Nondestructive evaluation of forced delamination in glass fiber-reinforced composites by terahertz and ultrasonic waves

Glass fiber-reinforced composite laminates in polyetherimide resin have been studied via terahertz imaging and ultrasonic C-scans. The forced delamination is created by inserting Teflon film between various layers inside the samples prior to consolidating the laminates. Using reflective pulsed terahertz imaging, we find high-resolution, low-artifact terahertz C-scan and B-scan images locating and sizing the delamination in three dimensions. Furthermore, terahertz imaging enables us to determine the thicknesses of the delamination and of the layers constituting the laminate. Ultrasonic C-scan images are also successfully obtained; however, in our samples with small thickness-to-wavelength ratio, detailed ultrasonic B-scan images providing quantitative information in depth cannot be obtained by 5 MHz or 10 MHz focused transducers. Comparative analysis between terahertz imaging and ultrasonic C-scans with regard to spatial resolution is carried out demonstrating that terahertz imaging provides higher spatial resolution for imaging, and can be regarded as an alternative or complementary modality to ultrasonic C-scans for this class of glass fiber-reinforced composites.     

基于概率损伤算法的复合材料板空气耦合Lamb波扫描成像

采用非接触空耦传感器在准各向同性复合材料板中激励出单一的Lamb波模态,用于分层缺陷的扫描检测。扫描时,激励和接收传感器置于复合材料板同侧并相对倾斜布置,传感器沿2个正交方向同步线性扫描,得到不同位置的检测信号。对不同扫描路径下的检测信号进行连续小波变换,提取激励频率下的小波系数包络信号,对分层缺陷进行成像。在此基础上,利用概率损伤算法定义损伤指数,结合不同方向的损伤指数实现分层缺陷成像。采用全加法和全乘法对2个正交扫描方向得到的成像结果进行数据融合,实现了分层缺陷的定位和重构。并在成像算法中引入阈值,进一步提高了分层缺陷的定位精度以及重构质量。     

小截面方管结构CFRP复合材料的超声检测方法

新型飞行器已采用小截面碳纤维增强树脂基(Carbon Fiber Reinforce Polymer, CFRP)复合材料方管结构。针对该类复合材料方管的材料组成和结构特点,分析和对比了常用的超声穿透法、超声反射法和整体超声穿透法对复合材料方管内部质量检测的适用性,提出了超声对面内壁反射法。采用超声对面内壁反射法对试块中的人工缺陷和实际产品进行检测。结果表明,采用该检测方法能够全部检出试样中的10 mm×10 mm的人工分层缺陷,且可有效地检测实际产品的分层和孔洞缺陷。     

基于自动种子区域生长的超声B图像缺陷分割方法

提出一种基于自动种子区域生长的超声图像缺陷分割方法。首先使用最大类间方差（Otsu）分割法对超声B图像进行一次预分割;其次寻到绝对背景区,并且在此区域内自动设置种子起始点;然后利用区域生长法将缺陷从背景中分割;最后通过数字形态学降噪法来进一步提高缺陷的识别度。实验结果表明:该方法不仅能准确地分割出缺陷,且具有较好的缺陷边界信息,提高了对超声B图像的处理效率,有效地抑制了大部分图像噪声。     

Damage resistance of single lap adhesive composite joints by transverse ice impact

This paper focuses specifically on the high velocity transverse impact of composite joints by hailstones. Impact tests with ice spheres onto composite lap joint specimens were conducted to determine the failure threshold energy describing damage initiation, and to investigate the modes of damage. The damage areas imaged by ultrasonic scanning were quantitatively measured and the specimens were also sectioned and observed with optical microscopy to determine the exact location of damage. The damage area versus impact kinetic energy was found to increase dramatically for impacts beyond the failure threshold. Delamination of the composite originated at the bond overlap termination facing away from the impact side. The damage usually occurred at specific ply locations and a transition of the delamination to other ply locations was also observed. Numerical simulation of the impact was conducted and the results show that the plies where delaminations were observed to occur have the highest peel and shear stresses.     

