Low-velocity Impact Damage Analysis of Composite Laminates Using Self-adapting Delamination Element Method

On the basis ofa 2D 4-node Mindlin shell element method, a novel self-adapting delamination finite element method is presented, which is developed to model the delamination damage of composite laminates. In the method, the sublaminate elements are generated automatically when the delamination damage occurs or extends. Thus, the complex process and state of delamination damage can be simulated practically with high efficiency for both analysis and modeling. Based on the self-adapting delamination method, linear dynamic finite element damage analysis is performed to simulate the low-velocity impact damage process of three types of mixed woven composite laminates. Taking the frictional force among sublaminations during delaminating and the transverse normal stress into account, the analytical results are consistent with those of the experimental data.

A Two-step Inverse Method for Quantitative Damage Evaluation of Plate-like Structures Using Vibration Approach

This study presents a novel two-step approach to assess plate-like structural laminar damages,particularly for delamination damage detection of composite structures.Firstly,a 2-D continuous wavelet transform is employed to identify the damage location and sizes from vibration curvature data.An inverse method is subsequently then used to determine the bending stiffness reduction ratio along a specified direction,enabling the quantification of the delamination severity.The method employed in this study is an extension of the one-dimensional inverse method developed in a previous work of the authors.The applicability of the two-step inverse approach is demonstrated in a simulation analysis and by an experimental study on a cantilever composite plate containing a single delamination.The inverse method is shown to have the capacity to reveal the detailed damage information of delamination within a constrained searching space and can be used to determine the effective flexural stiffness of composite plate structures,even in cases of complex delamination damage.

DETECTION OF DELAMINATION IN A COMPOSITE PLATE BY SEM

A numerical method of integration of Green's functions of strip element method (SEM) is proposed The response of ultrasonic source generated by a transducer on the surface of a multi ply composite plate containing a delamination is analyzed by the use of SEM The numerical results show that the scanning features of the ultrasonic waves may be used to identify the delamination inside the composite plate.

F E M ANALYSIS OF DELAMINATION BUCKLING IN COMPOSITE PLATES AND SHELLS

The purpose of the present study is to develop a new finite element method far analyzing buckling of delaminated composite plates and shells. This is achieved by establishing a new finite element called the reference-surface element. By use of the compatibility condition under Mindlin assumptions, the formulation of the reference-surface element was derived from whichever plate-element or shell-element being capable of analyzing composite plates and shells. This method assures a reasonable description of displacement field and the satisfaction of compatibility conditions for delamination problem. Numerical results for linear delamination buckling of axially compressed shells are presented to validate the method.

NUMERICAL ANALYSIS OF DELAMINATION GROWTH FOR STIFFENED COMPOSITE LAMINATED PLATES

A study of postbuckling and delamination propagation behavior in delaminated stiffened composite plates was presented. A methodology was proposed for simulating the multi-failure responses, such as initial and postbuckling, delamination onset and propagation, etc. A finite element analysis was conducted on the basis of the Mindlin first order shear effect theory and the von-Krmn nonlinear deformation assumption. The total energy release rate used as the criteria of delamination growth was estimated with virtual crack closure technique (VCCT). A self-adaptive grid moving technology was adopted to model the delamination growth process. Moreover, the contact effect along delamination front was also considered during the numerical simulation process. By some numerical examples, the influence of distribution and location of stiffener, configuration and size of the delamination, boundary condition and contact effect upon the delamination growth behavior of the stiffened composite plates were investigated. The method and numerical conclusion provided should be of great value to engineers dealing with composite structures.

Delamination Identification for FRP Composites with Emphasis on Frequency-Based Vibration Monitoring-A Review

Fibre reinforced polymer (FRP) composite laminates are now commonly usedin many structural applications, especially in the aerospace industry, where margins ofsafety are kept low in order to minimise weight. Timely detection and assessment ofdamage (in particular delaminations) in composite laminates are therefore critical, as theycan cause loss of structural integrity affecting the safe operation of the composite structures.The current trend is towards implementation of structural health monitoring (SHM)systems which can monitor the structures in situ without down time. In this paper, first, thecurrent available SHM techniques for delamination detection in FRP composites arebriefly reviewed, including acoustic emission, fibre optic sensors, Lamb wave-,impedance- and vibration-based methods. Among different vibration-based methods,frequency monitoring is the simplest to implement, requiring only single pointmeasurement, and is relatively accurate and reliable, thus it becomes the main focus ofpresent paper. A comprehensive review of frequency-based vibration monitoring isconducted in terms of the various aspects of delamination identification in FRPs throughfrequency shifts, including review of theoretical models for free vibration of delaminatedFRP beams, survey of finite element modelling of delaminated composite structures,summary of experimental modal analyses on FRP composites with delaminations, andinverse algorithms for frequency-based delamination assessment. This paper aims to helpthe readers to get an overview of the available SHM techniques for monitoring the integrityof FRP composites, with a special emphasis on delamination assessment throughfrequency-based vibration monitoring.

