Large Deformation Capability Research Articles

Simulating loading–unloading hysteretic behaviors of nematic-genesis polydomain nematic elastomers

Nematic elastomers (NEs) are lightly cross-linked elastomers with nematic mesogens integrated in their polymer networks. Combination of large deformation capability with nematic-isotropic phase transition enables NEs to be the most promising soft materials for impact attenuation, actuation and soft robotics. In this paper, we focus on nematic-genesis polydomain NEs (N-PNEs) where mesogens are cross-linked at nematic states. N-PNEs are capable of absorbing and dissipating energy and easy to synthesize. We present a Voronoi diagram-based finite element model for specimen-scale N-PNEs, and investigate the cyclic tensile and compressive behaviors of N-PNEs at different strain rates. Our simulations reveal a smooth polydomain-monodomain transition during loading, accompanied by a full recovery of polydomain texture after the load is removed, indicating a memory effect of initial disordered mesogen alignment. The predicted behaviors align well with experimental observations, which validates our model. Furthermore, we assess the energy absorption and dissipation capabilities of N-PNEs compared to monodomain NEs, identifying conditions where N-PNEs exhibit superior performance. This study not only enhances our understanding of polydomain-monodomain transitions in N-PNEs, but also lays the groundwork for the development of N-PNE-based energy absorbers.

Experimental and constitutive model investigation on the tensile mechanical properties of polyurea at wide strain-rate range

Polyurea is extensively utilized as a protective coating material for structures under impact and blast loadings, owing to its significant large tensile deformation capability and strain hardening effect. In this study, the tensile performance of polyurea was experimentally and theoretically investigated. Firstly, rational geometric shapes and dimensions were designed, and a series of quasi-static and dynamic uniaxial tensile tests were conducted to obtain the nonlinear stress-strain curves at different strain rates. The results indicated a notable strain-rate effect in polyurea, with an increase in elastic modulus and tensile stress as the strain rate increased. Moreover, as the strain rate escalates, the strain hardening effect becomes more pronounced, thereby enhancing the protective capabilities of polyurea. Subsequently, an existing hyperelastic model, namely the Mooney-Rivlin (MR) model, was modified to incorporate the strain rate effect, resulting in a non-linear visco-hyperelastic constitutive model. Furthermore, the proposed constitutive model and parameters were validated by simulating existing tensile and blast tests using the finite element (FE) program LS-DYNA. This work could provide a reference for the design of structural protection.

Effect of geometrical design variables on implantation configuration and fixation stiffness of titling bone anchors: A parametric finite element study

The mechanical interaction of a tilting anchor and cancellous bones of various densities was simulated using finite element modeling. The model enjoyed a sophisticated representation of the bone, as an elasto-plastic material with large deformation capability. The anchor's tilting action during implantation phase, as well as its fixation stiffness during pull-out test, were predicted by the model and a parametric study was performed to investigate the effects of the anchor's distal width and corner fillet radius, on these measures. The model predictions were validated against the results of an experimental test on ovine humerus specimens. The model could reasonably reproduce the tilting action of the anchor during the implantation phase. Comparison of the model predictions with the experimental results revealed similar trends during both the implantation and the pull-out phases, but smaller displacement magnitudes (end points: 1.4 vs. 2.1 mm and 4.6 vs. 5.2 mm, respectively). The results of the parametric study indicated substantial increase in the fixation stiffness with increasing bone density. Reducing the distal width and increasing the fillet radius improved the anchor's implantation configuration and fixation stiffness in low-density bones. For high-density bone applications, however, a larger distal width was favored for improving the fixation stiffness.

Highly Flexible and Superelastic Graphene Nanofibrous Aerogels for Intelligent Sign Language.

