Trend Decomposition for Temperature Compensation in a Radar-Based Structural Health Monitoring System of Wind Turbine Blades.

The compensation of temperature is critical in every structural health monitoring (SHM) system for achieving maximum damage detection performance. This paper analyses a novel approach based on seasonal trend decomposition to eliminate the temperature effect in a radar-based SHM system for wind turbine blades that operates in the frequency band from 58 to 63.5 GHz. While the original seasonal trend decomposition searches for the trend of a periodic signal in its entirety, the new method uses a moving average to determine trends for each point of a periodic signal. The points of the seasonal signal no longer need to have the same trend. Based on the determined trends, the measurement signal can be corrected by temperature effects, providing accurate damage detection results under changing temperature conditions. The performance of the trend decomposition is demonstrated with experimental data obtained during a full-scale fatigue test of a 31 m long wind turbine blade subjected to ambient temperature variations. For comparison, the well-known optimal baseline selection (OBS) approach is used, which is based on multiple baseline measurements at different temperature conditions. The use of metrics, such as the contrast in damage indicators, enables the performance assessment of both methods.

Nondestructive corrosion degradation assessment based on asymmetry of guided wave propagation field.

The article presents the results of numerical and experimental investigation of guided wave propagation in steel plates subjected to corrosion degradation. The development of novel procedures allowing for the assessment of the corrosion degradation level is crucial in the effective diagnostics of offshore and ship structures that are especially subjected to aggressive environments. The study's main aim is to investigate the influence of surface irregularities on wave propagation characteristics. The paper investigates wavefront asymmetry caused by the non-uniform thickness of damaged specimens. In the first step, the influence of thickness variability on the symmetry of the wave field has been investigated numerically. The corroded plates with variable degrees of degradation have been modeled using the random fields approach. The degree of degradation (DoD) varied from 0% to 40%. In the next step, the developed method was examined during experimental tests performed on specimens subjected to accelerated corrosion degradation. The experimental tests were conducted for intact and for corroded plates characterized by a DoD of 10%. It is demonstrated that the new approach based on wave field analysis can be used in structural state assessment.

An Efficient Neural Network Design Incorporating Autoencoders for the Classification of Bat Echolocation Sounds.

Bats are widely distributed around the world, have adapted to many different environments and are highly sensitive to changes in their habitat, which makes them essential bioindicators of environmental changes. Passive acoustic monitoring over long durations, like months or years, accumulates large amounts of data, turning the manual identification process into a time-consuming task for human experts. Automated acoustic monitoring of bat activity is therefore an effective and necessary approach for bat conservation, especially in wind energy applications, where flying animals like bats and birds have high fatality rates. In this work, we provide a neural-network-based approach for bat echolocation pulse detection with subsequent genus classification and species classification under real-world conditions, including various types of noise. Our supervised model is supported by an unsupervised learning pipeline that uses autoencoders to compress linear spectrograms into latent feature vectors that are fed into a UMAP clustering algorithm. This pipeline offers additional insights into the data properties, aiding in model interpretation. We compare data collected from two locations over two consecutive years sampled at four heights (10 m, 35 m, 65 m and 95 m). With sufficient data for each labeled bat class, our model is able to comprehend the full echolocation soundscape of a species or genus while still being computationally efficient and simple by design. Measured classification F1 scores in a previously unknown test set range from 92.3% to 99.7% for species and from 94.6% to 99.4% for genera.

Directional Multifrequency Guided Waves Communications Using Discrete Frequency-Steerable Acoustic Transducers.

A novel directional transducer based on guided waves (GWs) is introduced in this article, designed for use in structural health monitoring (SHM) and acoustic data communication applications, i.e., systems in which the elastic medium serves as a transmission channel and information is conveyed through the medium via elastic waves. Such systems can overcome difficulties associated with traditional communication methods like wire-based or radio frequency (RF), which can be complex and have limitations in harsh environments or hard-to-reach places. However, the development of these techniques is hampered by GW dispersive and multimodal propagation and by multipath interference. The shortcomings can be effectively addressed by employing frequency steerable acoustic transducers (FSATs), which leverage their inherent directional capabilities. This can be achieved through the exploitation of a frequency-dependent spatial filtering effect, yielding a direct correlation between the frequency content of the transmitted or received signals and the direction of propagation. The proposed transducer is designed to actuate or sense the A0 Lamb wave propagating in three orientations using varying frequencies and has three channels with distinct frequencies for each direction, ranging from 50 to 450 kHz. The transducer performance was verified through finite element (FE) simulations, accompanied by experimental testing using a scanning laser Doppler vibrometer (SLDV). The unique frequency-steering capability of FSATs is combined with the ON-OFF keying (OOK) modulation scheme to achieve frequency directivity in hardware, similar to ongoing research in 5G communications. The multiple-in-multiple-out (MIMO) capabilities of the transducer were finally tested over a thin aluminum plate, showing excellent agreement with the FE simulation results.

