The damage identification process provides relevant information about the current state of a structure under inspection, and it can be approached from two different points of view. The first approach uses data-driven algorithms, which are usually associated with the collection of data using sensors. Data are subsequently processed and analyzed. The second approach uses models to analyze information about the structure. In the latter case, the overall performance of the approach is associated with the accuracy of the model and the information that is used to define it. Although both approaches are widely used, data-driven algorithms are preferred in most cases because they afford the ability to analyze data acquired from sensors and to provide a real-time solution for decision making; however, these approaches involve high-performance processors due to the high computational cost. As a contribution to the researchers working with data-driven algorithms and applications, this work presents a brief review of data-driven algorithms for damage identification in structural health-monitoring applications. This review covers damage detection, localization, classification, extension, and prognosis, as well as the development of smart structures. The literature is systematically reviewed according to the natural steps of a structural health-monitoring system. This review also includes information on the types of sensors used as well as on the development of data-driven algorithms for damage identification.

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Damage Identification in Structural Health Monitoring: A Brief Review from its Implementation to the Use of Data-Driven Applications.

In-flight and wireless damage detection in a UAV composite wing using fiber optic sensors and strain field pattern recognition

This paper develops a high-sensitivity flexible eddy current array (HS-FECA) sensor for crack monitoring of welded structures under varying environment. Firstly, effects of stress, temperature and crack on output signals of the traditional flexible eddy current array (FECA) sensor were investigated by experiments that show both stress and temperature have great influences on the crack monitoring performance of the sensor. A 3-D finite element model was established using Comsol AC/DC module to analyze the perturbation effects of crack on eddy currents and output signals of the sensor, which showed perturbation effect of cracks on eddy currents is reduced by the current loop when crack propagates. Then, the HS-FECA sensor was proposed to boost the sensitivity to cracks. Simulation results show that perturbation effect of cracks on eddy currents excited by the HS-FECA sensor gradually grows stronger when the crack propagates, resulting in much higher sensitivity to cracks. Experimental result further shows that the sensitivity of the new sensor is at least 19 times that of the original one. In addition, both stress and temperature variations have little effect on signals of the new sensor.

A High-Sensitivity Flexible Eddy Current Array Sensor for Crack Monitoring of Welded Structures under Varying Environment.

Damage detection method based on Lamb waves for stiffened composite panels

Grout compactness monitoring of concrete-filled fiber-reinforced polymer tube using electromechanical impedance

A distributed fibre Bragg grating array called the AMAP (Acoustic Mode Assessment Photonic) sensor is applied to spectrally decompose a complicated multi-modal packet of Lamb waves propagating in an aluminium plate. Pass-band filtering of the complex spectrum is then used to achieve a complete time-domain separation of the constituent modes. The use of a compact surface-mounted sensor array to unscramble a multi-modal Lamb wave packet represents a novel development with potentially important implications for ultrasonic methods of structural health monitoring and non-destructive testing, in particular acousto-ultrasonics and acoustic emission. It provides in principle an opportunity to better exploit the significant diagnostic potential of modes above the first cut-off frequency, which traditionally has been difficult because of the time-domain complexity of mixed signals in this regime. More generally, it enables selective time-domain reconstruction of specific modes of interest, eliminating interference from other modes, boundary reflections and noise. An experimental case study involving a high frequency multi-modal Lamb wave inspection is used to validate and demonstrate the approach.

A distributed sensing capability for in situ time-domain separation of Lamb waves

