A study on completely computer-assisted kinematic analysis of planar link mechanisms

For the purpose of complete computerization of the kinematic analysis of planar mechanisms, an iterative process of analysis has been determined by means of incidence matrices of prime structures. The prime structures have been defined as the structures composed of pin-jointed links that cannot be derived from other structures, and some of them have been introduced together with positional relations of their joints. Moreover, it is shown that kinematic analyses can also be carried out by means of the prime structures.     

PVDF压电薄膜特性研究及其在装备状态监测中的应用

主要研究PVDF压电薄膜监测结构的基本特性及在装备状态监测中的应用。通过PVDF压电薄膜监测结构的动态响应实验,证明PVDF压电薄膜监测结构应变的灵敏度较高,适合于装甲装备状态检测的应用要求。     

周期性结构的石墨烯对太赫兹波的吸收特性研究

针对在六角孔形周期性结构阵列铜网上生长而成的石墨烯,对其在太赫兹波段的吸收进行了研究与讨论。用太赫兹时域光谱耦合系统对石墨烯样品进行检测,检测结果表明,在0.7~1.4THz范围内,因石墨烯样品含有的杂质增强了对太赫兹波的吸收,进而增大了整体的吸收率,所以片状石墨烯样品的吸收率约为4%,比以往文献中记载的2.3%高。因部分太赫兹波被石墨烯周期性结构形成的等离子带吸收,还有少部分太赫兹波被周期性结构干涉和散射,周期性结构石墨烯的吸收率增大了约1.5倍。     

即贴型单层带通FSS的传输特性

以狭缝阵列为例,利用模匹配法计算分析,并结合实验,针对即贴型频率选择表面(FSS)与介质板的复合结构进行研究,分析了不同厚度的聚酰亚胺基底膜和粘合胶层的影响,并深入研究了不同复合方式下的频率响应.结果表明,即贴型FSS结构中,基底膜与粘合胶层等细节的影响不可忽略,影响程度与复合方式和介质厚度均密切相关.复合结构中应当含有粘合胶层,避免空气间隙的影响.仅当基底膜和粘合胶层的介电常数小于介质板或与之相近,采用合适的复合方式,并且介质板厚度为基底膜和粘合胶层厚度的10倍以上,才能使基底膜和粘合胶层对复合结构传输特性的影响降至最低.一系列厚度的介质单侧加载时的FSS结构频率响应测试结果与计算结果的对比表明粘合工艺的缺陷会导致即粘型FSS结构的传输性能发生变化.FSS的设计中应当全面考虑上述因素的影响.     

A data-driven approach for real-time prediction of thermal gradient in engineered structures

Predicting thermal gradient of engineered structures with uncertain parameters is crucial for the safety. Many numerical computation methods have been developed in literatre; however, a practical approach is still pursued. To bridge this research gap, a data-driven approach is proposed to realize real-time prediction of thermal gradient in engineered structures with uncertain parameters. First, tetrahedral elements are used to build structures with complex geometry, and the face-based smoothed finite element method (FS-FEM) is employed to calculate the thermal response of the structures. By doing so, a good trade-off between the efficiency and accuracy for modelling the structures can be achieved. Then, the generated datasets are used for training a back propagation neural network (BPNN) model, and real-time prediction of thermal gradient in engineered structures can be performed without knowing the exact distribution functions of stochastic parameters. Lastly, two case studies were investigated to evaluate the performance of the proposed approach. The analysis results demonstrate that the proposed data-driven approach is able to offer accurate predictions of the thermal gradient for the engineered structures with uncertain parameters.

     

Complementary freeze-etch replicas by adapting the Denton apposed specimen tooling to the Balzers unit

It is postulated that there may be classes of microstructures, having anisotropic internal features, which are related to internally isotropic structures by a coordinate transformation. Such structures are known to occur, for instance, in metallography when the bulk material has been subjected to deformation and there are many biological analogies. The simplest case of coordinate transformation is investigated theoretically in this paper; that of a linear transformation in one dimension. Formulae have been found for the numerical constant which allows the density of the internal surface which separates two components of the material to be calculated from the border profile density on a plane section through the material. Measurements on trabecular bone have suggested that it may be a structure of this type, and, on this assumption, the errors due to incorrectly assuming isotropy, when calculating the area of the surface which separates the bone from the marrow, are discussed.     

