Non-Destructive Inspection of Impact Damage in Composite Aircraft Panels by Ultrasonic Guided Waves and Statistical Processing.

This paper discusses a non-destructive evaluation (NDE) technique for the detection of damage in composite aircraft structures following high energy wide area blunt impact (HEWABI) from ground service equipment (GSE), such as heavy cargo loaders and other heavy equipment. The test structures typically include skin, co-cured stringers, and C-frames that are bolt-connected onto the skin with shear ties. The inspection exploits the waveguide geometry of these structures by utilizing ultrasonic guided waves and a line scan approach. Both a contact prototype and a non-contact prototype were developed and tested on realistic test panels subjected to impact in the laboratory. The results are presented in terms of receiver operating characteristic curves that show excellent probability of detection with low false alarm rates for defects located in the panel skin and stringers

The role of evanescent modes in Global-Local analysis of UGW in plates with varying local zone-scatterer relations

In order to provide a reliable and robust SHM performance, Ultrasonic Guided Waves (UGWs) need to be analyzed and understood. Numerical modeling of UGW propagation and scattering by hybrid methods offers the possibility of simulating UGW interaction with waveguides of arbitrary cross-sections and discontinuities. Maximizing the accuracy of such methods is important to perform quantitative SHM, while maintaining minimum computational cost. This work investigates the role of evanescent modes in the numerical analysis of UGWs in aluminum and composite plates with defects, by the hybrid Global-Local method. The complex solutions to the UGW eigenvalue problem are found and the scattering spectra for A0 and S0 incident modes are calculated. The accuracy of the numerical solution is then studied by computing the error in terms of energy balance. Parametric studies with respect to the local zone size, defect dimensions and shape are conducted including and excluding evanescent modes in the analysis. Considerations are provided to obtain a solution with error no greater than 5%, in terms of varying local zone – scatterer relations within plate waveguides

Sequelae in adults at 12 months after mild-to-moderate coronavirus disease 2019 (COVID-19).

Severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) infection can cause a wide array of symptoms ranging from mild to severe or fatal forms of coronavirus disease 2019 (COVID-19). Furthermore, it has been observed that in a proportion of patients a variable range of symptoms may persist for a long time. An increasing number of studies have been focused on long COVID, but they have mainly been concentrated on previously hospitalized severe COVID-19 patients reporting symptoms up to 6-months after illness. The main aim of this study was to evaluate the prevalence of COVID-related symptoms 12-months after the onset of mild-to-moderate disease

Elastic and Thermal Wave Propagation based Techniques for Structural Integrity Assessment

Non Destructive Evaluation (NDE) is a fundamental step in several phases of the lifespan of structures, that aims at assessing their health status and ensuring their quality. NDE of aircraft structures, in particular, is a crucial process to ensure passenger safety, industry cost savings and technological advancement. Current visual inspection and lifespan estimation of aircraft are not able to properly assess the health status of structures, especially when damages are present at the interior level and can compromise the integrity of the overall assembly. Composite aircraft, in particular, are subjected to a wide variety of damage that are very difficult to avoid and visually detect. The need for a NDE tool that can help establish the requirement for further inspection following a GSE impact or similar event motivated this research and the development of inspection prototypes. The technique must be able to easily and rapidly inspect the structure, accessing it only from the outside, and detect defects at different levels of the assembly in a statistically reliable manner.Exploiting the physics of propagation of elastic and thermal waves and their interaction with material properties and discontinuities, combined to advanced signal processing, damage detection can be achieved and structural integrity assessment can be performed.In this work, elastic and thermal wave propagation are discussed and investigated in an NDE perspective. Both physics are approached from a first theoretical point of view, their propagation is further observed through simulation analyses and their application is finally implemented in the inspection of representative aerospace composite panels

Elastic and Thermal Wave Propagation based Techniques for Structural Integrity Assessment