Lamb-wave based structural health monitoring (SHM) approaches are typically constrained to operate below the first cut-off frequency to simplify the interpretation of the wave field in the time-domain. However from a diagnostic perspective, it is desirable to unlock the additional information encoded in the higher-order Lamb wave spectrum. Wave-mode decomposition is necessary for the extraction of useful information from multi-modal acoustic wave fields, which requires spatially dense sampling over the field. The instrument of choice for this task is the laser Doppler vibrometer, which is capable of producing detailed spectral decompositions. However vibrometry is not suited to in-situ measurement for SHM. Fibre Bragg gratings (FBGs) are capable of sensing Lamb waves and detection of higher order modes using FBGs has been previously demonstrated. The ability to multiplex multiple short-length gratings along a single fibre to create an FBG array gives rise to an in-situ sensor with sufficiently dense spatial sampling of an acoustic wave field to perform useful wave-mode decomposition. This paper explores some of the fundamental limits to modal decomposition resolution and bandwidth that exist for such sensors. Potential sources of noise and distortion encountered due to limitations of the sensor fabrication and interrogation methods are also discussed. In addition, modal decomposition of Lamb waves with frequencies up to 1.25 MHz is demonstrated in a laboratory experiment using an array of sixteen ~1 mm long gratings bonded to an aluminium plate. At least four modes are distinguishable in the resulting spectral decomposition.

Limits to acoustic sensing and modal decomposition using FBGs

A new acoustic sensing capability, consisting of a flexible high-density linear piezoelectric sensor array coupled to a high-bandwidth interrogation device, is developed and applied to in situ wave...

In situ wavenumber–frequency modal decomposition of acoustic emissions:

Acousto-ultrasonics offers a promising basis for broad-field detection of structural damage in aircraft and could provide a basis for the development of condition-based maintenance approaches for airframes. When applied in situ, the technique can offer both substantial cost savings and performance improvements over conventional ultrasonic nondestructive inspection (NDI). The application of the technique involves the insertion of piezoelectric elements and attendant electrical conductors in the host structure. The Defence Science and Technology Organisation (DSTO) has worked with a lo cal industry partner to develop the Acousto Ultrasonic Structural health monitoring Array Module (AUSAM), a compact device for the control of embedded piezoelectric transducer networks. The module, which has the footprint of a small tissue box, provides autonomous control of two send and four receive elements, and can operate synchronously with other modules to accommodate larger transducer numbers. The efficacy of the system is demonstrated through a case study involving fatigue crack monitoring in an F-111C lower wing skin test coupon. The results indicate that the system can be successfully used for crack growth monitoring, however they also highlight several issues that need to be resolved before the approach can be applied routinely in practice.

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Application of the AUSAM System for Fatigue Crack Monitoring in a Wing Skin: A Case Study

The concept of in situ Structural Health Monitoring (iSHM) involves incorporating a diagnostic capability to detect and monitor damage in structural ‘hot spots’. These systems allow the introduction of Condition Based Monitoring (CBM) approaches for ageing aircraft fleets and thus could help significantly reduce through-life support costs and increase aircraft availability. The Australian Defence Science and Technology Organisation (DSTO) has a program of work aimed at developing autonomous, robust, reliable iSHM systems with the specific aim of retro-fitment to existing aircraft. A current laboratory demonstrator of the technology is focused on the application of an acousto-ultrasonic (AU)-based iSHM system for monitoring fatigue cracks on generic structurally-detailed lower wing skin (LWS) panel specimens. This particular scenario, which simulates the LWS on an F-111C aircraft, is considered an ideal developmental platform and springboard for future iSHM applications, since it captures a number of key issues that AU-based iSHM systems need to address to ensure robust reliable diagnostics, viz., complex structural geometry, crack closure and high operational loads. This paper describes the design, application and assessment of an AU-based iSHM system to detect and monitor growth of fatigue cracking in the generic complexgeometry LWS panel specimens subject to representative spectrum loading. Its performance is discussed and compared to measurements furnished by thermoelastic stress analysis and visual observation. Testing associated with this complex realistic panel specimen revealed several deficiencies in the design of the iSHM system.

Cedric Rosalie

Papers

A distributed sensing capability for in situ time-domain separation of Lamb waves

Limits to acoustic sensing and modal decomposition using FBGs

In situ wavenumber–frequency modal decomposition of acoustic emissions:

Application of the AUSAM System for Fatigue Crack Monitoring in a Wing Skin: A Case Study

Acousto-Ultrasonic In Situ Monitoring of Fatigue Cracking in an Aircraft Wing Skin Specimen