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This paper describes the implementation of a structural health monitoring (SHM) method for mechanical components and structures in composite materials with a focus on carbon-fiber-overwrapped pressure vessels (COPVs) used in the aerospace industry. Two flex arrays of polyvinylidene fluoride (PVDF) interdigital transducers have been designed, realized, and mounted on the COPV to generate guided Lamb waves (mode A0) for damage assessment. We developed a custom electronic instrument capable of performing two functions using the same transducers: passive-mode detection of impacts and active-mode damage assessment using Lamb waves. The impact detection is based on an accurate evaluation of the time of arrival and was successfully tested with low-velocity impacts (7 and 30 J). Damage detection and progression is based on the calculation of a damage index matrix which compares a set of signals acquired from the transducers with a baseline. This paper also investigates the advantage of tuning the active-mode frequency to obtain the maximum transducer response in the presence of structural variations of the specimen, and therefore, the highest sensitivity to damage.

This paperexplores the application of the RASCAN holographic radar for non-destructive subsurface imaging of works of art and architecture. This radar provides high-resolution plan-view images of the shallow subsurface in dielectric materials.The radar is particularly sensitive to small metallic targets, but also to variations in moisture content. Originally developed for detection of hidden bugging devices, sounding of building construction details, and detection of landmines, here the utility of the RASCAN radar for art and architectural preservation studies is demonstrated by several bench-top experiments on stone andwooditemswith different subsurface defects and features, aswellas actual field tests on a decorative marble medallion in the floor of theTemple of San Biagio in Montepulciano, Italy, and Frescoes in the Church of San Rocco in Cornaredo, Italy. Historical research indicates that the medallion in San Biagio was laid circa1590 during the funeral ceremony of a Prelatio of the family Casata Cervini.The actualburialplace ofthe Prelatioisnot recorded, but a radar scan ofthe medallion, and follow-up scansofa bench-top modelsuggest the possibilityofa cavity that could containremains orrelics.In San Rocco, smalldelaminationswere detected behind the frescos

This review article is focused on the analysis of the state of the art of sensors for guided ultrasonic waves for the detection and localization of impacts for structural health monitoring (SHM). The recent developments in sensor technologies are then reported and discussed through the many references in recent scientific literature. The physical phenomena that are related to impact event and the related main physical quantities are then introduced to discuss their importance in the development of the hardware and software components for SHM systems. An important aspect of the article is the description of the different ultrasonic sensor technologies that are currently present in the literature and what advantages and disadvantages they could bring in relation to the various phenomena investigated. In this context, the analysis of the front-end electronics is deepened, the type of data transmission both in terms of wired and wireless technology and of online and offline signal processing. The integration aspects of sensors for the creation of networks with autonomous nodes with the possibility of powering through energy harvesting devices and the embedded processing capacity is also studied. Finally, the emerging sector of processing techniques using deep learning and artificial intelligence concludes the review by indicating the potential for the detection and autonomous characterization of the impacts.

This paper describes the design of a multifunctional transducer for structural health monitoring (SHM) applications that integrates an interdigital piezoelectric transducer (IDT), used for Lamb Wave generation and reception, a circular piezoelectric sensor, used for the detection and localization of low-velocity impacts, and a resistance temperature detector (RTD) in a single device. The three elements were fabricated on the same metallized piezoelectric polyvinylidene fluoride (PVDF) film using a laser etching process. Characterization of the resulting device proved the advantages of having different transducers on the same device, providing information useful to implement advanced SHM algorithms. Testing of the RTD highlighted some criticalities of the metallized PVDF film that prevented accurate temperature measurements due to spurious behavior under strain.