复合材料构件热压罐成型工艺质量的群子理论分析

基于热压罐成型复合材料构件的无损检测数据, 利用群子统计理论分析了复合材料构件结构形式与成型质量的关联性, 建立了分层面积的群子模型, 获得了反映分层面积倾向性的群子参数, 对热压罐成型复合材料构件的工艺质量进行了评价。结果表明, 在所统计的航空复合材料构件中, 构件的分层面积分布以小分层为主, 构形复杂（如工形件）、过薄（1~2 mm）或过厚（>5 mm）的复合材料构件产生大分层的倾向性增大, 同时分层面积分布的分散性也增大, 成型质量不易控制。      

Prediction of Delamination Onset and Critical Force in Carbon/Epoxy Panels Impacted by Ice Spheres

Polymer matrix composite structures are exposed to a variety of impact threats including hail ice. Internal delamination damage created by these impacts can exist in a form that is visually undetectable. This paper establishes an analysis methodology for predicting the onset of delamination damage in toughened carbon/epoxy composite laminates when impacted by high velocity ice spheres (hailstones). Experiments and analytical work focused on ice sphere impact onto composite panels have determined the failure threshold energy as a function of varying ice diameter and panel thickness, and have established the ability to predict the onset of delamination using cohesive elements in explicit dynamic finite element analysis. A critical force associated with damage onset was found to be independent of the ice diameter and thus can be expressed as a function of basic panel-describing parameters, namely bending rigidity and interlaminar fracture energy. Critical force can be used as a failure criterion in simpler models (e.g., shell elements) when predicting the onset of delamination by high speed spherical ice impact.     

复合材料层压板分层缺陷超声相控阵检测与评估

针对碳纤维增强树脂基复合材料分层缺陷的无损检测与评估问题,通过制备预埋分层缺陷的标准试样,利用超声相控阵技术对缺陷进行无损检测与定量评估,并对测量误差进行分析。首先,在层压板铺层中间埋入聚酰亚胺薄膜制备分层缺陷试样;然后,对试样进行超声相控阵检测,通过超声S扫和C扫图像对缺陷进行定性分析与定量测量,并结合声场仿真对检测误差进行分析。结果表明:所制备试样内分层缺陷形状规则、埋深及大小与预设一致;超声相控阵步进方向检测尺寸比较准确,而扫查方向尺寸误差较大;超声相控阵技术能够准确识别分层缺陷的形状、尺寸及位置,具有很高的检测精度,对较小缺陷具有很好的检测效果。     

Automated Quality Characterization for Composites Using Hybrid Ultrasonic Imaging Techniques

An enhanced technique using image processing has been developed for automated ultrasonic inspection of composite materials, such as glass/carbon-fibre-reinforced polymer (GFRP or CFRP), to ascertain their structural healthiness. The proposed technique is capable of identifying the abnormality features buried in the composite by image filtering and segmentation applied to ultrasonic C-Scan images. This work presents results performed on two composite samples with simulated delamination defects. A local gating scheme is applied to raw A-Scan data for improved contrast between defective and healthy regions in the produced C-Scan image. In this test campaign, different filtering and thresholding algorithms are evaluated and compared in terms of their effectiveness on defect identification. The accuracies of less than 3 mm and 1.11 mm were attained for the defect size and depth, respectively. The results demonstrates the applicability of the proposed technique for accurate defect localization and characterization of composite materials.     

玻璃纤维/环氧树脂复合材料叠层板冲击损伤特性的若干实验研究

用落重法对玻璃纤维/环氧树脂[0₂/90₂/+45₂/-45₂]_s叠层板作低速冲击试验。检测复合材料内部损伤特性时,探索出一种价廉易得的元素造影液──乙酸锰/丙酮溶液代替昂贵的氯化金/乙醚溶液,可获得满意的效果。用热解剥层技术揭示了复合材料冲击损伤区的形貌特征,并研究了冲击能量与分层损伤实际面积之间的关系,发现起始层间开裂有一冲击能量门槛值,层间分层损伤面积与冲击能量呈线性关系。用扫描电镜观察冲击损伤区的微观形态,发现平行于纤维方向的开裂是基体受拉开裂、层间分层是剪切开裂。      

Detecting Damage in Composite Material Using Nonlinear Elastic Wave Spectroscopy Methods

Modern aerospace structures make increasing use of fibre reinforced plastic composites, due to their high specific mechanical properties. However, due to their brittleness, low velocity impact can cause delaminations beneath the surface, while the surface may appear to be undamaged upon visual inspection. Such damage is called barely visible impact damage (BVID). Such internal damages lead to significant reduction in local strengths and ultimately could lead to catastrophic failures. It is therefore important to detect and monitor damages in high loaded composite components to receive an early warning for a well timed maintenance of the aircraft. Non-linear ultrasonic spectroscopy methods are promising damage detection and material characterization tools. In this paper, two different non-linear elastic wave spectroscopy (NEWS) methods are presented: single mode nonlinear resonance ultrasound (NRUS) and nonlinear wave modulation technique (NWMS). The NEWS methods were applied to detect delamination damage due to low velocity impact (<12 J) on various composite plates. The results showed that the proposed methodology appear to be highly sensitive to the presence of damage with very promising future NDT and structural health monitoring applications.     