Highly flexible and superelastic aerogels at large deformation have become urgent mechanical demands in practical uses, but both properties are usually exclusive. Here a trans-scale porosity design is proposed in graphene nanofibrous aerogels (GNFAs) to break the trade-off between high flexibility and superelasticity. The resulting GNFAs can completely recover after 1000 fatigue cycles at 60% folding strain, and notably maintain excellent structural integrity after 10000 cycles at 90% compressive strain, outperforming most of the reported aerogels. The mechanical robustness is demonstrated to be derived from the trans-scale porous structure, which is composed of hyperbolic micropores and porous nanofibers to enable the large elastic deformation capability. It is further revealed that flexible and superelastic GNFAs exhibit high sensitivity and ultrastability as an electrical sensors to detect tension and flexion deformation. As proof, The GNFA sensor is implemented onto a human finger and achieves the intelligent recognition of sign language with high accuracy by multi-layer artificial neural network. This study proposes a highly flexible and elastic graphene aerogel for wearable human-machine interfaces in sensor technology.

Measurement of tensile and shear strength of interfacial bonding in fiber reinforced soft composites combining experiments and simulation

Interfacial bonding strength plays a key role in the mechanical performance of fiber reinforced polymers. However, measurement of interfacial bonding strength in fiber reinforced soft composites (FRSCs) becomes a challenging issue, because the large deformation capability of soft matrix invalidates most of current testing methods. This paper demonstrates an advanced method combining experiments and simulation, which provides a convenient and reliable means to measure both tensile strength and shear strength of interfacial bonding for FRSCs. The specimen is made by embedding a fiber bundle in a soft plate for transverse tensile test. Various stress states along the fiber–matrix interface can be realized by controlling the inclination angle of fiber bundle to the stretching direction. Tensile strength and shear strength of interfacial bonding in FRSCs can be estimated based on the analysis of critical stress states. The comparison with the widely adopted fiber pull-out test indicates that the nominal shear strength obtained in pull-out test is severely underestimated.

A New Type Bionic Foldable Wing Design for High Maneuverable Unmanned Aerial Vehicle

With the improvement of aircraft requirements in civil and military fields, aircraft are developing towards the direction of high efficiency and multi-mission. Foldable wing aircraft with large deformation capability will be able to meet flight requirements and performance under different conditions. Based on the bionic design concept, a feather-like foldable morphing wing based on a multi-link mechanism from the flight characteristics of birds was designed. In order to validate the feasibility of the proposed morphing wing conception, a UAV with bionic foldable wings was fabricated. The aerodynamic performance of the prototype model was tested under different working conditions by wind tunnel test and flight test. The simulation and wind tunnel experimental test showed that the prototype has excellent longitudinal and transverse directional aerodynamics. When the wing is symmetrically morphing, the optimal lift-to-drag ratio can be maintained under different flow velocities. When the wing is asymmetrically morphing, it can replace the aileron to achieve an efficient roll maneuver.

Fabrication of a magnetic thin-film actuator using roll-to-roll gravure printing

Magnetic actuators are attractive for their fast response, large deformation capability, and low power requirement compared with other types of actuators. Roll-to-roll printing is a continuous process and offers high productivity. We fabricated the cantilever-structured magnetic actuators using the roll-to-roll gravure printing process. By applying a partial magnetic field while carrying out the roll-to-roll printing process, the vector of the magnetic layer was programmed instantaneously during printing. This technique improved productivity and reduced materials waste. The fabrication process for the actuator includes printing the sacrificial layer, printing and magnetizing the magnetic layer, and coating the structural layer. An air gap between the substrate and cantilever beam of the actuator was made by removing the sacrificial layer. The actuator samples were examined in terms of displacement, hysteresis, and durability. Our actuators can rotate up to 90° and can be actuated at 20 Hz under a magnetic field of 80 mT.

Structural performance evaluation of pultruded GFRP composite space truss: Experimental study and numerical simulation

This paper presents space truss structures assembled from pultruded square hollow glass fiber reinforced plastic (GFRP) members. The novel steel connectors are used to connect the GFRP members, which can significantly improve construction efficiency. The structural performance of the four-span space truss and space truss unit is evaluated by the four-point bending test and the static test, respectively. The test results show that the GFRP space truss structure exhibited satisfactory overall stiffness and load-bearing capacity. The failure modes mainly include shear failure of the bolt, local buckling of GFRP profiles, and local cracks at holes and cuts. The structural stiffness of the space truss unit increases at the post-failure stage, which is caused by the compression buckling of the pultruded GFRP profile. The structural stiffness of the four-span space truss decreases at the post-failure stage, and the large nonlinear deformation capability provides an early warning before the ultimate collapse. In addition, a finite element model considering material nonlinearity is developed to predict the failure mode, overall structural stiffness, and load-carrying capacity of the GFRP space truss structure.

