Development of smart composite structures with small-diameter fiber Bragg grating sensors for damage detection: Quantitative evaluation of delamination length in CFRP laminates using Lamb wave sensing

The authors and Hitachi Cable, Ltd. have recently developed small-diameter optical fiber and its fiber Bragg grating (FBG) sensor for embedment inside a lamina of composite laminates without strength reduction. The outside diameters of the cladding and polyimide coating are 40 and 52 μm, respectively. First, a brief summary is presented for applications of small-diameter FBG sensors to damage monitoring in composite structures. Then, we propose a new damage detection system for quantitative evaluation of delamination length in CFRP laminates using Lamb wave sensing. In this system, a piezo-ceramic actuator generates Lamb waves in a CFRP laminate. After the waves propagate in the laminate, transmitted waves are received by an FBG sensor attached on or embedded in the laminate using a newly developed high-speed optical wavelength interrogation system. This system was applied to detect interlaminar delamination in CFRP cross-ply laminates. When the Lamb waves passed through the delamination, the amplitude decreased and a new wave mode appeared. These phenomena could be well simulated using a finite element analysis. From the changes in the amplitude ratio and the arrival time of the new mode depending on the delamination length, it was found that this system could evaluate the delamination length quantitatively. Furthermore, small-diameter FBG sensors were embedded in a double-lap type coupon specimen, and the debonding progress could be evaluated using the wavelet transform.     

An NDT technique for composite structures using visualized Lamb-wave propagation

This study proposes a non-destructive testing (NDT) technique that visualizes the propagation of ultrasonic waves in solids, including composite laminates. This technique provides a moving diagram of traveling waves through the use of a pulsed laser that scans a test piece. A non-contact scan by the pulsed laser for ultrasound generation with reception at a fixed point enables us to inspect an arbitrarily shaped object and also facilitates easy operation of the measurement system. We applied the proposed technique to the inspection of CFRP laminates and successfully visualized the wave scattering due to impact-induced delamination as well as the propagation of the S₀ and A₀ Lamb modes. We also addressed disbonding detection in a composite skin-stringer structure and concluded that the reliable detection of damage and the advantages of the proposed technique are applicable to inspections of composite structures.     

Assessment of debonding in sandwich CF/EP composite beams using A0 Lamb wave at low frequency

The aim of this study is to quantitatively assess debonding in sandwich CF/EP composite structures with a honeycomb core using acoustic waves activated and captured by surface-mounted PZT elements. For experimental investigation, debonding was introduced at different locations in sandwich CF/EP composite beams. The fundamental anti-symmetric A₀ Lamb mode was excited at a low frequency. The transmitted and reflected wave signals in both surface panels were captured by PZT elements after interacting with the debonding damage and specimen boundaries. Aided by finite element analysis (FEA), the differences in wave propagation characteristics in sandwich composite beams and composite laminate (e.g. skin panel only) were investigated. The debonding location was assessed using the time-of-flight (ToF) of damage-reflected waves, and the severity of debonding was evaluated using both the magnitude of the reflected wave signal and the delay in the ToF of Lamb wave signals. Good correlation between the experimental and FEA simulation results was observed. The results demonstrate the effectiveness of Lamb waves activated and captured by surface-mounted PZT elements on either surface of sandwich composite structures in assessing debonding.     

Ultrasonic inspection of adhesively bonded CFRP/aluminum joints using pulsed laser scanning

In this study, we propose an ultrasonic inspection technique for detecting disbonds in adhesively bonded carbon fiber-reinforced plastics (CFRP)/aluminum joints using pulsed laser scanning. A specimen with artificially induced square disbonds was scanned by a pulsed laser for ultrasound generation, and the propagating waves were received by a transducer placed on its surface. A series of images of the traveling waves were obtained by processing the received signals. An initial, quick inspection was performed using low-frequency Lamb waves. Changes in the propagation of the Lamb waves were observed at the disbond regions, and disbonds larger than 5 × 5 mm² were successfully detected. A second, detailed inspection of the detected disbond regions was performed using the high-frequency through-transmission ultrasonic method. The shape of the disbond was precisely imaged, and the evaluated size of the disbond matched well with the actual size. We thus demonstrated the efficiency and the feasibility of the proposed technique for the inspection of adhesively bonded CFRP/Al joints.     

