A reliable damage diagnostic by ultrasonic guided wave (GW) based structural health monitoring (SHM) can only be achieved if the physical interactions between wave propagation, the SHM system and environmental factors are fully understood. The purpose of this research was to gain knowledge about the effects of high-amplitude low-frequency structural vibrations (HA-LFV) and audible sound waves (SW) on ultrasonic GW propagation. Measurements were performed on a stiffened panel of a full-scale composite torsion box containing barely visible impact damage. Time-domain analysis of the filtered GW signals revealed that the main effect of HA-LFV was the presence of coherent noise. This was interpreted as the consequence of superposition of multiple dispersive wave groups produced by mode conversion at the moment of reflection on the corrugated panel surfaces during propagation. It was also observed that the coherent noise amplitude depends on the amplitude of the HA-LFV, and on the ratio between the HA-LFV frequency and the ultrasonic excitation frequency. These relationships can potentially be explored for the development of a HA-LFV compensation mechanism to enable in-service GW based damage diagnostics. In contrast, GW signals in the cases with audible SW present were almost unaffected. It was concluded that there is strong evidence supporting the hypothesis that ultrasonic GW propagation with HA-LFV effects can be analysed under the assumption of a permanently corrugated structure.

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The successful utilization of guided wave-based structural health monitoring (SHM) for detailed quantitative diagnostic of damage in composite aircraft primary structures depends on the excitation frequency, geometry, and positioning of the piezoelectric transducers. This study proposes a novel methodology to consistently define those parameters, which is not tuned for a single damage size, does not resort to unrealistic usage of pure guided wave modes, and is applicable to a generic full-scale composite aircraft primary structure. The proposed criteria for designing the piezoelectric transducer network are based on sensor output, coupled electro-mechanical response of the transducer-structure assembly, energy transfer from the bonded piezoelectric transducer to the structure, wavefront coverage of the monitored area, and measurement equipment capabilities. The design methodology was successfully validated by testing the capabilities of the SHM system for the diagnostic of barely visible impact damage of different severities, applied in different locations on a full-scale thermoplastic composite aircraft stiffened panel.@en

The adoption of composites as aircraft primary structural material has created a real need for a new aircraft maintenance philosophy to enable almost real-time structural condition assessment. It is thus crucial to develop structural health monitoring (SHM) systems capable of performing damage diagnostic and remaining useful life prognostic. At the same time, the urge to reduce production costs has led to consistent developments in thermoplastic composite (TpC) technology. In particular, new possibilities have been unlocked for automated assembly processes based on welding. This context constitutes a unique opportunity for integrating research on SHM into the advances of TpC technology, in order to contribute to a combined reduction of production and maintenance costs, and thus to the development of a truly cost-effective composite airframe. Over the last three decades, ultrasonic guided waves (GWs) have been recognised as having a great potential for detailed quantitative diagnostic of damage in composite structures. However, there are still no certified GW based SHM (GW-SHM) applications for civil aircraft. The reason for that is a limited understanding about the interaction between measurement variability factors associated with real operational environments, damage types, materials and geometric complexity. Therefore, the aim of the research presented in this thesis was to accelerate the bridging of those knowledge gaps and thereby to improve the reliability of GW-SHM for composite aircraft. The research presented in this thesis has put forward three possible paths for improving the reliability of GW-SHM systems for composite aircraft. First, to the early detection of manufacturing defects by investigating the relationship between GW propagation and assembly process parameters at structural element scale. Second, to reduce the uncertainty in the damage diagnostic by increasing the systematisation of the GW-SHM system design. Third, to increase the robustness of damage diagnostic capabilities by studying the effects of real operational-environmental variability factors on GW propagation at real scale.@en

This article presents research about the propagation of ultrasonic guided waves in ultrasonically welded thermoplastic composite joints. The goal of the study was to understand the effect of weld manufacturing defects on guided wave transmission across the joint. Triangular energy-directors integrated into the lower composite adherends enabled the production of defective joints in a controlled manner. The produced defect types were unwelded areas and adherend fibre bundle distortion. The reference condition corresponded to the fully welded stage which showed the highest single-lap shear strength. It was possible to detect adherend fibre bundle distortion through the increase in the negative shift of the signal characteristic frequency. Evidence of the presence of unwelded areas was found in the increase of Time-of-Flight of the maximum amplitude Lamb wave group. The sensitivity of the diagnostic parameters was found to be dependent on the ultrasonic guided wave excitation frequency.