U-Type Honeycomb Based on Super-Elastic Shape-Memory Alloy for One-Dimensional Large Wing Morphing

Flexible skin is an important part of morphing wings, which must comply with the altered geometry during wing morphing. Especially for some applications of one-dimensional large morphing wing such as chord-change, flexible skin is demanded to be capable of large deformation. In this paper, we present a U-type honeycomb structure of shape-memory alloy (SMA) for flexible skin that can undergo large deformation owing to the super-elastic Nickel Titanium (NiTi)-SMA, and the mechanical properties are studied by numerical and experimental methods. In the stage of linear elasticity, U-type honeycomb structure is analyzed theoretically based on the virtual work principle, and its mechanical behavior in the stage of large deformation is studied by finite element method with considering geometric and material nonlinearity. In-plane tensile tests and out-of-plane load-bearing tests are performed, and the experimental results agree well with the numerical results. The experimental results have proved that the U-type honeycomb structure based on NiTi-SMA has large deformation capability, of which the recoverable global strain can reach at least 60%, and has great potential in the application of morphing wings.

Multi-objective shape optimization of large strain 3D helical structures for mechanical metamaterials

The need for mechanical metamaterials with large strain range and lightweight properties are evidenced to engineering applications. In this regard, novel helical structures are proposed as suitable unit cell’s components of mechanical metamaterials. Three-dimensional helical structures composed of varying coil numbers, defined in a cylindrical spatial domain are shape optimized through genetic algorithm in a finite element script for conflicting objectives of minimum mass and maximum tensile range. The superior performance of the shape optimized helical structure is highlighted in terms of structural rigidity, large deformation capability, buckling and vibrational modal analysis in compare to equivalent coil springs of identical weight and comparable domain. Deformation mechanism is analyzed carefully to justify the improved performances of proposed structure. Tensile and compressive experimental analysis are undertaken to validate the enhanced strain ranges. One dimensional metamaterials implementations with various tessellation arrangements are simulated. Results show that the proposed design can effectively generate lightweight substitutes of metamaterials unit cells ligaments to improve the strain range performance. Planar and lattice metamaterial concepts employing shape optimized helical structure are illustrated to demonstrate the possibilities of promoting lightweight structural integrities in the design of mechanical metamaterials.

Evaluation of the fire performance of unprotected composite beams with fin-plate joints

PurposeThis paper aims to investigate the fire performance of composite beams when considering the hogging moment resistance of the fin-plate beam-to-girder joints including the effect of continuity of reinforcements.Design/methodology/approachExperiments on composite beams with fin-plate joints protected only at the beam ends are conducted. The test parameter is the specification of reinforcement, which affects the rotational restraint of the beam ends. In addition, a simple method for predicting the failure time of the beam using an evaluation model based on the bending moment resistance of the beam considering the hogging moment resistance of the fin-plate joint and the reinforcement is also presented.FindingsThe test results indicate that the failure time of the beam is extended by the hogging moment resistance of the joints. This is particularly noticeable when using a reinforcing bar with a large plastic deformation capability. The predicted failure times based on the evaluation method corresponded well with the test results.Originality/valueRecent studies have proposed large deformation analysis methods using FEM that can be used for fire-resistant design of beams including joints, but these cannot always be applicable in practice due to the cost and its complexity. Our method can consider the hogging moment resistance of the joint and the temperature distribution in the axial direction using a simple method without requirement of FEM.

Performance-Based Evaluation of Energy Dissipating Hysteretic Infilled Reinforced Concrete Frame Based on Cyclic Tests

ABSTRACT The detachment of the infill with the RC frame starts at a low level of drift that triggers several undesirable modes of failure of the infill itself as well as within the frame. The proposed research intends to develop an alternative infill wall system with large deformation capability without collapse and a large energy dissipation capacity. It comprises slotted blocks with the energy dissipating link elements. The results show that the energy dissipating hysteretic infills can minimize the undesirable characteristic effects and achieve enhanced performance limit states even under the collapse prevention damage state.

