Application of ultrasonic guided waves for non-destructive testing of defective CFRP rods with multiple delaminations

One of the most important parts of gliders is a lightweight longeron reinforcement made of carbon fibre reinforced plastic (CFRP) rods. During manufacturing, in order to build the arbitrary spar profile, these small diameter (few millimetres) rods are glued together into epoxy filled matrix. Still, defects present in the rods, such as break of fibres, multiple delaminations due to lack of bonding and reduction in density affect construction strength markedly and are extremely complicated to eliminate. Therefore, appropriate non-destructive testing techniques intended for carbon fibre rods should be applied prior to gluing them together.The aim of the present paper is to analyse development possibilities of NDT technique based on application of ultrasonic guided waves and intended for CFRP rods that are used for aerospace applications and are defective with multiple delaminations.The regularities of ultrasonic guided wave propagation in a defective CFRP circular-shape rod with multiple delaminations have been investigated using 3D numerical simulations, finite difference and finite element models. The corresponding experiments have been conducted as well. Based on leaky wave suppression over a defective region due to the weak vertical component of particle displacements, the mechanism of guided wave interaction with the region of multiple delaminations is explained from the ultrasonic NDT viewpoint.     

Characterisation of rectangular type delaminations in composite laminates through B- and D-scan images generated using Lamb waves

The quantitative assessment of both, the length and width, of a delamination in quasi-isotropic composite laminates, using experimentally generated B- and D-scan imaging techniques employing ultrasonic Lamb waves was presented. Distinct zones were identified in B- and D-scans and the sizes of zones were expressed in terms of the width and length of delamination, the probe separation distance, and the probe diameter. B-scans generated through numerical simulations gave an insight and significance of each zone. B- and D-scans were used to estimate the width and length of delaminations, respectively. The estimated width and length of delaminations were found to be in concurrence with the actual values.     

激光超声技术在先进复合材料无损检测中的应用研究

为实现航空航天先进复合材料的非接触、高精度检测,研究激光超声技术在复合材料无损检测中的应用。制备预埋人工缺陷的碳纤维树脂基复合材料和陶瓷基复合材料试样,利用自主研制的激光激励、激光探测的全光学激光超声无损检测系统进行试验研究,实现碳纤维复合材料层压结构模拟分层缺陷检测,层压复合材料紧固孔边沿分层检测,以及陶瓷基复合材料分层检测。研究结果表明:激光超声检测技术可以有效检出碳纤维树脂基复合材料内部直径2 mm以上分层型缺陷,可检出碳纤维复合材料紧固孔边沿的小尺寸分层,可表征C/SiC复合材料内部直径5 mm以上分层,在航空航天工程领域应用前景广阔。     

Comparison of Lamb waves and pulse echo in detection of near-surface defects in laminate plates

One of the problems faced in ultrasonic nondestructive testing (NDT) of composite laminates is near-surface delaminations which may be due to impact damage. The normal incidence pulse echo technique has difficulty in resolving echoes from this type of defect since they often lie within the length of the transmitted ultrasonic pulse. Although a high frequency probe may be used, it has its limitations and could be problematic in composite materials in which ply reflections may interfere with defect reflections. The use of an acoustic delay, together with a high frequency probe, may improve the detection. In this paper, a different approach has been developed using the fundamental Lamb wave (S₀ mode) to detect delaminations in unidirectional fibre composite materials. It has been found that the Lamb wave amplitude decreases significantly over a delamination region. The decrease in amplitude is strongly dependent upon the depth of the delamination and is most sensitive to the delaminations near to the surface of the laminate. By scanning the transducer over the surface, it is possible to measure the size and depth of this kind of delamination. This technique is comparable to the delayed pulse echo technique and can be used to complement other techniques.     