Numerical and experimental investigation of guided ultrasonic wave propagation in non-uniform plates with structural phase variations.

The article presents the results of numerical and experimental investigations of guided wave propagation in aluminum plates with variable thickness. The shapes of plate surfaces have been specially designed and manufactured using a CNC milling machine. The shapes of the plates were defined by sinusoidal functions varying in phase shift, which forced the changes in thickness variability alongside the propagation path. The main aim of the study is to analyze the wave propagation characteristics caused by non-uniform thickness. In the first step, the influence of thickness variability on the time course of propagating waves has been analyzed theoretically. The study proves that the wave propagation signals can be determined based on knowledge about the statistical description of the specimen geometry. The histograms of thickness distribution together with the a priori knowledge of the dispersion curves were used to develop an iterative procedure assuming that the signal from the previous step becomes the excitation in the next step. Such an approach allowed for taking into account the complex geometry of the plate and rejecting the assumption about the constant average thickness alongside the propagation path. In consequence, it was possible to predict correctly the signal time course, as well as the time of flight and number of propagating wave modes in specimens with variable thickness. It is demonstrated that theoretical signals predicted in this way coincide well with numerical and experimental results. Moreover, the novel procedure allowed for the correct prediction of the occurrence of higher-order modes.

Performance Assessment for a Guided Wave-Based SHM System Applied to a Stiffened Composite Structure.

To assess the ability of structural health monitoring (SHM) systems, a variety of prerequisites and contributing factors have to be taken into account. Within this publication, this variety is analyzed for actively introduced guided wave-based SHM systems. For these systems, it is not possible to analyze their performance without taking into account their structure and their applied system parameters. Therefore, interdependencies of performance assessment are displayed in an SHM pyramid based on the structure and its monitoring requirements. Factors influencing the quality, capability and reliability of the monitoring system are given and put into relation with state-of-the-art performance analysis in a non-destructive evaluation. While some aspects are similar and can be treated in similar ways, others, such as location, environmental condition and structural dependency, demand novel solutions. Using an open-access data set from the Open Guided Waves platform, a detailed method description and analysis of path-based performance assessment is presented.The adopted approach clearly begs the question about the decision framework, as the threshold affects the reliability of the system. In addition, the findings show the effect of the propagation path according to the damage position. Indeed, the distance of damage directly affects the system performance. Otherwise, the propagation direction does not alter the potentiality of the detection approach despite the anisotropy of composites. Nonetheless, the finite waveguide makes it necessary to look at the whole paths, as singular phenomena associated with the reflections may appear. Numerical investigation helps to clarify the centrality of wave mechanics and the necessity to take sensor position into account as an influencing factor. Starting from the findings achieved, all the issues are discussed, and potential future steps are outlined.

Double-Stage DMAS With Fresnel Zone Filtering in Guided Waves Damage Imaging.

Digital beamforming methods in plate-like structures are widely exploited for Lamb waves-based damage imaging. Among them, the delay and sum (DAS) imaging technique is the most popular thanks to its low-computational cost and ease of implementation. However, the imaging outputs are low quality due to the high levels of side lobes and limited off-axis signal rejection, which leads to limited image resolution and contrast. Recently, the delay multiply and sum (DMAS) beamforming has been applied to nondestructive testing (NDT) field as a promising DAS alternative able to enhance the imaging reconstruction in terms of contrast and damage detectability. However, DMAS is still affected by high levels of artifacts. To tackle this aspect, literature offers a beamforming algorithm called double-stage DMAS (DS-DMAS), first introduced in photoacoustic imaging and medical ultrasound imaging. In this article, the DS-DMAS performance is analyzed for Lamb waves inspection, to provide an exhaustive comparison between DAS, DMAS, and DS-DMAS. As a further step, a filtering process addressed as Fresnel zone filtering (FZF) is used to restrict the beamforming partial sums in a physical way to the area around the scattering point. The proposed approach is an adaptation of a well-established technique in seismic data processing called Fresnel migration, able to suppress artifacts and enhance the quality of the imaging. The algorithms have been compared and characterized by exploiting an online free dataset for guided waves inspection (ht.tp://openguidedwaves.de/) which collects piezo pitch-catch signals traveling through a quasi-isotropic carbon fiber-reinforced plate (CFRP) at different actuated frequencies and damage positions.

