粘贴光纤检测复合材料梁的分层

将光纤干涉仪的一个臂粘贴在复合材料梁的表面,用逐点下压的方法实现了复合材料的分层检测.实验证明,在简单支撑下分层信息可能淹没在梁本身的变形中,但是在连续支撑下能够有效的检测出材料中是否存在分层以及分层的位置.实验测试和理论仿真获得了一致的结果.     

单向碳纤维复合材料分层缺陷垂直涡流检测有限元仿真研究

针对单向碳纤维复合材料分层缺陷难以检测的问题,设计了一个共面三矩形线圈探头,建立了利用该探头进行单向碳纤维复合材料分层缺陷垂直涡流检测的有限元模型,并利用该模型仿真计算了存在不同厚度分层缺陷时垂直涡流检测探头输出的感应电压信号变化情况。仿真结果表明,所设计的探头可灵敏地检测出单向碳纤维复合材料分层缺陷,并且在探头扫描检测分层缺陷的过程中,探头输出的感应电压虚部会出现成对的峰值和次峰值,峰值与分层厚度成正比,次峰值对的出现位置对应于分层缺陷边缘的位置,通过对峰值大小和次峰值对的出现位置进行分析,可以实现单向碳纤维复合材料分层缺陷厚度和范围的定量化评估,为实际工程应用提供了有价值的参考。     

有初始缺陷复合材料梁在湿热状态下混沌运动

在考虑剪切变形的影响基础上,研究了有初始缺陷复合材料梁在湿热状态下的混沌运动,并讨论分析了剪切变形、初始缺陷、温度、湿度等因素对混沌运动区域的影响,得到了以下结论：当温度升高时,非完善复合材料粱发生混沌运动区域增大;当初始缺陷程度增加,非完善复合材料梁的混沌运动区域越来越小;湿度升高时。非完善复合材料梁发生混沌运动区域增大;当考虑剪切变形影响时非完善复合材料梁的混沌运动区域变大;在湿热状态下理想复合材料梁的混沌运动区域要比非完善复合材料梁的混沌运动区域大。     

基于DSSPI的航空复合材料无损检测技术研究

复合材料在民用航空器结构上得到了广泛应用,但是复合材料与金属材料相比,受低能量冲击后会出现分层,产生肉眼看不到的损伤,极大地影响航空安全;如何快速准确地对其进行无损检测,并判断缺陷的位置大小是很有意义的;基于激光错位散斑干涉技术的航空复合材料无损检测技术方法,结合CCD剪切照相,并经过计算机进行数字图像处理来检测与识别航空复合材料的分层及损伤程度;为了验证技术的可行性,通过对预制损伤的蜂窝夹层复合材料标准试件进行了检测,很好地检测出了损伤位置及大小,因此技术是完全可行的,可以应用于复合材料的快速检测.     

复合材料整体成型过程分层缺陷动态扩展研究

分层是复合材料层合板最主要的缺陷/损伤形式,当前研究多集中于复合材料使用过程中的分层损伤,对制造过程中的分层缺陷研究较少.复合材料层合板的分层缺陷在整体成型过程中因不均匀温度场、非对称结构等原因易发生扩展,采用预埋隔离纸模拟分层缺陷,研究分层缺陷在整体成型过程中的扩展行为,对分层扩展的驱动力进行了计算分析,并通过分析热循环对T300/QY8911复合材料层合板界面性能的影响以及计算裂纹尖端能量释放率,从实验研究和有限元计算两方面证明了分层扩展是一个动态过程.研究结果表明,热残余应力是T300/QY8911复合材料层合板整体成型过程中发生分层缺陷扩展的主要驱动力;增加热循环次数和提高热循环温度会显著降低T300/QY8911复合材料层合板的层间剪切强度,当能量释放率降低到低于层间断裂韧性值时,分层动态扩展过程则停止.     

轴向可伸缩复合材料悬臂梁的非线性振动研究

以航空领域中可变体机翼的伸缩变形过程为研究对象,对可伸缩悬臂复合材料层合梁的时变非线性振动进行理论研究.建立可伸缩悬臂复合材料层合梁在外载荷作用下的非线性动力学模型;根据时变系数非线性动力学方程研究时变非线性振动特性.分析可伸缩悬臂复合材料层合梁在外伸与收缩变形过程中的非线性动力学特性.从数值结果上看:模型的外伸速度、飞行速度对振动的影响较大,初值对振动的影响较小.     

轴向运动复合材料薄壁梁的横向振动分析

研究可轴向运动复合材料薄壁梁的动力学特性.基于VAM（变分渐进法）复合材料薄壁梁理论,采用Euler Bernoulli梁模型并根据Hamilton原理来建立复合材料薄壁梁的动力学方程.应用假设模态法对薄壁梁进行自由振动分析,通过比较研究验证了该建模方法的正确性,并分析了几何参数和物理参数对薄壁梁固有频率的影响.推导了轴向运动复合材料薄壁梁的横向振动方程,借助四阶Runge-Kutta法进行数值计算,研究了不同纤维铺层方式和不同匀速度大小对可轴向运动复合材料薄壁梁横向振动末端位移响应的影响.     

