复合材料层压板钻孔分层激光超声检测方法

飞机复合材料结构钻孔分层的定位、定量检测是无损检测领域的难点之一,也是航空制造领域亟待解决的安全问题之一,激光超声检测技术是解决该问题的可能途径。试验验证利用激光超声检测复合材料钻孔分层的技术可行性。制备复合材料层压板钻孔试样,研究热弹性条件下脉冲激光在复合材料中产生超声波的宽频带特性,提取出满足检测灵敏度、分辨力要求并具有良好信噪比的超声信号;分析近孔边缘分层界面对声传播规律的影响,得出钻孔边缘分层缺陷的激光超声表征方法;采用脉冲反射法、透射法对复合材料钻孔试样进行激光超声C扫描检测,获得钻孔分层缺陷的形貌、尺寸和位置特征。研究结果表明,激光超声检测技术是解决飞机复合材料结构钻孔分层检测问题的有效手段。     

基于空耦超声共振法的复合材料测厚技术研究

为解决当前传统超声检测技术对复合材料的检测耦合剂污染和检测效率低等问题,提出空气耦合超声共振法来检测复合材料薄板的厚度,从而确定缺陷的大小。利用 COMSOL 有限元仿真软件对复合材料进行建模,设置不同厚度及不同大小缺陷的物理模型来对比实验,后处理求解并进行快速傅里叶变换,提取谐振频率计算出复合材料的厚度。结果表明,超声共振法可对复合材料进行定性、定量检测;当复合材料的厚度越薄时,超声信号产生的谐振频率越大,则复合材料中所含缺陷范围越广,分层现象越严重;其检测精度可达 0.1 mm 左右,相对误差范围分布在 5% 以下。实验证明了该测厚技术的可靠性,为超薄复合材料板缺陷厚度的测量提供一定的参考和借鉴。     

机器人辅助激光超声检测系统及参量匹配方法

为解决飞行器复合材料构件的非接触、高精度无损检测问题,提出基于关节型机器人的激光超声检测系统及光声学参量匹配方法。采用有限元方法建立层状复合材料模型,计算分析材料层状各向异性导致激光超声的非对称分布、声束倾斜和畸变特征,结合实验分析得出利用激光超声表征分层的光声学参量匹配方法。在系统设计上,利用1 064 nm波长的Nd:YAG脉冲激光器激励超声波,利用基于光折变效应的双波混合干涉测量系统探测超声信号,激励和探测激光由光纤传导并投射至被检测工件表面,采用精密六轴关节型机器人作为C型扫描装置,建立系统的实验室原型,实现碳/环氧复合材料试样的C型扫描检测,得到试样中模拟缺陷的分布、形状和尺寸特征,验证了检测系统及参量匹配方法的有效性。研究结果表明,研制的机器人辅助激光超声检测系统可以实现碳/环氧复合材料内部直径1 mm以上分层的检测与成像,在飞行器复合材料构件的无损检测方面具有应用前景。     

层合板低速冲击损伤的数值模拟

为了分析复合材料层合板在冲击载荷作用下,层合板的损伤演化发展情况,结合采用单元失效技术和铺层刚度退化技术建立层合板低速冲击的三维有限元分析模型。在该模型中,当有限元模型中的单元发生某种冲击失效形式时,定义该单元发生部分失效,并将其刚度适当的退化。计算发现冲击背面容易产生基体开裂,并由此导致分层发生,而且靠近冲击背面的界面所产生的分层面积要较靠近冲击正面的界面的分层面积要大;随着冲击能量的增大,分层面积也增大;当冲击能量很高时,铺层内会出现纤维断裂,同时在层合板的边界处也容易出现损伤,计算结果和试验结果吻合。     