Feature guided wave inspection of bond line defects between a stiffener and a composite plate

Adhesive bonding is widely used in aerospace composite structures. A continuous well-cured bond can offer good joint strength and improved fatigue and impact resistance, and is therefore crucial to the performance of the entire structure. This paper explores the feasibility of using feature guided waves (FGW) for rapid screening of the bond line between a stiffener and a carbon fiber reinforced polymer (CFRP) composite panel. Such FGWs are capable of focusing the wave energy along the stiffener and the bond layer, with limited radiation to the adjacent plate. The Semi-Analytical Finite Element (SAFE) approach is employed to understand the modal properties of FGWs that exist in the structure, and criteria are suggested to choose proper mode-frequency combination that is sensitive to adhesive defects. A shear horizontal type FGW mode is identified to be well suited, as it is easy to excite, and propagates with little dispersion and relatively low attenuation, while it retains sufficient energy around the bond layer. Both 3D Finite Element (FE) simulations and experiments are performed to study the interaction of the selected FGW mode with defects in the adhesive bond, and the results show excellent agreement. The reflection behavior and the wave-defect resonance phenomenon are investigated, which demonstrate the capability of the FGW for the bond line inspection.     

复合材料层板泄漏兰姆波检测技术研究

当一定频率的超声波以适当角度对置于浸液中的薄板材料斜入射时,在板中能激励产生兰姆波,同时在声束入射点附近产生泄漏波声场。采用泄漏兰母皮Ｃ扫描技术,可成功地检出复合材料层板中的分层及孔隙等缺陷。     

Experimental analysis of drilling fiber reinforced composites

In comparison with metals, long-fiber reinforced composites have a layered structure, with different properties throughout their thickness. When drilling such structures, internal defects like delamination occur, caused by the drilling loads and their uneven distribution among the plies. The current experimental analysis is focused towards determining the cutting loads distribution (axial and tangential) along the work-piece thickness and tool radius by analyzing the thrust and torque curves when drilling with 3 different drills carbon-fiber (CFRP) and glass-fiber (GFRP) reinforced composite plates. A wide range of cutting parameters is tested. The highest loads are found at the tool tip in the vicinity of the chisel edge for all cases. It is also found that the maximum load per ply varies mainly with the axial feed rate and tool geometry, while the spindle speed has little or no influence. The analysis is useful for selecting the cutting parameters for delamination free drilling and also for conducting drill geometry optimizations.     

Lamb wave based ultrasonic imaging of interface delamination in a composite T-joint

In this article, the complete characterization of the interface delamination in a composite T-joint using ultrasonic guided Lamb waves is discussed. Correlation between the numerically simulated and experimentally obtained B-scans was established. In the B-scan images, different Zones were identified and could be utilized for differentiating the healthy and delaminated T-joints. The length and the width of delamination were computed from traversing the transducers in two orthogonal directions (i.e. called the B-scans, and the D-scans). For the first time, backward propagating mode (called “Turning Lamb Modes”) that are generated at corner of the T-joint was observed and verfied. Using a single linear line scan of the “Turning Lamb Modes”, both the width and length of interface delamination could be computed. In the present study, the experiments were carried out employing air-coupled ultrasonic transducers.     

Design of a guided wave absorber for Structural Health Monitoring system development

The objective of this paper is to present and validate an efficient guided wave absorber for increased sensitivity of structural health monitoring systems based on guided wave, and especially for inspection close to structure boundaries. Such an absorber would also help with systematic characterization of guided waves interaction with defects in aerospace structures. A complete numerical parametric study is undertaken using the power flow in the structure to determine the reflection and transmission coefficients and thus the dissipated energy by the absorber. Starting with the determination of the geometry and dimensions using Finite Element Modelling (FEM), and following with an investigation on the influence of the absorber's mechanical properties, a practical and efficient absorber is identified. The dimensions of the developed absorber are minimized to provide optimal results for frequencies ranging from 50 kHz to 500 kHz for an incident A₀ mode. The resulting right triangle cross section absorber of Teflon has been machined and tested experimentally using a piezoelectric wave generator on one side of the absorber, and a tri-dimensional laser Doppler vibrometer (3D-LDV) to measure the transmitted wave on the other side. Using a spatial Fourier transform approach for the determination of the experimental reflection and transmission coefficients, the numerical results have been successfully validated.     

