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Experimental fatigue tests performed on riveted carbon fiber laminates showed that the specimens, after some thousands of cycles, heated up, reaching around the rivet holes high temperatures. These temperatures if not properly controlled could be detrimental for the joint health. In order to understand the relative importance of all the mechanical and geometric parameters involved in the joint manufacturing, a numerical model of the joined carbon fiber laminates was developed using a commercial finite elements code. A simplified analytical model of heat transfer was then used to simulate the rivets thermal behavior under the assigned operating conditions: this model allowed comparisons with finite elements and can be used for simple thermal simulations. The numerical and analytical results were found in a good agreement with the available experimental results. The present research conclusions can be used for drawing better design rules in the definition of riveted carbon fiber laminates.

Composite materials, in addition to the high specific mechanical properties, have properties enabling their applications in high-pressure, high-temperature (HPHT) and corrosive environment as occurs in deep water. Their use for manufacturing pipelines and offshore risers can provide relevant performance advantages over steel such as lower weight, improved fatigue capacity, corrosion resistance and higher strain limits. However, composite materials are more complex to use in design than metallic materials due to their anisotropic properties and lack of accurate failure prediction models. Thus, a continuous in-situ and in real-time Structural Health Monitoring (SHM) of composite components would be necessary and useful to promote their use in a wider range of operational conditions. In this work, an FRP pipe sample was instrumented with Piezoelectric Wafer Active Sensors (PWAS) used either as passive receivers or as transmitters of guided waves for active health monitoring in pitch-catch configuration. The propagation properties of guided waves in glass fibres reinforced composites were studied by developing MATLAB scripts, running FE simulation and experiments. The numerical and experimental signals were post-processed in MATLAB by Short Time Fourier Transform (STFT) in order to evaluate their frequency and time-frequency content. Furthermore, the guided waves were used to detect artificial defects imposed on the structure identifying their location. Several testcases were studied to find out the limitations and the most suitable conditions of using guided waves for defects monitoring in a composite pipe. The work proposes effective methods for pipe structural health evaluation by non-destructive techniques and ultrasonic guided waves.

The present paper is focused on the analysis of the adhesion integrity of Single Lap Joints (SLJ). The technique adopted for the monitoring is based on the employment of vibration signals generated and received by piezo sensors attached on the specimens surfaces on the two opposite sides of the adhesive. Undamaged joints, when subjected to harmonic excitation, behave linearly exhibiting a spectral response including just the excitation frequency. As soon as the adhesive fails, harmonic excitation induces a rich variety of spectral components in the joints response, including subharmonics and superharmonics. A new approach is presented based on the beating resulting from the superposition of harmonic waves. The interpretation of the spectral content of the joint response is based on an analytical model of the single lap joint with different amounts of debondings associated with a nonlinear behaviour. Local Defect Resonance (LDR) conditions are promoted with the consequent superposition of defect with main structure vibration. Analytical results are compared with experiments and Finite Elements (FE) simulations demonstrating the consistence of the formulation. An efficient method for the Structural Health Monitoring (SHM) of the bonded single lap joints is provided, able to correlate the joint vibrational response to the debonding extension through the analysis of the response spectral content.

In the present paper a method for the Structural Health Monitoring (SHM) of bonded lap joints is presented. This method is based on the interference of elastic waves generated by piezo sensors and travelling along thin bonded plates through the adhesion area. Tone bursts of different extension were generated and, when they encountered the debonded area, the wave speed changed. This affected the wave reflection at the boundary of the disbond and the subsequent interference of the reflected wave with the main wave travelling along the joint. Destructive interference conditions were promoted when the adhesive was partially debonded and this was related to the length of the disbond. In the present study numerical simulations based on finite elements were performed and compared with a simplified analytical model describing the Lamb waves propagation through the joint. The simulation results were subsequently compared with the experimental data and a good agreement was found: the proposed method was simple, straightforward and its application on thin single lap joints provided reliable results.