Non Destructive Evaluation (NDE) is a fundamental step in several phases of the lifespan of structures, that aims at assessing their health status and ensuring their quality. NDE of aircraft structures, in particular, is a crucial process to ensure passenger safety, industry cost savings and technological advancement. Current visual inspection and lifespan estimation of aircraft are not able to properly assess the health status of structures, especially when damages are present at the interior level and can compromise the integrity of the overall assembly. Composite aircraft, in particular, are subjected to a wide variety of damage that are very difficult to avoid and visually detect. The need for a NDE tool that can help establish the requirement for further inspection following a GSE impact or similar event motivated this research and the development of inspection prototypes. The technique must be able to easily and rapidly inspect the structure, accessing it only from the outside, and detect defects at different levels of the assembly in a statistically reliable manner.Exploiting the physics of propagation of elastic and thermal waves and their interaction with material properties and discontinuities, combined to advanced signal processing, damage detection can be achieved and structural integrity assessment can be performed.In this work, elastic and thermal wave propagation are discussed and investigated in an NDE perspective. Both physics are approached from a first theoretical point of view, their propagation is further observed through simulation analyses and their application is finally implemented in the inspection of representative aerospace composite panels

Improved Global-Local method for ultrasonic guided wave scattering predictions in composite waveguides and defects

As structures increase in complexity, both in the use of advanced materials and high-performing designs such as composite assemblies, their health assessment becomes increasingly challenging. Ultrasonic guided waves (UGWs) have shown to be very promising in the inspection of large (i.e. aerospace components) attenuating (i.e. composite materials) structures and have been successfully employed for damage detection in a variety of fields. The intrinsic complex nature of UGWs, due to their dispersive behavior, combined with the structural complexity of the applications, though, requires improved inspection solutions of higher resolution and accuracy to ensure efficient and safe operations. The numerical simulation of UGW propagation becomes crucial to this end and has been addressed by many researchers with fully numerical, semi-analytical and hybrid approaches. The capability of predicting scattering of the UGWs’ interaction with a variety of damages and structural configurations can inform experimental testing in optimizing the sensitivity of UGW inspections to specific waveguides and defects and in interpreting the acquired data for the identification and quantification of structural health. In this work, an improved computational tool for UGW scattering predictions is presented. The approach relies on the Global-Local method and exploits the efficiency of the semi-analytical finite element (SAFE) method for the “regular” waveguide region and the resolution of full FE discretization for the area of local structural complexity. 2D and 3D applications of the Global-Local approach for UGW scattering predictions in composite structures over a wide range of frequencies will be presented, together with the validation of the method on different waveguides and demonstrations of the improved computational performance. The computational efficiency of the approach promises feasible and reliable UGWs predictions in multi-layered complex assemblies and different damage scenarios, and paves the way to virtual UGWs inspections and future integration in NDE testing

Active vibration control of a composite sandwich plate

The active vibration control of a rectangular sandwich plate by Positive Position Feedback is experimentally investigated. The thin walled structure, consisting of carbon-epoxy outer skins and a Nomex paper honeycomb core, has completely free boundary conditions. A detailed linear and nonlinear characterization of the vibrations of the plate was previously performed by our research group [1, 2]. Four couples of unidirectional Macro Fiber Composite (MFC) piezoelectric patches are used as strain sensors and actuators. The positioning of the patches is led by a finite element modal analysis, in the perspective of a modal control strategy aimed at the lowest four natural frequencies of the structure. Numerical and experimental verifications estimate the resulting influence of the control hardware on the modal characteristics of the plate. Experimental values are also extracted for the control authority of the piezoelectric patches in the chosen configuration. Single Input – Single Output (SISO) and MultiSISO Positive Position Feedback algorithms are tested and the transfer function parameters of the controller are tuned according to the previously known values of modal damping. A totally experimental procedure to determine the participation matrices, necessary for the Multiple-Input and Multiple-Output configuration, is developed. The resulting algorithm proves successful in selectively reducing the vibration amplitude of the first four vibration modes in the case of a broadband disturbance. PPF is therefore used profitably on laminated composite plates in conjunction with strain transducers, for the control of the low frequency range up to 100 Hz. The relevant tuning procedure moreover, proves straightforward, despite the relatively high number of transducers. The rigid body motions which arise in case of free boundary conditions do not affect the operation of the active control

