Fatigue crack growth in plate structures is monitored with ultrasonic guided waves generated from piezoelectric transducers. Cracks initiate in the vicinity of fastener holes due to cyclic in-plane...

Structural health monitoring of fatigue crack growth in plate structures with ultrasonic guided waves

Fiber Bragg gratings (FBGs) have being seen for a growing application in the field of structural health monitoring (SHM) because of their lightweight features. This letter introduces an adaptive SHM sensing system based on the ultrasonic guided wave (UGW) and FBG sensor technology. First, the capability of the proposed SHM sensing system used to detect UGW is demonstrated by the frequency detection of Rayleigh wave that is generated by the piezoelectric transducers with a function generator. Furthermore, a nanosecond pulse laser is utilized to generate Lamb wave on an aluminum plate, while the SHM sensing system is employed to detect the wave in a long distance. Finally, the SHM sensing system is used to locate the source of Rayleigh wave that is created by a pulsed laser. The experiment results show the proposed SHM sensing system is capable of localizing and monitoring the UGW by using four FBG cascaded on a loaded aluminum plate.

A laser-based fiber bragg grating ultrasonic sensing system for structural health monitoring

Effects of composite lamina properties on fundamental Lamb wave mode dispersion characteristics

In this study, we methodologically compare and review the accuracy and performance of C0-continuous flat and curved inverse-shell elements (i.e., iMIN3, iQS4, and iCS8) for inverse finite element method (iFEM) in terms of shape, strain, and stress monitoring, and damage detection on various plane and curved geometries subjected to different loading and constraint conditions. For this purpose, four different benchmark problems are proposed, namely, a tapered plate, a quarter of a cylindrical shell, a stiffened curved plate, and a curved plate with a degraded material region in stiffness, representing a damage. The complexity of these test cases is increased systematically to reveal the advantages and shortcomings of the elements under different sensor density deployments. The reference displacement solutions and strain-sensor data used in the benchmark problems are established numerically, utilizing direct finite element analysis. After performing shape-, strain-, and stress-sensing analyses, the reference solutions are compared to the reconstructed solutions of iMIN3, iQS4, and iCS8 models. For plane geometries with sparse sensor configurations, these three elements provide rather close reconstructed-displacement fields with slightly more accurate stress sensing using iCS8 than when using iMIN3/iQS4. It is demonstrated on the curved geometry that the cross-diagonal meshing of a quadrilateral element pattern (e.g., leading to four iMIN3 elements) improves the accuracy of the displacement reconstruction as compared to a single-diagonal meshing strategy (e.g., two iMIN3 elements in a quad-shape element) utilizing iMIN3 element. Nevertheless, regardless of any geometry, sensor density, and meshing strategy, iQS4 has better shape and stress-sensing than iMIN3. As the complexity of the problem is elevated, the predictive capabilities of iCS8 element become obviously superior to that of flat inverse-shell elements (e.g., iMIN3 and iQS4) in terms of both shape sensing and damage detection. Comprehensively speaking, we envisage that the set of scrupulously selected test cases proposed herein can be reliable benchmarks for testing/validating/comparing for the features of newly developed inverse elements.

A Comparative and Review Study on Shape and Stress Sensing of Flat/Curved Shell Geometries Using C0-Continuous Family of iFEM Elements.

Lamb wave based structural health monitoring shows a lot of potential
for damage detection of composite structures. However, currently there is no
agreement upon optimal network arrangement or detection algorithm. The objective of this research is to develop a sparse network that can be expanded
to detect damage over a large area. To achieve this, a novel technique based
on damage progression history has been developed. This technique gives an
amplification factor to data along actuator-sensor paths that show a steady
reduction in transmitted power as induced damage progresses and is implemented with the reconstruction algorithm for probabilistic inspection of
damage (RAPID) technique. Two damage metrics are used with the algorithm and a comparison is made to the more commonly used signal difference coefficient (SDC) metric. Best case results show that damage is detected within 12 mm. The algorithm is also run on a more sparse network with no damage detection, therefore indicating that the selected arrangement is the most sparse arrangement with this configuration.

