Thermal flutter analysis of large‐scale space structures based on finite element method

During the orbital day–night crossing period, the suddenly applied thermal loading is apt to introducing vibration on flexible appendages of large‐scale space structures. This kind of thermally‐induced vibration is a typical failure of modern spacecrafts. However, owing to the complexity of this problem, many earlier researches study only the vibration of simplified beam models, which can hardly describe the performance of practical structures. This paper aims at using the finite element method to analyse the non‐linear vibration of practical thin‐walled large‐scale space structures subjected to suddenly applied thermal loading. In this study, the coupling effect between structural deformations and the incident normal solar heat flux is considered; the necessary condition of thermally‐induced vibration is verified; and the criterion of thermal flutter is established. Copyright © 2006 John Wiley & Sons, Ltd.     

Effects of Multi‐Scale Patterning on the Run‐In Behavior of Steel–Alumina Pairings under Lubricated Conditions

In nature, many examples of multi‐scale surfaces with outstanding tribological properties such as reduced friction and wear under dry friction and lubricated conditions can be found. To determine whether multi‐scale surfaces positively affect the frictional and wear performance, tests are performed on a ball‐on‐disk tribometer under lubricated conditions using an additive‐free poly‐alpha‐olefine oil under a contact pressure of around 1.29 GPa. For this purpose, stainless steel specimens (AISI 304) are modified by micro‐coining (hemispherical structures with a structural depth of either 50 or 95 μm) and subsequently by direct laser interference patterning (cross‐like pattern with 9 μm periodicity) to create a multi‐scale pattern. The comparison of different sample states (polished reference, laser‐patterned, micro‐coined, and multi‐scale) shows a clear influence of the fabrication technique. In terms of the multi‐scale structures, the structural depth of the coarser micro‐coining plays an important role. In case of lower coining depths (50 μm), the multi‐scale specimens show an increased coefficient of friction compared to the purely micro‐coined surfaces, whereas larger coining depths (95 μm) result in stable and lower friction values for the multi‐scale patterns.     

Topology optimization of multiscale elastoviscoplastic structures

This paper extends current concepts of topology optimization to the design of structures made of nonlinear microheterogeneous materials. The objective is to maximize the macroscopic structural stiffness for a prescribed material volume usage while accounting for the nonlinearity and the microstructure of the material. The resulting design problem considers two scales: the macroscopic scale at which the optimization is performed and the microscopic scale at which the material heterogeneities and the nonlinearities are observed. The topology optimization at the macroscopic scale is performed by means of the bi‐directional evolutionary structural optimization method. The solution of the macroscopic boundary value problem requires as inputs the effective constitutive response with full consideration of the microstructure. While computational homogenization methods such as the FE² method could be used to solve the nonlinear multiscale problem, the associated numerical expense (CPU time and memory) is highly unacceptable. In order to regain the computational feasibility of the computational scale transition, a recent model reduction technique of the authors is employed: the potential‐based reduced basis model order reduction with graphics processing unit acceleration. Numerical examples show the efficiency of the resulting nonlinear two‐scale designs. The impact of different load amplitudes on the design is examined. Copyright © 2015 John Wiley & Sons, Ltd.     

基于混合单元建模的复合板热诱导结构振动研究

针对复合板的热诱导结构振动问题，提出一种新的混合单元计算模型，将三维和二维单元组成混合单元，利用不同类型单元体现复合结构中不同的材料特性，实现对真实复合结构物理特征的高精度模拟。在此基础上基于虚功原理建立了三维形式的复合板瞬态热弹性动力学方程，利用该方法对空间大尺度复合薄板的热诱导扰动进行研究。仿真结果表明，结构固有频率和热容是决定热诱导振动发生的主要内囚，对于一些刚度较大的空间复合薄板，由于空间约束环境弱，整体频率低易发生热诱导结构振动。本研究对于空间大尺度复合平板结构的工程设计具有一定的参考价值。     

Microstructural characterization of transformable Fe–Mn alloys at different length scales

The as-annealed and deformed Microstructure of transformable Fe–Mn alloys were, comprehensively, characterized over a wide range of length scales. Differential interference contrast optical metallography, combined with a tinting etching method, was employed to examine the grain morphology. A new specimen preparation method, involving electro-polishing and electro-etching, was developed for scanning electron microscopy and electron back-scattered diffraction analysis. This method leads to a very good imaging contrast and thus bridges the length scale gap between optical metallography and transmission electron microscopy. Moreover, it enables simultaneous scanning electron microscopy and electron backscatter diffraction analysis which allows correlations among morphology, crystal orientation and phase analysis in the length scale of microns. Transmission electron microscopy investigations were also made to evaluate the thermal and mechanical transformation products as well as defect structures.     

