Search Results (319)

Soft robotic manipulators exhibit pronounced nonlinearities due to, for instance, hyperelastic materials and fluidic actuation. Hysteresis is one of the most challenging effects to model and compensate and can significantly degrade tracking accuracy. While data-driven and model-free techniques for hysteresis compensation have been explored, they require extensive experimentation that accelerates material fatigue and reduces long-term reliability. This work presents a model-based control scheme that estimates and compensates, in real time, the hysteretic elasticity of a soft manipulator. A state observer is designed to estimate the internal state in a Bouc–Wen hysteresis model. This is combined with a nonlinear controller designed with a Lyapunov-based approach. Simulation and experimental results demonstrate improved performance and better disturbance rejection compared to an adaptive baseline controller. Full article

Resilient wheels are widely employed in metro vehicles to mitigate vibration and noise, in which rubber damping components play a critical role in load transmission and fatigue resistance. However, stress concentration and cyclic loading can significantly compromise their durability and service life. In this study, the structural optimization and fatigue life of rubber damping components in resilient wheels are systematically investigated based on finite element analysis and in-service metro operational data. A three-dimensional finite element model incorporating hyperelastic material behavior is developed to evaluate stress distributions under three representative conditions: press-fit assembly, straight-line operation, and curved-track operation. Based on the resulting stress fields, critical high-stress regions within the rubber component are identified and selected as targets for structural optimization. The Design of Experiments (DOE) methodology, integrated with the Isight 2022 optimization platform, is employed to determine the optimal geometric parameters that minimize the von Mises equivalent stress. Furthermore, a fatigue life prediction framework is established using actual metro service mileage data. Fatigue performance is assessed using Fe-safe 2022 software in conjunction with rubber fatigue crack propagation theory, and the results before and after optimization are systematically compared. This study demonstrates that stress concentrations in resilient wheel rubber damping components predominantly occur at fillet transition regions, governed by load transfer characteristics under press-fitting and service conditions. Through DOE-based structural optimization, the critical geometric parameters are effectively refined, leading to a significant reduction in stress levels in key regions. As a result, the proposed approach markedly improves fatigue performance, extending the minimum fatigue life from 1300 days to 24,322 days, thereby substantially enhancing the durability and reliability of the resilient wheel system. Full article

Numerical predictions of the wrinkling behavior of biaxially fiber-reinforced elastomer sheets are carried out under consideration of finite deformations. The Holzapfel–Gasser–Ogden material model is used to account for the anisotropic hyperelastic material behavior of the sheets, where material parameters are identified based on experimental data of tensile tests from literature. A Finite Element Method-based simulation strategy is presented to extract critical loading conditions and to access the postbuckling response using geometrical imperfections. Depending on the layup and aspect ratio of the sheets, wrinkling onset was predicted for global stretches between 10% and 25%. For sheets with fiber orientations [±45°] wrinkling is predicted at larger global stretches than for sheets with fiber orientations of $[+{30}^{\circ }/-{60}^{\circ }]$ for the same aspect ratio. Furthermore, it is shown that short sheets have a tendency towards symmetric wrinkling patterns whereas for long sheets asymmetric wrinkles are more likely to occur. Comparison of the numerical predictions with experiments from the literature shows that the geometrical characteristics of the wrinkles, such as wavelengths and amplitudes, can be well predicted. Far into the postbuckling regime, the deviations of the predicted wrinkling amplitudes and their experimental counterparts are around 30% or less. Full article

Fibrous natural-origin materials are not only attractive as raw materials in various applications but are also often produced as waste products in some manufacturing processes. Despite their comprehensive implementation as thermal or noise isolation materials, their behavior under mechanical load is not yet fully understood, and there are no assignments of existing universal material models for the category of fibrous materials. The conducted experimental research provides a methodology with which to evaluate the structural behavior of fibrous materials under applied compression force and classify these materials according to their mechanical properties based on a certain material model. As a result of this research, we observed that the mechanical properties of the fibrous material during compression mode are determined by the fibrous structure, with insignificant influence from the physical nature of the material itself. This investigation provides an analysis of the application of a hyperelastic incompressible isotropic model to fibrous material of different origins. Hyperelastic material models of the Money–Rivlin, Ogden, Yeoh, and polynomial type were implemented. The fitting quality of the Yeoh third-order model obtained the best fitting results for animal wool and mineral wool. Cotton wool showed the best fitting results with the polynomial fifth-order model. The outcome of this research will help create finite element models for structural analysis, efficiently modelling structural responses to vibration or noise. For most animal and mineral wool samples, the best agreement with the experimental compression curves was obtained using the Yeoh third-order hyperelastic model, with coefficients of determination R² between 0.979 and 0.996, while fifth-order polynomial fits locally reached R² up to 0.9999 for aged cotton wool. Full article