基于分割环阵的CFRP分层缺陷超声全聚焦成像

碳纤维复合材料在钻孔加工时易产生分层缺陷,分层缺陷严重影响了构件的力学性能,存在严重的安全隐患。针对碳纤维复合材料孔边分层缺陷的检测,提出一种基于分割环形阵列的1/4矩阵全聚焦成像方法。设计了R4×S8、R3×S12、R3×S16和R4×S12四种分割环形阵列结构,通过数值仿真对比分析了各阵列的聚焦声场特点,最终确定分割环阵探头为5 MHz的R4×S12探头。使用该探头对碳纤维复合材料孔边分层试块的上层、中层、下层缺陷进行全矩阵采集,利用VTK工具包分别进行全矩阵三维成像和1/4矩阵三维成像。结果表明,1/4矩阵方法比全矩阵方法三维成像的缺陷对比度更高,1/4矩阵方法的缺陷尺寸表征误差更小,误差不超过6%。与全矩阵成像相比,1/4矩阵成像信噪比提升范围为3.43~7.61 dB,有效提升了图像质量。

陶瓷基复合材料涡轮动叶榫头元件静强度实验及缺陷影响

陶瓷基复合材料(ceramic matrix composites,CMC)涡轮动叶榫头是动叶装配与承受离心载荷的关键。为了研究熔渗工艺制备CMC榫头在叶片旋转径向拉伸载荷下的力学行为,验证榫头内部质量对其静拉伸强度与破坏模式的影响,设计并制备燕尾形CMC高压涡轮动叶榫头元件试件,对其进行单轴静态拉伸实验,采用X射线CT无损检测对实验前试件内部质量进行扫描,采用DIC与声发射方法对实验过程进行监测。结果表明:燕尾形CMC榫头在单轴静态拉伸下保持完好接触,其静态拉伸强度与破坏模式对其内部质量(尤其是分层缺陷)非常敏感。试件不含分层缺陷时,损伤起始于榫头颈部,并迅速扩展,损伤起始载荷与最大破坏载荷接近,断口呈横向锯齿状;试件内部含分层缺陷时,损伤起始于缺陷,分层逐渐扩展并导致试件断裂,损伤起始载荷下降99.05%,最大破坏载荷下降14.29%,断口位置与分层缺陷一致。

Analysis of energy release rate for composite delaminated cylindrical shells subjected to axial compression

In this paper, the postbuckling governing equations and the analytical expression of the energy release rates associated with delamination growth in a compression-loaded cylindrical shell are derived by using the variational principle of moving boundary and the Griffith fracture criterion. The finite difference method is used to generate the postbuckling solutions of the delaminated cylindrical shells, and with these solutions, the values of the energy release rates are determined. In simulational examples, the effects of a wide range of parameters, such as delamination sizes and depths, boundary conditions, geometrical parameters, material properties and laminate stacking sequences on the energy release rates of axisymmetrical laminated cylindrical shells are intensively discussed.

基于均匀网格的复合材料层合板分层损伤建模

基于扩展逐层法(XLWM,extended layerwise method)、多维最大熵原理和分层前缘跟踪算法,建立了一种用于分析复合材料层合板分层损伤的均匀网格快速建模和分析方法(FUGD,fast uniform-grid delamination)。利用该方法可通过C扫描(或X射线)图像和二维均匀有限元网格快速建立分层损伤分析模型。首先,基于多维最大熵原理和扫描图像上的像素点灰度值重建分层区域,处于分层区域的有限元节点被标记为分层节点;然后,通过分层前缘跟踪算法和已标记为分层的节点精确地跟踪分层前缘;最后,以含方形、圆形、双圆形分层区域的方形层合板为例,验证该方法的有效性。

超薄层羟镁铝石纳米片的制备和应用

羟镁铝石是水中有害阴离子组分的有效去除剂,且在处理过程中不引发二次污染,但难以经传统阴离子黏土合成技术制备。利用优化后机械化学方法合成了高纯羟镁铝石,并进一步通过片层剥离技术成功制备了超薄层羟镁铝石纳米片,避免了传统方法制备的羟镁铝石因颗粒团聚而吸附性能无法充分发挥的情况。文中尝试了6种分层剂(甲醇、乙醇、正丁醇、丙酮、天冬酰胺、甲酰胺),其中甲酰胺分层效果最佳,由其制备的超薄层羟镁铝石片层纤薄、孔隙发育,比表面积达163.3 m^(2)/g,对典型有害组分Cr(Ⅵ)、Sb(Ⅲ)、Sb(Ⅴ)的吸附容量则分别可达12.98、25.46、29.18 mg/L,显著高于未分层羟镁铝石及其他分层剂处理后的羟镁铝石。超薄层羟镁铝石纳米片在水污染处理领域有广阔应用前景。

Investigating the influence of delamination on the stiffness of composite pipes under compressive transverse loading using cohesive zone method

The effect of delamination on the stiffness reduction of composite pipes is studied in this research.The stiffness test of filament wound composite pipes is simulated using cohesive zone method.The modeling is accomplished to study the effect of the geometrical parameters including delamination size and its position with respect to loading direction on stiffness of the composite pipes.At first,finite element results for stiffness test of a perfect pipe without delamination are validated with the experimental results according to ASTM D2412.It is seen that the finite element results agree well with experimental results.Then the finite element model is developed for composite pips with delaminated areas with different primary shapes.Thus,the effect of the size of delaminated region on longitudinal and tangential directions and also its orientation with respect to loading direction on delamination propagation and stiffness reduction of the pipes is assessed.

