Nicola Testoni

Complete band gaps in a polyvinyl chloride (PVC) phononic plate with cross-like holes: numerical design and experimental verification

In this work the existence of band gaps in a phononic polyvinyl chloride (PVC) plate with a square lattice of cross-like holes is numerically and experimentally investigated. First, a parametric analysis is carried out to find plate thickness and cross-like holes dimensions capable to nucleate complete band gaps. In this analysis the band structures of the unitary cell in the first Brillouin zone are computed by exploiting the Bloch-Floquet theorem. Next, time transient finite element analyses are performed to highlight the shielding effect of a finite dimension phononic region, formed by unitary cells arranged into four concentric square rings, on the propagation of guided waves. Finally, ultrasonic experimental tests in pitch-catch configuration across the phononic region, machined on a PVC plate, are executed and analyzed. Very good agreement between numerical and experimental results are found confirming the existence of the predicted band gaps.

Localization of defects in irregular waveguides by dispersion compensation and pulse compression

In this work a pulse-echo procedure suitable to locate defect-induced reflections in irregular waveguides is proposed. In particular, the procedure extracts the distance of propagation of a guided wave scattered from a defect within the echo signal, revealing thus the source-defect distance. To such purpose, first, a Warped Frequency Transform (WFT) is used to compensate the signal from the dispersion of the guided wave due to the traveled distance in a portion of the waveguide that is assumed as reference. Next, a pulse compression procedure is applied to remove the additional dispersion introduced by the remaining irregular portion of the waveguide. Thanks to this processing the actual distance traveled by the wave in the regular portion of the irregular waveguide is revealed. Thus the proposed strategy extends pulse-echo defect localization procedures based on guided waves to irregular waveguides. Since the processing is based on Fast Fourier Transforms, the algorithm can be easily implemented in real time applications for structural health monitoring. The potential of the procedure is numerically demonstrated by processing Lamb waves propagating in an irregular waveguide composed by aluminum plates with different thicknesses and tapered portions.

Use of Augmented Reality in aircraft maintenance operations

This paper illustrates a Human-Machine Interface based on Augmented Reality (AR) conceived to provide to maintenance operators the results of an impact detection methodology. In particular, the implemented tool dynamically interacts with a head portable visualization device allowing the inspector to see the estimated impact position on the structure. The impact detection methodology combines the signals collected by a network of piezosensors bonded on the structure to be monitored. Then a signal processing algorithm is applied to compensate for dispersion the acquired guided waves. The compensated waveforms yield to a robust estimation of guided waves difference in distance of propagation (DDOP), used to feed hyperbolic algorithms for impact location determination. The output of the impact methodology is passed to an AR visualization technology that is meant to support the inspector during the on-field inspection/diagnosis as well as the maintenance operations. The inspector, in fact, can see interactively in real time the impact data directly on the surface of the structure. Here the proposed approach is tested on the engine cowling of a Cessna 150 general aviation airplane. Preliminary results confirm the feasibility of the method and its exploitability in maintenance practice.

Compressive Sensing with warped frequency models in lamb waves damage detection procedures

Compressive Sensing (CS) has emerged as a potentially viable technique for the efficient acquisition of high-resolution signals that have a sparse representation in a fixed basis. In this work, we have developed a general approach for low rate sampling and efficient CS impulse response recovery algorithms that exploits convolution signal models of dispersive ultrasonic guided waves with a sparse representation in the frequency warped basis. We apply our framework to both to lower the sampling frequency and to enhance defect localization performances of Lamb wave inspection systems. The reconstruction algorithm is based on both the iterative support estimation and alternating minimization algorithm to further improve localization accuracy, separating the contribution of the exciting wave. As a result, an automatic detection procedure to locate defect-induced reflections was demonstrated and successfully tested on experimental Lamb waves propagating in an aluminum plate.

Non-Minimum Phase Iterative Deconvolution of Ultrasound Images

The strongest limitation to ultrasound image quality is due to the blurring effect on the back-scattered echo produced by the echographic transducer response. This effect alters significantly the received echo, reducing the resolution of the echographic image. As under proper general assumptions a convolution-based model can be used to represent the radiofrequency incoming echo signal, fast and robust deconvolution algorithms can be successfully employed to improve image quality by means of attenuating the unwanted transducer effects.

Adaptive wavelet-based signal dejittering

Sampling is commonly retained as a critical step in any mixed-signal system. High-speed analog-to-digital converter sampling jitter limits all-over performance of these systems introducing a signal dependent noise in the sampled signal. In most environments it is desirable to reduce sampling clock jitter, however there are cases where designers are forced to introduce or cope with this undesirable noise effect. This work describes an innovative algorithm based on multiresolution analysis (MRA) which allows for the recovery of the original unjittered sampled signal in environments where clock jitter is unavoidable. We make use of a new versatile signal model and an MSE estimation in the wavelet domain which lead to an adaptive wavelet rescaling technique centered around a fully precalculable rescaling matrix. This technique has been successfully applied to other fields, like extracellular recording (ER) signal denoising, since it can be shown this problem can be reformulated into a signal dejittering problem.