Towards Interpretable Machine Learning for Automated Damage Detection Based on Ultrasonic Guided Waves.

Data-driven analysis for damage assessment has a large potential in structural health monitoring (SHM) systems, where sensors are permanently attached to the structure, enabling continuous and frequent measurements. In this contribution, we propose a machine learning (ML) approach for automated damage detection, based on an ML toolbox for industrial condition monitoring. The toolbox combines multiple complementary algorithms for feature extraction and selection and automatically chooses the best combination of methods for the dataset at hand. Here, this toolbox is applied to a guided wave-based SHM dataset for varying temperatures and damage locations, which is freely available on the Open Guided Waves platform. A classification rate of 96.2% is achieved, demonstrating reliable and automated damage detection. Moreover, the ability of the ML model to identify a damaged structure at untrained damage locations and temperatures is demonstrated.

Wavefield Analysis Tools for Wavenumber and Velocities Extraction in Polar Coordinates.

Experimental characterization of Lamb waves in plate-like structures overcomes the intrinsic limits of a priori semianalytical finite element simulations, where material inaccuracies and nonidealities cannot be easily considered. Unfortunately, the experimental extraction of guided wave dispersion curves, and especially their polar representation along different directions of propagation at a given frequency, is not trivial. In nonisotropic materials, such analysis is a key aspect for a reliable and robust characterization of the behavior of waves. In this work, by exploiting scanning laser Doppler vibrometer measurements with narrowband excitation, two different signal processing methods for the extraction of the wavenumber polar representation at the excitation frequency are investigated and characterized. The first method is based on a distance regularized level set (DRLSE) algorithm, widely used in image processing and computer vision but, to the best of the author's knowledge, never used in the Lamb waves' field. The second method is based on the 2-D sparse wavenumber analysis which exploits the wavefield sparse representation in the wavenumber domain. With a precise and reliable extraction of the wavenumber characteristic in the k -space, the polar representations at the excitation frequency of phase and group velocities can be estimated. The former, by exploiting the well-known wavenumber-frequency relation, the latter, instead, by computing numerical derivative among wavenumbers at multiple frequencies. The methodology has been validated on three different composite plates with different degrees of nonisotropy properties. The results show the effectiveness of the two methods, highlighting the advantages and disadvantages of both.

Feasibility of Model-Assisted Probability of Detection Principles for Structural Health Monitoring Systems Based on Guided Waves for Fiber-Reinforced Composites.

In many industrial sectors, structural health monitoring (SHM) is considered as an addition to nondestructive testing (NDT) that can reduce maintenance effort during the lifetime of a technical facility, structural component, or vehicle. A large number of SHM methods are based on ultrasonic waves, whose properties change depending on structural health. However, the wide application of SHM systems is limited due to the lack of suitable methods to assess their reliability. The evaluation of the system performance usually refers to the determination of the probability of detection (POD) of a test procedure. Up until now, only a few limited methods exist to evaluate the POD of SHM systems, which prevents them from being standardized and widely accepted in the industry. The biggest hurdle concerning the POD calculation is the large number of samples needed. A POD analysis requires data from numerous identical structures with integrated SHM systems. Each structure is then damaged at different locations and with various degrees of severity. All of these are connected to high costs. Therefore, one possible way to tackle this problem is to perform computer-aided investigations. In this work, the POD assessment procedure established in NDT according to the Berens model is adapted to guided wave-based SHM systems. The approach implemented here is based on solely computer-aided investigations. After efficient modeling of wave propagation phenomena across an automotive component made of a carbon-fiber-reinforced composite, the POD curves are extracted. Finally, the novel concept of a POD map is introduced to look into the effect of damage position on system reliability.

Combined two-level damage identification strategy using ultrasonic guided waves and physical knowledge assisted machine learning.

