Reconstruction Algorithm For The Probabilistic Inspection Of Damage Research Articles

Detection and localization of corrosion using identical-group- velocity Lamb wave modes

ABSTRACT Multimodal is the prominent characteristic of Lamb waves, which means a variety of Lamb wave modes propagate in the structure and each of them has unique propagation behaviour. For a wideband multimodal Lamb wave signal, it is possible that many different modes share the same group velocity at some frequency points, while their dispersion behaviours change differently to thickness variation. In this study, the diversity and difference of the interaction between the corrosion and Lamb wave modes are investigated. On this basis, a corrosion index (CI) is established to evaluate the time gap between two modes at their identical group velocity points. The simulation results show that the CI value of a corroded path is obviously larger than that of a healthy path, indicating the presence of the damage, which is independent of any base-line signal. Furthermore, the new reconstruction algorithm for the probabilistic inspection of damage (RAPID) was proposed, where the signal difference coefficient was replaced by the CI to visualise damage location. The experimental results demonstrate the effectiveness of the proposed multimodal Lamb wave baseline-free damage detection method.

Ultrasonic tomography imaging enhancement approach based on deep convolutional neural networks.

The containment liner plate (CLP) is a thin layer of carbon steel material applied as a base for concrete structures protecting nuclear material. The structural health monitoring of the CLP is critical to ensure the safety of nuclear power plants. Hidden defects in the CLP can be identified utilizing ultrasonic tomographic imaging techniques such as the reconstruction algorithm for the probabilistic inspection of damage (RAPID) methodology. However, Lamb waves have a multimodal dispersion feature, which makes the selection of a single mode more difficult. Thus, sensitivity analysis was utilized since it allows for the determination of each mode's level of sensitivity as a function of frequency; the S0 mode was chosen after examining the sensitivity. Even though proper Lamb wave mode was selected, the tomographic image had blurred zones. Blurring reduces the precision of an ultrasonic image and makes it more difficult to distinguish the dimensions of the flaw. To enhance the tomographic image of the CLP, deep learning architecture such as U-Net was utilized for the segmentation of the experimental ultrasonic tomographic image, which includes an encoder and decoder part for better visualization of the tomographic image. Nevertheless, collecting enough ultrasonic images to train the U-Net model was not economically feasible, and only a small number of the CLP specimens can be tested. Thus, it was necessary to utilize transfer learning and get the values of the parameters from a pre-trained model with a much larger dataset as a starting point for a new task, rather than training a new model from scratch. Through these deep learning approaches, we were able to eliminate the blurred section of the ultrasonic tomography, leading to images with clear edges of defects and no blurred zones.

Damage imaging of a U-shaped boom using improved periodic permanent magnet electromagnetic acoustic transducers

ABSTRACT In this research, to solve the problem of the ultrasonic guided wave inspection of a U-shaped boom, an improved periodic permanent magnet electromagnetic acoustic transducer (PPM EMAT) is proposed for defect detection for a U-shaped boom. The acoustic field distribution range of a traditional PPM EMAT is approximately ±20°, and the energy distribution is relatively concentrated. By adjusting the tilt angle between adjacent magnets and coils of a traditional PPM EMAT, an improved PPM EMAT is proposed. The acoustic field distribution range of the improved PPM EMAT is approximately ±30°, and the energy distribution in this range is relatively uniform. The simulation and experimental results show that the improved PPM EMAT has superior frequency response characteristics and acoustic field directivity, which is suitable for detecting large and complex structures. Furthermore, based on the traditional PPM EMAT and the improved PPM EMAT array, combined with the reconstruction algorithm for the probabilistic inspection of damage (RAPID), the damage locations of the double-hole defects of a U-shaped boom are studied. It is shown that the imaging results for defect location using the improved PPM EMATs are better than those found using the traditional PPM EMATs.