A novel method based on nonlinear Lamb waves behavior and Local Defect Resonance (LDR) is proposed for locating and evaluating disbonds in Single-Lap Joints (SLJ) typically used in aerospace industry. The presence of damages/defects such as disbonds leads to the presence of sub- and super-harmonics components in the frequency response. The maximum acoustic wave-damage interaction is reached by particular excitation frequencies that enhance the nonlinear response causing LDR. The LDR frequency is experimentally evaluated through the appearance of a single subharmonic component in the frequency spectrum of signals received by piezoelectric transducer (PZT) bonded on the structure. The nonlinear properties of Lamb waves are exploited to make defects generate subharmonic waves at LDR frequency. An algorithm is implemented for damage/defect localization that is accurately obtained by knowing (i) PZTs positions, (ii) Time of Flight (ToF) and (iii) propagation properties of subharmonics packet. Several disbonds with different dimensions are artificially reproduced on an aluminum SLJ: experimental and FE results show good accordance both in usual (single damage) and critical (multi-damage) scenario. The paper proposes a baseline-free method for the disbonds detection, characterization and localization in SLJs that uses the PZT signals without affecting adhesive interface, thus allowing for an active health monitoring.

An innovative method based on nonlinear Lamb waves behavior and Local Defect Resonance (LDR) is proposed for locating ad evaluating disbonds in Single-Lap Joints (SLJ) typically used in aerospace industry. The presence of damages/defects such as disbonds leads to the presence of sub-and super-harmonics components in the frequency response. Particular wave excitation frequencies promote nonlinear response of the structure causing LDR. The LDR frequency was experimentally evaluated through the appearance of a single subharmonic component in the frequency spectrum of signals received by piezoelectric transducers (PZTs) bonded on the structure. The nonlinear properties of Lamb waves were exploited to make defects generate subharmonic wave packets at LDR frequency. An algorithm was developed for damage/defect localization that was accurately performed by knowing (i) PZTs positions, (ii) Time of Flight (ToF) and (iii) propagation properties of subharmonics packet. Several disbonds with different dimensions were artificially reproduced on an aluminum SLJ: experimental results showed good accordance both in usual (single damage) and critical (multi-damage) scenario. The paper proposes a baseline-free method for the disbonds detection and localization in SLJs that uses the PZT signals without affecting adhesive interface, thus allowing an active health monitoring.

Structural Health Monitoring (SHM) gives a diagnosis of a structure assessing the structural integrity and predicting the residual life through appropriate data processing and interpretation. A structure must remain in the design domain, although it can be subjected to normal aging due to usage, action of the environment, and accidental events. SHM involves the integration of electronic devices in the inspected structure that sometimes are Piezoelectric Transducers (PZT). These are lightweight and small and can be produced in different geometries. They are used both in guided wave-based and electromechanical impedance-based methods. The PZT bonding requires essential steps such as preparation of the surfaces, application of the adhesive, and assembly that make the bonding process not so easy to be realised. Furthermore, adhesives are susceptible to environmental degradation. Transducer debonding or non-uniform distributed glue underneath the sensor causes the reduction of the performance and can affect the reliability of the SHM system. In this paper, a sensor diagnostic method for the monitoring of the PZT operational status is proposed in order to detect bonding defect/damage between a PZT patch and a host structure. The authors propose a method based on the nonlinear behaviour of the contact PZT/structure that allows the identification of the damaged PZT and the geometrical characterization of the debonding. The feasibility of the diagnostic procedure is demonstrated by numerical studies and experiments, where disbonds were created by inhibiting the adhesive action on a part of the interface through Teflon film. The proposed method can be used to evaluate the sensor functionality after an extreme loading event or over a long period of service time.

This work aims at presenting techniques for the damage identification in single lap joints (SLJs). The two proposed experimental approaches, exploiting particular interactions of the structure with vibrational waves produced by piezoelectric sensors, allow to perform a Structural Health Monitoring (SHM) without a baseline. The first technique involves the excitation of the structure by means of stationary sinusoidal waves: the presence of a subharmonic in the frequency response spectrum at a receiver point indicates the presence of damage in the joint. In addition, through a simplified analytical model it is possible to relate the frequency of this subharmonic to the size of the damage. The second technique is based on the use of a tone burst: the exciting sensor sends this transient signal that travels through the bonded area and is subsequently read by the receiving sensor; the information received is the result of an interaction between the sent wave and the reflection of the boundaries, sensitive to possible damages. The attenuation of the burst, studied through the wave equations, gives indications on the size of the damage. Both experimental campaigns were carried out on aluminum SLJs bonded with acrylic adhesive, using piezoelectric sensors (one exciting and one receiving). Simplified analytical models were used to validate the experimental results. The good analytical-experimental correlation confirms the validity of the proposed approaches.