A new algorithm for acoustic emission localization and flexural group velocity determination in anisotropic structures

This paper presents a new in situ Structural Health Monitoring (SHM) system able to identify the location of acoustic emission (AE) sources due to low-velocity impacts and to determine the group velocity in complex composite structures with unknown lay-up and thickness. The proposed algorithm is based on the differences of stress waves measured by six piezoelectric sensors surface bonded. The magnitude of the Continuous Wavelet Transform (CWT) squared modulus was employed for the identification of the time of arrivals (TOA) of the flexural Lamb mode (A₀). Then, the coordinates of the impact location and the flexural wave velocity were obtained by solving a set of non-linear equations through a combination of global Line Search and backtracking techniques associated to a local Newton’s iterative method. To validate this algorithm, experimental tests were conducted on two different composite structures, a quasi-isotropic CFRP and a sandwich panel. The results showed that the impact source location and the group speed were predicted with reasonable accuracy (maximum error in estimation of the impact location was approximately 2% for quasi-isotropic CFRP panel and nearly 1% for sandwich plate), requiring little computational time (less than 2 s).     

Impact localisation with FBG for a self-healing carbon fibre composite structure

We demonstrated the localisation of impacts in orthotropic carbon fibre reinforced polymer (CFRP) composite materials to within a few centimetres, using a sparse array of fibre Bragg grating (FBG) sensors. This type of sensor is easily embedded in composites permitting the development of structures with integrated sensing capability. Impact location was determined by measuring the differences in time-of-flight of ultrasonic Lamb waves at three surface-mounted sensors. An algorithm was developed taking into account the angle dependence of the optical fibre sensor sensitivity and the variation of Lamb waves propagation velocity with direction and wave mode. The performance of FBG sensors for impact localisation was compared to that of standard piezoelectric transducers (PZTs), which are already widely used for that purpose. The FBG-based system showed promising potential for a non-intrusive impact detection system applied to self-healing composite structures.     

SHM of CFRP-structures with impedance spectroscopy and Lamb waves

Impedance spectroscopy and application of Lamb waves are attractive methods for permanent monitoring of integrity in Structural Health Monitoring (SHM). Investigations of CFRP structures (Carbon Fibre Reinforced Polymers) with embedded or attached piezoceramic elements are presented for both methods. Examples regard impact damage detection as well as estimation of influencing factors (e.g. degradation of contacts or piezoceramic) and important characteristics (e.g. generation, velocity and attenuation of Lamb waves).     

Impact localisation with FBG for a self-healing carbon fibre composite structure

We demonstrated the localisation of impacts in orthotropic carbon fibre reinforced polymer (CFRP) composite materials to within a few centimetres, using a sparse array of fibre Bragg grating (FBG) sensors. This type of sensor is easily embedded in composites permitting the development of structures with integrated sensing capability. Impact location was determined by measuring the differences in time-of-flight of ultrasonic Lamb waves at three surface-mounted sensors. An algorithm was developed taking into account the angle dependence of the optical fibre sensor sensitivity and the variation of Lamb waves propagation velocity with direction and wave mode. The performance of FBG sensors for impact localisation was compared to that of standard piezoelectric transducers (PZTs), which are already widely used for that purpose. The FBG-based system showed promising potential for a non-intrusive impact detection system applied to self-healing composite structures.     

Instantaneous delamination detection in a composite plate using a dual piezoelectric transducer network

This study proposes a new damage detection technique so that delamination in composite plates can be detected by comparing pitch-catch Lamb wave signals obtained from a piezoelectric transducer (PZT) network without using their own baseline signals obtained from the pristine condition. The proposed technique is based on the premise that the fundamental anti-symmetric (A₀) mode slows down when it passes through a delamination area while the speed of the fundamental symmetric mode is little affected by delamination. First, the A₀ mode in each path is isolated using a mode extraction technique. This mode extraction technique is able to isolate the A₀ mode without frequency or transducer size tuning using dual PZTs composed of concentric ring and circular PZTs. Once the A₀ modes are extracted from all paths in the transducer network, the relative time delay of the A₀ mode in each path with respect to the other paths is defined as a delamination sensitive feature. Then, an instantaneous outlier analysis is developed and performed on the damage sensitive feature to identify the path(s) affected by the delaminated region(s). Because the relative time delays of the A₀ modes are instantaneously compared, robust delamination detection is achieved even under varying temperature conditions.     

A New Inverse Algorithm for Tomographic Reconstruction of Damage Images Using Lamb Waves

Lamb wave tomography (LWT) is a potential and efficient technique for non-destructive tomographic reconstruction of damage images in structural components or materials. A new two-stage inverse algorithm with a small amount of scanning data for quickly reconstructing damage images in aluminum and CFRP laminated plates was proposed in this paper. Due to its high sensitivity to damages, the amplitude decrease of transmitted Lamb waves after travelling through the inspected region was employed as a key signal parameter related to the attenuation of Lamb waves in propagation routes. A through-thickness circular hole and a through-thickness elliptical hole in two aluminum plates, and an impact-induced invisible internal delamination in a CFRP laminated plate were used to validate the effectiveness and reliability of the proposed method. It was concluded that the present new algorithm was capable of reconstructing the images of the above mentioned various damages successfully with much less experimental data compared with those needed by some traditional techniques.     

