Buckling of shear deformable laminated composite plates

A shear deformable finite element is developed for the buckling analysis of laminated composite plates. The finite element formulation is based on Mindlin's theory in which shear correction factors are derived from the exact expressions for orthotropic materials. A variety of problems on uniaxial and shear bucklings of laminated composite plates are solved. The effects of material properties, plate aspect ratio, length-to-thickness ratio, number of layers and lamination angle on the buckling loads of symmetrically and antisymmetrically laminated composite plates are investigated. Optimal lamination arrangements of layers for maximizing the buckling loads of the plates are determined.     

发展形状记忆材料的展望

根据马氏体相变的特征:代位原子无扩散切变,以不变平面应变进行形状改变,以及按群论应用于马氏体相变的表述式,导出材料具有形状记忆效应的条件,即只要形成单变体或接近单变体马氏体,防止阻碍形状记忆效应因素如位错的形成和材料通过马氏体相变及其逆相变就能显示形状记忆效应。     

Simulation of fading path shape memory in the theory of simple materials with elastoplastic behavior and initial loading surface

A mathematical theory is proposed rigorous construction and specialization of constitutive relations for simple (in the Noll’s sense) strain-hardening elastoplastic materials with an initial loading surface and a fading path shape memory at the active deformation segment. Strains and symmetry type of the material are taken to be arbitrary. Physical equations are derived for materials with no path shape memory, with a weak fading memory, and with a fading memory of the nth order. Based on the proposed constitutive relations, physical equations are constructed for isotropic materials. In the context of the fading path shape memory, a definition of an elastic-perfectly plastic material is given. Assuming the condition of smallness of measures of strains throughout the entire “past” history, a theory has been developed for rigorous construction and specialization of constitutive relations for materials with a first-order fading path shape memory for infinitesimal strains. Special emphasis is placed on isotropic materials. __________ Translated from Problemy Prochnosti, No. 4, pp. 5–18, July–August, 2007.     

A General Magnetoelastic Coupling Theory of Deformable Magnetized Medium Including Magnetic Forces and Magnetostriction Effects

From the viewpoint of energy, a general magnetoelastic coupling theory including magnetic forces and magnetostriction effects is proposed for deformable magnetized medium. Firstly, a Taylor series expansion of independent variables of stress and magnetization in the elastic Gibbs free energy function is applied to obtain a polynomial expression; and then based on the magnetoelastic coupling mechanism, appropriate transcendental functions are substituted for some terms in a polynomial constitutive relationship derived by way of substituting the polynomial Gibbs free energy function in thermodynamic equations to achieve a more compact magnetostrictive constitutive relationship. The numerical simulation exhibits that the predicted magnetostrictive strain and magnetization curves under various pre-stresses are in good agreement with the experimental data given by Kuruzar et al (1971) and Jiles et al (1984). Secondly, based on the above magnetization constitutive relationship, a general magnetic forces expression is presented according to the variational principle for the total energy functional of the coupling system of the 3-d deformable magnetized materials. It is found that for the case of linear isotropic ferromagnetic materials, the magnetic forces expression can be degenerated into the Zhou-Zheng Model (1999). Combining the above nonlinear magnetostrictive constitutive relationship and magnetic forces expression, a general nonlinear magnetoelastic coupling theory is presented in this paper for deformable magnetized medium.     

Thermoelectroelasticity theory for materials with memory

In this work the extended Tiersten's theory of thermoelectroelasticity for materials with memory is presented. Constitutive equations are derived for nonlinear dielectric materials that possess fading memory of past fields. The consequences of the second law of thermodynamics is studied. The analysis is based on Coleman's work, the entropy inequality proposed by Green and Lindsay and the extended Tiersten's theory of thermoelectroelasticity.     

On the effect of residual birefringence in anisotropic photoelastic materials

The effect of residual birefringence on isoclinic angle and fringe order is considered for mechanically and optically anisotropic materials. Toupin's⁴ theory of deformable dielectrics forms the basis of the investigation. Equations are derived for fringe order, stress fringe value and isoclinic angle in linear orthotopic photoelastic materials. The principal effect of residual birefringence relates to the determination of the isoclinic angle. This is particularly so for certain fibre–orientations. Unlike the isotropic case, the isoclinic angle depends on the stress at which it is measured.     

纤维增强形状记忆聚合物复合材料及其航天应用

形状记忆聚合物(SMP)是一种能够保持临时形状,并在外界刺激下自发回复到其初始形状的智能材料,具有高形状固定率、高形状回复率、转变温度可调、变形能力强、质量轻等优点,但其应用受到响应方式单一和承载能力差的限制,通过向聚合物中添加功能颗粒或增强纤维制成形状记忆聚合物复合材料(SMPC),可有效解决这一问题。首先介绍了SMP形状记忆效应的原理,然后阐述了纤维增强型SMPC有限变形过程中纤维的微屈曲行为。最后对可变形结构在航天领域的应用进行了论述。      

Bending-stretching analysis of thick functionally graded micro-plates using higher-order shear and normal deformable plate theory

In the present article, higher-order shear and normal deformable plate theory together with modified couple stress theory are developed to study the bending analysis of thick functionally graded rectangular micro-plates. One material length scale parameter is used for capturing the size effects. Utilizing the variational approach and also a principle of virtual displacement, a new form of equilibrium equations and the corresponding boundary conditions are derived. It is assumed that material properties vary through the thickness according to the power law function. Finally, an analytical solution for the bending problem of a simply supported FG rectangular micro-plate is presented.     