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This paper describes the test campaign of an ultrasonic guided wave (GW) based structural health monitoring system (SHM) deployed on a full-scale stiffened panel of a thermoplastic composite horizontal stabilizer torsion box. The diagnostic capabilities of the SHM system were successfully evaluated by testing barely-visible impact damage of different severities, applied in different critical areas of the structure. The test campaign allowed the successful validation of a novel procedure to consistently and reliably design a piezoelectric transducer network. It also allowed the gathering of strong evidence that piezoelectric transducer technology has appropriate durability characteristics for real SHM applications. Finally, it enabled the study of the influence of audible structural vibrations on GW signals.@en

One of the advantages of thermoplastic composites relative to their thermoset counterparts is the possibility of assembling components through welding. Ultrasonic welding in particular is very promising for industrialization. However, uncertainty in the fatigue and fracture behaviour of composites is still an obstacle to the full utilisation of these materials. Health monitoring is then of vital importance, and Lamb wave techniques have been widely recognised as some of the most promising approaches for that end. This paper presents the first experimental study about the influence of welding travel on the transmission of Lamb waves across ultrasonically welded thermoplastic composite joints in single-lap configuration. The main aim of this research is to start to understand how guided waves interact with the internal structure of ultrasonic welds, so that benign, manufacturing-related structural features can be distinguished from damaging ones in signal interpretation. The power transmission coefficient and the correlation coefficient proved to be suitable for analysing the wave propagation phenomena, allowing quantitative identification of small variations of weld-line thickness and intermolecular diffusion at the weld interface. The conclusions are used to develop a tentative damage detection criterion which can later on assist the design of a Lamb wave based structural health monitoring system for thermoplastic composite structures. The Lamb wave test results are backed up by phased-array inspections, which also provide some extra insight on the internal structure of ultrasonic welds.

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Certification is the key step towards the implementation of structural health monitoring (SHM) as part of condition based maintenance programmes of aircraft fleets. That can only be accomplished by demonstration of system performance in multiple scenarios and in a statistically relevant way. This paper describes a fully computational approach for reliability assessment of an ultrasonic guided wave (GW) based SHM system for a full-scale stiffened panel of a horizontal stabilizer torsion-box entirely made of carbon fibre reinforced thermoplastic. A pseudo-stochastic nature is attributed to the damage index values calculated from the finite-element signals, allowing hit-miss data to be generated. That data is in turn employed for the computation of probability of detection curves and relative operating characteristics. The results of this research show that detailed damage diagnostics in full-scale complex composite structures can be achieved by using ultrasonic GWs under the hotspot monitoring philosophy. Moreover, it is demonstrated that properly validated models of the monitoring environment can be used for reliability analysis of GW based SHM systems for full-scale complex composite structures, thereby reducing the need for extensive and costly experimental test campaigns, which in turn can contribute to an acceleration of the establishment of a certification framework for SHM systems.
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Ultrasonic welding is a very promising technique for joining thermoplastic composite (TpC) components in aircraft primary structures [1, 2]. The potential introduction of new lightweight structures in civil aviation has been driving the change towards condition-based maintenance (CBM) as an alternative to the regular inspection interval approach [3]. In turn, CBM has been pushing forward the development of structural health monitoring (SHM) technology capable of reliably assessing the structural integrity of a component during flight or at pre-flight checks. Among all of them, Lamb wave techniques are widely recognised as some of the most promising approaches for SHM of composite structures [4]. Accurate quantitative damage assessment can only be performed if ultrasonic response changes can be unambiguously correlated to specific damages. That correlation requires the distinction of benign features from damaging ones. Therefore, before designing an SHM system for any component it is first necessary to understand the influence of several intrinsic characteristics of the undamaged joints on Lamb wave propagation. In the case of TpC ultrasonically welded (UW) joints, the cross-section is characterised by a very thin (~ 0.05 mm) polymeric weld-line between the laminated adherends, and a region within the two laminate layers adjacent to the weld-line which is affected by partial fusion of the matrix during the welding process [1, 2]. Although this heat-affected zone does not have a separation surface as the weld-line, it is expected to have different elastic properties to the rest of laminate. Despite several computational and experimental investigations about Lamb-wave-based SHM of different types of composite structures, the interaction of Lamb waves with the unique properties of TpCUW joints has not been addressed yet. This paper presents the first experimental study about the influence of weld quality and adherend edge tapering on the propagation of Lamb waves through TpCUW single-lap joints. The main aim of this research is to understand how the guided wave interactions with the internal structure of TpC ultrasonic welds influence the scattering at a single-lap geometry. The conclusions can later be used to develop a model which will assist the design of a Lamb-wave-based SHM system for a TpC structure. The results from this study are also expected to provide some reverse-engineering insight on different weld qualities, ultimately contributing to the improvement of the manufacturing process of TpCUW joints.@en