基于超声复解析信号的碳纤维层压复合材料检测方法研究

采用超声波检测碳纤维层压复合材料时,换能器将接收到层间产生的反射信号。为了有效利用层间反射信号表征碳纤维层压复合材料内部树脂及纤维铺层的几何状态,开展基于超声复解析信号的检测方法研究。介绍超声复解析信号的基本理论,阐述其相对原始A型信号的优势;定义用于成像的复解析信号表征参量,设计基于表征参量的检测成像算法,采用简化声学模型分析碳纤维层压复合材料微观结构超声响应对表征参量的影响;针对包含冲击损伤的碳纤维增强复合材料层压板开展阵列超声检测试验,试验结果表明基于超声复解析信号的检测成像方法可清晰观测出碳纤维层压复合材料微观结构内部的纤维铺层几何形态,基于原始A型信号的检测成像方法无法有效观测,超声复解析信号具有用于表征评价复合材料脱层、褶皱等缺陷的巨大潜力。     

Energy release rate of postbuckled laminates with a through-width delamination

The strain energy release rate is calculated for buckled one-dimensional delamination (through-width delamination) in composite laminates subjected to in-plane compression. A crack closure method based on plate finite elements is used in this analysis. For some laminates containing a one-dimensional delamination in cylindrical bending, closed form solutions are available. The present finite element solutions show excellent agreement with the analytical solutions. The strain energy release rate for various types of laminates is also calculated using the present finite element method. The results show that the strain energy release rate strongly depends on the type of laminate.     

超声铣削复合材料应力场分析与试验研究

为解决复合材料难加工问题,本文采用超声铣削和普通铣削加工方法对复合材料进行试验研究。试验结果表明,在超声加工条件下,由于外加的周期性振动使铣削力大大减小,使得已加工表面产生较小的应力场,则纤维束和碳基体交界处微裂纹扩张速度减慢和分层不明显;而在普通加工条件下,由于产生较大的铣削力,则纤维束和基体交界处裂纹扩展迅速,且分层也十分明显,工件表面纹理质量很差。     

Vibration analysis of delaminated composite beams and plates using a higher-order finite element

In order to analyze the vibration response of delaminated composite plates of moderate thickness, a FEM model based on a simple higher-order plate theory, which can satisfy the zero transverse shear strain condition on the top and bottom surfaces of plates, has been proposed in this paper. To set up a C⁰-type FEM model, two artificial variables have been introduced in the displacement field to avoid the higher-order derivatives in the higher-order plate theory. The corresponding constraint conditions from the two artificial variables have been enforced effectively through the penalty function method using the reduced integration scheme within the element area. Furthermore, the implementation of displacement continuity conditions at the delamination front has been described using the present FEM theory. Various examples studied in many previous researches have been employed to verify the justification, accuracy and efficiency of the present FEM model. The influences of delamination on the vibration characteristic of composite laminates have been investigated. Especially the variation of ‘curvature of vibration mode’ (i.e., the second-order differential of deflections in vibration mode) caused by delamination has been studied in detail to provide valuable information for the possible identification of delamination. Furthermore, two approaches have been investigated to detect a delamination in laminates by employing this information.     

多元变参数振动钻削复合材料的研究

根据复合材料力学和线弹性断裂力学，假设钻削轴向力为一集中作用载荷，从而构造了正交纤维束增强复合材料的钻削分层临界力的数学模型，仿真结果表明，随着待钻削深度的减小，分层临界力急剧降低。基于这一特点，提出了多元变参数振动钻削纤维增强复合材料的新思路，并进行了实验验证。     

Buckling analysis of a laminated composite plate with delaminations using the enhanced assumed strain solid shell element

The paper deals with the validation of a recently proposed hexahedral solid-shell finite element in the buckling analysis of a laminated composite plate with delaminations. The object is to study the buckling behavior of structures with delaminations using the enhanced assumed strain (EAS) solid shell element with 5, 7 and 9 parameters. The EAS three-dimensional finite element formulation presented in this paper is free from shear locking and leads to accurate results for distorted element shapes. The developed FE model is used to study the effects of some parameters in the buckling load, such as the stacking sequences, delamination size, aspect ratio, width-to-thickness ratio. The feasibility of the proposed method is confirmed by numerical examples. Results show that using hexahedral solid-shell finite element in the buckling analysis is more efficient than using the enhanced solid finite element.     