/pdf/damage-detection-of-composite-plates-by-lamb-wave-ultrasonic-4u6u1t8np7.pdf

Damage detection of composite plates by Lamb wave ultrasonic tomography with a sparse hexagonal network using damage progression trends

The paper presents a structural health monitoring system based on propagation of tuned Lamb waves and their interference with discontinuities. The dispersion curves are studied to determine the appropriate type and dimension of transducers and to select the optimum scanning frequencies and relevant propagation modes. A piezoelectric sensor network was implemented in an aluminum plate in order to generate and to sense the wave propagation and associated reflections. The algorithm developed for damage detection relies on the comparison of undamaged and damaged responses of the structure. Combinations of filters and statistical methods were applied to detect differences in the sensor signals acquired for the two different states (damaged and undamaged), corresponding to damage reflections. In order to eliminate the false positives due to noise, a probability analysis is performed to obtain the final damage position. The software designed for the current application allows the automatic calculation of dispersion curves, it executes the scans, performs data processing, executes the detection algorithm and presents the probable damages and their positions in a graphical form. Experiments were performed with the introduction of cumulative damages in the plate such as surface and through-the-thickness holes and cuts, ranging from 7 mm to 1 mm in diameter. Additionally, a stringer was attached to the plate by a single rivet line to simulate an aircraft skin structure. Cuts originating from rivet holes and connecting adjacent rivets, as well as loosened rivets were detected by the system. The introduction of the stringer resulted in a loss of precision in the determination of the radial position of the damages near it. Also, the network revealed significant difficulties in the detection of damages beyond the stringer.

/pdf/structural-health-monitoring-system-using-piezoelectric-2gyln33luy.pdf

Structural Health Monitoring System Using Piezoelectric Networks with Tuned Lamb Waves

With the application of newer materials, such as composite materials, and growing complexity and capacity of current aircraft structures, reliably and completely assess the condition of the total structures in real time is then of growing and utmost importance. PZT Network and Phased Array, Lamb wave based Structural Health Monitoring (SHM) systems were developed to be applied to thin panels. The selection of transducers, their size and selected locations for their installation are described. The development and selection of the signal generation and data acquisition systems is also presented in detail. The requirements conducing to the development and selection of these systems are laid and particularly the selection of the actuation signal applied is justified. The development of a damage detection algorithm based in the comparison of the current structural state to a reference state is described, to detect damage reflected Lamb waves. Such method was implemented in software and integrated in the SHM system developed. Subsequently the detection algorithm, based in discrete signals correlation, was further improved by incorporating statistical methods. For phased arrays, a novel damage location algorithm is presented based on the individual sensors response. A visualization method based concurrently in the statistical methods developed and superposition of the different results obtained from a test set was implemented. These tests conducted to the successful and repeatable detection of 1mm damages in a multiple damaged plate with great confidence. Finally, a brief comparison and a hybrid system implementation is presented.

https://iopscience.iop.org/article/10.1088/1742-6596/305/1/012087/pdf

PZT Network and Phased Array Lamb Wave Based SHM Systems

Presently, aircraft structures are maintained and revised on a scheduled basis. Each aircraft manufacturer establishes, for each model, a strict inspection and revision calendar that must be followed firmly that leads to high operation costs. Parts can be replaced just because they exceeded their predicted lifetime, while others are inspected without revealing any kind of damage. Related disassembling and assembling processes are also time-consuming, and sometimes redundant. Structural Health Monitoring is an emerging study field that seeks to address these issues. The advantages of having SHM are the possible reduction of safety factors during the components design process, which will lead to higher payloads and lower fuel consumptions, thus turning aviation more environmentally friendly. There are several approaches being study, all with the same objective in mind: to develop a system capable of monitoring the structural integrity in real time by detecting damage presence, shape, size, growth study, etc. It is possible to achieve accurate component available life-time in real-time by using well established predictive algorithms for damage growth. Another requirement is that such systems must be reliable and avoid false warnings. The method presented here is based on Lamb waves, which are particularly susceptible to interferences caused by damages or boundaries. Several actuation wave functions were investigated. The algorithm for damage detection relies on the assumption that if one is able to compare the response for the undamaged structure with the one presenting damage, when subtracting one from the other, a difference will emerge at a specific time. Such propagation is strongly influenced to interferences caused by damages or boundaries. The actuation system and wave generation requires a careful and detailed approach. The algorithm for damage detection relies on the assumption that if one is able to compare both undamaged and damaged response of the structure, by subtracting one from the other, a difference will emerge at a specific time. The damage is then assumed to work as a source of reflections. After detecting a damage reflection, since the propagation velocity is known for the host medium, a distance for each actuator can be determined and three probability curves can be defined. The point at which the three curves intersect, pinpoints the damage location. To improve the accuracy of the detection method, test runs are repeatedly performed, filters are applied to sensor data and a statistical scheme is put in place. Test runs were successfully performed with the plate undamaged and with different types of damage (surface holes and cracks, through holes and cracks) of different sizes, ranging from 5 mm to as small as 1mm.

