Dedicated Solutions for Structural Health Monitoring of Aircraft Components

Aircraft structures, like any other mechanical structure, are subjected to various external factors that influence their lifetime. Mechanicalnbsp;and the environment are only some of the factors that can degrade the structure of aircraft components. Monitoring of these degradations by regular inspections or automated data recording is vital for the structural health of the critical components of an aircraft. This research proposes a number of dedicated solutions for structural health monitoring (SHM) of aircraft components helping the detection of structural degradation. The dedicated solutions for SHM presented in this study include the following; 1. Flat coil sensors inspired by eddy current technology, 2. Electrical crack gauges for crack monitoring, 3. Optical fibres used for impact detection on composite materials, 4. SHM using ultrasonic Lamb waves testing with pseudo defects for signal validation as well as 5. Percolation sensors used for the detection of corrosion liquids in confined parts. The testing systems are applied in aircraft components made of aluminium, maraging steel and monolithic and sandwich carbon fibre reinforced polymers (CFRP). The flat coil sensors are glued on aluminium 2024-T3 plates and are able to detect fatigue cracks in critical areas. The critical area is a hole in the middle of the plate which is a stress concentrator. The sensors are based on eddy currents that are induced into the material. Structural discontinuities under the surface of the material alter the electromagnetic properties of the coil showing an impedance change relevant to the crack size. It was found that a 2% increase of the relative difference of impedance corresponds to a crack size of 10 mm within the materials investigated. The electrical crack gauges are glued on aluminium 2024-T3 plates and on an Airbus A320 slat-track. The crack gauges are made of an electrically conductive component and are connected to an ohmmeter. It was found that by exposing the plates to fatigue, simulating flight conditions, the electrical gauges,nbsp;to the loss of their electrical conductivity, can reveal the crack growth. The optical fibres are attached to monolithic and sandwich CFRC of floor panels and in a EC 135 tailboom. Lamb waves generated from a forced impact arrive at the optical fibres leading to an anisotropic refractive index, called birefringence. The pressure levels from the incident elastic waves result in a change of intensity due to a polarization rotation of the light propagating through the optical fibre. It was found that optical fibres offer some advantages compared to conventional piezoelectric transducers and the differentiation of the signal between input and output can reveal information on the impact force and on delaminations presence. The pseudo defects are used for the validation of ultrasonic Lamb waves testing. It is well known that piezoelectric transducers can be used to excite and receive Lamb waves. These elastic waves can propagate through thin plates. Pseudo defects of different size and shape are placed on the plate in various angles and positions and the acoustic response with and without defect is compared and validated for SHM purposes. It wasnbsp;that a correlation of the signals of the real defects and the pseudo defects is feasible and it can lead to a mapping for the probability of detection (POD) of real defects at different positions. The percolation sensors are used in confined spaces in order to detect aqueous liquids or high humidity levels that can cause corrosion. The sensors are made of an organo-ceramic composite that consists of a conductive Titanium Carbonitride (TiCN) powder and Polyvinyl Alcohol (PVA). The concept of this sensor relies on the collapse of the percolation conductivity of the sensor due to swelling from aqueous absorption. This results in a sudden change in the electrical resistance of the conducting part. The sensor was validated concerning its temperature behaviour and its exposure in different levels of humidity with the aid of 10 salt solutions. It was found that the sensors can be used successfully, they were embedded now into three operational airplanes and due to their innovativenbsp;they have been patented. The tests were conducted in the laboratories of the Department of Materials Engineering (MTM) and the Department of Physics and Astronomy (FYS) of KU Leuven, Belgium as well as in the German Aerospace Centre (DLR) in Braunschweig, Germany.nbsp;aircraft components were obtained from ASCO Industries, Belgium and Eurocopter (Airbus Helicopters), France. The research conducted for this PhD thesis was a part of the Aircraft Integrated Structural Health Assessment (AISHA II) European project from the 7th Framework Programme of the European Commission.nrpages: 228status: publishe