The Global-Local approach for damage detection in composite structures and rails

Structural components with waveguide geometry can be probed using guided elastic waves. Analytical solutions are prohibitive in complex geometries, especially in presence of structural discontinuities or defects. The Global-Local (GL) approach provides the solution by splitting the waveguide in “local” and “global” regions. The “local” region contains the part of the structure responsible for the complex scattering of an incident wave. What happens in this region cannot be reproduced analytically. The “global” region is regular and sufficiently far from the scatterer, in order to exploit known analytical wave propagation solutions. The proposed GL approach discretizes the local region by finite elements, and utilizes the efficient SAFE method in the global region. Kinematic and mechanical constraints ensure the displacements and stresses continuity at the global-local interfaces. The sum of reflected and transmitted waves energy is used to check the before-after scattering energy balance. Numerical results regard the specific cases of a composite skin-to-stringer assembly used in modern aircraft construction and a rail-road track with a common section. The effects of different damage configurations are analyzed in both cases. The results can be useful to select the incident mode-frequency range in order to best identify specific defects in these structures

Global-Local Modeling of Guided-wave Scattering for Quantitative NDE

Among the several NDE and SHM techniques, ultrasonic guided waves are very suitable for the inspection of wide structures and complex geometries. Their behavior and interaction with geometrical or potential defective discontinuities needs to be understood to assist the experimental set-up of NDE tests and to interpret the collected data for quantitative damage detection and structural characterization. The Global- Local method is utilized here to investigate the guided-wave scattering in presence of very complex geometries, involving multi-layered materials and various types of defects. The standard Finite Element (FE) approach discretizes the region with discontinuities, while the Semi-Analytical Finite Element (SAFE) method discretizes the cross-section only of the waveguide geometry and propagates the solution along the wave propagation direction, analytically through eigenvector decomposition. The two problems are coupled at the interacting boundaries in terms of tractions and displacements, to guarantee energy conservation. The scattering coefficients due to an incident wave mode are calculated by least square method and will be used in terms of reflected and transmitted energy and cross-sectional Poynting vectors to understand frequencymode sensitivity to defects. Results will be shown for the skin-to-stringer assembly of composite aircraft structures that are affected by various types of impact damage that are relevant to aircraft safety

Guided Wave Techniques for Damage Detection in Composite Aerospace Structures

Composite materials make up an increasing portion of today’s aerospace structures (see, e.g. Boeing 787 and Airbus 380). These aircrafts’ fuselage, for example, is composed of a laminated composite skin connected to composite stringers and C-frames. Of primary importance is the detection of damage in these built-up structures, whether caused by the manufacturing process or in service (e.g. impacts). A related issue is the characterization of the composite (visco)elastic mechanical properties, that can also be related to the quantification of potential damage. Guided elastic waves propagating in the ~100s kHz regime lend themselves to provide the necessary sensitivity to these two conditions (damage and mechanical properties). This presentation will discuss the use of these waves to provide information on both damage and mechanical properties of composite structures that are typically used in modern commercial aircraft fuselages. In particular, a scanning system using air-coupled ultrasonic transducers and statistical processing will be presented for the detection and the quantification of impact-induced damage in laboratory test panels representative of fuselage construction. A Semi-Analytical Finite Element (SAFE) technique will be presented to identify the layer-by-layer properties of the composite skin laminate by observation of the guided wave dispersive behaviour. Finally, a technique to use SAFE analysis in structures of non-uniform cross-section (able, for example, to model the guided wave propagation from the composite skin into the skin-to-stringer assembly) will be proposed by utilizing concepts of Global-Local modelling that use normal-mode expansion and boundary continuity