In the field of orthopedic surgery, particularly distraction osteogenesis (DO), the mechanical environment plays a decisive role in the quality of bone regeneration and the safety of the soft tissue envelope. The continuous monitoring and accurate prediction of distraction resisting forces (DRF) are critical for preventing soft tissue complications such as nerve ischemia, joint contractures, and mechanical failure of the lengthening device. However, current clinical practice relies heavily on subjective assessment or passive monitoring tools that lack predictive capabilities. To address this gap, this study proposes a comprehensive solution combining a custom mechanical acquisition system with a high-fidelity finite element (FE) prediction method. The system design features a novel “double-ring” sensor interface specifically engineered to decouple axial distraction forces from parasitic bending moments generated by asymmetric muscle tension. Furthermore, a patient-specific FE model utilizing the Ogden hyperelastic constitutive law was derived, explicitly based on the patient’s muscle volume from preoperative CT imaging, to predict the non-linear force evolution. The feasibility and accuracy of the system were validated in a pilot in vivo study using a single ovine model ($N=1$). To isolate the soft tissue resistance from callus formation, distraction was performed immediately postoperatively up to a total length of 4 cm. Experimental results demonstrated the system’s high linearity ($R^2>0.999$) and its ability to capture the characteristic viscoelastic relaxation of living tissues. The FE model successfully predicted the peak distraction forces, showing improved agreement with experimental data at larger distraction magnitudes. By integrating mechanical sensing with predictive modeling, this framework lays the foundation for future closed-loop, patient-specific control in distraction osteogenesis. Full article

Silicone rubber is extensively used in engineering applications due to its toughness and impact resistance; however, traditional characterisation methods fail to capture its nonlinear deformation characterisation and triaxial mechanical behaviour. To address this, we derived a constitutive model within the framework of continuum mechanics that assumes a condition of near incompressibility and conducted uniaxial, planar, and equibiaxial tension tests to fit the model parameters. Through systematic analysis of triaxial mechanical responses under these three loading modes, we determined the material’s nonlinear large-deformation behaviour and sensitivity to the biaxiality ratio. Comparative analyses with classical hyperelastic models show that the proposed model achieves a good balance between the number of parameters and fitting accuracy. After the parameter-fitting process, we performed finite element simulations of the three loading modes. The simulation results show good agreement with experimental data in terms of deformation patterns and stress–strain curves. This study provides a novel theoretical tool for evaluating the mechanical properties and structural designs of soft materials. Full article

Highly deformable adhesive materials are increasingly employed in engineering applications where flexibility, energy dissipation and damage tolerance are required. The mechanical characterisation of these materials, however, presents significant challenges due to their pronounced non-linear behaviour, large deformations, and, in many cases, time-dependent effects. This paper provides a critical review of experimental, constitutive and numerical approaches used for the characterisation of highly deformable adhesive materials, considered here as bulk materials independently of a specific joint configuration. The review covers mechanical testing methods under large strains, hyperelastic and visco-hyperelastic constitutive models, and the application of fracture mechanics concepts and numerical techniques as exploratory tools for material analysis and comparison. Particular attention is given to the capabilities and limitations of the different approaches, their domains of applicability and the assumptions involved in their use. By highlighting current practices, open challenges and recent developments, this work aims to support the selection of appropriate characterisation methodologies and modelling strategies for highly deformable adhesive materials. Full article

Elastomer-based nanocomposites combining polymer flexibility with conductive nanofillers provide lightweight, stretchable systems with tunable electromechanical properties for wearable electronics, soft robotics, and self-powered sensors. However, predicting their nonlinear response remains challenging because the observed piezoelectric-like response arises from strain-dependent interfacial polarization and evolving piezoresistive conduction pathways within heterogeneous microstructures. We introduce a continuum electro-hyperelastic framework combining the Mooney–Rivlin model for large-strain elasticity with a Helmholtz free-energy approach for electrostatic coupling. Analytical expressions for stress, electric displacement, and apparent piezoelectric coefficients are derived and implemented in finite element simulations. The model accurately reproduces the experimental mechanical, dielectric, and electromechanical behaviour of polydimethylsiloxane (PDMS) nanocomposites with 0.1–1 wt% graphene. These show increased stiffness, relative permittivity (from 3.4 to 4.0, ≈18%), and quasi-static d₃₃ coefficients (from −5.6 to −10.0 pC N⁻¹, ≈80% enhancement). Analytical and finite element method (FEM) results show consistent trends across the full deformation range, with Maxwell stress agreement within 10% at lower deformation levels, while deviations of 33–40% for coupled electromechanical quantities at an axial displacement u_z = ~−1 mm (~16.7% compressive strain) are attributable to three-dimensional shear effects absent from the uniaxial analytical assumption. Simulations reveal that graphene boosts Maxwell stress, yielding a four-fold increase at lower stretch ratios. This reframes PDMS–graphene composites as electro-hyperelastic materials, offering a predictive, extensible framework. It highlights apparent piezoelectricity as an emergent, tunable effect from charge redistribution in a compliant hyperelastic matrix—guiding the design of next-generation flexible devices leveraging field-induced coupling over intrinsic polarization. Full article