Robotic fiber placement process analysis and optimization using response surface method

Unlike traditional materials, composites are carefully designed materials suitable for specific applications. Conventional methods of fabrication of composite structures have proven to be labor intensive and time-consuming. Robotic fiber placement is a composite fabrication technique that increases the flexibility of fiber placement process and allows for the fabrication of more complex structures. This study is aimed at analyzing and optimizing the robotic fiber placement process parameters. Many experiments have been conducted to analyze gas torch temperature, fiber laying head speed, and fiber compaction force and the process is optimized using response surface method.     

Preservation and visualization of molecular structure in detergent-extracted whole mounts of cultured cells.

Today's electron microscopes have a resolution sufficient to resolve supramolecular structures. However, the methods used to prepare biological samples for electron microscopy often limit our ability to achieve the resolution that is theoretically possible. We use whole mounts of detergent-extracted cells grown on Formvar-coated gold grids as a model system to evaluate various steps in the preparation of biological samples for high resolution scanning electron microscopy (SEM). Factors that are important in determining the structure and composition of detergent-extracted cells include the nature of the detergent and the composition of the extraction vehicle. Chelation of calcium is extremely important to stabilize and preserve the cytoskeletal filaments. We have also demonstrated both morphologically and by gel electrophoresis that treatment of cells with bifunctional protein crosslinkers before or during extraction with detergent can significantly enhance the preservation of both proteins and supramolecular structures. The methods used to dry samples are a major determinant of the quality of structural preservation. For cytoskeletons freeze-drying (FD) is superior to critical point-drying (CPD), one reason being that CPD samples have to be dehydrated, thereby causing more shrinkage as compared to FD samples. The high pressures to which samples are exposed during CPD may also cause increased shrinkage, and water contamination during CPD causes severe structural damage. We have obtained the best structural preservation of detergent-extracted and fixed cells by manually plunging them into liquid propane and drying over night in a freeze-dryer. The factor that most limits achievement of high resolution in SEM is the metal coat, which has to be very thin, uniform, and free of grain in order not to hide structures or to create artifactual ones. We have found that sputter-coating with 1-3 nm of tungsten (W) or niobium (Nb) gives extremely fine-grained films as well as satisfactory emission of secondary electrons. These samples can also be examined at high resolution by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The best preservation and visualization of supramolecular structures have been obtained using cryosputtering, in which the samples are freeze-dried and then sputter-coated within the freeze-dryer while still frozen.     

Thermal expansion of functionally graded and wafer-layered structures produced by laser direct metal deposition

A range of engineering alloys was selected to create two distinct sets of structures. One was functionally graded materials (FGM)—using pairs of these alloys—and the second type was a series of wafer-layered structures using pairs of these alloys in different combinations. The aim of this investigation was to identify unique sets of structures of alloys which provide very different coefficients of thermal expansion (CTE) compared to those of individual elements. The process used to create these structures was laser direct metal deposition (DMD) additive manufacturing technology. The linear thermal expansion coefficients of these samples were measured and the results show that specific sets of FGM and wafer type structures of specific constituent metal alloys can be fabricated by DMD, in which the overall coefficient of thermal expansion of these new structures is significantly different from that of each alloy when measured individually. FGM and wafer type structures of specific constituent metal alloys have lower CTE than those of original alloys.     