合成绝缘子的超声检测方法

提出以临界角入射的纵波检测合成绝缘子横截面上裂纹。在模拟绝缘子芯棒的断裂过程中，用超声探测裂纹的大小和扩展速度来评价绝缘子的质量和各类材料绝缘子的承载能力，从而研究合成绝缘子可能发生的内绝缘故障以及发展机理。阐述了超声检测的理论依据，试验结果说明该方法可用于检测合成绝缘子芯棒裂纹。     

Estimation of impact-induced damage in CFRR laminates through ultrasonic imaging

Ultrasonic C-scans are used frequently to detect defects in composite components caused during fabrication and damage resulting from service conditions. Evaluation and characterization of defects and damage require experience and good understanding of the composites. Under low-velocity impact loading delamination is observed to be a major failure mode. In this paper, the results of extensive experimental work on damage of carbon fiber reinforced plastic (CFRP) laminates due to low-velocity impact are presented. The study was carried out on 10 different layups over four different thickness. Both drop-weight and projectile impacts simulating practical conditions like tool-drop and runway debris, respectively, were carried out at energy levels varying from 3 to 30 J. The ensuing delamination damage was determined by ultrasonic: C-scans using the pulse-echo immersion method for both projected and layer-wise distribution. While projected delamination was obtained by placing a gate over the backwall echo, layerwise distribution was obtained by successive time delay from the frontwall to the backwall echo covering each interface. Delamination areas were quantified accurately by processing the raw image data using a digital image processing technique. Based on the data obtained an empirical relation was established between the delamination area and the impact energy.     

碳纤维/树脂基复合材料铣削表面粗糙度及表面形貌研究

目的研究了CFRP材料铣削加工过程中,部分主要工艺对CFRP材料加工表面质量的影响规律,为工艺参数优化,提高此类零件的表面质量提供依据。方法设计了CFRP材料铣削中的切削参数、刀具结构、加工方法与加工表面粗糙度及表面形貌之间的单因素试验。通过单调改变一个切削参数而其余切削参数不变,得到了工件表面粗糙度和表面形貌随切削参数、刀具结构、加工方法的变化规律。结果当铣削速度增大时,工件的表面粗糙度变化不大,表面微坑缺陷的数量却有所增加,但变小、变浅。当进给速度增大时,工件表面粗糙度呈上升趋势,表面缺陷也随之增加。无涂层多齿刀具铣削后的工件表面粗糙度最大,其次是金刚石涂层多齿刀具铣削的工件,最小的是金刚石涂层交错齿刀具铣削的工件。多齿刀具加工后的表面有较多的微坑缺陷,但普遍深度较浅且面积较小。交错齿刀具对分层缺陷的抑制作用最明显,但在左旋和右旋刀齿交错处容易出现较严重的加工缺陷。与普通机械加工方法相比,超声振动加工方法得到的工件表面质量较好,可以有效减少表面微坑缺陷,改善CFRP铣削加工表面质量。结论 CFRP材料铣削加工时,为了获得较好的加工表面质量,切削参数应选用较高的切削速度和较低的进给速度,切削刀具宜选用多齿带涂层刀具。和普通机械加工方法相比,超声振动铣削加工方法更为有利于获得好的表面质量。     

Some experimental investigations in the drilling of carbon fiber-reinforced plastic (CFRP) composite laminates

In this paper, a concept of delamination factor F_d (i.e. the ratio of the maximum diameter D_max in the damage zone to the hole diameter D) is proposed to analyze and compare easily the delamination degree in the drilling of carbon fiber-reinforced plastic (CFRP) composite laminates. Experiments were performed to investigate the variations of cutting forces with or without onset of delamination during the drilling operations. The effects of tool geometry and drilling parameters on cutting force variations in CFRP composite materials drilling were also experimentally examined. The experimental results show that the delamination-free drilling processes may be obtained by the proper selections of tool geometry and drilling parameters. The effects of drilling parameters and tool wear on delamination factor are also presented and discussed.Cutting temperature has long been recognized as an important factor influencing the tool wear rate and tool life. An experimental investigation of flank surface temperatures is also presented in this paper. Experimental results indicated that the flank surface temperatures increase with increasing cutting speed but decreasing feed rate. Optimal cutting conditions are proposed to avoid damage from burning during the drilling processes.     

Interaction of shear horizontal bend (SHB) guided mode with defects

The authors have recently reported on the attractive properties of shear-horizontal type feature guided (SH_B) waves for the inspection of 90° plate bends. Further to this, here the interaction of the SH_B mode with transverse and longitudinal defects in the bend region is studied using 3D finite element simulations validated by experiments. Trends are discussed in light of the underlying wave mechanics principles. The results show that the SH_B mode yields improved through-thickness defect resolution capabilities as compared to regular plate guided SH waves in view of mode confinement and higher frequencies used.     

