A sparse-array structural health monitoring (SHM) system based on guided waves was applied to the door of a commercial shipping container. The door comprised a corrugated steel panel approximately 2.4 m by 2.4 m surrounded by a box beam frame and testing was performed in a nonlaboratory environment. A 3-D finite element (FE) model of the corrugations was used to predict transmission coefficients for the A0 and S0 modes across the corrugations as a function of incidence angle. The S0 mode transmission across the corrugations was substantially stronger, and this mode was used in the main test series. A sparse array with 9 transducers was attached to the structure, and signals from the undamaged structure were recorded at periodic intervals over a 3-week period, and the resulting signal database was used for temperature compensation of subsequent signals. Defects in the form of holes whose diameter was increased incrementally from 1 to 10 mm were introduced at 2 different points of the structure, and signals were taken for each condition. Direct analysis of subtracted signals allowed understanding of the defect detection capability of the system. Comparison of signals transmitted between different transducer pairs before and after damage was used to give an initial indication of defect detectability. Signals from all combinations of transducers were then used in imaging algorithms, and good localization of holes with a 5-mm diameter or above was possible within the sparse array, which covered half of the area of the structure.

Evaluation of the damage detection capability of a sparse-array guided-wave SHM system applied to a complex structure under varying thermal conditions

A new approach for structural health monitoring using guided waves in plate-like structures has been developed. In contrast to previous approaches, which mainly focused on isotropic or quasi-isotropic plates, the proposed algorithm does not assume any simplifications regarding anisotropic wave propagation. Thus, it can be used to improve the probability of detection. In this paper the mathematical background for damage localization in anisotropic plates will be introduced. This is an extension of the widely known ellipse method. The formalism is based on a distributed sensor network, where each piezoelectric sensor acts in turn as an actuator. The automatic extraction of the onset time of the first waveform in the differential signal in combination with a statistical post-processing via a two-dimensional probability density function and the application of the expectation-maximization algorithm allows a completely automatic localization procedure. Thus, multiple damages can be identified at the same time. The present study uses ultrasonic signals provided by the spectral element method. This simulation approach shows good agreement with experimental measurements. A local linear neural network is used to model the nonlinear dispersion curves. The benefit of using a neural network approach is to increase the angular resolution that results from the sparse sensor network. Furthermore, it can be used to shorten the computational time for the damage localization procedure.

https://iopscience.iop.org/article/10.1088/0964-1726/19/4/045022/pdf

Multi-site damage localization in anisotropic plate-like structures using an active guided wave structural health monitoring system

Advanced post-processing for scanned ultrasonic arrays: Application to defect detection and classification in non-destructive evaluation

Efficient finite element modelling of ultrasound waves in elastic media

This paper deals with quantifying the performance of a technique for detection, location, and sizing of circumferential crack-like defects in pipelines using synthetically focused guided waves. The system employs a circumferential array of piezoelectric transducer elements. A torsional probing guided wave is excited using the array, which subsequently interacts with the reflecting features of the pipe, such as defects or weld caps. The recorded backscattered signals are synthetically focused to every point of interest in the pipe wall, to form an image of the reflecting features of the pipe. The defect image amplitude is used to estimate the defect depth, and the full width at half maximum of the defect image circumferential profile is used to estimate the circumferential extent of the defect. The imaging system is tested with data from finite element simulations and from laboratory experiments. It is found that reliable sizing of circumferential cracks in finite element simulations and experiments can be achieved if the circumferential extent of the defect is greater than 1.5 lambdaS, where lambdaS is the shear wavelength at the frequency of inspection. This result is theoretically valid for any pipe size, any axial defect location, and any inspection frequency. Amplitude gains of around 18 dB over an unfocused system have been observed experimentally in an 8-inch pipe with a 9 dB SNR improvement.

The application of synthetic focusing for imaging crack-like defects in pipelines using guided waves

Synthetically focused imaging has been used for some time in the NDE community. The techniques have primarily been directed towards imaging using bulk waves. There has recently been use of SAFT (Synthetic Aperture Focusing Technique) using guided waves in plates. Here, we review three different synthetically focused imaging algorithms for a linear array aperture: CSM (Common Source Method), SAFT and TFM (Total Focusing Method). The resolution of the different techniques is obtained from scalar diffraction theory and then validated by means of a low frequency (50kHz) steel plate experiment using PZT excitation and laser reception of the A0 mode. Imaging of through thickness slits parallel to the array is then discussed.

Review of Synthetically Focused Guided Wave Imaging Techniques With Application to Defect Sizing

Synthetically focused guided wave imaging techniques have previously been employed for plate like structures. Much work has been has been done using algorithms such as the synthetic aperture focusing technique (SAFT), the total focusing method (TFM) and common source method (CSM) using both linear and circular arrays. The resolutions for such algorithms for the plate case are well known. We have attempted to use these algorithms for imaging defects in pipes using an array of piezoelectric shear transducers clamped around the pipe circumference. We show that the SAFT and the TFM methods both suffer from coherent noise in the image caused by circumferentially propagating wave modes. It is shown that the common source method (CSM) method does not suffer from this problem though the resolution obtained is poorer. Results from the different imaging algorithms are presented for an 8 inch pipe using 50 kHz excitation, both from finite element simulations and laboratory experiments.

The Application of Synthetically Focused Imaging Techniques for High Resolution Guided Wave Pipe Inspection

We have employed synthetic focusing of guided waves for imaging defects in pipes using an array of piezoelectric shear transducers clamped around the pipe circumference. The performance of the focused system is tested and compared to an unfocused system using data from finite element simulations of an 8 inch pipe with circumferentially orientated cracks and laboratory experiments with circumferential slots, both excited at 50 kHz. The amplitude of the reflection from smaller features is increased by around 17 dB with the focused system. It is also possible to estimate the circumferential extent of the feature if it is larger than two wavelengths of the interrogating signal, in this case, the shear horizontal wavelength. The robustness of the focused system to possible setup errors, such as coupling or phase errors is then thoroughly investigated by synthetically corrupting finite element data. The system is shown to be very robust against realistic levels of setup errors.

J. Davies

Papers

The application of synthetic focusing for imaging crack-like defects in pipelines using guided waves

Review of Synthetically Focused Guided Wave Imaging Techniques With Application to Defect Sizing

The Application of Synthetically Focused Imaging Techniques for High Resolution Guided Wave Pipe Inspection

Synthetic focusing for high resolution guided wave pipe inspection: further results and robustness studies