Irradiation hardening and ductility loss of Eurofer97 steel variants after neutron irradiation to ITER-TBM relevant conditions

Ten Eurofer97 steel variants, produced by non-standard fabrication-processing routes and modified alloying chemistries, were studied by neutron irradiations in the high flux isotope reactor. The irradiations were performed to ITER-TBM relevant conditions of ∼255–350 °C, 2.94–3.24 dpa. We quantified the irradiation-induced degradation of the steels using mechanical property tests. All the steels suffered from irradiation hardening, where a significant increase in Vickers microhardness and yield stress (σYS) occurred, accompanied with severe loss of tensile elongation. The extent of hardening was material dependent. For Tirr = 300±30 °C, most steels showed σYS increase in the range of ∼30% to as high as ∼66%, except for a low temperature tempered steel with σYS increase below 15%. Despite large losses in elongation, most failures were ductile. Significant post-necking ductility was retained with reduction in area (RA) between 65–75%, but <50% for low temperature tempered steels. The ultimate tensile stress to yield stress (σUTS/σYS) ratios decreased significantly after irradiation, highlighting irradiation-induced strain hardening capacity reduction. No major effect of irradiation on the plastic instability stress (σPIS) and true fracture stress of the steels was observed. By comparing the tensile stresses in true stress units and with literature, the results suggest that RAFM steel designing should target materials with a large separation between σPIS and σYS, to ensure the materials can maintain large work hardening and uniform deformation capability after irradiation. The tensile data of the steels additionally revealed a compelling evidence of an inverse trend between the change in RA and increase in σYS of the neutron irradiated Eurofer97 type steels.

Proof of Concept Novel Configurable Chipless RFID Strain Sensor.

This paper contains two main areas of research: First, this work outlines a novel, highly sensitive strain sensor design that should support various levels of deformation, depending on the substrate type used. Physical implementations in this work have focused on proving its large deformation capabilities, and simulations have been used to assess its more general electromagnetic response. The other part of this paper focusses on exploring other effects that will impact the sensing of strain of resolutions below 10 με, which is a capability achieved by other aerospace-grade strain sensor technologies. These effects are limited to mechanical swelling and sensor orientation in the azimuth and elevation planes, as these appear to be unexplored and highly relevant issues to the topic of chipless RFID-based strain sensing. From this exploration, it is apparent that the effects of mechanical swelling and sensor orientation (amongst others) will need to be addressed in any real-life implementation of the sensor, requiring a strain resolution below 10 με.

Harnessing post-buckling deformation to tune sound absorption in soft Helmholtz absorbers

Helmholtz resonator (HR) has always been an important part of artificial sound-absorbing materials, most of which however cannot be tuned in real time and hence have a limited scope of applications. In this work, we integrate for the first time the soft hyperelastic material into the main structure of HR to design a star-shaped soft Helmholtz absorber. The soft HR exhibits different post-buckling deformation behavior when its wall-thickness varies, which further yields different acoustic characteristics. By combining two star-shaped HRs with different wall-thickness, we are able to achieve asymmetric sound absorption when specific and respective compression loads are applied to the two HRs. In addition, high sound absorption at various frequencies can be obtained via different combinations of the applied loads. Due to the perfect capability of reversible large deformation of soft hyperelastic materials, the sound absorption of the proposed pair of HRs can be real-time tuned effectively by mechanical loading and unloading. In other words, the acoustic switch controlled by mechanical load can be realized. The proposed soft absorber has an obvious practical application value, and also provides an important illustration for the design of soft and tunable acoustic devices.

Shape memory alloy-spring damper for seismic control and its application to bridge with laminated rubber bearings

This study introduces a shape memory alloy (SMA)-spring damper which is composed of SMA bars and elastic springs arranged in perpendicular. The damper depicts a curved flag-shape hysteretic behavior that is endowed with self-centering capacities and large deformation capabilities but uses reduced amount of SMA material. A design procedure is proposed to apply the SMA-spring damper to the bridge with laminated rubber bearings which would slide under seismic excitations. Analytical results validate the effectiveness of SMA-spring dampers in seismic control of the bridge: (1) The damper provides trivial stiffness to the bridge at small displacement, and the isolation efficiency of the bridge is maintained; (2) large horizontal force is provided for the structures at large deformation of the bearings, which alleviates the excessive displacement of bearings and prevents span collapse; and (3) the damper helps recenter the bearings and reduce the residual displacement of the bridge.