一种适用于社交网络分析的分层社区检测算法

分层社区在社交网络中普遍存在,笔者针对社交网络分析中分层社区检测中容易将分层社区判定为独立社区的问题,提出一种基于层次压缩和扩展的分层社区检测算法。该算法定义了社区判定系数和聚类系数来描述节点的社区分层倾向,描述节点与社区的关系,缩小了社区检测的计算范围;通过迭代压缩、扩展等操作更新节点相关系数指标,最终算法复杂度为O(m+n)。在不损失检测质量的前提下,可以得到较好的检测效率。     

基于Python的复合材料船体梁极限强度分析

为在复合材料船体结构数值仿真分析中进行快速建模和分析,采用Python脚本语言和Abaqus前处理模块,开发出针对复合材料船体梁极限强度计算的通用模块.该模块考虑复合材料的各向异性、失效准则及铺层方式等因素.实例表明：所开发的程序界面友好、可操作性强,能有效提高复合材料船体梁极限分析的效率.     

复合材料磁悬浮列车车体结构数值模拟(III)——车体结构性能的参数化数值模拟

在磁悬浮列车车体参数化数值模型的基础上,开展参数变化对车体结构性能影响的数值试验,研究复合材料梁截面几何参数对车体刚度和频率的影响。在典型荷载工况下,研究关键设计参数对车体结构性能、结构部件连接模型的力学性能、车体频率和振型、车体结构线性屈曲性能的影响,确定关键设计参数对复合材料车体结构性能的影响趋势,为车体优化设计奠定基础,验证将参数化车体数值模型作为车体结构性能研究的有效性。     

不同脱层位置下脱层屈曲梁振动实验研究

通过实验对一端固定一端夹支脱层屈曲梁在轴向周期激励作用下的非线性动力响应进行了实验研究.利用位移时间历程图,相图和频谱图,对多组不同脱层位置下脱层屈曲梁的非线性动力响应进行了分析.实验表明脱层梁结构存在倍周期以及混沌运动等非线性动力学行为.同时实验还表明,在相同的脱层长度下,脱层位置对脱层梁的动力学特性有明显影响,即脱层区域中心越靠近梁结构的中心位置,脱层梁的一阶自然频率越低,且越容易在较低的激励频率和激励荷载下发生周期分叉和混沌等行为.     

Probabilistic free vibration analysis of beams subjected to axial loads

A stochastic finite-element-based algorithm for the probabilistic free vibration analysis of beams subjected to axial forces is proposed in this paper through combination of the advantages of the response surface method, finite element method and Monte Carlo simulation. Uncertainties in the structural parameters can be taken into account in this algorithm. Three response surface models are proposed. Model I: star experiment design using a quadratic polynomial without cross-terms; Model II: minimum experiment design using a quadratic polynomial with cross-terms; Model III: composite experiment design using a quadratic polynomial with cross-terms.A separate set of finite element data is generated to verify the models. The results show that the Model II is the most promising one in view of its accuracy and efficiency. Probabilistic free vibration analysis of a simply supported beam is performed to investigate the effects of various parameters on the statistical moments of the frequency response of beams. It is found that the geometric properties of beams have significant effects on the variation of frequency response.     

Dynamic finite element analysis of laminated beams with delamination cracks using contact-impact conditions

A new finite element modeling technique is presented to investigate the static and dynamic behavior of laminated composite beams with partial delamination. In this study, a recently developed rectangular beam element is used. The element has lateral and axial displacements as degrees of freedom, but not rotation. For simplicity, linear shape functions are used for the beam element. As a result, the element has six degrees of freedom, four of which are the axial nodal displacements at the corner points and the other two are the lateral displacments at the ends. In addition, contact-impact conditions are applied to the finite element modeling to avoid overlapping of the upper and lower portions of a delaminated section. The numerical study shows that, depending on existence of an embedded delamination crack and its size, the response is different for a beam with a crack and subjected to a short impulse load. Hence, the present modeling technique may be used for detection of an embedded delamination crack.     

Thermoelastic dissipation in stepped-beam resonators

In recent years, stepped beam resonators have found broad application in MEMS/NEMS devices. A beam resonator with an undercut at the support, produced due to isotropic etching of the supporting substrate during fabrication, has also been characterized as stepped beam in the literature. The present study deals with thermoelastic dissipations of clamped–clamped stepped beam resonators under adiabatic surface thermal conditions having j (j = 1, 2, …., n) number of sections defined by (j ? 1) number of steps along the length. Numerical results are obtained for three different types of stepped beams of rectangular cross-section having single step such as beams with cross-sectional change at the step only in lateral direction (type-1), in bending direction (type-2), and in both lateral and bending directions (type-3) where the section on the right of the step possesses smaller cross-sectional size compared to the other. The obtained results show that Q-factors vary significantly with step positions in all the three types of stepped beams. For constant length, the Q-factor increases in the type-1 while it decreases in other two types of stepped beams as the step position moves from the left support to the right along the length. Moreover, Q-factors in a type-1 stepped beam depend on the widths of different sections and can be higher than a uniform beam of same thickness for some particular step positions. For most common lengths of stepped beams in real applications with the step close to the left support, type-1 stepped beams provide higher quality factors than the other stepped beams provided that they have the same cross-sectional area.     