压力容器断裂韧度要求的预测

在弹塑性断裂双参数分析方法的基础上，导出了预测压力容器等重要工程结构断裂韧度ＣＴＯＤ要求的理论表达式。只要在实际结构服役条件下，材料断裂韧度高于该表达式要求的韧度值，就可防止结构的弹塑性断裂失效。同时，在该表达式中考虑了应力集中及焊接残余应力的影响，并表明断裂韧度要求随作用应力及板厚的增加而增加。最后给出了两个实例，以考核该理论表达式的实用性。     

A general optimality criterion for strength and stiffness of dual-material-property structures

An optimality criterion is established for combined strength and stiffness requirements per unit weight of structures utilizing materials which may have different strength, stiffness or density for the tension and compression members. In the extreme case this covers the use of different materials for tension and compression members; referred to here as dual-material structures. It is shown that additional considerations are required for statically determinate structures and proof of optimality is given for statically indeterminate and statically determinate cases. In all cases, the optimal structures are based on Michell structural layouts, in which the cross-sectional areas of the structural members are determined to give constant strain energy per unit mass. Material density may be replaced by material cost per unit volume to optimize combined strength and stiffness requirements for minimum material cost.     

A new approach to the design and optimisation of support structures in additive manufacturing

Support structures are required in several additive manufacturing (AM) processes to sustain overhanging parts, in particular for the production of metal components. Supports are typically hollow or cellular structures to be removed after metallic AM, thus they represent a considerable waste in terms of material, energy and time employed for their construction and removal. This study presents a new approach to the design of support structures that optimise the part built orientation and the support cellular structure. This approach applies a new optimisation algorithm to use pure mathematical 3D implicit functions for the design and generation of the cellular support structures including graded supports. The implicit function approach for support structure design has been proved to be very versatile, as it allows geometries to be simply designed by pure mathematical expressions. This way, different cellular structures can be easily defined and optimised, in particular to have graded structures providing more robust support where the object’s weight concentrate, and less support elsewhere. Evaluation of support optimisation for a complex shape geometry revealed that the new approach presented can achieve significant materials savings, thus increasing the sustainability and efficiency of metallic AM.     

High-resolution cryo-scanning electron microscopy study of the macromolecular structure of fibronectin fibrils

High-resolution cryo-scanning electron microscopy was used to examine fibronectin fibrils formed in culture by human skin fibroblasts and in a cell-free system by denaturing soluble plasma fibronectin with guanidine. These studies indicate that the conformation of fibrils formed in culture and in a cell-free system appeared to be similar and that fibronectin fibrils have at least two distinct structural conformations. Fibronectin fibrils can be very straight structures with smooth surfaces or highly nodular structures. The average diameter of the nodules in these fibrils is 12 nm. Both conformations can be seen within an individual fibril indicating that they are not different types of fibronectin fibrils but rather different conformational states. Immunolabeling studies with a monoclonal antibody, IST-2, to the heparin II binding domain of fibronectin revealed that the epitope was buried in highly smooth fibrils, but it was readily exposed in nodular fibrils. We propose, therefore, that fibronectin fibrils are highly flexible structures and, depending on the conformation of the fibril, certain epitopes on the surface may be buried or exposed.     

Experimental study on the high-damping properties of metallic lattice structures obtained from SLM

Modern additive manufacturing technologies allow the creation of parts characterized by complex geometries that cannot be created using conventional production techniques. Among them the Selective Laser Melting (SLM) technique is very promising. By using SLM it is possible to create lightweight lattice structures that may fill void regions or partially replace bulk regions of a given mechanical component. As a consequence, the overall mechanical properties of the final component can be greatly enhanced, such as the resistance to weight ratio and its damping capacity against undesired vibrations and acoustic noise. Nevertheless, only a few research works focused on the characterization of the dynamic behavior of lattice structures, that were mainly investigated in the low frequency range or directly tested on some specific applications. In this work the dynamic behavior of lattice structures in the medium-high frequency range was experimentally investigated and then modelled. For this purpose, different types of lattice structures made of AlSi10Mg and AISI 316L were measured. Experimental modal analysis was performed on the obtained specimens in order to assess the influence of lattice material and unit cell geometry on their global dynamic behavior. Experimental results revealed that lattice structures have superior damping characteristics compared to solid materials having an equivalent static stiffness. Eventually, the classic Rayleigh model was found to be adequate - with some approximation - to explain the damping behavior of a generic lattice structure.     