The compressive behavior of CFRP laminates with delamination of different shapes using numerical and experimental approaches

Composite laminates often produce delamination due to a series of factors during the manufacture and service process. In order to research the effect of containing oblique elliptical and circular initial delamination damage on the compressive strength of composite laminates, numerical and experimental methods are used in this paper. Finite element models (FEMs) and the progressive damage subroutine USDFLD are developed to predict the damage initiation and extension behavior of the intralaminar of the laminate. Interlaminar damage is predicted based on cohesive zone models (CZM). At the same time, four compression tests containing different initial delamination damage are performed. The results show that experimental measurements of compression tests concur with the numerical predictions and validate the FEMs. The strength and stiffness of the specimens gradually decrease with the increase of the initial delamination area. It reveals that, for the compression process, the delamination damage extends from the edge of the initial damage to the surroundings. The bearing capacity of the composite laminates is mainly determined by the material properties of the fibers and matrix, rather than the initial delamination.

     

Suppression of spurious vibration of cantilever in atomic force microscopy by enhancement of bending rigidity of cantilever chip substrate

To improve the precision of dynamic atomic force microscopy (AFM) using cantilever vibration spectra, a simple but effective method for suppressing spurious response (SR) was developed. The dominant origin of SR was identified to be the bending vibration of the cantilever substrate, by the analysis of the frequency of SR. Although a rigid cover pressing the whole surface of the substrate suppressed SR, the utility was insufficient. Then, a method of enhancing the bending rigidity of the substrate by gluing a rigid plate (clamping plate, CP) to the substrate was developed. This chip can be used with an ordinary cantilever holder, so that the reproducibility of SR suppression when attaching and detaching the cantilever chip to the holder was improved. To verify its utility, the evaluation of a microdevice electrode was performed by ultrasonic atomic force microscopy. The delamination at a submicron depth was visualized and the detailed variation of the delamination was evaluated for the first time using clear resonance spectra. The CP method will particularly contribute to improving dynamic-mode AFM, in which resonance spectra with a low quality factor are used, such as noncontact mode AFM in liquid or contact resonance mode AFM. The effect of the CP can be achieved by fabricating a substrate with a thick plate beforehand.     

Low-velocity impact-induced delamination detection by use of the S0 guided wave mode in cross-ply composite plates: A numerical study

Finite element method based numerical simulations are performed to identify low-velocity impact-induced asymmetrically-located delamination in the [0/90₃]_S and [0/90]_2S composite plates, respectively, using a fundamental symmetric guided wave mode (S₀). The wave attenuation effect due to the viscoelasticity of the composite material is modeled by calculating the Lamb wave attenuation constants and using the Rayleigh proportional damping model. The estimated sizes and locations of the delamination in both plates were in good agreement with the experimental measurements. Moreover, the analysis of wave structure of the impacted plates shows that when the S₀ mode propagates through the damaged region, the delamination mouth opens up due to the presence of standing waves, which are generated as a consequence of multiple reflections of trapped waves with the delamination boundaries.     

复合材料结构损伤联合定位法试验研究

提出了复合材料结构损伤联合定位法,该法需首先获得复合材料结构在随机激励下的振动响应信号,计算结构的互相关函数幅值向量,通过对损伤结构的互相关函数幅值向量进行光滑拟合作为参考向量,再对损伤情况下的互相关函数幅值向量和参考向量分别进行连续小波变换,得到各自的小波系数,进而求得小波系数差的模,根据小波系数差的模的极值进行损伤的定位。该方法无需进行结构建模和模态识别,在无完好结构信息的情况下就可准确进行损伤定位。最后,还通过蜂窝夹层梁和玻璃纤维层合板的损伤检测试验,验证了该方法的有效性。     

CF/Al复合丝的超声浸渗制备工艺与性能

探讨了超声液相浸渗法在制备碳（石墨）／铝复合丝过程中的适应性,通过选取适当的工艺参数和,包括纤维去胶及预热温度、铝液温度、浸渗速度和引入的超声能量等,可以得到复合质量较好、具有较高力学性能的复合丝。复合丝拉伸断口的扫描电镜（ＳＥＭ）观察可怕面有一定起伏,部分纤维拔出但长度较短,表明纤维与基体具有适当的界面结合。     

Examination and modification of equivalent delamination factor for assessment of high speed drilling