Shear bond of composites-to-brick applied with highly deformable,in relation to resin epoxy,interface materials

The paper presents comparison of a work of stiff and flexible bonds fastening composite strengthening to masonry. In the first approach (traditional), barely deformable interface material made of stiff epoxy resin is used as shear bonds of composites-to-brick. In the second one (innovative), highly deformable interface material made of flexible polymer is used as repair shear bonds of composites-to-brick. Behavior of both materials was compared using single-lap shear tests made on four kinds of fiber fabrics (glass, carbon, basalt and steel) applied to clay brick units. The results indicated that highly deformable interface materials allow increasing load capacity, because deformable adhesive layers reduce shear stress concentrations in bond, redistributing stress more evenly along the whole lap joint. Usefulness of the theory which allows calculating the bond shear stress–slip characteristic was also discussed in accordance to the highly deformable interface materials.     

Advanced calculation of the room-temperature shapes of thin unsymmetric composite laminates

It is well known that the room-temperature shapes of unsymmetric laminates do not always conform to the predictions of classical lamination theory. Instead of being saddle shaped, as classical lamination theory predicts, the room-temperature shapes of unsymmetrically laminated composites are often cylindrical in nature. In addition, a second cylindrical shape can sometimes be obtained from the first by a simple snap-through action. Hyer developed for the class of all square unsymmetric cross-ply laminates which can be fabricated from four layers i.e., [0₃/90], [0₂/90/0], [0/90/0/90], [0₂/90₂], an extended classical lamination theory to predict whether these laminates have a saddle shape or one or two cylindrical shapes. The Finite Element Analysis (FEA) has just recently been used for the calculation of the room-temperature shapes of unsymmetric laminates, because more sophisticated finite element codes are now available and the calculations can be made in an acceptable time. The hope is to get more accurate results for the shape and the stresses and forces that occur during the snap through action. These results are needed for the development of active deformable composite structures based on unsymmetric laminates and incorporated shape memory alloy wires [Schlecht M. & Schulte K., Development of active deformable structures due to thermal residual stresses and incorporating shape memory alloys. In Proc. ECCM Smart Composites Workshop, ECCM6, Bordeaux, 1993, pp. 20–115.] Results for different lay-ups are presented and compared.     

Multilayered optical memory with bits stored as refractive index change. I. Electromagnetic theory

With the Lippman-Schwinger equation, dyadic Green's functions, and the vector coherent transfer function method, an electromagnetic theory of a waveguide multilayered optical memory is first developed for the static case, from which a theory describing a conventional multilayered optical memory with bits stored as a refractive index change is also derived. In addition, the formulas for readout signals and cross talk are given, and some problems of numerical calculations are discussed. The theories can be used effectively for optimum design of a multilayered optical memory with bits stored as a refractive index change.     

Discrete element modeling of granular hopper flow of irregular-shaped deformable particles

Many natural and engineered granular materials have relatively deformable particles. Besides particle size and shape, particle deformability is another salient factor that significantly impacts the material’s flow behavior. In this work, the flow of irregular-shaped deformable particles in a wedge-shaped hopper is investigated using discrete element simulations. A bonded-sphere model is developed to simultaneously capture irregular particle shapes and particle-wise deformations (e.g., compression, deflection, and distortion). Quantitative analysis of the effects of irregular shapes and particle deformations shows that the increase in particle stiffness tends to increase initial packing porosity and decrease the flow rate in the hopper. Rigid particles tend to have clogging issues, whereas deformable particles have less chance to, indicating particle deformation reduces the critical bridging width in the hopper flow. Detailed analysis of stress fields is also conducted to provide insights into the mechanism of particle flow and clogging. Stresses and discharge rates calculated from numerical simulations are compared and show good agreement with Walker’s theory and the extended Beverloo formula. Simulations with various particle shape combinations are also performed and show that the initial packing porosity decreases with an increasing percentage of fibers while the discharge rate has a complex dependency on particle shapes.     

Shape Memory Polymers for Body Motion Energy Harvesting and Self‐Powered Mechanosensing

Growing demand in portable electronics raises a requirement to electronic devices being stretchable, deformable, and durable, for which functional polymers are ideal choices of materials. Here, the first transformable smart energy harvester and self‐powered mechanosensation sensor using shape memory polymers is demonstrated. The device is based on the mechanism of a flexible triboelectric nanogenerator using the thermally triggered shape transformation of organic materials for effectively harvesting mechanical energy. This work paves a new direction for functional polymers, especially in the field of mechanosensation for potential applications in areas such as soft robotics, biomedical devices, and wearable electronics.     

Strength of the Layered Cylindrical Shell of Composites Under Internal Pressure with Regard to External Damage

Strength of Materials - The strength of a layered shell structure of composite materials is investigated by combining modern strength criteria, the theory of deformable solid orthotropic medium...     