超声C扫描设备定量评价方法研究

超声C扫描系统在超声成像检测、缺陷识别等无损检测领域获得了广泛应用。但是,对C扫描图像的缺陷进行精确分析和表征一直是超声领域的难点之一。基于超声C扫描缺陷图像,给出了一种结合K-means聚类与Graham算法的图像特征参数定量估计方法,通过定量估计的参数能够有效地评价超声C扫描系统的检测质量。实验结果表明该方法能够有效描述标准圆形人工缺陷区域特征,有利于进一步评价超声C扫描设备。     

Defect Detection in Carbon-Fiber Composites using Lamb-Wave Tomographic Methods

Lamb-wave tomography (LWT) offers a powerful nondestructive technique for the health assessment of large structures as their propagation properties depend on the thickness and the mechanical properties of the material. Development of a fast and accurate algorithm for defect detection is of paramount importance in any structural-health-monitoring (SHM) system. The present study explores the prospects of LWT as a SHM technique with an accent on developing a suitable algorithm for real-time inspection. Projection data is collected by electronically scanning an array of ultrasonic sensors arranged in a modified cross-hole geometry. The data thus collected is investigated to extract energy profile of the traveling waves. Multiplicative algebraic reconstruction technique (MART) algorithms are used as a tool for tomographic reconstruction from a set of multiple independent measurements. The performance of algorithms is evaluated from the point of view of the cost of algorithm, achievable resolution, and accuracy of results. Experimental results show that MART is capable of characterizing defects in thin isotropic and composite plates within a reasonable error band (±26% normalized, ±2.6 RMS) and is suitable for application to LWT of large structures such as aircraft skins.     

Smart structures — vibration of composites with piezoelectric materials

A manufacturing technique is developed for embedding piezoelectric material in composite laminates while maintaining the structure strength and piezoelectric effectiveness. An ultrasonic C-scan test is applied to screen out the specimen with possible delamination along the interface of the piezoelectric material and glass fiber layer. It is shown that the problem of electrical insulation and piezoelectric material cracking can be prevented. In addition, tensile and static tests are conducted to validate the manufacturing technique. An analytical model is also presented to predict the natural frequencies and mode shapes of a composite structure with embedded piezoelectric materials, and the predictions are verified by modal testing.     

Health monitoring of aerospace composite structures – Active and passive approach

Impact damage is one of the major concerns in maintenance of aircraft structures built from composite materials. Damage detection in composite materials can be divided into active and passive approaches. The active approach is usually based on various non-destructive techniques utilizing actuators and/or receivers. In contrast passive approaches do not involve any actuators; receivers are used to “sense and/or hear” any perturbations caused by possible hidden damage. Often strain data are used to localize impacts and estimate their energy. The assumption is that damage occurs above well-defined energy of impacts. The paper illustrates one active and one passive method recently developed for impact damage detection. The first method, based on guided ultrasonic waves, utilises 3-D laser vibrometry and does not require any signal processing. Simple laser scans, revealing the change in Lamb wave response amplitudes, have been used to locate delamination and estimate its severity in a composite plate. In contrast, the second method does not require any sophisticated instrumentation but relies on advanced signal processing. An array of piezoceramic sensors has been to detect strain waves transmitted from an impact applied to the composite aircraft structure. The modified multilateration procedure with Genetic Algorithms has been used to locate impact position.     

Measurement of Interfacial Fracture Toughness of Surface Coatings Using Pulsed-Laser-Induced Ultrasonic Waves

This research develops a new technique for the measurement of interfacial fracture toughness of films/surface coatings using laser-induced ultrasonic waves. Using pulsed laser ablation on the bottom substrate surface, strong stress waves are generated leading to interfacial fractures and coating delamination. Simultaneously, a laser ultrasonic interferometer is used to measure the normal (out-of-plane) displacement of the top surface coating in order to detect coating delamination in a non-destructive manner. We can thus determine the critical laser energy for delamination, yielding the critical stress (that is, the interfacial strength). Subsequently, to examine the interfacial fracture toughness, additional pulsed laser irradiation is applied to a pre-delaminated specimen to show that the delamination area expands. This type of interfacial crack growth can be visualized using laser ultrasonic scanning. Furthermore, the calculation of elastic wave propagation was carried out using a finite-difference time-domain method) in order to accurately estimate the interfacial stress field. In this calculation, the stress distribution around the initial delamination is calculated to obtain the stress intensity factor. Based on the experimental and computational results, interfacial fracture toughness can be quantitatively evaluated. Since this technique relies on a two-laser system in a non-contact approach, it may be useful for a quantitative evaluation of adhesion/bonding quality (including both interfacial fracture strength and toughness) in various environments.     