Structural Health Monitoring of composite structures is one of the significant challenges faced by the aerospace industry. A combined two-level damage identification viz damage detection and localization is performed in this paper for a composite panel using ultrasonic guided waves. A novel physical knowledge-assisted machine learning technique is proposed in which domain knowledge and expert supervision is utilized to assist the learning process. Two supervised learning-based convolutional neural networks are trained for damage detection (binary classification) and localization (multi-class classification) on an experimental benchmark dataset. The performance of the trained models is evaluated using loss curve, accuracy, confusion matrix, and receiver-operating characteristics curve. It is observed that incorporating physical knowledge helps networks perform better than a direct deep learning approach. In this work, a combined damage identification strategy is proposed for a real-time application. In this strategy, the damage detection model works in an outer-loop and predicts the state of the structure (undamaged or damaged), whereas an inner-loop predicts the location of the damage only if the outer-loop detects damage. It is seen that the proposed technique offers advantages in terms of accuracy (above 99% for both detection and localization), computational time (prediction time per signal in milliseconds), sensor optimization, in-situ monitoring, and robustness towards the noise.

Detection of Pin Failure in Carbon Fiber Composites Using the Electro-Mechanical Impedance Method.

This paper presents a proof of concept for simultaneous load and structural health monitoring of a hybrid carbon fiber rudder stock sample consisting of carbon fiber composite and metallic parts in order to demonstrate smart sensors in the context of maritime systems. Therefore, a strain gauge is used to assess bending loads during quasi-static laboratory testing. In addition, six piezoelectric transducers are placed around the circumference of the tubular structure for damage detection based on the electro-mechanical impedance (EMI) method. A damage indicator has been defined that exploits the real and imaginary parts of the admittance for the detection of pin failure in the rudder stock. In particular, higher frequencies in the EMI spectrum contain valuable information about damage. Finally, the information about damage and load are merged in a cluster analysis enabling damage detection under load.

Bone Fracture Sensing Using Ultrasound Pitch-Catch Measurements: A Proof-of-Principle Study.

The most common imaging method used to diagnose and monitor bone fractures and healing is multiple radiographic images performed by highly trained professionals with expensive equipment that can expose patients to high levels of ionizing radiation. Here we introduce and illustrate proof-of-concept of a potential alternative method for measuring bone fractures: ultrasound pitch-catch measurement technique. Measurements are performed with two piezoelectric transducers, housed in standard orthopedic screws and fixed on either side of simulated fractures, with and without an orthopedic plate, in ex vivo pig limb bones. Using this measurement method, we were able to detect significant differences between uncut and 2-, 5- and 10-mm-deep bone cuts using a two-sided t-test with an α level of 5%.

Temperature affected guided wave propagation in a composite plate complementing the Open Guided Waves Platform.

The influence of temperature is regarded as particularly important for a structural health monitoring system based on ultrasonic guided waves. Since the temperature effect causes stronger signal changes than a typical defect, the former must be addressed and compensated for reliable damage assessment. Development of new temperature compensation techniques as well as the comparison of existing algorithms require high-quality benchmark measurements. This paper investigates a carbon fiber reinforced plastic (CFRP) plate that was fully characterized in previous research in terms of stiffness tensor and guided wave propagation. The same CFRP plate is used here for the analysis of the temperature effect for a wide range of ultrasound frequencies and temperatures. The measurement data are a contribution to the Open Guided Waves (OGW) platform: http://www.open-guided-waves.de . The technical validation includes initial results on the analysis of phase velocity variations with temperature and exemplary damage detection results using state-of-the-art signal processing methods that aim to suppress the temperature effect.

Combined Inspection and Data Communication Network for Lamb-Wave Structural Health Monitoring.

Guided ultrasound waves have long been studied in the context of structural health monitoring (SHM). More recently, they have also been proposed for the acoustic data communication. This paper aims at a joint approach combining guided-wave damage inspection with the acoustic data communication. In this work, a network of autonomous transceiver nodes is modeled that represents a part of an SHM system where the available bandwidth is divided into inspection and communication frequencies. The presented communication protocol is insensitive to dispersion and multipath interference that commonly hamper Lamb-wave signal processing. Experimental results include the successful detection of different damage types at several positions on a metallic plate. Moreover, the communication of the data, namely, damage indicators, across the distributed nodes to a central downstream port is shown. Finally, a numerical study regarding the dynamic behavior of autonomous sensor networks is presented.

Ultrasound Bone Fracture Sensing and Data Communication: Experimental Results in a Pig Limb Sample.

Approximately 6.3 million fractures occur each year in the United States alone. Accurately monitoring the progression of fracture healing is essential to be able to advise patients when it is safe to return to normal activity. The most common method used to confirm and monitor fracture healing is the acquisition of multiple radiographic images over the many months required for healing. This imaging method uses large expensive equipment and exposes patients to high levels of ionizing radiation. In the study described here, we tested another technology for monitoring fracture healing that could minimize the need for multiple radiographic images. We tested a piezoelectric transducer fixed to the surface of a bone that uses electromechanical impedance spectroscopy to measure simulated fractures and transmits the measurement data to an acoustic receiver located externally on the skin surface.