Shear tests on highly thermal insulating clay unit masonry walls with thin layer mortar

The paper presents the results of a series of 6 shear tests on full scale highly thermal insulating clay unit masonry walls. The walls consisted of units with large voids filled with mineral wool with a thermal conductivity of λ = 0,07 W/(m · K). The aim of the investigations was the verification of the in‐plane‐shear resistance of this type of thermal insulating clay unit masonry in addition to the tests reported in [1]. The current design rules for clay unit masonry in DIN EN 1996‐1‐1/NA are rather conservative compared to the test results for thermal insulating units.     

Karhunen–Loève decomposition of random fields based on a hierarchical matrix approach

The simulation of the behavior of structures with uncertain properties is a challenging issue, because it requires suitable probabilistic models and adequate numerical tools. Nowadays, it is possible to perform probabilistic investigations of the structural performance, which take into account a space‐variant uncertainty characterization of the structures. Given a structural solver and the probabilistic models, the reliability analysis of the structural response depends on the continuous random fields approximation, which is carried out by means of a finite set of random variables. The paper analyzes the main aspects of discretization in the case of 2D problems. The combination of the well‐known Karhunen–Loève series expansion, the finite element method and the hierarchical matrices approach is proposed in the paper. Copyright © 2013 John Wiley & Sons, Ltd.     

Exact response bound analysis of truss structures via linear mixed 0‐1 programming and sensitivity bounding technique

In the present paper, finding the exact bound of structural response for truss structures is considered under bounded interval type uncertainty. This problem is challenging since seeking the exact bound corresponds to locating the global optima of a multivariate function (generally nonconvex). Traditional treatment of this problem involves the solution of a linear mixed 0‐1 programming problem, which is a highly computationally demanding task especially when large‐scale structures are taken into consideration. In order to alleviate the computational effort, a sensitivity bounding technique is developed in this work using the tools from convex analysis to disclose the monotonicity of concerned structural response function with respect to 0‐1 variables. It is shown that this technique can not only reduce the number of 0‐1 variables substantially but also change the computational complexity of the considered problem from nondeterministic polynomial–hard to nondeterministic polynomial–hard in some cases. The proposed approach provides the possibility of finding the exact bound of structural response for large‐scale truss structures within a reasonable time, and its effectiveness is demonstrated through several numerical examples.     

Modelling and fatigue life assessment of complex structures

This paper presents some of the motivations and main conclusions from a series of joint Nordic research initiatives in which an integrated research approach to the development of future generations of advanced fabricated structures have been employed. The integrated research approach includes coordinated efforts in several key technologies: high‐speed welding processes, high strength materials, cost‐effective NDE, post‐weld treatments and FE‐based design assessment tools. Traditionally, fatigue assessment methods for welded structures have been developed based on small‐scale test specimens and verification studies for large structures are rarely published. Applications on complex structures have led to several new assessment concepts and areas for future work. A modified structural stress method that proposes a multi‐linear stress distribution through the plate thickness is introduced. Also, a crack growth assessment method in which the constraint equations of a sub‐model are linked to the global model is presented. Both these new methods are promising for complex structures. The crucial role of boundary conditions for complex structures is highlighted as is the future challenge of understanding and making use of the residual stress state for welded structures.     

Light‐Emitting Nanophotonic Designs Enabled by Ultrafast Laser Processing of Halide Perovskites

Nanophotonics based on resonant nanostructures and metasurfaces made of halide perovskites have become a prospective direction for efficient light manipulation at the subwavelength scale in advanced photonic designs. One of the main challenges in this field is the lack of large‐scale low‐cost technique for subwavelength perovskite structures fabrication preserving highly efficient luminescence. Here, unique properties of halide perovskites addressed to their extremely low thermal conductivity (lower than that of silica glass) and high defect tolerance to apply projection femtosecond laser lithography for nanofabrication with precise spatial control in all three dimensions preserving the material luminescence efficiency are employed. Namely, with CH₃NH₃PbI₃ perovskite highly ordered nanoholes and nanostripes of width as small as 250 nm, metasurfaces with periods less than 400 nm, and nanowire lasers as thin as 500 nm, corresponding to the state‐of‐the‐art in multistage expensive lithographical methods are created. Remarkable performance of the developed approach allows to demonstrate a number of advanced optical applications, including morphology‐controlled photoluminescence yield, structural coloring, optical‐ information encryption, and lasing.     

Analysis,design, and optimization of structures with integral compliant mechanisms for mid‐frequency response

A multi‐scale paradigm is proposed that utilizes periodic, small‐scale, integral compliant mechanisms within larger‐scale structures for the attenuation of vibro‐acoustic response. Amplification principles serve as the basis for the design of these mechanisms in achieving reduced energy transmission. The spectral finite element method is exploited for a force–velocity and energy flow analysis of the resultant truss‐like structures. A genetic algorithm is employed to optimize structure size for greatest effectiveness in the frequency range of interest. This study demonstrates that a significant decrease in structural vibration is achievable and suggests promising applications including the design of acoustic isolation panels for broadband vehicle noise reduction. Copyright © 2007 John Wiley & Sons, Ltd.     

