L. Maio

Guided waves in a stiffened composite laminate with a delamination

The integrity of safety-critical structural composites can be enhanced by the use of innovative ultrasonic nondestructive evaluation techniques. Among the various existing techniques, guided wave methods provide a good promise in terms of sensitivity to a variety of damage types or defects and the extent of the area that can be monitored, given the ability of these waves to travel relatively long distances within the structure under investigation. In comparison with isotropic metallic structures, wave propagation in composite structures presents additional complexity for effective damage identification. The material inhomogeneity, anisotropy, and the multilayered construction of composite materials lead to significant dependence of wave modes on laminate layup configurations, direction of propagation, frequency, and interface conditions. In this article, a specific structure will be analyzed with different levels of complexities in an effort to determine the propagation characteristics of the waves. The investigated structure is a stiffened composite plate of finite thickness and infinite lateral dimensions. The work is carried out using theoretical analysis, numerical modeling, and laboratory experiments. Numerical (finite element) simulations are used for more realistic models, where the geometric and material complexities present practical difficulties in direct analysis using theoretical constructs only.

Model assisted probability of detection for a guided waves based SHM technique

Guided wave (GW) Structural Health Monitoring (SHM) allows to assess the health of aerostructures thanks to the great sensitivity to delamination and/or debondings appearance. Due to the several complexities affecting wave propagation in composites, an efficient GW SHM system requires its effective quantification associated to a rigorous statistical evaluation procedure. Probability of Detection (POD) approach is a commonly accepted measurement method to quantify NDI results and it can be effectively extended to an SHM context. However, it requires a very complex setup arrangement and many coupons. When a rigorous correlation with measurements is adopted, Model Assisted POD (MAPOD) is an efficient alternative to classic methods. This paper is concerned with the identification of small emerging delaminations in composite structural components. An ultrasonic GW tomography focused to impact damage detection in composite plate-like structures recently developed by authors is investigated, getting the bases for a more complex MAPOD analysis. Experimental tests carried out on a typical wing composite structure demonstrated the effectiveness of modeling approach in order to detect damages with the tomographic algorithm. Environmental disturbances, which affect signal waveforms and consequently damage detection, are considered simulating a mathematical noise in the modeling stage. A statistical method is used for an effective making decision procedure. A Damage Index approach is implemented as metric to interpret the signals collected from a distributed sensor network and a subsequent graphic interpolation is carried out to reconstruct the damage appearance. A model validation and first reliability assessment results are provided, in view of performance system quantification and its optimization as well.

Investigation on fundamental modes of guided waves propagating in symmetric and nonsymmetric composite laminates

A quasi-isotropic composite laminate is constructed in an attempt to create a structure that behaves like an isotropic plate. Its membrane behavior is similar to that of the isotropic plate while the bending behavior is quite different from the latter. Moreover, the laminae may or may not be arranged symmetrically with respect to the midplane thereby resulting in a different mechanical response. In this work, guided wave propagation along multiple directions in symmetric and not symmetric quasi-isotropic plates is evaluated. Experimental and numerical results for the fundamental modes A0 and S0 are analyzed for the symmetric and nonsymmetric layups. An eight-node brick type element based on the three-dimensional theory is used in modeling to predict numerically the velocity of wave modes propagating in the graphite/epoxy composite plates. Agreement between experimental and numerical approaches is found and interesting dependencies between velocity of propagating modes and laminate stacking sequence are discussed. A final comparison with analytical dispersion curves obtained by the implementation of the global matrix method is discussed.

Detecting delaminations and disbondings on full-scale wing composite panel by guided waves based SHM system

A full-scale lower wing panel made of composite material has been designed, manufactured and sensorised within the European Funded research project named SARISTU. The authors contributed to the whole development of the system, from design to implementation as well as to the impacts campaign phase where Barely Visible and Visible Damages (BVID and VID) are to be artificially induced on the panel by a pneumatic impact machine. This work summarise part of the experimental results related to damages production, their assessment by C-SCAN as reference NDT method as well as damage detection of delimitations by a guided waves based SHM. The SHM system is made by customized piezoelectric patches secondary bonded on the wing plate acting both as guided waves sources and receivers. The paper will deal mostly with the experimental impact campaign and the signal analyses carried out to extract the metrics more sensitive to damages induced. Image reconstruction of the damages dimensions and shapes will be also described based mostly on the combination of metrics maps over the plate partial surfaces. Finally a comparison of damages maps obtained by the SHM approach and those obtained by “classic” C-SCAN will be presented analyzing briefly pros and cons of the two different approached as a combination to the most effective structural maintenance scenario of a commercial aircraft.

