International Journal of Solids and Structures最新文献

In this paper, the dynamic shear response and failure of Ti-6Al-4V using four different test specimen geometries, viz. Hat-Shaped Specimen (HSS), Flat Hat-Shaped Specimen (FHSS), Chip Hat-Shaped Specimen (CHSS) and Double Shear Specimen (DSS), are critically examined and compared. Through a combination of experiments (using the standard Split-Hopkinson Pressure Bar system), finite-element simulations and metallographic examinations of their fracture morphology, the dynamic shear characteristics (strain hardening, strain rate strengthening effect and failure strain) of Ti-6Al-4V obtained using the different specimen geometries are critically examined, compared and analyzed. It will be shown that differences in the stress/strain uniformity, the plastic deformation zone, and the stress state induced by the different specimen geometries lead to discrepancies in the measured shear response and failure that were observed. The shear stress–strain curve obtained using the DSS will be shown to be more precise than the other specimen geometries.

This work focuses on dynamic continualization of multifield multilayered metamaterials in order to obtain energetically-consistent models able to provide an accurate description of the dispersive behavior of the corresponding discrete system. Continuum models, characterized by constitutive and inertial non-localities, have been identified through a recently proposed enhanced continualization scheme. They are identified by governing equations both of the integro-differential and higher-order gradient-type, whose regularization kernel or pseudo-differential functions accounting for shift operators are formally expanded in Taylor series. The adopted regularization kernel exhibits polar singularities at the edge of the first Brillouin zone, thus assuring the convergence of the frequency spectrum to the one of the Lagrangian system in the entire wave vector domain. The validity of the proposed approach is assessed through the investigation of multilayered discrete lattices with an antitetrachiral topology, where local resonators act as rigid links among the layers. The convergence of dispersion curves of the continuum model to the ones of the Lagrangian model is proved in the whole first Brillouin zone as the adopted continualization order increases, both considering the propagation and the spatial attenuation of Bloch waves inside the metamaterial. A low frequency continualization is also provided, leading to the identification of a first-order medium.

The modeling of the visco-electro-elastic behavior of functionally graded beam benders with PZT constituents is accomplished via a hierarchical framework that is based on the homogenization technique for composite layers and the laminate theory for a composite laminate. The representation of a bulk PZT constituent is based on linear visco-electro-elastic constitutive equations. The resulting bending displacements of PZT-graded bimorph and multimorph are obtained under the assumption of the Euler–Bernoulli beam. The experimental data of the bending displacements versus applied voltage are compared with the predictions for a bimorph and a multimorph, resulting in a good agreement. The responses of a bender to a complete cycle of applied voltage are shown in order to reveal the critical hysteretic actuation due to the presence of a visco-electro-elastic PZT material in a functionally graded piezoelectric beam bender which is made by functionally graded piezoelectric materials.

Architectured materials are engineered materials with specific geometries and arrangements to exhibit desired mechanical properties. One class of architectured material is the Topologically interlocked material (TIM) system. In TIM systems, a plurality of convex particles is assembled under specific geometric constraints such that a load-carrying structure is obtained without the need for an adhesive bond between building blocks. Past investigations have considered TIM systems with building blocks and assemblies with a high degree of symmetry. Here the geometric constraints commonly imposed on the geometry of the system are relaxed. Two new types of skewed building blocks are introduced: one with only a rotational symmetry and no mirror symmetry, and one without rotational or mirror symmetry. These blocks are used to build even and odd-numbered assemblies and to create TIM systems with both mirror and rotational symmetry, rotational symmetry only, and no symmetry. A vector field representing the material architecture is introduced and demonstrated to connect architecture and mechanical behavior. It is demonstrated that load transfer patterns in the TIM system closely match the geometric symmetry. This allows for the demonstration of achiral and chiral mechanical behavior as represented by the presence of reaction moments for the centrally loaded square TIM assembly. The chirality of the building blocks manifests itself in the mechanical behavior of the TIM system only once the deflection of the system opens the contacts between building blocks such that building blocks accommodate deformation. Chiral building blocks diffuse the load from the central load path dominant in the TIM systems built from achiral blocks. This construction concept allows for simultaneous improvements in mechanical properties (strength and stiffness) solely from geometry.