Effect of support compliance and residual stress on the shape ofdoubly supported surface-micromachined beams

Doubly supported silicon-micromachined beams are increasingly used to study the mechanical properties of materials. Residual stresses in the beams and support compliance cause significant vertical deflections, which affect the performance of these micromachined devices. We present here both experimental results for doubly supported polysilicon surface-micromachined beams, and an elastic model of the devices that takes into account the compliance of the supports and the geometrical nonlinear dependence of the vertical deflections on the stress in the beam. An elastic one-dimensional model was used for the beams, and the response of the supports to forces and moments was obtained using finite-element method simulations. The model explains a previously observed gradual increase of the maximum vertical deflections of the beams with increasing length at a given constant residual stress and, in agreement with experimental observations, predicts two stable states for compressively stressed beams: one with the beam bent up, the other down. We introduce a critical biaxial residual stress σr_c, above which there are significant deflections of the beams, σ_rc can be used in practice to determine the maximum allowable compressive biaxial residual stress as a function of the beam length. The effect of variations of the compliance of the supports is reported     

Free vibration analysis of beams on elastic foundation

A simple finite element method is developed and applied to treat the free vibration analysis of beams supported on elastic foundations. The entire analysis is programmed to run on a microcomputer and with few elements modelling the beam, gives quick and reliable results. Numerical examples pertaining to the free vibration of beams in some special situations are considered, such as a stepped beam on an elastic foundation, beam on a stepped elastic foundation and a continuous beam on an elastic foundation. Present results compare very well with those obtained from existing solutions, wherever possible.     

A novel method to fabricate single crystal nano beams with (111)-oriented Si micromachining

Single crystal beams made by (111)-oriented Si micromachining are usually anchored to the substrate directly. It is difficult to use the beams as resonators, since they are electrically connected to the substrate. This paper presents a modified process to fabricate single crystal nano beams which are electrically isolated from the substrate. In this process, the single crystal nano beams are fully released from the substrate and mechanically supported by metal wires, which also serve as electrical connections. The metal wires are much stiffer than the beams and do not degrade the mechanical properties of the beam according to simulations. The length and width of the beams are determined by photolithography. The thickness of the beam and the gap between the beam and the substrate are determined by the dry etching and KOH etching processes. The influence of KOH etching on the beam thickness is documented through ongoing experiments. At present, the double clamped beams and the cantilever beams have been fabricated. The thinnest beam to date was measured to be 47 nm. The resistance between the beam and the substrate was measured to be 214GΩ, while the resistance of a 147 nm-thick beam was measured to be 816 Ω. The surface roughness of the (111) plane is also discussed. The RMS surface roughness of the nano beam was measured to be 1.08 nm in an area of 5 μm × 5 μm, which was etched with 45%wt. KOH at 50°C.     

Simulation of the dynamic response of concrete beams externally reinforced with carbon-fiber reinforced plastic

Laboratory-scale concrete beams 3 × 3 × 30 in (7.62 × 7.62 × 76.2 cm) in size were impulsively loaded to failure in a drop-weight impact machine. The beams had no internal steel reinforcement, but instead were externally reinforced on the bottom or tension side of the beams with one-, two- and three-ply unidirectional carbon-fiber reinforced plastic (CFRP) panels. In addition, several of the beams were also reinforced on the sides, as well as the bottom, with three-ply CFRP. The beams were simply supported and loaded at beam midspan, and sustained dynamic loads with amplitudes up to 10 kips (44.5 kN) and durations less than 1 ms. Experimental measurements included total load, midspan displacement and strains, and a high-speed framing camera (10 000 frames s⁻¹) which gave insight into the failure mechanisms. Dynamic beam behavior was also studied numerically using the finite-element method. The numerical simulations gave insight into the localized displacement behavior of the beams when subjected to an intense, impulsive load, as well as the phenomenon known as the ‘traveling plastic hinge.’ Results showed that the numerical simulations did very well in predicting the displacement-time behavior of two beam types studied in detail; a plain concrete beam and a beam reinforced with three-ply CFRP on the bottom.     

Beam element with spatial variation of material properties for multiphysics analysis of functionally graded materials

In this paper, two different methods for modelling of functionally graded material (FGM) beam with continuous spatially varying material properties will be presented and compared, namely the multilayering method and the direct integration method. Both the methods are related to homogenization of spatially varying material properties of real FGM beam and to calculation of the secondary variables of the FGM beams. The multilayering method is based on the laminate theory, which is very often used by modelling of the multilayer composite beams. The direct integration method transform spatial continuous varying material properties to the effective ones by direct integration of derived homogenization rules. In next part of the paper, new multiphysical beam finite element will be presented, which in conjunction with the proposed homogenization methods can be used for very effective analysis of the FGM beam structures. The numerical experiment will be presented concerning the multiphysical (electro–thermal–structural) analysis of the chosen FGM beams with spatial continuous variation of material properties.