Methodologies for Guided Wave-Based SHM System Implementation on Composite Wing Panels: Results and Perspectives from SARISTU Scenario 5

Within the SARISTU project, the Application Scenario 5 (AS05) was devoted primarily to the development of methodologies based on ultrasonic guided waves for Structural Health Monitoring (SHM) implementation on wing structural elements made of composite materials for detecting BVID or hidden flaws. These methodologies have been mainly developed by the authors of this paper, technologically integrated, and applied on small-scale structural elements within Scenario 5 (unstiffened and stiffened plates) focusing, at the end of the work, also on statistical assessment of damage thresholds levels for each methodology propaedeutic to a probability of detection (POD) evaluation of each approach. The paper will shortly present the methodologies developed and implemented, the main experimental and numerical results in terms of damage detection, and the statistical assessment of threshold damage detection levels. Finally, a short comparison about pros and cons of the methodologies as well as the migration strategy of the methodologies to the Integration Scenario 12 for full-scale wing implementation will be presented.

Air-coupled guided wave detection and wavenumber filtering to full-field representation of delamination in composite plates

Delamination faults in composite plates are considered dangerous as they can cause catastrophic failure before being visually assessed. Effects of delaminations are particularly relevant in guided waves scattering, local resonances and mode conversion. Detecting and analyzing these phenomena is relevant for plate characterization. In this work, leaky guided waves are used to detect delamination in composite plates. To such purpose, a hybrid ultrasonic set-up and a dedicated signal processing are proposed. An air-probe with a proper lift-off is used to detect the leakage in terms of air pressure wave over the plate surface. A piezoelectric transducer is used to generate acoustic guided waves in the composite plate. Multiple acquisitions are averaged to increase the SNR for each position of the air-probe. Curvelet Transform (CT) domain processing of the projection coefficients of the acquired elastic wave is exploited to decompose waves that are overlapped both in the time/space and in the frequency/wavenumber domain. In fact, CT is a special member of the family of multiscale and multidimensional transforms whose spatial and temporal localization is very well suited for processing signals which are sparse in the above mentioned domains. In this work this sparsity is exploited to emphasize the information of leaky guided waves scattered by the delamination by removing from the data the information related to the incident wave field. As an application, the presence of a delamination generated by a 21 Joule impact performed on a 4.9 mm thickness composite laminate was detected contactless by exploiting guided wave leakage.

Improving prostate biopsy protocol with a computer aided detection tool based on semi-supervised learning

Prostate cancer is one of the most frequently diagnosed neoplasy and its presence can only be confirmed by biopsy. Due to the high number of false positives, Computer Aided Detection (CAD) systems can be used to reduce the number of cores requested for an accurate diagnosis. This work proposes a CAD procedure for cancer detection in Ultrasound images based on a learning scheme which exploits a novel semi-supervised learning (SSL) algorithm for reducing data collection effort and avoiding collected data wasting. The ground truth database comprises the RFsignals acquired during biopsies and the corresponding tissue samples histopathological outcome. A comparison to a state-of-art CAD scheme based on supervised learning demonstrates the effectiveness of the proposed SSL procedure at enhancing CAD performance. Experiments on ground truth images from biopsy findings show that the proposed CAD scheme is effective at improving the efficiency of the biopsy protocol.

rtCAB: A tool for reducing unnecessary prostate biopsy cores

With more than 110.000 new cases/year in Europe, prostate cancer (PCa) is one of the most frequent neoplasy. When suspects arise from standard diagnostic methods (i.e. Digital Rectal Exam, Transrectal Ultrasonography (TRUS), PSA level) a prostate biopsy (PBx) is mandatory. As patient discomfort and adverse event probability both grows with core number, it is desirable to reduce the number of PBx cores without negative impinging on diagnose accuracy. The work describes an innovative processing technique called real-time Computer Aided Biopsy (rtCAB) which enhances TRUS video stream with a false color overlay image, and suggests the physician where to sample thus reducing the total number of cores. Our proposal consists in a real-time non-linear classifier which processes the output of an original Maximum Likelihood estimator of Nakagami parameters based on Pade´ Approximant. The resulting algorithm, implemented making full use of CUDA parallel processing capabilities, is capable to deliver frame rates as high as 30 fps. Classification model was trained on a prostate gland adenocarcinoma database (400 PBx cores, 8000 ROIs). Ground truth for each core was established by an expert physician, providing tissue description and illness percentage for each core. The system was tuned for reducing the number of false positives while preserving an acceptable number of false negatives. Comparing to a classical double sextant PBx, the positive prediction value (PPV) of our method is 65% better, with an overall sensitivity of 100%.