A two‐scale generalized finite element approach for modeling localized thermoplasticity

Predicting localized, nonlinear, thermoplastic behavior and residual stresses and deformations in structures subjected to intense heating is a prevalent challenge in a range of modern engineering applications. The authors present a generalized finite element method targeted at this class of problems, involving the solution of intrinsically parallelizable local boundary value problems to capture localized, time‐dependent thermo‐elasto‐plastic behavior, which is embedded in the coarse, structural‐scale approximation via enrichment functions. The method accommodates approximation spaces that evolve in between time or load steps while maintaining a fixed global mesh, which avoids the need to map solutions and state variables on changing meshes typical of traditional adaptive approaches. Representative three‐dimensional examples exhibiting localized, transient, nonlinear thermal and thermomechanical effects are presented to demonstrate the advantages of the method with respect to available approaches, especially in terms of its flexibility and potential for realistic future applications in this area. Parallelism of the approach is also discussed. Copyright © 2016 John Wiley & Sons, Ltd.     

Entwicklung von Werkstoffmodellen zur Beschreibung der lokalen Eigenschaften der Wärmeeinflusszone von hochfesten Feinblechen

The analysis of the complete process chain is an important requirement in the fine sheet metal working for the improvement of strength evaluation of structures and components. Particularly at applications of high‐strength steel sheets the development of material properties as well as the production process play a decisive role, because of the complex requirement profile these steels have to satisfy. In the context of the represented research the material reactions were experimental examined on the basis of the thermal effect of different joining processes at multiphase steel. The investigations were accomplished on HCT600XD, HCT780XD, SZBS800 and LH800. Exemplarily the article discusses the procedures and the results of the research of HCT600XD and SZBS800.     

Consideration of influence factors between small‐scale specimens and large components on the fatigue strength of thin‐plated block joints in shipbuilding

Shipbuilding in blocks, as being usual on all larger shipyards, requires that the blocks will finally be welded together manually or semi‐automatically, that is, with butt‐welds in transverse direction that have to withstand relatively high dynamical loads. Modern shipbuilding aims at lightweight construction with thin plates that may have a plate thickness down to 4 mm. Previous investigations showed that manually produced butt‐welds in such thin structures did not reach the calculated fatigue life as required in the rules. Up to the present, this problem has not yet been solved, and it is questioned if all influence factors on the fatigue behaviour of real structures are correctly considered as no damage cases at butt joints that are known yet. In the investigation described here, results from small‐scale specimens tested with cyclic loads will be transferred to large components, considering the effects of recorded pre‐deformations induced by welding as well as measured differences in residual stresses between small‐scale specimens and large components, thus clarifying how far for instance a detrimental stress ratio should be taken into account by the rules for thin plates.     

Fatigue strength investigations of welded details of stiffened plate structures in steel ships

Ships are prone to fatigue due to high cyclic loads mainly caused by waves and changing loading conditions. Therefore, fatigue is an important criterion during design. Different approaches are applied to the fatigue strength assessment. However, the results are varying and the validity of results from small-scale fatigue tests for real structures is sometimes unclear. Therefore, deeper fatigue strength investigations were performed within a German industry-wide joint research project aiming at the harmonization of the approaches. Regarding ship structures, two types of structures were selected for the investigations. The first concerns web frame corners being typical for ro/ro ships from which three models were tested. The associated small-scale specimen is a cruciform joint. The second type is the intersection between longitudinals and transverse web frames, which recently showed fatigue failures in containerships. Five models were tested, three under constant and two under variable amplitude loading. The associated small-scale specimen is a longitudinal attachment on a stiffener top. All large-scale tests showed a relatively long crack propagation phase after first cracks had appeared, calling for a reasonable failure criterion. For the numerical analysis, the nominal, the structural hot-spot as well as the effective notch stress approach have been applied. The latter allows the consideration of the weld shape which could partly explain differences in the observed and calculated failure behaviour. The applicability of the different approaches is quite good if some specific aspects are observed. Insofar the investigations give a good insight into the strength behaviour of complex welded structures and into current problems and opportunities offered by numerical analyses.     

