We present a structural health monitoring system based on the simultaneous use of passive and active sensing. The passive approach is based on acoustic emission, whereas the active approach uses the electromechanical impedance and the guided ultrasonic wave methods. As all these methods can be deployed with the use of wafer-type piezoelectric transducers bonded or embedded to the structure of interest, this article describes a unified structural health monitoring system where acoustic emission, electromechanical impedance, and guided ultrasonic wave are integrated in the same hardware/software unit. We assess the feasibility of this multimodal monitoring in a large flat aluminum plate instrumented with six transducers. Acoustic emission events are simulated by exciting a tone burst or by using the conventional pencil lead break test, and the detected signals are processed with a source localization algorithm to identify the position of the source. For the active sensing, damage is simulated by adding a small mass to the plate: the raw waveforms are processed with a delay-and-sum algorithm to create an image of the plate, whereas the electrical admittance of each transducer is analyzed using the statistical index of the root-mean-square deviation. The results presented in this article show that the proposed system is robust, mitigates the weaknesses of each method considered individually, and can be developed further to address the challenges associated with the structural health monitoring of complex structures.

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Abstract:We describe a nondestructive evaluation (NDE) method based on the propagation of highly nonlinear solitary waves (HNSWs) to determine the excess of water on the surface of existing concrete structures. HNSWs are induced in a one-dimensional granular chain placed in contact with the concrete to be tested. The chain is part of a built-in transducer designed and assembled to exploit the dynamic interaction between the particles and the concrete. The hypothesis is that the interaction depends on the stiffness of the concrete and influences the time-of-flight of the solitary pulse reflected at the transducer/concrete interface. Two sets of experiments were conducted. In the first set, eighteen concrete cylinders with different water-to-cement (w/c) ratios were cast and tested in order to obtain baseline data to link the ratio to the time of flight. Then, sixteen short beams with fixed w/c ratio, but subject to water in excess at one surface, were cast. The novel NDE method was applied along with the conventional ultrasonic pulse velocity technique in order to determine advantages and limitations of the proposed approach. The results show that the time of flight detected the excess of water in the beams. In the future, the proposed method may be employed in the field to evaluate rapidly and reliably the condition of existing concrete structures and, in particular, concrete decks.

The structural health monitoring/nondestructive evaluation method based on the generation and detection of highly nonlinear solitary waves is emerging as a cost-effective technique to monitor or inspect a variety of structures and materials. These waves possess unique characteristics not seen in conventional ultrasounds. Outlier analysis is a statistic tool able to identify anomalies in data that diverge from a set of baseline data. Although outlier analysis has received considerable attention for defect detection using modal data, guided ultrasonic waves, or other nondestructive approaches, its application for the analysis of solitary waves has never been explored. In the study presented in this article, the use of outlier analysis in terms of discordancy test and Mahalanobis squared distance was investigated to enhance the damage detection capability of a monitoring system based on highly nonlinear solitary waves. Two experiments were performed to demonstrate the procedure. In the first experiment, a thick steel plate was probed with a solitary wave transducer placed above the plate, and damage was simulated in terms of a foreign object magnetically attached to the bottom of the plate, at different distances from the transducer. In the second experiment, two aluminum plates were placed above each other in dry contact with the top plate subjected to localized, mostly hidden, defects. The transducer used in the first experiment was in this second test encased in a small cart with wheels to scan the sample at discrete positions. For both experiments, a few features were extracted from the time waveforms and fed to a univariate and a multivariate analysis that compared the testing data to a set of baseline data. The results show that the outlier analysis significantly improves the ability to detect damage using solitary waves.

This article presents a nondestructive evaluation (NDE) method to infer the neutral temperature and the axial stress in thick beams. The method relies on the propagation of highly nonlinear solitary waves generated at one end of a chain of spherical particles in a dry point contact with the beam to be evaluated. The waves are reflected back to the chain and the research hypothesis is that the axial stress, which influences the beam's stiffness, affects the amplitude and speed of the reflected waves. To verify this hypothesis a general finite element model of thermally stressed beams was developed and coupled to a discrete particle model able to predict the propagation of the waves along an L-shaped granular medium. The models were validated experimentally to quantify the repeatability of the setup, the sensitivity of the wave features on the thermal stress, and the independence of the wave features on the neutral temperature of the beam. The hypothesis was proven valid by both the numerical and the experimental results. In the future, these findings may be used to refine a NDE method to assess stress in columns, to infer the neutral temperature of continuous welded rails, and to prevent thermal buckling of critical structures.