Study of concrete de-bonding assessment technique for containment liner plates in nuclear power plants using ultrasonic guided wave approach

In this work, the guided wave de-bonding area-detecting technique was studied for application to containment liner plates in nuclear power plant areas. To apply this technique, an appropriate Lamb wave mode, symmetric and longitudinal dominance, was verified by the frequency shifting technique. The S0 2.7 MHz mm Lamb wave mode was chosen to realize quantitative experimental results and their visualization. Results of the bulk wave, longitudinal wave mode, and comparison experiments indicate that the wave mode was able to distinguish between the de-bonded and bonded areas. Similar to the bulk wave cases, the bonded region could be distinguished from the de-bonded region using the Lamb wave approach. The Lamb wave technique results showed significant correlation to the de-bonding area. As the de-bonding area increased, the Lamb wave energy attenuation effect decreased, which was a prominent factor in the realization of quantitative tomographic visualization. The feasibility of tomographic visualization was studied via the application of Lamb waves. The reconstruction algorithm for the probabilistic inspection of damage (RAPID) technique was applied to the containment liner plate to verify and visualize the de-bonding condition. The results obtained using the tomography image indicated that the Lamb wave-based RAPID algorithm was capable of delineating debonding areas.

Non-contact phase coded excitation of ultrasonic Lamb wave for blind hole inspection

The combination of air-coupled ultrasonic testing (ACUT) and ultrasonic Lamb wave is featured with long-distance propagation and high sensitivity to discontinuities, which is a promising method for rapid and accurate inspection of plate-like materials and lightweighted structures. However, dispersive nature of Lamb wave, signal attenuation plus inevitable noises would lead to low signal-to-noise ratio (SNR). To address this problem, phase coded excitation and pulse compression technique are proposed in this paper to achieve higher SNR by over 10 dB in received signals. 13-bit and 1-carrier-period Barker code is employed as both main lobe peak and Peak Side-lobe Level (PSL) are relatively high. It is demonstrated that A0 mode Lamb wave has good localization ability for defects based on these SNR-enhanced signals. Furthermore, Damage Index (DI) and modified Reconstruction Algorithm for the Probabilistic Inspection of Damage (RAPID) are applied to realize ultrasonic imaging based defect evaluation. Results show that the imaging results agree well with the actual artificial defects in terms of size and shape. Lamb-wave-based air-coupled ultrasonic testing, combined with DI and ultrasonic imaging algorithm, could be a potential way in the NDT of lightweighted structures.

Quantitative damage imaging in plates based on SH0 mode tomographic approach

Exploring the non-dispersive property, the fundamental shear horizontal mode SH0 can be widely used in the detection and monitoring of defects in plate like structures. To combine an approach of ultrasonic guided waves and a tomographic technique by adapting a reconstruction algorithm for the probabilistic inspection of damage (RAPID) using a double-turn coil omnidirectional shear horizontal wave magnetostrictive patch transducer (OSH-MPT) with three different kinds of array arrangements is presented here for imaging. SH0 can propagate in a composite material through bonding double-turn coil OSH-MPT. To get the information about a damage, RAPID can be utilized based on direct wave of SH0 mode. It is advantageous to monitor a composite plate where the velocity of direct waves depends on the propagation direction. In this paper, a surface simulated defect, using a small cylinder, in a composite material is quantitatively evaluated by using shear horizontal mode tomographic approach. Using modified approaches, to eliminate the effect of uneven probability distribution of the array (UPDA) in RAPID, imaging is obtained. Three different array setups of transducers have been tested to monitor the influence on the images. It has been presented that the image generated by the proposed approach with the optimal transducer array and shape factor, β, calculation method has great influence on the real defect shape. To improve the imaging results threshold is selected based on the receiver operating characteristic (ROC) curve. The proposed approach can be used to detect, locate and image the surface defects in plate like structures.

Quantification of defects in an aluminum plate using direction-tunable shear horizontal wave imaging

The quantification of damage, in plates, pipes and such like structures, is one of the current research areas. In this study, to estimate the notch size, shape, and orientation was carried out experimentally based on fundamental shear horizontal mode, SH0 mode. Using least number of transmitters, the reconstruction algorithm for the probabilistic inspection of damage (RAPID) has been used to carry out fast and efficient investigation of scattered waves using SH0wave interaction with notch at various incident angles. An approach of ultrasonic guided waves and (RAPID) algorithm, using a direction-tunable shear-horizontal mode array magnetostrictive patch transducer (DT-SHMA-MPT), the notch in a thin aluminum plate is experimented. Firstly, the RAPID algorithm, based on SH0mode, is used here to quantitatively study a notch in a thin aluminium plate. Secondly, the imaging application using DT-SHMA-MPT is achieved. Experiments were conducted using an array of eight DT-SHMA-MPTs based on a pitch-catch mode to utilize the scattered waves at the notch. The results obtained through experiments reveal the effect of directionality of the scattered waves and have find out the optimal transducers configuration. The localization, quantification and orientation at the same time are obtained using only two or three transmitters. To improve the accuracy of imaging, a threshold value was selected. The presented approach can be used to detect, locate and image the surface defects in aluminum plate.

