Angela Brindisi

Acoustic emission (AE) is a phenomenon that follows the initiation of a crack in structural materials, i.e. after a loading sequence particularly intense. This phenomenon is strictly related to fatigue, intended as the creation of microcracks that, bit by bit, threaten the material and therefore the structural resistance. Since 1950, this crack generation has been demonstrated to be linked to acoustic waves travelling in the material. Therefore, AE shall not be confounded with sound emission. The research aims at detecting this kind of waves directly in the material and not after the very high frequency waves they can produce in the air, with the related losses in terms of travelling energy, eventually too small to be easily detected. The sensors that aim at detecting these waves, are called Acoustic Sensors, and shall not be confused with microphones, aiming at detecting sound pressure, travelling in the air. AE is currently getting new attention from the scientific and technological community for a number of applications that need a prompt identification of the health status of a certain structure. For instance, they allow getting info on the released energy after a crack event, making it possible to extrapolate indications on its residual life and support other structural health monitoring (SHM), systems to identify maps of the existing, current damage easily. Such potentialities are particularly interesting for critical structures, like the hydrogen tanks, recently recognized as a novel component on-board commercial aircraft, following the greening wave that is investing all the means of transport, and even all-day life. In this perspective, the widely used piezoelectric ceramics, excellent for their very large bandwidth, are hard to use for the possibility of spark insurgence after being exposed to strain fields. In this view, optical sensors could be a viable means for overcoming those limitations. Within the H2ELIOS, an EU Clean Aviation Project, CIRA is starting to develop its expertise, with the support of the consortium, to deploy a suitable AE sensor network for SHM purposes. In this activity, preliminary results are shown, mainly dealing with the characterization of typical crack signals by a common piezoelectric sensors network for individuating its main features, in view of replacing it with an optical acoustic system. Different coupons have been tested on a standard mechanical rig. At first, a simple growing damage was imposed, and the strain history was recorded and post-processed in detail, highlighting some peculiar properties of the acquired signal. In a second case, the signals were detected at the event insurgence, irrespective of the magnitude and variability of the quasi-static load therein applied, in the perspective of a full real-time application. A correlation analysis was then performed among the successive acquisitions, and some preliminary conclusions were drafted. Preliminary evidence of the equivalence between optical and piezoelectric systems was explored.

This study aims to develop an integrated monitoring system using a fiber Bragg grating sensor network to record the structural response of a landing gear system under operational loads to detect hard landing conditions on soils with different absorbing characteristics and to differentiate between soil types during landings. This paper refers to drop tests carried out at a drop tower of the test article, an integrated leaf spring landing gear with fiber Bragg grating sensors, measuring strain to evaluate landings from different heights on different soil types: hard soil, sand, and gravel. Cross-correlation and fast Fourier transform analyses can help to assess the repeatability of the impact tests, to assess the developed system as very reliable in detecting landing conditions and ensure very low error in the accuracy of the sensor placement, or to assess whether different impacts under different conditions produce consistent responses.

This study focuses on a system constituted of two piezoelectric transducers installed on a slat representative element, with ice protection purposes. The waves generated by these actuators can cause, in fact, shear actions between the slat panel and the ice accretion, with the final effect of breaking and detaching it. A property of the system is, however, the possibility of regulating the phase between the excitation signals of the two transducers. This capability can be exploited to produce local advantageous wave interference with a consequent amplification of the shear actions. Benefits can be obtained in terms of: (1) reduction of needed power; (2) recovery of signal intensity losses due to distance, geometric, and mechanic discontinuities; (3) recovery of non-optimal functionality due to off-design conditions. The work starts with an overview of the impact of the ice on the aeronautic and other sectors. Then, attention is paid to the systems currently used to protect aircraft, with a specific focus on ultrasounds generated by piezoelectric transducers. The concept proposed in this work is then presented, illustrating the main components and the working modality. On this basis and considering the specific nature of the physical phenomenon, the modeling approach was defined and implemented. At first, the impact of some critical parameters, such as the temperature and the thickness of the ice, was investigated. Then, the impact of the phase delay parameter was considered, estimating the increase of magnitude potentially reachable by means of optimal tuning. Finally, a preliminary experimental campaign was organized and a comparison with the numerical predictions was performed.

The Smart Landing Gear system to be developed in the framework of the ANGELA project provides the strain measurements on landing gear structure at landings, and this system should be maintained efficiently under operational conditions. It is intended to assess the relevance of Fiber Bragg Gratings for in-flight testing. To assess the capabilities of the FBG bonding and to analyze the strain transmission conditions from the host structure to the FBG through the bonding layer during the operational phases of landing gears, a long-period cyclic loading test campaign on the bonding layer itself was performed. The primary objective of this fatigue-like test was to prove the ability of FBG sensors to withstand the operational life-cycle of landing gear while providing the same strain transfer function throughout the entire cycle; the secondary objective was to select the most suitable fiber-coating and bonding agents for this application. This document describes the execution and results of the fatigue-like test, intended as a preparatory test campaign to support the preliminary design activities of the Smart Landing System.

