Francesco Lanza di Scalea

Performance assessment and validation of piezoelectric active-sensors in structural health monitoring

A sensor diagnostics and validation process that performs in situ monitoring of the operational status of piezoelectric (PZT) active-sensors in structural health monitoring (SHM) applications is presented. Both degradation of the mechanical/electrical properties of a PZT transducer and the bonding defects between a PZT patch and a host structure could be identified by the proposed process. This study also includes the investigation into the effects of the sensor/structure bonding defects on high-frequency SHM techniques, including Lamb wave propagations and impedance methods. It has been found that the effects are significant, modifying the phase and amplitude of propagated waves and changing the measured impedance spectrum. These changes could lead to false indications on the structural conditions without an efficient sensor-diagnostic process. The feasibility of the proposed sensor diagnostics procedure is then demonstrated by analytical studies and experimental examples, where the functionality of the surface-mounted piezoelectric sensors was continuously deteriorated. The proposed process can provide a metric that can be used to determine the sensor functionality over a long period of service time or after an extreme loading event. Further, the proposed method can be useful if one needs to check the operational status of a sensing network right after its installation.

Macro-fiber composite piezoelectric rosettes for acoustic source location in complex structures

An approach based upon the employment of piezoelectric transducer rosettes is proposed for passive damage or impact location in anisotropic or geometrically complex structures. The rosettes are comprised of rectangular macro-fiber composite (MFC) transducers which exhibit a highly directive response to ultrasonic guided waves. The MFC response to flexural (A0) motion is decomposed into axial and transverse sensitivity factors, which allow extraction of the direction of an incoming wave using rosette principles. The wave source location in a plane is then simply determined by intersecting the wave directions detected by two rosettes. The rosette approach is applicable to anisotropic or geometrically complex structures where the conventional time-of-flight source location is challenging due to the direction-dependent wave velocity. The performance of the rosettes for source location is validated through pencil-lead breaks performed on an aluminum plate, an anisotropic CFRP laminate and a complex CFRP-honeycomb sandwich panel.

Temperature effects in ultrasonic Lamb wave structural health monitoring systems

There is a need to better understand the effect of temperature changes on the response of ultrasonic guided-wave pitch-catch systems used for structural health monitoring. A model is proposed to account for all relevant temperature-dependent parameters of a pitch-catch system on an isotropic plate, including the actuator-plate and plate-sensor interactions through shear-lag behavior, the piezoelectric and dielectric permittivity properties of the transducers, and the Lamb wave dispersion properties of the substrate plate. The model is used to predict the S(0) and A(0) response spectra in aluminum plates for the temperature range of -40-+60 degrees C, which accounts for normal aircraft operations. The transducers examined are monolithic PZT-5A [PZT denotes Pb(Zr-Ti)O3] patches and flexible macrofiber composite type P1 patches. The study shows substantial changes in Lamb wave amplitude response caused solely by temperature excursions. It is also shown that, for the transducers considered, the response amplitude changes follow two opposite trends below and above ambient temperature (20 degrees C), respectively. These results can provide a basis for the compensation of temperature effects in guided-wave damage detection systems.

Propagation of ultrasonic guided waves in lap-shear adhesive joints: Case of incident a0 Lamb wave

This paper deals with the propagation of ultrasonic guided waves in adhesively bonded lap-shear joints. The topic is relevant to bond inspection by ultrasonic testing. Specifically, the propagation of the lowest-order, antisymmetric a0 mode through the joint is examined. An important aspect is the mode conversion at the boundaries between the single-plate adherents and the multilayer overlap. The a0 strength of transmission is studied for three different bond states in aluminum joints, namely a fully cured adhesive bond, a poorly cured adhesive bond, and a slip bond. Theoretical predictions indicate that the dispersive behavior of the guided waves in the multilayer overlap is highly dependent on bond state. Experimental tests are conducted in lap-shear joints by a hybrid, broadband laser/air-coupled ultrasonic setup in a through-transmission configuration. The Gabor wavelet transform is employed to extract energy transmission coefficients in the 100 kHz 1.4 MHz range for the three different bond states examined. The cross-sectional mode shapes of the guided waves are shown to have a substantial role in the energy transfer through the joint.

Ultrasonic guided wave monitoring of composite wing skin-to-spar bonded joints in aerospace structures

The monitoring of adhesively bonded joints by ultrasonic guided waves is the general topic of this paper. Specifically, composite-to-composite joints representative of the wing skin-to-spar bonds of unmanned aerial vehicles (UAVs) are examined. This research is the first step towards the development of an on-board structural health monitoring system for UAV wings based on integrated ultrasonic sensors. The study investigates two different lay-ups for the wing skin and two different types of bond defects, namely poorly cured adhesive and disbonded interfaces. The assessment of bond state is based on monitoring the strength of transmission through the joints of selected guided modes. The wave propagation problem is studied numerically by a semi-analytical finite element method that accounts for viscoelastic damping, and experimentally by ultrasonic testing that uses small PZT disks preferably exciting and detecting the single-plate s0 mode. Both the models and the experiments confirm that the ultrasonic ene...