钛合金蠕变损伤的非线性Lamb波检测

金属材料在高温高压服役过程中会发生蠕变损伤,检测和评价金属材料的早期蠕变损伤具有重要工程意义。针对Ti60钛合金蠕变损伤采用非线性Lamb波进行检测,分别选择Lamb波S_1-S_2模式对和A_4-S_8模式对开展钛合金蠕变损伤试样的实验测量,并采用归一化非线性参数来表征钛合金的蠕变损伤状态。研究结果表,明两种模式对的归一化非线性参数随着材料蠕变损伤程度的加剧均表现出"上升-下降"的变化趋势,且A_4-S_8模式对归一化非线性参数变化率比S_1-S_2模式对更大,说明该模式对对Ti60钛合金蠕变损伤更为敏感。     

Ultrasonic based method for damage identification in composite materials

We present an ultrasonic-based Lamb wave propagation method for identifying and measuring the damage location in a material as a basis for structural health monitoring (SHM). Lamb waves can propagate in a structure via mode conversion and reflection from the surfaces of the structure, and can lead to interference patterns as a resulting wave vector propagates along the structure. We determined the experimental and analytical effects of various parameters on the sensitivity of damage detection. A methodology is proposed for estimating and measuring the location of damage in test specimens. An experimental setup is used for generating A_o? Lamb waves by calibrating ultrasonic pulse generation for optimal values of parameters. Materials with different damage levels are tested in their undamaged and damaged conditions, and the effects of the parameters on the generated waves in test specimens are observed experimentally.     

基于多维阵列和空间滤波器的损伤无波速成像定位方法

随着复合材料在航空结构中的广泛应用,基于压电传感器（PZTs）阵列和Lamb波的结构健康监测成像方法已经成为复合材料结构健康监测技术的研究热点,但是复合材料的各向异性特点导致依赖于信号传播速度的延迟-累加、相控阵等成像方法难以实现其损伤的准确监测。鉴于此,研究了一种与信号传播速度无关的空间滤波器损伤成像定位方法,该方法利用Lamb波在结构中传播时的空间-波数域特征,通过设置空间权重函数使压电传感器阵列形成波数带宽为[k_min,k_max]的空间-波数域滤波器对特定空间方位的Lamb波进行滤波,得到损伤的角度图像; 然后,利用多维线性压电传感器阵列各自针对损伤得到的角度图像进行融合,得到损伤的坐标图像,从而实现了在不依赖传播速度的情况下对损伤的成像定位。在碳纤维层合板上对该方法进行了实验验证。实验结果表明：基于多维线性压电传感器阵列和空间-波数域滤波算法的无波速成像定位方法可以对复合材料结构损伤进行不依赖信号传播速度的成像定位,定位误差在1 cm以内。     

On the influence of moisture absorption on Lamb wave propagation and measurements in viscoelastic CFRP using surface applied piezoelectric sensors

The understanding of the impact of environmental influence factors on propagation and damping of Lamb waves in composite materials is a topic of great interest for both design and utilization of structural health monitoring (SHM) systems. In this work, the influence of humidity absorption on the dispersive behavior of Lamb waves propagating in viscoelastic composite materials is investigated. Using a transversely isotropic material model and DMA measurements, the changes in the viscoelastic material properties due to water absorption are characterized. By means of a higher order plate theory and those mechanical properties, the dispersion curves for unconditioned and hot/wet-conditioned UD reinforced CFRP plates are then predicted. Both the changes in Lamb wave velocity and Lamb wave damping are investigated and compared with experimental values. Additionally, the changes of the sensor response, which are related to both the changes of the material properties and that of the adhesive layer, are investigated. The large impact of moisture absorption on Lamb wave excitation and propagation and its relevance for structural health monitoring (SHM) applications is shown and discussed.     

Lamb wave tomography and its application in pipe erosion/corrosion monitoring

Ultrasonic Lamb wave techniques are widely used in a number of NDE applications. To excite Lamb waves, mode conversion of bulk waves or photoacoustic excitation often are used. Both of these approaches suffer from the need for liquid couplant or ablation of materials to reach a good signal-to-noise ratio. In this paper, we propose a novel technique that utilizes point source excitation and detection of Lamb waves through dry, elastic contacts to monitor velocity changes. A pair of pin transducers is used to excite and detect theA ₀ mode Lamb wave in the pipe wall, and the wave velocity is obtained by time-of-flight measurement. Any change in the pipe wall thickness can be detected by the change in the Lamb wave velocity due to the dispersive nature of theA ₀ mode. We demonstrate the power of this approach in ultrasonic pipe erosion/corrosion monitoring and its potential application in aircraft skin defect imaging. We present results of measurements of plate thickness and erosion/corrosion in a section of pipe that was removed from service, as well as imaging of defects in an aluminum thin plate.     