Size-dependent behaviour of electrically actuated microcantilever-based MEMS

In this paper, the nonlinear size-dependent static and dynamic behaviours of a microelectromechanical system under an electric excitation are investigated. A microcantilever is considered for the modelling of the deformable electrode of the MEMS. The governing equation of motion is derived based on the modified couple stress theory (MCST), a non-classical model capable of capturing small-size effects. With the aid of a high-dimensional Galerkin scheme, the nonlinear partial differential equation governing the motion of the deformable electrode is converted into a reduced-order model of the system. Then, the pseudo-arclength continuation technique is used to solve the governing equations. In order to investigate the static behaviour and static pull-in instabilities, the system is excited only by the electrostatic actuation (i.e., a DC voltage). The results obtained for the static pull-in instability predicted by both the classical theory and MCST are compared. In the second stage of analysis, the nonlinear dynamic behaviour of the deformable electrode due to the AC harmonic actuation is investigated around the deflected configuration, incorporating size dependence.     

On the bending of viscoelastic plates made of polymer foams

Considering the viscoelastic behavior of polymer foams a new plate theory based on the direct approach is introduced and applied to plates composed of functionally graded materials (FGM). The governing two-dimensional equations are formulated for a deformable surface, the viscoelastic stiffness parameters are identified assuming linear-viscoelastic material behavior. The material properties are changing in the thickness direction. Solving some problems of the global structural analysis it will be demonstrated that in some cases the results significantly differ from the results based on the Kirchhoff-type theory.     

Optimum distribution of smart stiffeners in sandwich plates subjected to bending or forced vibrations

The problems of optimum distribution of active stiffeners manufactured from piezoelectric or shape memory alloy materials and bonded to or embedded within the facings of a sandwich plate are considered. The sandwich plate consists of thin composite or isotropic facings which are in the state of plane stress and a thick shear deformable core. The amplitude of forced vibrations of the plate is reduced using symmetric couples of piezoelectric stiffeners subjected to out-of-phase dynamic voltages. Shape memory alloy stiffeners are used to reduce bending deformations. In the latter case, a desirable effect is achieved by activating the stiffeners on one side of the middle surface. Optimum design is considered based on the requirement of minimal transverse static or dynamic deflections subject to a constraint on the volume of smart stiffeners. The variables employed in the process of optimization are the ratios of the cross-sectional areas of the stiffeners in each direction to their respective spacings. It is shown, that, dependent on the load, materials, and geometry, optimum design can significantly reduce deflections, i.e. enhance the strength, of sandwich plates.     

Orthogonal fibre composites as micromorphic materials

The analogy between the static linearly elastic theory of first order micromorphic media and a model of a matrix reinforced with orthogonal interlocking deformable fibres is established. Corresponding interpretations of some of the non-classical stresses are brought to light. Reductions of the constitutive equations are carried out for isotropy and for further reduction to a micropolar material (whereupon the physical interpretation of constitutive coefficients becomes impractical). An 8-node, 4-sided, isoparametric, displacement finite element is developed for plane stress and plane strain. With 4 degrees of freedom at each node. it is capable of modelling the micromorphic material and through a reduction to 3 degrees of freedom it becomes an element of a micropolar medium. Finite element results are obtained for the case of a circular hole in a uniform tension field and agree well with exact results for a micropolar medium. Corresponding results for 2 classes of micromorphic materials are also given.     

Plane stress and plate bending coupling in BEM analysis of shallow shells

In this paper the shear deformable shallow shells are analysed by boundary element method. New boundary integral equations are derived utilizing the Betti's reciprocity principle and coupling boundary element formulation of shear deformable plate and two‐dimensional plane stress elasticity. Two techniques, direct integral method (DIM) and dual reciprocity method (DRM), are developed to transform domain integrals to boundary integrals. The force term is approximted by a set of radial basis functions. Several examples are presented to demonstrate the accuracy of the two methods. The accuracy of results obtained by using boundary element method are compared with exact solutions and the finite element method. Copyright © 2000 John Wiley & Sons, Ltd.     

Controlling Malleability of Metamaterials through Programmable Memory

Malleability, the ability to adapt materials to specific shapes, is necessary in applications where a form closure is requested. The material should be easily deformable between desired stable shapes. Such stability can be obtained through bistable elements that act as memory in metamaterials. Herein, a material with memory behavior programmed by the local temperature is presented. The behavior can be switched between a permanent shape change and a complete elastic recovery after removing an applied mechanical load. Additionally, a deformed material can be forced to recover its shape by heating. Through an adaption of the mesostructure and the used polymers, the characteristic behavior (switching time and temperature) can be adjusted. Furthermore, heating can be applied locally that only certain parts are able to change. A unit cell design based on analytical and numerical analyses is demonstrated that considers not only the mesostructure but also the combination of polymeric materials with specific thermoresponsive mechanical behavior. Unit cells and structures of several cells are additively manufactured to validate the programmable behavior. The concept is extended to indirect heating with an alternating magnetic field, using a compound made from a polymeric material and magnetic particles.