Elisabetta Pistone

Noncontact monitoring of immersed plates by means of laser-induced ultrasounds

This article presents the results of an experimental and numerical study where guided ultrasonic waves were used for the structural health monitoring/nondestructive evaluation of an immersed aluminum plate. Leaky Lamb waves were generated by means of a pulsed laser and detected by an array of immersion transducers. The signals were then processed using continuous wavelet transform to extract few damage-sensitive features that were fed to an unsupervised learning algorithm based on outlier analysis. The experimental setup was simulated numerically using a commercial finite element software to predict the time of arrival of the propagating modes. In order to assess the capability of the monitoring system to detect damage, four defects were devised on the plate prior to the immersion in water. We found that the noncontact probing system and the signal processing enable the detection of cracks and holes.

Guided Ultrasonic Wave Imaging for Immersed Plates Based on Wavelet Transform and Probabilistic Analysis

We propose a nondestructive evaluation method for immersed structures based on the propagation of ultrasonic waves induced by means of laser pulses and detected with an array of immersion transducers. In the study presented in this article, a laser operating at 532 nm is employed to excite leaky guided waves on an aluminum plate immersed in water. An array of immersion transducers is used to record the waves radiating from the laser-illuminated point. The detected signals are processed with an imaging algorithm based on continuous wavelet transform and probabilistic analysis to localize the presence of artificial defects machined in the plate. With respect to the existing imaging methods for plates, the proposed algorithm is pseudo baseline free, because it does not require data recorded from a pristine plate, but it requires that only a portion of the plate is free from defects. We find that the proposed algorithm enables the detection of surface cracks. Advantages and limitations of the algorithm for the nondestructive evaluation of underwater structures are discussed.

On the use of an array of ultrasonic immersion transducers for the nondestructive testing of immersed plates

This paper presents the results of an experimental study in which guided ultrasonic waves were used for the nondestructive evaluation of an aluminium plate immersed in water. Leaky Lamb waves were generated using a spherical focused transducer, and detected by five immersion transducers arranged in a semi-circular array. The signals were processed to extract seven damage-sensitive features from the time, frequency and joint time–frequency domains. These features were then fed to an unsupervised learning algorithm based on the outlier analysis to identify the presence of defects artificially devised on the plate. We found that the array of immersion transducers and the signal processing technique enable the detection of the defects with a success rate close to 95%.

Signal processing for the inspection of immersed structures

In this paper, we present a non-destructive inspection method for immersed waveguide. A laser operating at 532 nm is used to excite leaky guided waves on an aluminum plate immersed in water. The plate has a few artificial defects. An array of immersion transducers is used to detect the propagating waves. A signal processing based on continuous wavelet transform is utilized to extract a few damage-sensitive features that are used in an outlier analysis and in a probabilistic-based imaging method. The experimental results show that the proposed system can be used for the inspection of underwater waveguides.

Ultrasonic waves for the inspection of underwater waveguide structures

The non destructive inspection of immersed structures is popular as it minimizes unexpected and costly failures of important marine structures. In this paper we present a non-contact laser/immersion transducer technique for the inspection of underwater waveguide structures. The technique uses laser pulses to generate leaky guided waves and conventional immersion transducers to detect these waves. To prove the feasibility of the proposed methodology, a laser operating at 532 nm is used to excite leaky guided waves in a plate subjected to different damage scenarios. The plate is immersed in water at constant temperature and damage is first simulated using different weights located in the region of interest, i.e. between the point of the laser illumination and the immersion transducers. Damage is also simulated by engraving a series of notches on the face of the plate exposed to the probing system. The waveforms are then processed using the joint time-frequency analysis of the Gabor wavelet transform, statis...

Performance, Reliability and Validation of Mass Spring Systems by Means of Vibration-based Excitation Sources

This paper presents the experimental evaluation of insertion loss of mounted track systems. Research outcomes can be implemented for the performance monitoring of metro lines and of high-speed railway infrastructures. Measurement-based identification of the systems has been developed following the “left and right test procedure” according to DIN SPEC 45673-3 on three mockups under loaded and unloaded conditions. The mock-up represented a standard track, a light mass spring system (LMSS) and a heavy mass spring system (HMSS) of a metro line. An impact hammer with mass of 8.0 kg capable of exciting a maximum force of 22.5 kN is used as excitation source and nine vibration receivers are used according to several protocols. Time, frequency and time-frequency analysis of all the collected data is presented. Advanced signal processing coupled with filtering techniques has been applied to guarantee the reliability of the data. Scope of the tests is the determination of ground transfer and of natural frequency of loaded and unloaded tracks in order to calculate the characteristic insertion loss. Ground correction curves for the LMSS and the HMSS system are computed and implemented for the final results. Environmental and operational effects are also considered to properly prognose and manage the performance and reliability of the systems. Theoretical insertion loss curves for the HMSS and LMSS systems have been computed according to a 2D 6-degree of freedom numerical model and compared to experimental ones. In all cases, the experimental values are higher than theoretical ones.

On the processing of leaky guided waves propagating in immersed plates

We present a non-destructive inspection method for the structural health monitoring of underwater structures. A laser operating at 532 nm is used to excite leaky guided waves on an aluminum plate immersed in water. The plate has a few artificial defects namely vertical notch, horizontal notch, corrosion, and small hole. An array of five immersion transducers arranged in half-circle is used to detect the propagating waves. A signal processing technique is implemented to assess the presence of damage; the method is based on continuous wavelet transform to extract a few damagesensitive features fed to an artificial neural network for damage classification. The experimental results show that the proposed system can be employed for the inspection of underwater plates.

Inspection of Underwater Metallic Plates by means of Laser Ultrasound

Guided Ultrasonic Waves (GUWs) are increasingly considered in the nondestructive testing of engineering systems. This paper shows a non-contact laser/immersion transducer system for the detection of damage in submerged structures. A pulsed laser was used to generate stress waves in an aluminium plate immersed in water; a pair of immersion transducers was used to detect the signals. The waveforms were processed using wavelets to extract information about the attenuation of the propagating modes. Damage was simulated by devising a notch and a circle on the face of the plate exposed to the probing system.

Bulk Waves for the Nondestructive Inspection of Immersed Structures

Laser-generated ultrasonic guided and bulk waves are increasingly considered for the nondestructive testing (NDT) and structural health monitoring of engineering systems. Methods based on the use of pulsed laser or continuum laser are ideal when a non-contact approach for the generation and detection of stress waves is desired. This paper presents the initial progresses of an ongoing study where pulsed laser is used to generate stress waves in underwater structures. In particular, in this paper we show the results of few experiments where stress ultrasonic waves are generated in an aluminum block. Owing to the geometry of the specimen, ultrasonic bulk waves are generated and detected by means of either a dry or an immersion transducer. The study presented here aims at investigating the effect of certain water parameters on the ultrasonic energy propagating through the specimen. The results of three experiments are presented. In the first experiment the effect of water level on the signal-to-noise ratio of laser generated bulk waves was evaluated. Then, the effect of laser energy was studied. Finally, the effect of water temperature on the amplitude of the bulk waves was investigated. With the exception of the latter experiment, we used laser pulses at 1064 nm and 532 nm wavelengths.