金属夹心CFRP复合材料超声检测方法

新型机翼、弹翼等结构已采用金属夹心碳纤维增强（Carbon Fiber Reinfored Polymer，CFPR）复合材料结构。针对该类复合材料结构的材料组成和结构特点，分析和对比常用的超声穿透法和超声反射法能否对金属夹心CFPR复合材料内部质量进行检测，提出了先采用超声C扫穿透法确定缺陷的平面位置，然后采用高分辨率超声A扫反射法确定复合材料中分层缺陷及采用超声相位法确定复合材料与金属间的脱粘缺陷的方法。采用提出的方法检测带有预置缺陷的试样和实际产品。检测结果表明，提出的方法可准确检测试样中大小为5 mm×5 mm的预置人工分层和脱粘，且能准确检测实际产品中的缺陷。     

The Calibration and Sensitivity Aspects of a Self-Referencing Routine When Applied to Composites Inspection: Using a Pulsed Thermographic Setup

This text analyzes the effects of calibration settings on a self-referencing processing routine, when applied to inspecting a set of Carbon Fiber composite structures via pulsed thermographic apparatus. The Self-referencing algorithm operates on acquired thermograms by tracking each pixel location contrast-temporal cooling curve, to expose any potential deviation and acquire quantitative assessment of the host material subsurface condition. The calibration factors in this study include; the kernel size that represents the sound (or defect-free) or reference, temperature-time history, the threshold in each center pixel signal relative to its surrounding (average of the kernel pixel population). The study quantifies the effect of each factor using a pulsed thermographic test campaign; first applied to a set of artificially designed samples; in addition to actual blind samples. The controlled coupons are 3D printed out of plastics; while the defects are embedded (via 3D printing) to represent different and controlled shapes and configurations. The 3D printed samples are used to help represent different fill-rates to imitate honeycomb structures, and to embed defects of finite thickness. The study findings highlights the effect of each of the calibration settings and demonstrate tangible improvement in the processed frames.     

Air-Coupled Ultrasonic Signal Processing Method for Detection of Lamination Defects in Molded Composites

In order to improve signal-to-noise ratio (SNR) of air-coupled ultrasonic signal in the detection of lamination defects in molded composite, the pulse compression and wavelet filtering hybrid signal processing method is proposed. The selection principle of parameters of the hybrid signal processing method is studied. The actual detection results of molded composite show that the hybrid method is very effective in improving the SNR of air-coupled ultrasonic signal when selecting reasonable parameters (The experiment results demonstrate that the optimal parameters are 13-bit Barker code sequences signal with three-cycle per sub-pulse, db9 wavelet, six decomposition levels, and soft threshold function.). An improvement in SNR up to 18.81 dB is attained compared with the original received signal. The quantitative accuracy of defects in C-scan image based on the hybrid method is also very high, and defects as small as \(\emptyset \)5 mm can be easily identified.     

Imaging of Barely Visible Impact Damage on a Complex Composite Stiffened Panel Using a Nonlinear Ultrasound Stimulated Thermography Approach

Thermosonics, also known as ultrasonic stimulated thermography, is a rapid non-destructive evaluation technique that uses an infrared camera to visualise material defects by detecting the frictional heating at crack surfaces when a part under inspection is vibrated. These vibrations are usually produced by an ultrasonic horn being pressed against the surface of the test sample, which result in uncontrolled generations of frequency components and excitation amplitude. This makes thermosonics highly non-reproducible and unreliable. This paper presents a novel thermographic method, here named as nonlinear ultrasound stimulated thermography, for the detection and imaging of real material defects such as impact damage on a complex composite stiffener panel. This technique combines nonlinear ultrasonic techniques with thermography. A nonlinear ultrasonic approach was used as signature for a reliable frequency-selective excitation of material defects, while an infrared camera was employed to reveal the damage location and severity. A nonlinear narrow sweep excitation method was employed to efficiently excite the local resonance frequencies of the damaged region in order to give rise to the highest nonlinear harmonic response in the material leading to a high heat generation at the crack surface. The experimental tests were carried out with a laser vibrometer in order to better understand the interaction of elastic waves with nonlinear scattering. An ad-hoc nonlinear thermal-structural finite element and crack model was developed to study the heat generation caused by the movement of the crack surfaces when elastic waves with a particular frequency impinges on the crack interphase with good agreement with the experimental results. The proposed new method allows to detect single and multiple barely visible impact damage in a quick, reliable and reproducible manner and overcomes the main limitations of classical thermosonics.