复合固体层中兰姆波的声场分析

基于弹性导波的部分平面波分析方法,对复合固体层中兰姆波的声场分布问题进行了研究。在对称的复合固体层中传播的兰姆波模式可以是对称的或反对称的;在非对称的复合固体层中传播的兰姆波模式是非对称的。针对常见的固体层－粘结层－固体层结构中兰姆波的色散关系及声场分布等进行了数值分析,所得结果对兰姆波应用于复合固体层的无损评价具有实际意义。     

大厚径碳纤维复合材料三维钻削有限元仿真及试验研究

针对CR929远程宽体客机承力构件制孔出口分层缺陷预测难、制孔载荷预测试验成本高等问题,开展大孔径碳纤维增强树脂基复合材料(CFRP)三维钻削仿真及试验研究。首先基于ABAQUS用户自定义子程序接口,利用Fortran语言编写CFRP宏观力学本构模型;随后建立大孔径CFRP三维钻削仿真过程有限元模型,并验证其正确性;最后利用有限元模型预测不同加工参数下的制孔轴向力、扭矩及出口分层损伤。研究结果表明:基于三维实体单元建模的复合材料,三维钻削有限元仿真模型可以可靠地预测制孔过程中的轴向力、扭矩。在CFRP出口处嵌入黏结单元可以预测制孔出口分层的形状。在相同工艺参数条件下, 制孔的轴向力、扭矩、出口分层的仿真预测与试验结果最大相对误差分别为15.0%、19.0%、12.4%。      

玻璃纤维复合材料分层缺陷的红外热波检测

基于红外热波无损检测技术的基本原理,运用脉冲式主动红外热波检测技术对玻璃纤维试件分层缺陷进行了检测试验,在对试件分层缺陷的热图序列进行深入研究的基础上,对图像进行了校正、滤波、增强、分割等处理,提取了丰富的有用信息,进而应用相关算法对缺陷的大小和深度进行定量。试验研究结果表明,该技术可对玻璃纤维复合材料的分层缺陷进行快速有效的检测和判别。     

Effects of exit back-up on delamination in drilling composite materials using a saw drill and a core drill

Machining of composites has caught greater attention in manufacturing of structural parts in aerospace, automobile and sporting goods. Composite materials have advantageous features in strength and stiffness coupled with lightweight compared to the conventional metallic materials. Amongst all machining operations, drilling is the most commonly applied method for generating holes for riveting and fastening the structural assembly. Delamination is one of the serious concerns in drilling holes in composite materials at the bottom surface of the workpiece (drill exit). Quite a few references of the drilling of fiber-reinforced plastics report that the quality of cut is strongly dependent on drilling parameter as well as the drill geometry. Saw drills and core drills produce less delamination than twist drills by distributing the drilling thrust toward the hole periphery. Delamination can be effectively reduced or eliminated by slowing down the feed rate when approaching the exit and by using back-up plates to support and counteract the deflection of the composite laminate leading to exit side delaminations. The use of the back-up does reduce the delamination in practice, which its effects have not been well explained in analytical fashion. This paper predicts the effects of backup plate on delamination in drilling composite materials using saw drill and core drill. The critical drilling thrust force at the onset of delamination is calculated and compared with that without backup. The well known advantage of industrial use of backup can be understood fundamentally by the fact that the threshold thrust force at the onset of delamination is increased making the delamination less induced.     

An infrared line scanning technique for detecting delaminations in carbon fibre tubes

A thermal nondestructive testing method based on an infrared line scanner for cylindrical specimens is presented. A seven-layered carbon-glass fibre tube with an artificial delamination was used as a test sample. A finite difference computational model of the heating of the tube was constructed in order to simulate the measurement and evaluate the capability of detecting delaminations. The numerical results show the temperature difference caused by the delamination to be well above the practical detection limit, which was verified by the actual measurements.     

空心抽油杆的超声导波检测

利用超声纵向导波对抽油杆缺陷进行检测。将空心抽油杆简化为一管状波导,利用管道中的导波频散曲线,并结合抽油杆的缺陷检测要求,设计出适合抽油杆缺陷检测的传感器,选择合适的导波模态对抽油杆表面上宽1 mm、深1 mm的周向槽进行检测,试验结果表明将抽油杆简化为管状波导是可行的,利用设计的环状传感器激发出纵向轴对称L(0,2)导波模态,有效检出了抽油杆表面的缺陷。