连续油管分层缺陷磁导率扰动电磁检测方法与检测系统

板材的分层缺陷检测通常采用超声测厚法实现,但超声测厚法对近表层检测存在盲区,且高速检测系统复杂、成本高。本研究提出一种连续油管分层缺陷的磁导率扰动电磁检测新方法,并设计了相应的检测系统。该系统利用永磁体沿钢管或钢板的法向施加磁力,管板内分层产生内部磁场的畸变,进而引起局部区域的磁导率扰动,这一扰动沿厚度方向传递到表层,在空间上差动的涡流探头测量表层磁导率的相对变化,即可实现内部分层缺陷的检测与评估。从机理上阐明了分层缺陷的埋深对测点处相对磁导率变化的影响;随后,基于有限元模型研究了分层的长度、埋深、分层厚度等因素引起的测点处磁导率变化;最后,试验验证了理论分析和仿真结果的有效性。

轮胎结构中层间裂纹扩展过程的数值模拟

轮胎是一个十分复杂的橡胶基复合材料结构,在使用过程中常常发生层间脱层破坏。提出了一种新的复合材料结构中层间脱层裂纹扩展模型,并利用Irwin的虚拟裂纹闭合技术和有限元分析技术对轮胎结构带束层中的层间脱层裂纹扩展过程进行了数值模拟。结果表明,提出的模型能够比较真实地揭示复杂复合材料结构层间脱层裂纹扩展复杂性的特点。

流媒体同步机制的研究

多媒体同步是多媒体应用中的一个关键问题,介绍了较流行的几种多媒体同步方法,并对这些方法进行了简单的比较,在此基础上提出了一种基于流媒体传输技术和RTP/RTCP协议的同步模型,该模型有机地结合了分层同步法和时间戳同步法,实现了媒体间不同粒度的同步。

有限元方法对SCSP粘结剂溢出问题的研究

利用有限元分析方法对 SCSP器件内部粘结剂的溢出问题进行了研究 .对粘结剂不同溢出高度的模型进行有限元建模分析 ,模拟结果能很好地和实验结果相吻合 .为了有效减少由热应力引发产生的分层 ,模拟得到了粘结剂溢出高度的最佳控制范围 .

含椭圆分层损伤复合材料层合板分层前缘接触效应研究

采用基于 Mindlin一阶剪切理论的四节点板单元 ,分析了分层前缘闭合接触效应对含椭圆分层复合材料层合板前后屈曲行为的影响。通过在分层区域内引入虚杆单元来防止上、下子板发生相互重叠 ,并假设接触区内没有摩擦。结果表明 ,分层前缘的闭合接触效应受分层几何形状的影响很大 ,当椭圆分层子板的长轴与压缩载荷方向一致时 ,闭合接触效应对含分层层合板前后屈曲行为最大。

高耗能企业关口平衡问题的双目标规划模型

针对高耗能企业关口平衡问题的多解性特点,提出了关口平衡双目标规划模型,以研究关口平衡合格率、关口流量和机组出力波动性之间的关系。采用线性规划对关口平衡问题进行建模分析,以整个调度周期内不满足关口平衡条件的关口流量惩罚值总和最小作为模型的第一优化目标,并引入整个调度周期内机组调节量惩罚值总和最小作为第二优化目标,分别采用线性加权求和法和分层序列优化法对该双目标优化模型进行求解。基于国内某大型钢铁企业数据的实验结果表明,双目标规划模型能够在不增加关口流量惩罚值总和的情况下,有效减少机组出力波动。

多分层对复合材料层合板自振特性的影响

基于Mindlin一阶剪切理论分项等参插值的有限元法,建立了含多个分层损伤复合材料层合板自由振动分析的有限元模型和分析方法,并采用线性接触模型模拟分层区域上、下子板的相互作用。通过典型数值算例,讨论了分层位置、数目及板的边界条件诸参数对其振动特性的影响。结果表明:分层位置沿板长方向变化时,中间分层的频率变化范围较大,表面分层变化较小,但变化趋势基本相同;沿层合板厚度方向,多分层中最长分层的位置越靠近层合板中面,则其对振动特性的影响越大;多个分层位置较靠近层合板表面,且板边界条件约束较弱时,多分层与单分层对振动特性影响的差别不大,此时,可将多分层损伤层合板简化为单分层损伤层合板来进行振动分析。

二元结构土层分布区工程降水诱发地面沉降变形研究

基于南宁市地铁1号线广西大学站上部为透水性较差的黏性土层,下部为透水性好的砂砾石土层,在分析基坑降水诱发地面沉降机理的基础上,探讨二元结构土层中附加应力和压缩土层分层厚度的确定方法,最后通过分层总和法计算得到因降水引起的地面沉降。研究结果表明:计算得到的地面沉降与实测沉降基本吻合,表明所提出的沉降参数确定方法合理、可靠,对类似工程具有借鉴作用。