This work introduces an integrated Material Point Method (MPM) framework for optimizing tendon-driven hyperelastic robots under extreme 3D deformations. To overcome the mesh distortion limitations of the traditional FEM at large strains, we develop a coupled MPM–tendon hyperelastic model that integrates Yeoh constitutive laws with discrete tendon actuation forces. The model enables robust simulation of anisotropic stress propagation through Lagrangian particle tracking and Eulerian grid discretization, eliminating mesh entanglement artifacts. A strain-gradient-driven tendon path algorithm ensures mechanical efficiency using Fréchet distance-based similarity metrics and curvature smoothness screenin, enforcing spatial continuity in complex topologies. Validation demonstrates: (1) Sub 3 mm geometric errors and about 89% volumetric overlap in worm-inspired deformations; (2) optimal computational efficiency at 0.4–0.6 mm grid densities, balancing accuracy and resource overhead; and (3) projected alignment errors of 0.8 mm (XY), 1.3 mm (XZ), and 2.9 mm (YZ) in multi-view spatial analyses. The framework achieves about 89% ± 2% volumetric overlap in quadrupedal morphing via agonist–antagonist tendon optimization, demonstrating efficacy for extreme 3D deformation control. Full article

This study presents a finite element (FE) investigation of intervertebral disc (IVD) degeneration in the human lumbar spine (L1–L3 segment). The model, based on CT-derived geometry and isotropic hyperelastic representation of disc tissues, incorporates controlled simplifications, detailed in the limitations section. Degenerative changes were parametrically simulated across healthy, mild, moderate, and severe stages by reducing disc height (up to 60%), nucleus pulposus volume (up to 70%), and adjusting tissue stiffness to reflect dehydration and fibrosis. Displacement-controlled compressive loading was applied to assess von Mises stress distributions, reaction forces, and load transfer mechanisms. Results indicate significant load redistribution: annulus fibrosus stresses increased by up to 175% in severe degeneration, while nucleus pulposus stresses decreased by ~70%, indicating a diminished compressive load-bearing contribution of the nucleus. Model predictions were validated against cadaveric and in vivo data, confirming trends in intradiscal pressure (IDP) reductions (40–70%) and stress elevations. The parametric framework elucidates interactions between geometric and material changes, providing clinicians with insights into degeneration progression and guiding biomedical engineers in implant design and interventions. Full article

Hyperelastic models for the deformation of hydrogels were evaluated as alternatives to the widely used neo-Hookean model. Poly(ethylene glycol diacrylate) (PEGDA) was synthesized via photopolymerization, with precursor molecular weights from 700 to 4000 Da and synthesis concentrations between 5 and 30 wt% in water. Hydrogels are often modeled as neo-Hookean solids; this model holds only over a limited strain range. To model deformation over a broader range and seek additional insight into gel network structures, the Mooney–Rivlin, Ogden, Rubinstein–Panyukov, and Localization models were applied to uniaxial compression data and their fits assessed against “Mooney plots” of reduced stress versus the inverse extension ratio. The Ogden model best fits the stress–strain curves to higher ratios and the reduced stress plots over the broadest range of formulations. The Localization and Rubinstein–Panyukov models fit well above c*, the overlap concentration, capturing low-strain behavior and the observed maxima under compression in Mooney plots. The Mooney–Rivlin model fit the stress–strain curves but was unable to fit the reduced stress plots. The Localization and Rubinstein–Panyukov model parameters suggest that entanglements play a significant role at all concentrations, with their contribution decreasing as the network concentration increases. This demonstrates the potential of using two-parameter models to understand the deformation of hydrogels. Full article