High-Precision Noncontact Guided Wave Tomographic Imaging of Plate Structures Using a DHB Algorithm

The safety diagnostic inspection of large plate structures, such as nuclear power plant containment liner plates and aircraft wings, is an important issue directly related to the safety of life. This research intends to present a more quantitative defect imaging in the structural health monitoring (SHM) technique by using a wide range of diagnostic techniques using guided ultrasound. A noncontact detection system was applied to compensate for such difficulties because direct access inspection is not possible for high-temperature and massive areas such as nuclear power plants and aircraft. Noncontact systems use unstable pulse laser and air-coupled transducers. Automatic detection systems were built to increase inspection speed and precision and the signal was measured. In addition, a new Difference Hilbert Back Projection (DHB) algorithm that can replace the reconstruction algorithm for the probabilistic inspection of damage (RAPID) algorithm used for imaging defects has been successfully applied to quantitative imaging of plate structure defects. Using an automatic detection system, the precision and detection efficiency of data collection has been greatly improved, and the same results can be obtained by reducing errors in experimental conditions that can occur in repeated experiments. Defects were made in two specimens, and comparative analysis was performed to see if each algorithm can quantitatively represent defects in multiple defects. The new DHB algorithm presented the possibility of observing and predicting the growth direction of defects through the continuous monitoring system.

A Study on Defect Tomographic Imaging in Pressure Vessel With Liquid Medium

Abstract Pressure vessels are widely utilized in many areas of industrial production and daily life for medium storage, which causes performance degradation in pressure vessels, such as crack and corrosion, and lead to serious safety and financial consequences. Reconstruction Algorithm for the Probabilistic Inspection of Damage (RAPID) is a kind of guided wave-based tomography method which is suitable to evaluate structure integrity of pressure vessels. In this article, the effect of liquid level on guided wave propagation and imaging results of RAPID algorithm is investigated, and an optimal baseline matching method based on amplitude variance is proposed to improve the imaging accuracy of RAPID algorithm with liquid-contained condition. The attenuation effect of liquid on guided wave amplitude is investigated. The damage signals are matched with baseline signals recorded at different liquid levels, and the effect of liquid on RAPID algorithm is discussed based on the results. The experiment of image reconstruction for pressure vessel using the optimal baseline matching method based RAPID algorithm is conducted as well. The experimental results show that the optimal baseline matching method can effectively select the best baseline signal, and the reconstructed images can accurately locate the defects on pressure vessels with considering the change of liquid level.

Influence of the PZT Sensor Array Configuration on Lamb Wave Tomography Imaging with the RAPID Algorithm for Hole and Crack Detection.

The tomography technique is an effective way to quantitatively evaluate damage from reconstruction imaging in structure health monitoring (SHM). The reconstruction algorithm for the probabilistic inspection of damage (RAPID) algorithm based on the signal difference coefficient (SDC) feature is a promising approach due to its superior performance. This paper focuses on the influence of different patterns of PZT (Lead Zirconate Titanate) sensor array configurations, i.e., the circular, square, and parallel array, on reconstruction image qualities for evaluating hole and crack damage. Variable shape parameters are applied to account for the unequal damage distances of different actuator-sensor pairs. Considering the directionality scattering fields of cracks, the angular scattering pattern of the SDC values are studied by simulation. The SDC variations for different groups of sensing paths at the same actuator are applied to predict the crack orientation. An improved RAPID algorithm is proposed by defining an additional SDC value of 1 in the path along the predicted crack orientation, which is determined by the point of the actuator causing the minimal SDC variation and the center point of the initial reconstruction image of the crack. The results show that the improved RAPID algorithm is effective for the evaluation of crack damage. Reconstruction image qualities with three PZT sensor array configurations for both holes and cracks are compared. The research is significant for selecting the PZT sensor array configuration in SHM.

