purpose The purpose of this study is to provide a novel multi-fingered hand made of hyperelastic material. This kind of hand has the advantage of less mechanical parts, simpler control system. It can greatly cut down the complexity and cost of the hands under conditions of ensuring enough flexibility of grasping.Design/methodology/approach Based on the principle of virtual work, the equations of pulling force and grasping force are derived. To get the max grasping force, the optimal structural dimensions of the hand are obtained by finite element simulations. Hand’s grasping experiment is conducted.Findings The factors influencing grasping force and grasping stability are identified, and they are the length between short poles around the knuckles and the height of short poles. Experimental results show that the max strain of knuckles is less than the elastic limit of hyperelastic material, and the presented hand is practicable. The adaptive ability and grasping stability of the presented hand are demonstrated.Originality/value A novel multi-fingered hand made of hyperelastic material is presented in this paper. By designing the thickness of every section of a hyperelastic plate, the knuckle sections will bend and other sections of the plate will remain straight, and thus, the multi-fingered hand will grasp. [ABSTRACT FROM AUTHOR]

Structural bonding with so-called elastic adhesives like polyurethane or silicone adhesives is increasingly being used for various applications in the automotive, transportation, and building industry. Elastic adhesives can bond dissimilar materials such as steel, glass or plastics together. Elastic adhesives can withstand large levels of deformation at failure and are therefore very useful to compensate movements coming from crash, fatigue or thermal loadings which can weaken the surrounding structure. In order to design elastic adhesives using numerical methods such as Finite Element analysis, it is crucial to be able to predict the mechanical behavior of the adhesive joint accurately. This implies to characterize the behavior of elastic joints submitted to real situations such as multi-axial loadings and large deformations. This paper presents the development of experimental methods based on Digital Image Correlation techniques to measure the local multi-axial deformation behavior of a polyurethane adhesive. Experimental results are then used to identify constitutive parameters of hyperelastic materials models that make it possible to predict the non-linear elastic behavior of the adhesive up to large levels of deformations under multi-axial loading conditions and prior to failure using FE simulation. The methodology is validated by testing and simulating specimens which geometry are representative of bonded joints under service conditions. [ABSTRACT FROM AUTHOR]

Biological cell studies have many applications in biology, cell manipulation, and diagnosis of diseases such as cancer and malaria. In this study, Inverse Finite Element Method (IFEM) combined with Levenberg-Marquardt optimization algorithm has been used to extract and characterize material properties of mouse oocyte and embryo cells at large deformations. Then, the simulation results have been validated using data from experimental works. In this study, it is assumed that cell material is hyperelastic, isotropic, homogenous, and axisymmetric. For inverse analysis, FEM model of cell injection experiment implemented in Abaqus software has been coupled with Levenberg-Marquardt optimization algorithm written in Matlab; through this coupling, the optimum hyperelastic coefficients, which give the best match between experimental and simulated forces, are extracted. Results show that among different hyperelastic material models, Ogden material is suitable for characterization of mouse oocyte cell and Mooney-Rivlin or polynomial is suitable for characterization of mouse embryo cell. Moreover, the evaluated Poisson ratio of the cell is obtained to be equal to 0.5, which indicates that the structural materials of mouse oocyte and embryo are compressible. [ABSTRACT FROM AUTHOR]

Atomic force microscopy (AFM) indentation is the most suitable way to characterize micromechanical properties of soft materials such as bio tissues. However, the mechanical data obtained from force-indentation measurement are still not well understood due to complex geometry of the bio tissue, nonlinearity of indentation contact, and constitutive relation of hyperelastic soft material. Poly-N-isopropyl acrylamide (PNIPAM) filled with 5 wt% polystyrene (PS) sphere particles material system can be utilized as a simplified model for mimicking a whole host of soft materials. Finite element model has been constructed to simulate indentation as in AFM experiments using colloidal probes for a parametric study, with the main purpose of understanding the effect of particles on overall behavior of mechanical data and local deformation field under indentation contact. Direct comparison between finite element simulation and indentation data from AFM experiments provides a powerful method to characterize soft materials properties quantitatively, addressing the lack of analytical solutions for hard-soft composites, both biological and synthetic ones. In this framework, quantitative relations are found between the depth, at which the particle was embedded, the particle size and the elastic modulus of the overall composite. Comprehensive characterizations were established to distinguish indentation on a particle residing on top of the hydrogel from a particle embedded inside the hydrogel matrix. Finally, different assumptions of interface friction at the boundary between the particle and the hydrogel have been tested and directly compared with experimental measurements. [ABSTRACT FROM AUTHOR]

In insulation joints, elastomeric U-shaped monolithic seals (UMSs) are replacing O-ring systems because of their enhanced sealing capabilities for the oil and gas industries. UMSs are compressed axially during assembly and radially when pressurized in operation. The reliability of UMSs due to the displacement imposed during assembly and the internal pressure in operation is influenced by the axial compression ratio, thickness ratio (TR), and geometric complexity. In this study, the hyperelastic behavior of elastomeric UMSs under axial and radial compressions is investigated using axisymmetric finite-element analysis. Twelve examples of UMSs with three geometric restraints (open grooves on both sides (type 1), an open groove on one side only (type 2), and no groove (type 3)) and four thickness ratios (TR = 0.25, 0.50, 1.00, and 1.50) are evaluated. To analyze nonlinear elastomeric materials, neo-Hookean constitutive equations are applied and the UMSs are considered as being a nearly incompressible hyperelastic material with a Poisson's ratio of 0.499. The failure and detachment risks of UMSs are analyzed in terms of the equivalent stress, gap distance, contact pressure, and strain energy density. It is advantageous that the smaller the TR, the smaller the stress distribution. However, the generation of broader detachment regions is observed. Type 1 symmetrically shows the lowest stress distribution and the smallest detachment region, whereas type 3 symmetrically shows the highest values. Type 3 (TR = 0.25) shows the broadest detachment region in the arc-length range from -15.7 to 15.7 mm, whereas the largest gap of 0.7 mm is observed in type 2 (TR = 0.5). For all types, the detachment region disappears completely at TR = 1.0 or higher, which implies that full sealing is occurring. The average contact pressure increases exponentially during axial compression (in assembly) and linearly during radial compression (in operation). The largest contact pressure of 31.5 MPa is observed in type 3 (TR = 1.5), while the lowest is observed in type 1 (TR = 0.25). As for the strain energy density, type 3 at TR = 0.25 shows the largest increase in the strain energy density with 1.75 MJ/m3, while type 1 shows the most stable values of all cases. In conclusion, the lowest risk of failure of a nonlinear hyperelastic UMS was investigated numerically with minor equivalent stress and detachment region with higher contact pressure, which can be taken into account to ensure the reliability of the UMS. [ABSTRACT FROM AUTHOR]

The problem of nonlinear bending of a curved tube made of incompressible rubber-like material is considered. The tube shaped like a portion of a thin-walled toroidal shell between two radial planes is inflated by pressure and then subjected to in-plane bending moments. To investigate nonlinear response and stability of the tube under these loading conditions, a finite-element approach is proposed. A special shell finite element is formulated under the assumption of uniform deformation along the tube length. The effect of wrinkling on nonlinear response of the tube is described using the tension-field theory. A change in the inflating pressure resulting from deformation of the tube due to bending is taken into account in the formulation of the governing equations. The effect of pressure on the bending stiffness, stability, and deformations of a curved tube is examined and discussed. [ABSTRACT FROM AUTHOR]