Guided waves based SHM systems: parameters selection for better identification and localisation of damages in composites stiffened plates

Structural Health Monitoring deals mainly with structures instrumented by secondary bonded or embedded sensors that, acting as both signal generators and receivers, are able to “interrogate” the structure about its “health status”. This innovative approach to the damage analysis is particularly promising for reducing the maintenance costs and eventually the weight of aerospace composite structures, without any reduction of the safety level required. These structures are currently designed and employed with significant reduction of the pristine material allowables to account certain failure mechanisms frequently inducing relatively small hidden damages called Barely Visible Damages, consisting among others in delaminations and/or debondings and being detectable only by specific instruments operated by trained personnel. It has been proved that the propagation of guided waves is affected by the presence of such type of damages, but their effective identification and localization depends on the accurate “tuning” of the wave characteristic (frequency, amplitude, velocity, mode) as well as on the proper selection of the best parameter of the specific wave mode selected and data analysis algorithm. The intent of this paper is to summarize the experiences gained by the authors in selecting the most sensitive parameters according to the type of damage to be investigated in several typology composite plate-like structures.

Development of a de-icing system for aerodynamic surfaces based on ultrasonic waves

The paper presents a preliminary study about a de-icing system using ultrasonic waves. The activity has been developed within the project “SMart On-Board Systems” (SMOS), which is part of Italian Aerospace National Research Program, funded by the Italian Ministry of Education and Research and coordinated by CIRA. Conceived for an aircraft wing leading edge, the system shall be extended to other aircraft components, once its efficiency and reliability will be demonstrated. Herein, the results of a preliminary numerical work on a NACA 0012 profile are presented. Guided waves are generated by a piezoelectric transducer bonded on the structure and they cause shear stresses that induce ice delamination and fracture. The investigation is focused on the selection of most suitable excitation frequency for the actuator. Finite element analyses are performed to demonstrate the effectiveness of this approach.

Hybrid guided wave based SHM system for composite structures for impact and delamination detection combining fiber Bragg grating sensing and piezoelectric patches

Structural Health Monitoring deals mainly with structures instrumented by secondary bonded or embedded sensors. Sensors, acting passively or actively as both signal generators and receivers, are able to “listen” to any event happening in the structure (passive SHM) and to “interrogate” the structure to check its “health status” (active SHM). Structures embedded with sensors appear promising for reducing the maintenance costs and the weight of aerospace composite structures, without any reduction of the safety level required. Among many actuators/sensors technologies under investigation for active SHM systems, the combination of piezoelectric patches employed as guided wave exciters or impact sensors and optical fiber Bragg gratings (FBG) as stress wave detectors look promising for their distributed sensing capability as well as weight reduction compromise in a so-called “hybrid structural component”. FBGs have been employed only recently as stress ultrasonic wave sensors due to the reduced number of high-frequency optical interrogators available. One such device, a multi-channel fiber optic acoustic emission (FAESense™) system developed by Redondo Optics, has been employed by the authors for this purpose. Hybrid SHM systems employing FBGs as sensor arrays could provide more distributed data about the local integrity of the structure with less weight addition compared to other sensor types. Typical diameter of fiber optics could allow the embedding of sensor arrays within the composite laminate. Finally, FBGS can provide simultaneously high frequency data characterizing guided wave propagation as well as low frequency local deformations permitting an SHM approach combining global and local impact and damage detection. Intent of this paper is to summarize the first experience gained by the authors in developing SHM systems for composite plate-like hybrid structures for impact detection.

Laser Doppler velocimetry and PZT sensing for the study of guided waves in a stepped aluminum plate

Lamb waves propagating in thin plates and shells are being widely studied for their potential applications in nondestructive inspection of large-scale structures. These structures are generally characterized by the presence of geometrical discontinuities such as stiffeners, mechanical joints or variable thicknesses that affect the propagation characteristics of Lamb waves that can be very similar to those from defects occurring in service (delamination, disbond, etc.). Therefore, the knowledge of the effects of such discontinuities on the propagation of guided waves is essential to avoid their false identification as defects. In this work Lamb waves propagating in a metal plate with a downward step are studied through laboratory experiments. A single 10 mm piezoceramic disk (PZT) bonded to the host structure using cyanoacrylate gage adhesive is utilized for Lamb waves generation and the responses are measured at multiple locations, along a line crossing the step, using a scanning laser Doppler vibrometer (LDV). The interaction of the fundamental Lamb mode A0 with the geometrical discontinuity in the isotropic plate is investigated and discussed.

Numerical simulations and experimental experiences of impact on composite structures

Abstract The composite material and the qualification/certification of composite structures require rigorous analysis validated by the experiments, due to the sensitivity of composites to out-of-plane loads, the multiplicity of composite failure modes, and the lack of standard analytical methods. Because of the need to compare measured properties and performance on a common basis, users and producers of materials use standardized test methods such as those developed by the American Society for Testing and Materials (ASTM) and the International Organization for Standardization (ISO). These standards prescribe the method by which the test specimen should be prepared and tested, as well as how the test results should be analyzed and reported. The chapter contains information about mechanical tests and numerical simulations in general as well as tension, compression, shear, and impact tests in particular; then the composite application about the aerospace structure and the evaluation of the dynamic response of the composite materials is discussed.