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•Three types of disordered perforated systems are designed to investigate their mechanical properties.•The disordered perforated systems exhibit great robustness in auxetic ...behavior.•The rotating mechanism still contributes to the auxeticity of the perforated systems.•The geometric perturbations greatly affect the mechanical properties of the perforated system.•The disordered perforated auxetic system exhibits slight anisotropic feature.
Previous discussions about perforated auxetic metamaterials primarily focused on the ordered systems with high degree of geometric symmetry. However, it is difficult to manufacture or retain the perfect auxetic systems in practical applications. In this paper, three types of disordered perforated auxetic systems including orientated disordered system, dimensional disordered system and complete-disordered system are explored thoroughly. The perforations of interest are oval holes which are advantageous in ensuring auxeticity, reducing stress level and improving material distribution. The designed disordered systems are fabricated by 3D printing technology and then are tested by the uniaxial tension to reveal their mechanical properties and verify the related finite element models. Thereafter, the evolution of mechanical properties of these disordered systems is investigated numerically for the varied perturbed geometric parameters such as the orientation and dimension of oval hole. The results reveal that the disordered systems still show great robustness in auxetic behavior, although the disorder in orientation and dimension exists. A high degree of symmetry in microstructure is not necessary for designing perforated auxetic systems. This provides a great convenience for the flexible and practicable design and application of perforated auxetic metamaterials.
Due to the existence of highly ordered hierarchical chiral structures, biological fibers and bundles such as tendons, ligaments, and climbing tendrils usually have excellent mechanical properties. ...How the hierarchical chiral structures affect the mechanical properties of fibers and bundles, however, remains unclear. In this paper, a theoretical model of the biological fibers and bundles with self-similar hierarchical chiral structures is developed by virtue of a simple homogenization treatment. Using the built model, the load transfer behavior and effect of material chirality, the chiral assembly forms, and structure level number on the mechanical properties are investigated. The results show that the mechanical properties of fibers and bundles are significantly dependent on the material chirality of fibrils and hierarchical chiral structures. The material chirality of fibrils can make the fiber much softer. And the hierarchical chiral structure provides the fibers and bundles with superior capability to endure large elongation and storage energy. The work is not only helpful for understanding the structure-property of biological fibers and bundles, but also for the design of artificial muscles and soft devices such as strain sensors.
Slender chiral filaments are ubiquitous in both artificial and biological materials. Due to their chiral microstructures, chiral filaments usually exhibit favorable properties such as superior ...elasticity and unusual stretch-twist coupling deformation. However, how these chiral microstructures affect the elastic behavior of filaments remains unclear. In this paper, a refined Cosserat rod model with misfit or mismatching of chirality induced by inhomogeneous arrangement of chiral microstructures incorporated is developed. Using the refined rod model, the force-displacement relationships and variation of structural chirality during the tensile processes of two typical helical structures, i.e., single-strand helix and double-strand helix, are investigated. The results show that the misfit of chirality can lead to a bend-twist deformation with a high coupling degree, which makes the rod much “soft” when stretched. The chiral filaments undergo an unusual twist when stretched, corresponding to an obviously nonlinear variation of structural chirality. The work suggests that the misfit of chirality can be used to tune the elastic behavior of chiral filaments, which is helpful in guiding the design of flexible actuators and soft devices.
The significant increase in speed of high-speed train will cause the dynamic contact force of the pantograph-catenary system to fluctuate more severely, which poses a challenge to the study of the ...pantograph-catenary relationship and the design of high-speed pantographs. Good pantograph-catenary coupling quality is the essential condition to ensure safe and efficient operation of high-speed train, stable and reliable current collection, and reduction in the wear of contact wires and pantograph contact strips. Among them, the dynamic parameters of high-speed pantographs are crucial to pantograph-catenary coupling quality. With the reduction of the standard deviation of the pantograph-catenary contact force as the optimization goal, multi-parameter joint optimization designs for the high-speed pantograph with two contact strips at multiple running speeds are proposed. Moreover, combining the sensitivity analysis at the optimal solutions, with the parameters and characteristics of in-service DSA380 high-speed pantograph, the optimization proposal of DSA380 was given.
As a traditional numerical simulation method for pantograph–catenary interaction research, the pantograph–catenary finite element model cannot be applied to the real-time monitoring of ...pantograph–catenary contact force, and the computational cost required for the multi-parameter joint optimization of the pantograph–catenary system with the finite element model is very high. In this paper, based on the selective crow search algorithm–radial basis function (SCSA-RBF) network, the time-domain signal of the panhead acceleration, which can be obtained in real-time through non-contact test technology, is taken as the boundary condition to directly solve the pantograph dynamic equation and a data-physics coupling model that can quickly predict the pantograph–catenary interaction is proposed. The prediction model is trained and verified using the dataset generated through the finite element model. Furthermore, the prediction model is applied to the multi-parameter joint optimization of six pantograph dynamic parameters and nine pantograph dynamic parameters, considering nonlinear panhead stiffness, and optimization suggestions under various speeds and filtering frequencies are given.
Many biological materials, such as wood and bone, possess helicoid microstructures at microscale, which can serve as reinforcing elements to transfer stress between crack surfaces and improve the ...fracture toughness of their composites. Failure processes, such as fiber/matrix interface debonding and sliding associated with pull-out of helical fibers, are responsible mainly for the high energy dissipation needed for the fracture toughness enhancement. Here we present systemic analyses of the pull-out behavior of a helical fiber from an elastic matrix via the finite element method (FEM) simulation, with implications regarding the underlying toughening mechanism of helicoid microstructures. We find that, through their uniform curvature and torsion, helical fibers can provide high pull-out force and large interface areas, resulting in high energy dissipation that accounts, to a large extent, for the high toughness of biological materials. The helicity of fiber shape in terms of the helical angle has significant effects on the force-displacement relationships as well as the corresponding energy dissipation during fiber pull-out.