The Mullins effect is a highly anisotropic damage phenomenon exhibited by filled rubbers among other soft materials. When filled rubbers are subjected to uniaxial tension, their apparent stiffness drops in the direction of stretching but is essentially unaltered in the transverse directions. However, micromechanical full-network models where Mullins softening is described at the level of individual chains often predict that uniaxial deformations induce transverse softening in addition to softening in the stretching direction. Moreover, these approaches typically require the storage of damage state variables for each chain, which is computationally expensive. Taking an alternative approach, we present a full-network model for the Mullins effect where the damage state is described by a single macroscopic damage tensor from which the damage state in each direction can be calculated. The evolution of damage is specified through damage surfaces and damage flow rules, which depend on the directions of principal stretches. The model is shown to reproduce experimental data for filled rubbers sequentially subjected to uniaxial tension in different directions. The model is also implemented in the finite element software ABAQUS as a user subroutine UMAT to illustrate the suitability of the model to simulate non-homogeneous deformation states.

We report on circular buckles experimentally observed by optical and atomic force microscopy on gold ductile thin films deposited by physical vapor deposition on silicon wafers. It is shown that, whatever the radius blister dimensions, their maximum deflections are higher than those expected by the elastic theory. It suggests that plastic events may take place in the film, impacting on the blister morphology as a result. Based on nanoindentation experiments carried out on our gold films, a proper plastic hardening law has been determined by calculations using the finite elements method. The influence of this elasto-plastic behavior on the buckled circular profiles has been then numerically studied, compared to the experimental observations and discussed.

The multilevel helical structures in various engineering and natural fields offer excellent deformation flexibility and load bearing capabilities. Understanding the interplay between the local frictional contact and the geometric characteristics of the helical structure under complex external loads has attracted considerable interest. In this work, the effect of local frictional contact behaviors on the bending in multilevel helical structures is investigated by using a combination of theoretical modeling, finite element (FE) simulations, and experiments. In the case of pure bending, the kinematic parameters of the bent multi-stage helix are derived concisely by the idea of the kinematic analogy. The bending stiffness of the multi-stage helix is further obtained. In the case of the combined tension/torsion and bending, the 3D thin rod model incorporating Coulomb’s friction is established to describe the mechanical responses. It is found that the relationship between equivalent bending stiffness and the laying angle exhibits nonlinearity. A comparison with the classical Papailiou model reveals that, for helical structures at large laying angles, the influence of friction is primarily determined by the internal force in the tangential direction, which is the core assumption of the Papailiou model. However, in the case of small laying angles, the helical twisting characteristics and the contribution of the internal forces and moments in the other two directions (normal and binormal directions) to the friction cannot be ignored. Subsequently, a multilevel frictional contact transmission formulation is proposed according to the force action–reaction principle. Based on the above formulation, the non-simplified thin rod equations with Coulomb’s friction are extended to describe the multilevel stick-slip bending behaviors of the second stage cable (3*3). The dissipation capacity of helical structures is evaluated quantitatively under the hysteretic bending. Finally, the theoretical model is verified by FE simulations and experimental results. This work provides insights for unveiling the intrinsic relationship between the nonlinear bending and local frictional contact behaviors in the multilevel helical structures.

Biodegradable polymer-based stents simultaneously provide scaffolding, drug release, and biodegradation to eliminate chronic inflammation. The most important factors hindering the wide use of these stents are thick struts, low radial strength, and large footprints formed on the inner wall of the artery as a result of stent expansion. Negative Poisson’s Ratio (NPR), also known as the Auxetic design, has shown great potential to provide radial strength with less strut thickness. However, a detailed mechanical evaluation proving improvement in stent performance parameters is not available in the literature. In this study, the performance parameters of two stent designs based on the Auxetic geometry with PLLA were analyzed under in-vivo conditions using an in-silico model consisting of the artery, crimper, and expander FE model. For this purpose, one design utilizes Auxetic unit cell, which is already available in the literature, while the other uses a newly proposed Hybrid design combining Auxetic and Chevron type geometries. Additionally, a specially heated coaxial balloon-catheter system was considered as a deployment tool between glass transition and body temperature, and carried out for thin-strut stent simulations. The Hybrid design is shown to resolve the foreshortening problem of Auxetic design and collapse pressure of commercial PLLA stents. In this present study validates the potential of Hybrid design to overcome problems for polymer-based biodegradable stents.

Elastomeric solid materials typically exhibit a pronounced viscoelastic response. In this paper we consider a large deformation viscoelasticity theory for isotropic elastomeric materials which uses a multi-branch multiplicative decomposition of the deformation gradient. We then describe the numerical implementation of the theory in the open-source finite element program FEniCSx. Several example simulations which demonstrate the capability of the theory and its numerical implementation to model stress-relaxation, creep, stretch-rate sensitivity, hysteresis, damped inertial oscillations, and dynamic column buckling are shown. The source codes for these simulations are provided. The theory and the codes presented in this paper lay the foundation for future extensions of the theory and its numerical implementation to include the effects of coupling with thermal, electrical, and magnetic fields — extensions which are of central importance in modeling the response of soft-active materials.