Identification of Crack Length and Angle at the Center Weld Seam of Offshore Platforms Using a Neural Network Approach

The reconstruction algorithm for the probabilistic inspection of damage (RAPID) is aimed at localizing structural damage via the signal difference coefficient (SDC) between the signals of the present and reference conditions. However, tomography is only capable of presenting the approximate location and not the length and angle of defects. Therefore, a new quantitative evaluation method called the multiple back propagation neural network (Multi-BPNN) is proposed in this work. The Multi-BPNN employs SDC values as input variables and outputs the predicted length and angle, with each output node depending on an individual hidden layer. The cracks of different lengths and angles at the center weld seam of offshore platforms are simulated numerically. The SDC values of the simulations and experiments were normalized for each sample to eliminate external interference in the experiments. Then, the normalized simulation data were employed to train the proposed neural network. The results of the simulations and experimental verification indicated that the Multi-BPNN can effectively predict crack length and angle, and has better stability and generalization capacity than the multi-input to multi-output back propagation neural network.

Quantitative imaging of surface cracks in polymer bonded explosives by surface wave tomographic approach

One of the effective ways to quantitatively inspect or monitor an industrial structure is to obtain images via either scanning or tomography. A combination of ultrasonic guided wave approach and tomographic technique by employing a reconstruction algorithm for the probabilistic inspection of damage (RAPID) can be used to inspect effectively the damage area via visual images. In this paper, surface cracks in polymer bonded explosives (PBXs) were quantitatively evaluated by a Rayleigh surface wave tomographic approach. Considering the relative low wave velocity in the material tested, special surface wave transducers with silicone wedge were designed and produced. The measured signals of Rayleigh surface wave propagation in the specimen with and without cracks were used in a tomographic algorithm to reconstruct images of defects. Results indicate that the setup of the array of transducers has significant influence on the appearance of images. Optimal transducer array setups were also proposed to measure the ultrasonic wave signals for irregular shaped defects to quantitatively image irregular surface cracks in PBXs. It was shown that the image generated by the proposed approach with the optimal transducer array and shape factor calculation method has good agreement with the real defect shape. The present approach provides an alternative tool that can be applied to detecting, locating and imaging the surface defects in PBXs.

Lamb Wave Interaction at Delamination and Debondings Due to Impact Damage in Complex Stiffened CFRP Structures

The increased usage of carbon fiber reinforced plastics (CFRP) for primary aerospace structures involves dealing with the susceptibility of composite laminates to impact loads as well as the occurrence of barely visible impact damages. One special case among impact sources is the so-called blunt impact, which may cause damage primarily to the internal structure. Therefore, the assessment of debonding of stiffening elements in CFRP structures poses an attractive application case for structural health monitoring by guided ultrasonic waves. Wave propagation phenomena at impact damages as well as the signal processing utilized to extract a damage related feature (i.e., damage index (DI)) contribute to the sensitivity, and thus, to the reliability of structural health monitoring (SHM) systems. This work is based on data from the EU-funded project SARISTU, where a generic CFRP door surrounding fuselage panel with an integrated sensor network has been built and tested by introducing a large number of impact damages. Wave interaction of delaminations and stringer debondings of different size and morphology in omega-stringer stiffened structures are examined to highlight the factors contributing to the sensitivity. Common damage indicator formulations for the use with imaging algorithms, such as the reconstruction algorithm for the probabilistic inspection of damage (RAPID), are applied on data from various damage cases. Furthermore, the difference in detectability of delaminations and debondings as well as the implications on imaging algorithms is examined.

Local interrogations for detection of multiple defects using an active sensor network

With the application of a clock-like sensor network, a local interrogation strategy is established to extend the reconstruction algorithm for the probabilistic inspection of damage (RAPID) into multiple defects identification. In this strategy, the monitoring area is divided into several monitoring subareas, and each subarea is covered by a particular sensor network. Local interrogations are then employed in each subarea using the associated network of sensing paths. Benefit from that, the artifacts arise from the intersection of key sensing paths associated with different defects may be absent completely from some monitoring subarea. With the application of a proper image fusion strategy, the artifacts would be reduced and the damage indications could be preserved. The algorithm is applied to an aluminum plate with two artificial through-thickness holes introduced. The results demonstrated that the RAPID algorithm with the application of local interrogations is capable of identifying multiple defects in plate-like structures.