次镜支撑结构的力学性能分析

次镜支撑结构性能的好坏直接影响光学系统的成像质量。针对几种典型的支撑结构,利用I-DEAD软件建立有限元模型,进行动力学分析计算。在比较典型结构的基础上,改进得到了2种新型偏置结构,即三翼偏置支撑结构和新型的四翼偏置支撑结构。通过分析表明,在同等条件下,2种新型结构能更大程度地减小遮拦比,提高光学系统质量;或是说,在遮拦比一致以及偏置距离相同的条件下,2种新型支撑结构的一阶谐振频率均大于传统的四翼偏置支撑结构,稳定性能更好,更适用于大口径的光学系统。     

Image analysis of electron micrographs relating to mineralization in calcifying cartilage: Theoretical considerations

Biological mineralization is a cell-mediated process which is believed to be triggered by a “nucleating agent.” Various matrix structures, such as matrix vesicles, collagen fibrils and macromolecules, have been claimed to be the source of this substance, since these components have been found by transmission electron microscopy (TEM) of thin sections to be associated with early mineral crystals. Systematic image analysis of the relationships revealed in electron micrographs between specific matrix components and early mineral deposits has shown that unequivocal image interpretation is not possible. This is due principally to the problems posed by overprojection and truncation phenomena, since the structures being analyzed lie within the same dimensional range as thin section thickness. Various examples are illustrated and discussed. The site at which mineral crystals are initially laid down thus cannot be identified with any matrix structure using thin section TEM. Possible technical approaches to resolve this problem of image analysis are discussed.     

老龄飞机MSD结构剩余强度评定技术

讨论了多部位损伤(MSD)对结构安全性的影响和MSD结构安全性评定所赋予的新内容。通过等距等长共线孔孔边裂纹平板和等距等长共线裂纹平板的剩余强度计算,研究分析了MSD结构剩余强度评定中裂纹之间的相互作用、材料塑性、裂纹连通准则和破坏准则等关键技术。     

Contribution to the study of the vasculature of submandibular and sublingual glands and lymph nodes of rats by corrosion cast technique combined with scanning electron microscopy

The study of anatomical structures in their normal state allows the identification of pathological changes that can occur in them. Angiogenesis and the vasculature have been widely studied, mainly because of their association with the development of neoplasms. One of the methods applied for such purposes is the corrosion cast technique, which provides a copy of the vessels with normal as well as pathological structures. The replica of the vasculature provided by this technique allows the three-dimensional analysis of vessels by means of scanning electron microscopy. The aim of the present study was to demonstrate, by means of corrosion casts, the angioarchitecture of the submandibular and sublingual glands and lymph nodes. Scanning electron microscopy showed that the three structures have distinct vascular patterns. The corrosion cast technique can be employed in the study of the angioarchitecture of the submandibular and sublingual glands and lymph nodes, but requires specific precautions. The removal of the structures en bloc and the handling of the replicas with the aid of a stereoscopic magnifier reduce the risk of fractures.     

On bounding the life of structures subjected to steady load and operating within the creep range

The paper offers a simplified life assessment for structures nominally subjected to constant load and operating within the creep range. It is shown that the life of any structure, subjected to conditions where creep occurs, may be bounded by two procedures.In the particular case where the plastic yield surface is similar to the isochronous creep failure surface, the upper bound on life is obtained from a knowledge of the plastic collapse load. The lower bound approach requires a knowledge of the stationary state stress distribution. The authors argue that the actual time of failure relative to these bounds is governed by a material property rather than the geometry of the structure.The bounds on life have been applied to some simple structures where a more complete analysis or experimental data are available. It is shown that the upper bound solution gives a close approximation to the life of these structures provided that there is adequate ductility.