The response of rectangular piezoelectric sensors to Rayleigh and Lamb ultrasonic waves

A fundamental understanding of the response of piezoelectric transducer patches to ultrasonic waves is of increasing interest to the field of structural health monitoring. While analytical solutions exist on the interaction of a piezoelectric actuator with the generated Lamb waves, the behavior of a piezoelectric sensor has only been examined for the limited case of a piezo-actuated Lamb wave in a pitch-catch configuration. This paper focuses on the fundamental response of surface-bonded piezoelectric sensors to ultrasonic waves. The response to both Rayleigh waves and Lamb waves is examined, starting with harmonic excitation fields and moving to broadband and narrowband excitation fields. General oblique incidence of the wave on rectangular sensors is treated first; parallel incidence is then derived as a particular case. The solutions are developed analytically for the harmonic and the narrowband excitations, and semianalytically for the broadband excitation. The results obtained can be used to design u...

Ultrasonic inspection of multi-wire steel strands with the aid of the wavelet transform

The non-destructive detection of structural defects in multi-wire strands used as post-tensioned tendons and cable stays is a challenging, yet critical task. A promising method under investigation is based on the use of ultrasonic stress waves that propagate within the strand and interact with structural discontinuities. The waveguide-like geometry of the strands lends itself to the monitoring of long lengths at a time (long range). The topic of this paper is the enhancement of ultrasonic monitoring of strands by a joint time–frequency analysis based on the discrete wavelet transform (DWT). The test set-up uses magnetostrictive sensors for the excitation and the detection of ultrasonic guided waves in the strands. The main advantage of the DWT is an unmatched de-noising performance. Effective de-noising becomes necessary for the detection of small defects located far away from the inspection probes as desirable in the field. When compared to the traditional signal averaging, the DWT can be used in real time owing to its computational efficiency. The theory of the DWT filter bank decomposition is first revised. The effectiveness of the wavelet processing is then demonstrated for the detection of small notches of varying depths located in the free portion of the strands as well as in the critical anchored areas. The study also shows the importance of selecting the proper mother wavelet function for best performance. The DWT proves effective in eliminating the need for signal averaging and in reducing the power supply required by the monitoring system. Both outcomes make the guided wave inspection method for strands more suitable for field use.

Guided-wave Health Monitoring of Aircraft Composite Panels under Changing Temperature

This study deals with the health monitoring of fiber-reinforced composite panels using ultrasonic guided waves and flexible piezocomposite transducer patches in a changing temperature environment corresponding to normal aircraft operations (—40°C to +60°C). The wave propagation problem is first studied analytically by a model that accounts for temperature effects on the transducer piezo-mechanical properties, the transducer-panel interaction, and the panel wave dispersion properties. Experiments are also conducted on a Carbon-fiber Reinforced Plastic (CFRP) [0/±45/0]S laminate subjected to the —40°C to +60°C temperature excursion. Both model and experiment indicate substantial changes in the detected guided wave amplitude solely due to the temperature excursion. The second part of the study presents an application to bond defect detection in a simulated CFRP skin-to-spar joint of Unmanned Aerial Vehicle wings. It is shown that a statistical outlier analysis based on multiple guided-wave amplitude features and on a baseline partition is effective in detecting bond defects (poorly cured adhesive and two sizes of disbonds) despite the —40°C to +60°C temperature change. The results encourage the development of a continuous health monitoring system for composite aircraft wings during their normal operations.

Defect Classification in Pipes by Neural Networks Using Multiple Guided Ultrasonic Wave Features Extracted After Wavelet Processing

This paper casts pipe inspection by ultrasonic guided waves in a feature extraction and automatic classification framework. The specific defect under investigation is a small notch cut in an ASTM-A53-F steel pipe at depths ranging from 1% to 17% of the pipe cross-sectional area. A semi-analytical finite element method is first used to model wave propagation in the pipe. In the experiment, reflection measurements are taken and six features are extracted from the discrete wavelet decomposition of the raw signals and from the Hilbert and Fourier transforms of the reconstructed signals. A six-dimensional damage index is then constructed, and it is fed to an artificial neural network that classifies the size and the location of the notch. Overall, the wavelet-based multifeature analysis demonstrates good classification performance and robustness against noise and changes in some of the operating parameters.

Feature Extraction for Defect Detection in Strands by Guided Ultrasonic Waves

Multi-wire strands are widely used in civil structures as tensioning members in prestressed concrete, cable-stayed and suspension bridges. In recent years, various researchers including the authors have investigated guided ultrasonic waves as a tool for monitoring the health of the strands. Magnetostrictive ultrasonic transducers have been successfully used to excite and detect the guided waves in these components. This study improves the general guided-wave technique with magnetostrictive transducers for the detection and sizing of defects in strands. The improvement consists of extracting sensitive and robust features of the wave signals, and then using these features to construct a damage index (D.I.). The proposed D.I. is unaffected by accidental variations in the excitation power or by changes in the electromechanical coupling efficiency of the transducers. The specific defect under investigation is a notch cut at varying depths in a strand that is subjected to a typical operational load. Features extracted after discrete wavelet processing of the wave signals result in a D.I. that is robust against noise and is linearly related to the notch depth in a logarithmic scale. In particular, the variance of the signal’s wavelet coefficients results in the largest sensitivity to notch depth.