Template‐Free Growth of Well‐Ordered Silver Nano Forest/Ceramic Metamaterial Films with Tunable Optical Responses

Currently, the limitations of conventional methods for fabricating metamaterials composed of well‐aligned nanoscale inclusions either lack the necessary freedom to tune the structural geometry or are difficult for large‐area synthesis. In this Communication, the authors propose a fabrication route to create well‐ordered silver nano forest/ceramic composite single‐layer or multi‐layer vertically stacked structures, as a distinctive approach to make large‐area nanoscale metamaterials. To take advantage of direct growth, the authors fabricate single‐layer nanocomposite films with a well‐defined sub‐5 nm interwire gap and an average nanowire diameter of ≈3 nm. Further, artificially constructed multilayer metamaterial films are easily fabricated by vertical integration of different single‐layer metamaterial films. Based upon the thermodynamics as well as thin film growth dynamics theory, the growth mechanism is presented to elucidate the formation of such structure. Intriguing steady and transient optical properties in these assemblies are demonstrated, owing to their nanoscale structural anisotropy. The studies suggest that the self‐organized nanocomposites provide an extensible material platform to manipulate optical response in the region of sub‐5 nm scale.     

Uncertain static plane stress analysis with interval fields

Uncertain static plane stress analysis of continuous structure involving interval fields is investigated in this study. Unlike traditional interval analysis of discrete structure, the interval field is adopted to model the uncertainty, as well as the dependency between the physical locations and degrees of variability, of all interval system parameters presented in the continuous structures. By implementing the flexibility properties of some common structural elements, a new computational scheme is proposed to reformulate the uncertain static plane stress analysis with interval fields into standard mathematical programming problems. Consequently, feasible upper and lower bounds of structural responses can be effectively yet efficiently determined. In addition, the proposed method is adequate to deal with situations involving one‐dimensional and two‐dimensional interval fields, which enhances the pertinence of the proposed approach by incorporating both discrete and continuous structures. In addition, the proposed computational scheme is able to establish the realizations of the uncertain parameters causing the extreme structural responses at zero computational cost. The applicability and credibility of the established computational framework are rigorously justified by various numerical investigations. Copyright © 2016 John Wiley & Sons, Ltd.     

Combining Bottom‐Up Self‐Assembly with Top‐Down Microfabrication to Create Hierarchical Inverse Opals with High Structural Order

Colloidal particles can assemble into ordered crystals, creating periodically structured materials at the nanoscale without relying on expensive equipment. The combination of small size and high order leads to strong interaction with visible light, which induces macroscopic, iridescent structural coloration. To increase the complexity and functionality, it is important to control the organization of such materials in hierarchical structures with high degrees of order spanning multiple length scales. Here, a bottom‐up assembly of polystyrene particles in the presence of a silica sol–gel precursor material (tetraethylorthosilicate, TEOS), which creates crack‐free inverse opal films with high positional order and uniform crystal alignment along the (110) crystal plane, is combined with top‐down microfabrication techniques. Micrometer scale hierarchical superstructures having a highly regular internal nanostructure with precisely controlled crystal orientation and wall profiles are produced. The ability to combine structural order at the nano‐ and microscale enables the fabrication of materials with complex optical properties resulting from light–matter interactions at different length scales. As an example, a hierarchical diffraction grating, which combines Bragg reflection arising from the nanoscale periodicity of the inverse opal crystal with grating diffraction resulting from a micrometer scale periodicity, is demonstrated.     

An adaptive multi‐scale approach to the modelling of masonry structures

We present an adaptive multi‐scale approach for predicting the mechanical behaviour of masonry structures modelled as dynamic frictional multi‐body contact problems. In this approach, the iterative splitting of the contact problem into normal contact and frictional contact is combined with a semismooth Newton/primal‐dual active‐set procedure to calculate deformations and openings in the model structures. This algorithm is then coupled with a novel adaptive multi‐scale technique involving a macroscopic scale, which is the size of the masonry structure, and a mesoscopic scale, which is the size of the constituents (bricks, stone‐blocks), to predict appearance of dislocations and stress distribution in large‐scale masonry structures. Comparisons of the numerical results with data from experimental tests and from practical observations illustrate the predictive capability of the multi‐scale algorithm. Copyright © 2008 John Wiley & Sons, Ltd.     

Meshfree simulation of failure modes in thin cylinders subjected to combined loads of internal pressure and localized heat

This paper focuses on the non‐linear responses in thin cylindrical structures subjected to combined mechanical and thermal loads. The coupling effects of mechanical deformation and temperature in the material are considered through the development of a thermo‐elasto‐viscoplastic constitutive model at finite strain. A meshfree Galerkin approach is used to discretize the weak forms of the energy and momentum equations. Due to the different time scales involved in thermal conduction and failure development, an explicit–implicit time integration scheme is developed to link the time scale differences between the two key mechanisms. We apply the developed approach to the analysis of the failure of cylindrical shell subjected to both heat sources and internal pressure. The numerical results show four different failure modes: dynamic fragmentation, single crack with branch, thermally induced cracks and cracks due to the combined effects of pressure and temperature. These results illustrate the important roles of thermal and mechanical loads with different time scales. Copyright © 2008 John Wiley & Sons, Ltd.