The anatomy and mechanical strength of aortic valve leaflets are critical determinants of their biomechanical behavior and long-term structural integrity. The embryonic developmental period, when valves are forming, is critical to establish baseline leaflet properties. However, fetal stages of valve development, when valve leaflets are still forming and remodeling, are not well understood. The goal of this study is to investigate the biomechanical stress and deformation modes of developing valve leaflets during systole, and how leaflet biomechanics are affected by anatomy and material properties. To this end, the study employs a parametric approach to model the leaflet anatomy of an HH40 chick embryo, used here as a model of fetal cardiac development. To perform biomechanical analysis, a pressure profile derived from in ovo Doppler ultrasound measurements was applied, and an Ogden hyperelastic material model was employed following a sensitivity analysis. To determine the effect of valve anatomy on leaflet tissue deformation and stresses, we changed the leaflet midline curve (belly curve) from its native curvature to a linear profile and quantified biomechanical responses. Our analysis revealed a strong decrease in average leaflet effective stress as the belly curvature was shifted towards a linear profile. However, this reduction in average stress was at the expense of a biomechanical trade-off. The shift induced a progressive localization of stress concentration at the leaflet tips and commissures, and a distinct bending deformation mode at the tip under peak load. Our findings demonstrate that while the belly curve of the leaflet modulates tissue stress during valve opening, a low-stress anatomy does not align with hemodynamic performance. This work characterizes competing leaflet biomechanical responses (stress reduction versus failure modes) that shape valve leaflet formation, providing fundamental insights into developmental valve biomechanics. Full article

Phenyl-silicone rubber is the elastomer of choice for cryogenic and high-temperature static seals, yet quantitative links between thermo-oxidative aging and sealing reliability are still lacking. Here, sub-ambient (−70 °C to 25 °C) and room-temperature mechanical tests, compression set aging, SEM, FT-IR, and finite-element simulations are integrated to trace how aging translates into contact-pressure decay of an Omega-profile gasket. Compression set rises monotonically with time and temperature; an Arrhenius model derived from 80 to 140 °C data predicts 34 d (10% set) and 286 d (45% set) of storage life at 25 °C. SEM reveals a progressive shift from ductile dimple fracture to brittle, honeycomb porosity, while FT-IR confirms limited surface oxidation without bulk chain scission. Finite element analyses show that contact pressure always peaks at the two lateral necks; short-term aging increases in the shear modulus C₁₀ from 1.87 to 2.27 MPa, raising CPRESS by 8~21%, yet this benefit is ultimately offset by displacement loss from compression set (8.0 mm to 6.1 mm), yielding a net pressure reduction of 0.006 MPa. Critically, even under the most severe coupled condition (56 days aging with compression set), the predicted CPRESS remains above the 0.1 MPa leak-tightness criterion across the entire cryogenic service envelope. This framework provides deterministic boundaries for temperature, aging duration, and allowable preload relaxation, enabling risk-informed maintenance and replacement scheduling for safety-critical phenyl-silicone seals. Full article

This study first heat-treats the surface of plain-woven carbon fibers to remove the surface sizing. The treated carbon fibers were then hot-pressed with polypropylene films to produce a carbon fiber/polypropylene prepreg. The resulting prepreg was subjected to uniaxial and off-axis tensile tests, providing fundamental data for constructing a constitute model for the carbon fiber/polypropylene prepreg. The relative error between the model predictions and experimental data is maintained within ±10%. Based on the experimental results, a temperature-dependent viscoelastic–hyperelastic constitutive model for carbon fiber/polypropylene is proposed. This model decomposes the unit volume strain energy function into four components: matrix isochoric deformation energy, fiber tensile strain energy, fiber–fiber shear strain energy, and fiber-matrix shear strain energy. The matrix energy is strain rate-dependent, exhibiting viscoelastic mechanical behavior. The material parameters of the constitutive model were identified by fitting the experimental data. The model was implemented in MATLABR2024a, and off-axis tensile tests were performed at temperatures ranging from 423 K to 453 K. Numerical simulations were compared with experimental results to validate the model. This work provides guidance for the development and validation of constitutive models for thermoplastic polypropylene prepregs. Full article

Skin meshing is widely used to treat extensive burn injuries due to its cost-efficiency and capacity to cover large wound areas. As biomimetics focuses on deriving engineering principles from biological structure–function relationships, this review examines how to optimize skin-meshing expansion and investigates factors contributing to reported discrepancies between clinical and manufacturer-reported expansion ratios. The biology and mechanical behavior of skin layer are discussed, emphasizing the anisotropic properties govern by collagen fiber orientation associated with Langer’s lines in the dermis. The epidermis and hypodermis show isotropic properties and therefore have minimal influence on load-bearing capacity. Surveying 111 studies, the review evaluates which constitutive equations employed for skin modelling is suitable to replicate mechanical behavior of skin meshing undergoing large expansion. Elastic models fail to capture large expansion ratios. Viscoelastic and QLV are excluded due to negligible sliding of collagen fibers at slow strain rates and limited importance of hysteresis. Consequently, hyperelastic models are recognized as more suitable for predicting large deformations. Among these, the structural GOH model, which represents fiber dispersion through a probability-density function, demonstrates strong agreement with experimental data using few parameters; its damage extensions improve prediction of mesh tearing. Additionally, emerging auxetic mesh geometries with negative Poisson ratios are examined, highlighting their potential to achieve greater expansion when combined with suitable structural anisotropic constitutive models, e.g., GOH. Full article