Chiral microstructures exist widely in natural biological materials such as wood, bone, and climbing tendrils. The helical shape of such microstructures plays an important role in stress transfer ...between fiber and matrix, and in the mechanical properties of biological materials. In this paper, helical fiber fragmentation behavior is studied numerically using the finite-element method (FEM), and then, the effects of helical shape on fiber deformation and fracture, and the corresponding mechanical mechanisms are investigated. The results demonstrate that, to a large degree, the initial microfibril angle (MFA) determines the elastic deformation and fracture behavior of fibers. For fibers with a large MFA, the interfacial area usually has large values, inducing a relatively low fragment density during fiber fragmentation. This work may be helpful in understanding the relationship between microstructure and mechanical property in biological materials, and in the design and fabrication of bio-inspired advanced functional materials.
Due to the existence of microstructures involving characteristic length scales, micropolar materials such as bones, rocks and foam materials usually have complex fracture behaviors. The near-tip ...fields of an anti-plane crack in micropolar media and the local cracking modes, however, remain unclear. In this paper, the Williams asymptotic solutions of the near-tip fields of an anti-plane crack in micropolar elasticity are derived. A force stress intensity factor (FSIF) and two couple stress intensity factors (CSIFs) are the key fracture parameters characterizing the anti-plane crack-tip fields. And the energy release rate for an anti-plane crack is similar to the Rice's J-integral for classical continua. Then, six crack modes are introduced for a three-dimensional micropolar solid, i.e., three global modes controlled by the FSIFs and three local modes controlled by the CSIFs. Different from classical isotropic elasticity, a material constant appears in the singular terms of the crack-tip force stress and couple stress fields of isotropic micropolar materials. In addition, there are six constant terms in the asymptotic solutions of the crack-tip fields, i.e., three constant force stresses and tree constant couple stresses similar to the T-stress in classical fracture mechanics. Sequentially, the J-integral for three-dimensional micropolar solids is expressed concisely by the FSIFs and CSIFs. It can be observed that the CSIFs normalized by the torsion and bending characteristic lengths are dimensionally equivalent to the FSIFs. This work provides a theoretical basis for studying the fracture performance and crack propagation criterion of micropolar materials.
•The asymptotic solutions of the near-tip fields of an anti-plane crack for micropolar elasticity are provided.•Three global and three local crack modes are deeply elucidated for micropolar elasticity.•The J-integral is expressed concisely by three force stress intensity factors and three couple stress intensity factors.
•A new I-integral is proposed for inhomogeneous piezoelectrics with initial strains.•The I-integral is domain-independent for complex initial strain and interfaces.•The line integral along the ...interface is derived for discontinuous initial strains.•The SIFs jump markedly for piezoelectric bi-materials with various substrates.
Initial strain in piezoelectric composites is usually formed during the manufacturing process, especially in piezoelectric layer/film and elastic substrate structures. In this work, a novel interaction integral (I-integral) is developed to determine the stress intensity factors (SIFs) and the electric displacement (EDIF) of the interfacial crack between inhomogeneous piezoelectric bi-materials considering the effect of initial strain, and the theoretical derivation rigorously demonstrates that the proposed I-integral does not need to concern the derivatives of any material parameter. Further, complex material interface distribution near the crack tips in piezoelectric composites is considered in the integration domain , and the domain-independence of the established I-integral is theoretically demonstrated. This study derives the effect terms of inhomogeneous and discontinuous initial strain distributions on the I-integral in the interface cracking model of piezoelectric bi-materials. Numerical results show that the graded degree of the inhomogeneous piezoelectric bi-material has a significant effect on both the SIFs and the EDIF of an interfacial crack. As the graded parameter increases from zero to five, the normalized mode-I SIF increases significantly and the relative increment can reach 26.9% for inhomogeneous piezoelectric bi-materials. For the quadratic initial strain, the normalized mode-I SIF can be reduced by 37.5% compared with that for a homogeneous initial strain. The presented I-integral shows excellent domain-independence for continuously inhomogeneous and discontinuous material properties as well as for complex initial strain distribution (relative derivation <1.0%). For piezoelectric/elastic bi-materials, the IFs of the interface crack can be modulated by designing various distributions of material property and initial strain of the elastic substrate, which can provide guidance to reduce interface cracking in engineering.
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•A fracture mechanics model for the hierarchical chiral structure is developed.•The stress and couple stress near the crack tip in chiral fiber layer are obtained and verified by FEM.•The energy ...release rate of the twisting crack in the hierarchical chiral structure is obtained.•The toughening mechanism of macroscopic and microscopic chiral structures is revealed.
Numerous biological composites exhibit superior mechanical properties owing to their intricate hierarchy of chiral structures. Nonetheless, the mechanisms underlying the toughening of hierarchical chiral structures remain enigmatic. In this paper, we present a fracture mechanics model for the biological composites featuring a hierarchical chiral structure, aiming at elucidating the twisting behavior of crack and its corresponding toughening impact. We meticulously investigate the effects of chirality of structure elements at various levels on the energy release rate. The theoretical analysis reveals that by the fine-tunning chirality across multiple levels, the hierarchical chiral structure induces a discernible reduction in the energy release rate during crack propagation in a twisted way, significantly bolstering fracture toughness of biological composites. This study provides novel insight into the structure–property relationship of biological materials.
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