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.     

An empirical fatigue-life model for high-performance fibre composites with and without impact damage

This article follows earlier work on the development of a life-prediction method for carbon-fibre/epoxy laminates. For comparison with the behaviour of a number of different CFRP laminates already studied, further constant-life fatigue data have now been obtained for a further CFRP composite and a GRP laminate of similar construction – a 16-ply [(±45,0₂)₂]_S lay-up. Fatigue tests have been carried out on these materials in both the virgin condition and after damage by low-velocity impacts. Following analysis of these new data and a re-examination of the older data base, the constant-life model has been appropriately modified. It now offers a prediction procedure for the fatigue response of composite materials in the virgin and impact-damaged conditions which requires, in the first instance, only the tensile and compressive strengths of the composite in question. The model is equally applicable to both GRP and CFRP, despite the fact that the fatigue response of a GRP laminate is different from that of an equivalent CFRP material.     

Structural Health Monitoring Using Lamb Wave Reflections and Total Focusing Method for Image Reconstruction

This investigation aimed to adapt the total focusing method (TFM) algorithm (originated from the synthetic aperture focusing technique in digital signal processing) to accommodate a circular array of piezoelectric sensors (PZT) and characterise defects using guided wave signals for the development of a structural health monitoring system. This research presents the initial results of a broader study focusing on the development of a structural health monitoring (SHM) guided wave system for advance carbon fibre reinforced plastic (CFRP) composite materials. The current material investigated was an isotropic (aluminium) square plate with 16 transducers operating successively as emitter or sensor in pitch and catch configuration enabling the collection of 240 signals per assessment. The Lamb wave signals collected were tuned on the symmetric fundamental mode with a wavelength of 17 mm, by setting the excitation frequency to 300 kHz. The initial condition for the imaging system, such as wave speed and transducer position, were determined with post processing of the baseline signals through a method involving the identification of the waves reflected from the free edge of the plate. The imaging algorithm was adapted to accommodate multiple transmitting transducers in random positions. A circular defect of 10 mm in diameter was drilled in the plate, which is similar to the delamination size introduced by a low velocity impact event in a composite plate. Images were obtained by applying the TFM to the baseline signals, Test 1 data (corresponding to the signals obtained after introduction of the defect) and to the data derived from the subtraction of the baseline to the Test 1 signals. The result shows that despite the damage diameter being 40 % smaller than the wavelength, the image (of the subtracted baseline data) demonstrated that the system can locate where the waves were reflected from the defect boundary. In other words, the contour of the damaged area was highlighted enabling its size and position to be determined.     

A digital image analysis to evaluate delamination factor for wind turbine composite laminate blade

Kinetic energy of wind is converted into useful power by the rotation of blades that are fabricated using glass fiber reinforced composite. Bolted connections are used to fasten the spares with composite shells of the blades. In order the effect this fastening, holes of appropriate size are drilled in the composite laminates. Drilling in composite material is a common phenomena. However the damage caused in the material during drilling results in a drastical reduction of the performance of the component. A newer quantitative digital measurement model is generated using Buckingham’s π theorem to characterize the damage level, termed as refined delamination factor (F_DR). Along with F_DR, a comparison of F_D and F_DA is presented. Experimental results are used to validate the accuracy of proposed criteria.     

Investigations on rheological strains of compressed concrete elements strengthened with external composite reinforcement CFRP

The results of experimental studies on rheological strains of compressed concrete elements strengthened with surface CFRP materials are presented in this paper. The objective of the investigations was to estimate the influence of long-term load on strains and load-bearing capacity of compressed elements strengthened with CFRP materials. The studies were performed on specimens with various types of reinforcement. The specimens were strengthened with external composite reinforcement – longitudinal segments of CFRP strips and transverse confinement executed with CFRP sheets. The experimental studies were divided into two stages, in which specimens were subjected to long-term axial compression. The objects of the investigations were cylindrical specimens with a diameter of 113 mm and height of 350 mm. The plain concrete specimens were loaded with the level of about 1/3f_cm,cyl (mean cylinder compressive strength of the concrete). After the period of initial loading the specimens were strengthened and the level of effort was increased to about 2/3f_cm,cyl. The tests were conducted in constant thermal and moisture conditions. In the paper, the dependences of long-term strains increase in time are presented for the experimental elements. Additionally, theoretical studies connected with rheological model identification were performed.