Composite components suffer delamination at the entrance and exit of drilled holes. Many measures have been suggested by different researchers to assess such delamination damage. These include delamination factor, two-dimensional delamination factor, damage ratio, adjusted delamination factor, refined delamination factor, equivalent delamination factor, and minimum delamination factor. Among all these various assessment factors, the equivalent delamination factor looks simple and able to take into account the different features of delamination. However, the method of calculation of the equivalent delamination factor may not provide accurate values for delamination resulting from high speed drilling. The aim of this paper is to evaluate the equivalent delamination factor in high speed drilling of a composite laminate using a twist drill and develop a new approach to determine equivalent delamination factor which can be used for both conventional and high speed drilling conditions. This new method is applied to calculate the equivalent delamination factor in trials of drilling composite specimens at different speeds and feed rates and is found suitable.

     

基于模态分析和BP神经网络的复合材料脱层损伤监测研究

基于假定自然应变法建立了一个新的八节点压电固体单元，并采用具有相同坐标但不同节点号的节点对模拟脱层，分析了含不同脱层损伤梁的模态特性；进而提出了一种将计算力学、神经网络和实验模态分析相结合的复合材料结构脱层损伤检测的新方法。该方法通过数值模拟的手段为神经网络提供充足的训练样本，以实验模态结果作为神经网络的输入来预测复合材料结构的脱层损伤，实验结果证明了这一方法的可行性。     

Post failure analysis of 0°/90° ultra high molecular weight polyethylene composite after ballistic testing

The ballistic performance of a 0°/90° ultra high molecular weight polyethylene (UHMWPE) composite was investigated. Two areal densities were considered: one for fragmentation protection and one for high velocity bullet protection.
The role of microscopy in the post-failure analysis of the UHMWPE was explored. Stereo, optical and environmental scanning electron microscopy were utilized and attempts were made to correlate the results obtained with other characterization techniques available. These included high speed photography, visual inspection and ultrasonic inspection.
Visual inspection revealed the gross effects of ballistic testing. These primarily included deformation and delamination of the composite plate. The result obtained corresponded with the predicted impact event and, for high velocity bullet impact, corresponded well to the images obtained using high speed photography. Optical, stereo and electron microscopy allowed for fibre and matrix characterization. Fibre displacement and pull-out were also observed on impact and exit faces.     

A comparative study on free vibration analysis of delaminated torsion stiff and bending stiff composite shells

This paper presents a finite element method to compare the effects of delamination on free vibration of graphite-epoxy bending stiff and torsion stiff composite pretwisted shallow conical shells. The generalized dynamic equilibrium equation is derived from Lagrange’s equation of motion neglecting the Coriolis effect for moderate rotational speeds. An eight noded isoparametric plate bending element is employed incorporating rotary inertia and effects of transverse shear deformation based on Mindlin’s theory. The multipoint constraint algorithm is utilized to ensure the compatibility of deformation and equilibrium of resultant forces and moments at the delamination crack front. The standard eigen value problem is solved by applying the QR iteration algorithm. Mode shapes for typical configurations are also depicted. Numerical results obtained are the first known non-dimensional frequencies which could serve as reference solutions for the future investigators.     

含离散源损伤Z向增强复合材料加筋板压缩特性研究

以含穿透中央筋条的切口模拟离散源损伤,对无增强、Z-pin增强、改进锁式缝合增强、Tufting缝合增强复合材料加筋板进行轴向压缩试验,研究含离散源损伤Z向增强加筋板的损伤扩展模式与破坏特征。结果表明,壁板和筋条间的Z向增强有效控制了壁板与筋条的脱粘,提高了加筋板的屈曲载荷。切口前端的分层只引起局部的屈曲,沿切口方向未切断筋条的断裂和壁板边缘的突然压溃导致加筋板的最终破坏。三维有限元渐进损伤分析结果显示,选用Hashin判据作为失效判据,可以很好地模拟含离散源损伤复合材料加筋板的轴向压缩渐进损伤过程。采用线约束模拟壁板与筋条翼缘之间的Z向增强是合理的,线约束的引入在损伤扩展至筋条下方壁板区域后有效控制了损伤的扩展。