Surface Acoustic Wave-Based Microfluidic Coagulation Device for Monitoring Anticoagulant Therapy.

An universal coagulation test that reliably detects prolonged coagulation times in patients, regardless of which anticoagulant is administered, is not yet available. The authors developed a novel, miniaturized device utilizing surface acoustic waves (SAW) to detect clotting, and used it to develop a novel universal microfluidic coagulation test. Results from this assay were compared with results from standard coagulation assays to detect classical anticoagulants (unfractionated heparin, argatroban) and direct oral anticoagulants (dabigatran, rivaroxaban). The SAW-clotting time (SAW-CT) of this novel device was prolonged in a dose-dependent manner for heparin, argatroban, dabigatran, and rivaroxaban, comparable to standard assays. The authors confirmed the clinical utility of this device in a small patient population admitted to a stroke unit. Preliminary clinical data prove the suitability of the SAW-CT in patients receiving warfarin, rivaroxaban, or dabigatran. The device could be particularly useful as a point-of-care test to assess whole blood coagulation (e.g., in stroke units or in other emergency settings).

Comparison of X-ray-Mammography and Planar UWB Microwave Imaging of the Breast: First Results from a Patient Study.

Hemispherical and cylindrical antenna arrays are widely used in radar-based and tomography-based microwave breast imaging systems. Based on the dielectric contrast between healthy and malignant tissue, a three-dimensional image could be formed to locate the tumor. However, conventional X-ray mammography as the golden standard in breast cancer screening produces two-dimensional breast images so that a comparison between the 3D microwave image and the 2D mammogram could be difficult. In this paper, we present the design and realisation of a UWB breast imaging prototype for the frequency band from 1 to 9 GHz. We present a refined system design in light of the clinical usage by means of a planar scanning and compare microwave images with those obtained by X-ray mammography. Microwave transmission measurements were processed to create a two-dimensional image of the breast that can be compared directly with a two-dimensional mammogram. Preliminary results from a patient study are presented and discussed showing the ability of the proposed system to locate the tumor.

Damage localization in composite structures with smoothly varying thickness based on the fundamental antisymmetric adiabatic wave mode.

This work is based on the experimental observation that the phase and group velocity of the fundamental antisymmetric wave mode in a composite structure with linearly varying thickness changes as it propagates along the nonuniform waveguide (Moll et al., 2015). This adiabatic wave motion leads to systematic damage localization errors of conventional algorithms because a constant wave velocity is assumed in the reconstruction process. This paper presents a generalized beamforming approach for composite structures with nonuniform cross section that eliminates this systematic error. Damage localization results will be presented and discussed in comparison to existing techniques.

Micro-optical prototyping of a surface acoustic wave-based point-of-care coagulation assay and first application in anticoagulated patients.

OBJECTIVE: Clinicians demand for methods to monitor effects of direct anticoagulants in the emergency setting. We recently described a coagulation assay based on surface acoustic waves (SAW) technology, which quantifies anticoagulant effects by image processing. Here we describe the first step in miniaturizing this laboratory method and provide a portable prototype that contains the optical illumination and automatic on-board image processing. METHODS: A device about the size of a shoebox was realized that contains the SAW-chip, the signal generator, the LED illumination, as well as the necessary lenses, aperture, and CCD sensor. The microspheres in the blood were mixed by SAW, and the movement of the microspheres was quantified by on-board image processing. Upon contact with activation induced coagulation, this movement ceases, and coagulation times were measured and compared to the manual methods obtained by standard fluorescent microscopy. A major advantage of our method is the low amount of blood (~ 6 µL) necessary for testing. RESULTS: Results from the prototype correlated accurately with manual methods (Pearson correlation coefficient r = 0.9644). SAW-induced clotting time under anticoagulant treatment with dabigatran or rivaroxaban was well correlated with physicochemically determined plasma concentrations of these DOACs in anticoagulated patients. Compared to manual alignment of the chip under the fluorescence microscope, the prototype had a lower coefficient of variation. CONCLUSIONS: The last evolution step towards a point-of-care (POC)-device would be the development of a cartridge (containing calcium chloride and fluorescent label) such that a drop of blood can be introduced into the reaction vessel by a fluid actuator system.