Deployable scissor mechanisms derived from non-crossing angulated structural elements

This paper focuses on exploring and generalizing a scissor structural element, termed non-crossing angulated element (NCAE), in the context of unlocking its unexplored design potential and further broaden its applicability to deployable scissor mechanisms. Three types of NCAE assemblies are proposed and termed Types I, II, and III NCAE. The geometrical conditions of these NCAE assemblies are established to maintain an invariable subtended angle during motion. Based on Type I NCAE, a new construction method of double-ring mechanism that can transform the shape between two specified polygons with an equal number of sides is proposed. Owing to the scissor structure of NCAE, a novel deployable polyhedral mechanism with radially reciprocating motion but no singularity is synthesized. Relying on the advantage of the shape transformation property from NCAE, we incorporate this element with a straight scissor element and construct deployable polyhedral mechanisms with radial expansion motion to obtain a large deformation capability. To validate all the formed mechanisms, both computer model simulations and constructed prototype demonstrations are conducted. The results demonstrate the feasibility of the proposed mechanisms and synthesis approaches.

Microstructure-sensitive large-deformation model for thermomechanical processing simulations

Current computational mechanics methods lack coupling between remeshing and microstructure-sensitive models. However, this combination is necessary to consider microstructural evolution during large-deformation, thermomechanical forming operations. A model focused on improving microstructure prediction under forming operations through the advancement of remeshing capabilities integrated with crystal plasticity finite element (CPFE) is developed. To enable coupling between remeshing and a large deformation crystal plasticity model, the current 3D adaptivity (i.e., remeshing) capabilities in LS-DYNA were enhanced to be compatible with a CPFE model. These enhancements include developing remapping techniques that properly account for crystallographic texture evolution during deformation. The techniques used for mapping of microstructure-related variables differ from the smooth interpolation schemes typically used for mapping of other field variables such as stress or displacement. It is demonstrated that combining the nonlinear large deformation capabilities of LS-DYNA, microstructure-sensitive remapping, and a CPFE model yields a simulation framework capable of accurately predicting evolution of location-specific forged part microstructures and shear band formation. Greater microstructural accuracy in large-deformation models can lead to improved strength, life, and manufacturing yield of lightweight forged parts.

On bifurcation behavior of hard magnetic soft cantilevers

Hard magnetic materials belong to a novel class of soft active materials with the capability of quick, large, and complex deformation via applying an external actuation. They have an extensive range of potential applications in soft robots, biomedical devices, wearable devices, and stretchable electronic devices. Recently, investigation of experimental and theoretical nonlinear mechanics of hard magnetic soft cantilevers has received notable considerations. In the available analyses, the most attention was paid to the non-bifurcation-type nonlinear mechanics of the system, and bifurcation was considered for a specific case. In the current study, the general trend of bifurcation-type nonlinear mechanics of hard magnetic soft cantilevers is introduced and the effective parameters on the bifurcation behavior are identified. Additionally, new trajectories due to the bifurcation and the corresponding workspace are presented. Eventually, a comprehensive comparative study between the new trajectories and workspace with those reported in the prior studies is carried out.

Soft topological lattice wheels

We investigate the large deformation and extreme load-management capabilities of a soft topologically polarized kagome lattice mapped to a cylindrical domain through the problem of a lattice wheel rolling on an irregular surface. We test the surface–lattice interaction experimentally by subjecting a 3D-printed topological lattice wheel prototype to localized and distributed boundary loads. This investigation reveals a dichotomy in the force transfer between the two loading scenarios, whereby localized loads are absorbed with limited stress penetration into the bulk and small force transfer to the wheel axle, compared to distributed loads. Through numerical simulations, we compare the lattice wheel against a baseline solid wheel to highlight the unique stress management opportunities offered by the lattice configuration. These findings promote the design of rolling objects enabled by topological mechanics, in which a surplus of softness, activated by local asperities, can coexist with a globally stiff response to distributed loads that ensures satisfactory load-bearing capabilities.