Weight-on-Wheels (WoW) systems are aimed at indicating if the aircraft weight is loading onto the landing gear and its wheels, even partially. These systems are an integral part of the actuation system for safety-critical applications and shall provide reliable information on the actual operational status of the LG. That information reveals if the vehicle is in flight or on the ground. In this way, several kinds of accidents may be prevented, relating for instance, to the incorrect deployment of the landing gear, or even manoeuvres to a certain extent, therefore protecting the aircraft from dangerous damage. There are different architectures that have been proposed in the bibliography, some of them based on strain gauges deployed on the structure, or on proximity sensors installed on the wheels. Being this device and considered critical for safety, it is convenient to couple it with complementary measurements, recorded and processed by different sources. In general, it can be stated that such an intelligent sensor network may be seen as a fundamental support for proper landing gear deployment. The presented paper reports the results of a preliminary investigation performed by the authors to evaluate the possibility of deploying fibre optics on the landing gear structure as part of a WoW system to retrieve the required information. This choice would have a remarkable effect in terms of significant cabling reduction (a single array of sensing elements could be deployed over a single line), and cost abatement from both a manufacturing and operational point of view. There are many other benefits also when referring to an optical instead of a standard electrical sensor system. Due to its small size and ease of integration into different families of materials, it could be considered a system for monitoring the operating status of most actuators on board modern aircraft.

In aeronautics, hard landing is a critical condition as the aircraft approaches the runway with a vertical velocity that exceeds 2 m/s. Beyond that level, the energy that should be then absorbed by the whole structure could cause severe damage to the landing gear and the whole structural system. This document reports on the set-up, execution and results of a preparatory test campaign performed on a small landing gear (LG) demonstrator instrumented with a fibre-optic sensor system. In detail, a leaf spring landing gear was released from a drop tower to detect information about the strain state and the related acceleration history of some specific components during the impact. The objective of the present research is the development of a method for assessing whether hard landing is experienced, and to what extent. Deformation measurements through an integrated Fibre-Bragg Grating (FBG) network allowed retrieving impact velocity by a devoted, original algorithm. The proposed preliminary methodology is the base for assessing a more complex procedure to correlate structural response to the energy entering the structure during the touchdown event.

With the prospective of developing an integrated monitoring system aimed at assessing whether a landing gear has experienced hard impact during the approach, a dedicated method is developed aimed at determining vertical speed by means of strain measurement via FBG strain sensors. Representative impacts on simple structural elements have been reproduced in laboratory, as aluminum slender beams of different lengths were dropped from given heights onto a steel plate base. Contact velocities have been estimated by deformations detection.

We report on the use of Fiber Bragg Grating (FBG) sensors integrated onto an aircraft landing gear for remote and realtime load monitoring. Several FBGs strain sensors, both in a linear and tri-axial configuration, have been integrated on different locations of true landing gears (both Main and Nose gears) based on their load condition derived from FEM numerical analysis and exposed to numerous qualification lab tests where the load applied to the gears was varied in the range 0-20kN. To this aim, the gears were mounted on a 25kN hydraulic press, that changed the shock absorber route from 0 mm up to 200 mm (corresponding to the maximum take-off weight,~4600 kg). Obtained results are in good agreement with those provided by reference electrical strain gauges located very close to their optical counterparts, and demonstrate the great potentialities of FBG sensors technology to be employed for remote and real time load measurements on aircraft landing gears.

The work at hand focuses on an adaptive system aimed at improving the soundproof performance of car door seals at specific regimes (cruise), without interfering with the conventional opening and closing operations. The idea addresses the necessity of increasing seal effectiveness, jeopardized by aerodynamic actions that strengthen as the speed increases, generating a growing pressure difference between the internal and the external field in the direction of opening the door, and then deteriorating the acoustic insulation. An original expansion mechanism driven by a shape memory alloy (SMA) wire was integrated within the seal cavity to reduce that effect. The smart material was activated (heated) by using the Joule effect; its compactness contributed to the realization of a highly-integrable and modular system (expanding cells). In this paper, the system development process is described together with the verification and validation activity, aimed at proving the functionality of the realized device. Starting from industrial requirements, a suitable solution was identified by considering the basic phenomenon principle and the allowable design parameters. The envisaged system was designed and its executive digital mock-up (CAD, computer-aided design) was released. Prototyping and laboratory tests showed the reliability of the developed numerical models and validated the associated predictions. Finally, the system was integrated within the reference car. To demonstrate the insulation effect, the experimental campaign was carried out in an anechoic room, achieving significant results on the concept value.