A Structural Health Monitoring Approach based on a PZT Network Using a Tuned Wave Propagation Method

In this paper we set up a model of regional banking competition based on Bresnahan (1982), Lau (1982) and Nakane (2002). The structural model is estimated using data from eight Brazilian states and a dynamic panel. The results show that on average the level of competition in the Brazilian banking system is high, even tough the null of perfect competition can be rejected at the usual significance levels. This result also prevails at the state level: Rio Grande do Sul, Sao Paulo, Rio de Janeiro, Pernambuco and Minas Gerais have high degree of competition.

/pdf/estimating-a-theoretical-model-of-state-banking-competition-12gq5dqp2a.pdf

Estimating a theoretical model of state banking competition using a dynamic panel: the Brazilian case

The development of a structural health monitoring (SHM) strategy based on a PZT phased array system is proposed. The objective is to increase the low signal-to-noise ratio (SNR) compared to PZT networks for Lamb wave-based SHM systems. This is achieved by constructive interference—beamforming—of the different waves generated by the different transducers in the array. By carefully selecting and changing the delays in between the actuation of consecutive transducers in the array, a generated wave front can be steered to different selected directions in the plane of the plate component being scanned. By increasing the amplitude of generated waves, through beamforming, potential damage reflected waves present also an increased amplitude and higher SNR, facilitating their assessment in sensor signals and consequently damage detection. The developed automatic system was designed based on the use of the fast propagating fundamental symmetric Lamb wave mode (S
 0). The accuracy of the method is strongly dependant on a precise multiple actuation system and particularly in the accuracy at which the diminutive time delays are introduced in between the actuation of the different array elements. This problem was addressed by developing a dedicated multiple actuation system. The developed system merits were explored particularly and more importantly considering the application of the fast propagating and dispersive fundamental Lamb wave mode, requiring alternative actuation strategies with respect to conventional NDI systems based on non-dispersive body waves. This is done also to considerably decrease the threshold for minimum detectable damage dimensions which can be verified by conventional NDI systems and by similar state-of-the-art SHM systems, in an attempt to benefit a subsequent damage repair and/or prognosis procedure. Tests using the developed system were performed with the successful and repeatable detection of the 1 mm damages applied cumulatively into both aluminum and composite plates, subjected to different boundary conditions. Damages were simulated by surface and through thickness holes and cuts with different orientations. Finally, a network and phased array were also applied to a more complex composite panel with embedded fiber Bragg grating (FBG) optical sensors. An FBG interrogation technique, based in a tunable laser and photo-detector, was developed. The laser is tuned just before test execution and it is not changed during scans. With no moving components, this technique does not impose intrinsically a maximum sampling frequency. At the same time, the influence of temperature and operational induced strains (both static and due to low frequency vibration) are eliminated. This technique proved to be robust and of great potential for a future aircraft implementation.

/pdf/development-of-a-pzt-phased-array-and-fbg-network-for-2g2snp9rrd.pdf

Carlos Silva

Papers

Structural Health Monitoring System Using Piezoelectric Networks with Tuned Lamb Waves

PZT Network and Phased Array Lamb Wave Based SHM Systems

A Structural Health Monitoring Approach based on a PZT Network Using a Tuned Wave Propagation Method

Estimating a theoretical model of state banking competition using a dynamic panel: the Brazilian case

Development of a PZT phased array and FBG network for structural health monitoring based on guided Lamb waves