Multi-defect tomographic imaging with a variable shape factor for the RAPID algorithm

Guided wave inspection technique is a well-known large-area inspection method for industrial applications. However, guided wave inspection signals are hard to analyze due to mode complexity. Guided wave tomography is one of the solutions for wave signal interpretation. Guided wave tomography methods are an alternative for online nondestructive inspection and structural health monitoring. In this paper, RAPID (Reconstruction Algorithm for the Probabilistic Inspection of Damage) is applied to visualize guided wave defect signals in plate-like structures. Guided wave signal visualization is complicated by multi-defect complexity. To overcome the lack of sufficient guidelines for the shape factor β, a variable shape factor was applied. A guided wave pulse-echo inspection method is performed to find multiple defects. The results of this experiment show good agreement with real defects and images.

Ultrasonic Guided Wave Tomography for Damage Detection in Harsh Environment

Guided wave tomography is an attractive tool for the detection and monitoring of the critical area in a structure. Using signal difference coefficient (SDC) as the tomographic feature, RAPID (Reconstruction Algorithm for the Probabilistic Inspection of Damage) is an effective and flexible tomography algorithm. In this algorithm, signal changes are exclusively attributed to the structural variation. However, external environment factors like water loading or oil loading also change signals significantly. The presence of anti-symmetric mode with a predominant out of plane displacement makes it very sensitive to these interferences and leads to false alarms. In this paper, Lamb wave is excited in the low-frequency domain, where only the fundamental modes A0 and S0 exist. The significant difference in group velocity between the two modes makes it possible to separate them in time domain. A new method is proposed to extract pure S0 mode signal as valid measurement data to improve the algorithm in addressing false alarm caused by water loading. The results of the experiment demonstrate that the improved algorithm has the capability of providing accurate identification of damage in the presence of water loading.

Interference resisting design for guided wave tomography

RAPID (reconstruction algorithm for the probabilistic inspection of damage) is a new promising tomography approach for the detection and monitoring of critical areas in a structure. With the sensors permanently installed on or embedded in structures, changes in effective thickness and material properties caused by structural damage can be detected and mapped to the tomogram. However, in this method, the tomographic feature SDC (signal difference coefficient) captures the overall change of the received ultrasonic signals, which makes it sensitive to environmental factors (e.g. rain, changes in temperature and humidity). As a result, the approach is restricted in the laboratory environment. In this paper, the influence of measurement data length on the SDC and the tomogram are investigated, and a new strategy is established on how to choose the measurement data to obtain good reconstruction by matching the coverage zone of each transmitter–receiver pair with the corresponding affected zone. The proposed method is then applied to identify defects of the specimen in the presence of external sources of interference, such as water droplets and structural variations outside the critical area. The results demonstrate its capability of improved robustness in the presence of external sources of interference.

A study on quantitative lamb wave tomogram via modified RAPID algorithm with shape factor optimization

Guided wave tomography is a very attractive technique for online nondestructive inspection and structural health monitoring (SHM). RAPID (Reconstruction Algorithm for the Probabilistic Inspection of Damage) is an effective tomography algorithm for detecting, locating and imaging defect in plate-like structures. However, it is not complete for evaluating a damage area, because of the insufficient guideline for the shape factor called β. The work presented here introduces the calculation of shape factor for the evaluation of a damage area, as well as the variable β parameter technique to correct a damage shape. Experiment is performed using the guided wave pulse-echo method to find the length of damage on a ray path. In the images resulting from the advanced RAPID algorithm, the defect area and shape showed good agreement. The different shapes of the defect require further study.

Guided-wave tomographic imaging of defects in pipe using a probabilistic reconstruction algorithm

There has been much interest recently in monitoring the structural integrity of predetermined critical zones in pipeline using leave-in-place sensors. Ultrasonic guided waves have significant advantages over vibration and impedance methods for structural health monitoring. With guided waves it is possible to locate and size defect regions. The work presented here introduces a Reconstruction Algorithm for the Probabilistic Inspection of Damage (RAPID), which is used to construct tomographic images of multiple defects in a 16-diameter schedule 30 steel pipe. Imaging is completed using an array of 16 low-cost transducers. It is shown that defect location and severity are accurately predicted by employing the RAPID algorithm with multifrequency data sets. A sparse array study is completed in which images are reconstructed using only eight of the 16 available transducers. It is found that damage can still be detected using the sparse array, though defect sizing and location accuracy suffer. It is shown that multiple defects can still be resolved using 13 of the original 16 transducers chosen at random.